Human impact on aquatic environments

WEBVTT
Kind: captions
Language: en

00:00:00.230 --> 00:00:01.200
- [Narrator] When you go to the beach

00:00:01.200 --> 00:00:04.630
and you look at the ocean, it
oftentimes might look fine.

00:00:04.630 --> 00:00:05.910
But as we'll see in this video,

00:00:05.910 --> 00:00:09.210
we, human beings have been
stressing aquatic environments.

00:00:09.210 --> 00:00:12.850
And if we're not careful, we
might completely ruin them.

00:00:12.850 --> 00:00:17.850
For example, this is what a
healthy coral reef looks like.

00:00:18.160 --> 00:00:21.500
And coral are fascinating organisms.

00:00:21.500 --> 00:00:25.050
You can view them as little
animals that are fixed in place

00:00:25.050 --> 00:00:28.280
because they're releasing
this calcium carbonate

00:00:28.280 --> 00:00:30.430
the same things that eggshells are made of

00:00:30.430 --> 00:00:31.680
that they're fixed too

00:00:31.680 --> 00:00:34.550
and then that builds the coral reef.

00:00:34.550 --> 00:00:37.640
But they can exist, they
have their homeostasis

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in a particular temperature range

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and a given amount of various chemicals

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that are in the water.

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Now we know that human beings

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were causing the climate to warm,

00:00:49.300 --> 00:00:52.570
and that's all also causing
ocean temperatures to warm

00:00:52.570 --> 00:00:54.640
and as ocean temperatures warm,

00:00:54.640 --> 00:00:59.320
many of our coral reefs are
not able to be as sustainable.

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For example, this is a
less healthy coral reef.

00:01:03.410 --> 00:01:07.020
Some of the coral is still
alive and seemingly doing okay,

00:01:07.020 --> 00:01:10.790
but in other places, you see
what is known as bleaching,

00:01:10.790 --> 00:01:12.830
where the reef is now white

00:01:12.830 --> 00:01:15.030
and that's because in those situations,

00:01:15.030 --> 00:01:17.380
the coral is essentially dying off.

00:01:17.380 --> 00:01:19.010
Now you might say, well,
this is unfortunate

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because coral reefs are very beautiful

00:01:21.490 --> 00:01:23.760
and now it is less beautiful.

00:01:23.760 --> 00:01:25.550
But what are the other impacts?

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Well, all sorts of organisms and animals

00:01:28.240 --> 00:01:31.000
live in coral reefs,
get their food from it,

00:01:31.000 --> 00:01:32.150
get their shelter from it

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and if the coral reef start to die off,

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then the animals are going to die off.

00:01:35.630 --> 00:01:37.490
On top of that, these coral reefs

00:01:37.490 --> 00:01:40.700
that the coral are essentially
building as they live,

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they prevent erosion on the coastlines.

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So one change that affects one organism

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or one part of an ecosystem

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can have a lot of follow on effects

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on other parts of the ecosystem.

00:01:52.080 --> 00:01:54.130
Another perhaps more obvious way

00:01:54.130 --> 00:01:57.580
that we've been not being
nice to aquatic environments

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is things like oil spills.

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And you've probably seen this on the news

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when you have major oil spills,

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they tend to be pretty disturbing images,

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but this right here is a
bird that is covered in oil.

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And you can imagine when
a bird is covered in oil,

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it's not going to be able to fly,

00:02:12.250 --> 00:02:13.760
it's not going to be able to swim,

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it's not going to be able to have food

00:02:15.450 --> 00:02:18.110
and in a lot of cases,
it is likely to die.

00:02:18.110 --> 00:02:20.350
And just as we described
at the coral reef,

00:02:20.350 --> 00:02:22.030
this doesn't just affect the bird,

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it affects the entire ecosystem,
including human beings.

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And this is just the effect on a bird,

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it has affects on the fish,

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it has effects on the just natural balance

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that occurs in that aquatic environment.

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Now, another idea that
is less talked about

00:02:38.380 --> 00:02:42.240
is this notion of oceanic dead zones.

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And this right over here is
a picture of the gulf coast

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right off the coast of Louisiana,

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I was actually born right around there.

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And what it shows is every
year, this hypoxic zone,

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which is a zone of low oxygen
levels in the water form

00:02:59.210 --> 00:03:01.350
off the coast, and this
shows how bad it is.

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The red areas are the
really bad, very low oxygen.

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Sometimes you might forget,
and you might say, okay,

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for all of us who live in
the land or live on the land

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or in the air, we breathe oxygen,

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but organisms in the
water need oxygen as well.

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Oxygen that has been
dissolved in the water.

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And what's interesting is why this forms,

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it's actually a little
bit counter-intuitive.

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It turns out that chemicals
from human runoff,

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especially fertilizer.

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So you might not realize it,

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but this is the Delta of
the Mississippi river.

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And that has run off farm runoff

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from as far north as Minnesota
and Chicago and sewage

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and that farm runoff and that
fertilizer and that sewage,

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as it comes into the Gulf,

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it actually promotes algae formation.

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And you might say, well, that's good.

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Some more life is growing,
but so much algae gets formed

00:03:48.890 --> 00:03:52.570
and when that algae finally
dies and it gets decomposed,

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the decomposers actually use the oxygen.

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Remember, when you are actually
trying to metabolize things,

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you're using oxygen in order
to extract that energy.

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And so the oxygen in the ocean
in that area gets depleted,

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and then you have a situation

00:04:06.180 --> 00:04:08.460
where almost nothing can live

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in these zones right over here.

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And this is just a sample of the things

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that we are doing to
our aquatic environment.

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And there's other things,

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there's elemental sources of mercury

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that we throw into aquatic environments,

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and it makes the water highly toxic.

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There's obviously other
forms of trash, pollution,

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that we've put into the water

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but this is just to give you a sense

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and to start giving you an appreciation

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about how imbalanced everything is

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and how in one part of
the country say in Chicago

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waste that's going into
the Mississippi River

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can affect aquatic environments,
thousands of miles away.


Dividing complex numbers in polar form

WEBVTT
Kind: captions
Language: en

00:00:00.330 --> 00:00:02.620
- [Narrator] So we are given
these two complex numbers

00:00:02.620 --> 00:00:03.453
and we want to know

00:00:03.453 --> 00:00:07.010
what W sub one divided by W sub two is.

00:00:07.010 --> 00:00:09.760
So pause this video and see
if you can figure that out.

00:00:10.620 --> 00:00:13.760
All right, now let's work
through this together.

00:00:13.760 --> 00:00:15.760
So the form that they've written this in

00:00:15.760 --> 00:00:18.080
it actually makes it
pretty straightforward

00:00:18.080 --> 00:00:20.270
to spot the modulus

00:00:20.270 --> 00:00:23.670
and the argument of each
of these complex numbers.

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The modulus of W sub one

00:00:25.920 --> 00:00:28.780
we can see out here is equal to eight.

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And the argument

00:00:30.180 --> 00:00:31.870
of W sub one

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we can see

00:00:32.703 --> 00:00:34.960
is four Pi over three

00:00:34.960 --> 00:00:36.760
if we're thinking in terms of radians.

00:00:36.760 --> 00:00:41.360
So four Pi over three
radians, and then similarly

00:00:41.360 --> 00:00:43.620
for W sub two

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its modulus

00:00:45.060 --> 00:00:46.570
is equal to

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two

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and its argument

00:00:49.620 --> 00:00:50.810
is equal to

00:00:50.810 --> 00:00:53.163
seven Pi over six.

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Seven

00:00:55.800 --> 00:00:56.633
Pi

00:00:57.590 --> 00:00:59.250
over six.

00:00:59.250 --> 00:01:00.730
Now, in many videos

00:01:00.730 --> 00:01:02.320
we have talked about when you multiply

00:01:02.320 --> 00:01:04.220
one complex number by another

00:01:04.220 --> 00:01:05.780
you're essentially transforming it.

00:01:05.780 --> 00:01:08.680
So you are going to
scale the modulus of one

00:01:08.680 --> 00:01:10.030
by the modulus of the other.

00:01:10.030 --> 00:01:12.500
And you're going to
rotate the argument of one

00:01:12.500 --> 00:01:14.780
by the argument of the
other, I guess you could say

00:01:14.780 --> 00:01:16.930
you're going to add the angles.

00:01:16.930 --> 00:01:18.840
So another way to think about it is

00:01:18.840 --> 00:01:23.840
if you have the modulus of W
sub one divided by W sub two.

00:01:23.840 --> 00:01:26.810
Well then you're just going
to divide these moduli here.

00:01:26.810 --> 00:01:29.550
So this is just going to be eight over two

00:01:29.550 --> 00:01:31.380
which is equal to four.

00:01:31.380 --> 00:01:33.340
And then the argument

00:01:33.340 --> 00:01:34.610
of

00:01:34.610 --> 00:01:35.460
W

00:01:35.460 --> 00:01:36.660
sub one

00:01:36.660 --> 00:01:38.900
over W sub two.

00:01:38.900 --> 00:01:42.360
This is, you could imagine
you're starting at W sub one

00:01:42.360 --> 00:01:46.260
and then you are going
to rotate it clockwise

00:01:46.260 --> 00:01:48.540
by W sub two's argument.

00:01:48.540 --> 00:01:51.330
And so this is going to be four Pi

00:01:51.330 --> 00:01:52.310
over three

00:01:52.310 --> 00:01:53.300
minus

00:01:53.300 --> 00:01:54.970
seven Pi

00:01:54.970 --> 00:01:55.920
over six.

00:01:55.920 --> 00:01:57.230
And let's see what this is going to be.

00:01:57.230 --> 00:02:00.440
If we have a common
denominator four Pi over three

00:02:00.440 --> 00:02:04.010
is the same thing as eight Pi over six

00:02:04.010 --> 00:02:06.080
minus seven Pi

00:02:06.080 --> 00:02:07.260
over six

00:02:07.260 --> 00:02:10.550
which is going to be equal to Pi over six.

00:02:10.550 --> 00:02:12.340
And so we could write this,

00:02:12.340 --> 00:02:13.410
the

00:02:13.410 --> 00:02:14.560
quotient

00:02:14.560 --> 00:02:17.110
W one divided by W two

00:02:17.110 --> 00:02:18.260
is going to be equal to

00:02:18.260 --> 00:02:19.700
if we wanted to write it in this form

00:02:19.700 --> 00:02:22.140
its modulus is equal to four.

00:02:22.140 --> 00:02:23.720
It's going to be four times

00:02:23.720 --> 00:02:24.930
cosine

00:02:24.930 --> 00:02:26.870
of Pi over six

00:02:26.870 --> 00:02:27.890
plus

00:02:27.890 --> 00:02:28.740
i

00:02:28.740 --> 00:02:30.330
times sine

00:02:30.330 --> 00:02:32.090
of Pi

00:02:32.090 --> 00:02:33.960
over six.

00:02:33.960 --> 00:02:36.950
Now cosine of Pi over
six, we can figure out

00:02:36.950 --> 00:02:40.280
Pi over six is the same
thing as a 30 degree angle.

00:02:40.280 --> 00:02:42.270
And so the cosine of that

00:02:42.270 --> 00:02:44.160
is square root of three over two

00:02:44.160 --> 00:02:46.060
square root three over two.

00:02:46.060 --> 00:02:48.930
And the sine of Pi over six

00:02:48.930 --> 00:02:52.120
we know from our 30, 60, 90 triangles

00:02:52.120 --> 00:02:53.720
is going to be one half.

00:02:53.720 --> 00:02:55.380
So this is one half.

00:02:55.380 --> 00:02:56.980
And so if you distribute this four

00:02:56.980 --> 00:02:57.990
this is going to be equal to

00:02:57.990 --> 00:02:59.960
four times square root of three over two

00:02:59.960 --> 00:03:02.290
is two square roots of three

00:03:02.290 --> 00:03:04.520
and then four times one half is two.

00:03:04.520 --> 00:03:05.890
So plus

00:03:05.890 --> 00:03:06.980
two

00:03:06.980 --> 00:03:07.900
i

00:03:07.900 --> 00:03:09.453
and we are done.


Multiplying complex numbers in polar form

WEBVTT
Kind: captions
Language: en

00:00:00.420 --> 00:00:02.480
- [Instructor] We're given
two different complex numbers

00:00:02.480 --> 00:00:05.310
here, and we want to figure
out what is the product?

00:00:05.310 --> 00:00:08.610
Pause this video and see
if you can figure that out.

00:00:08.610 --> 00:00:11.910
All right, now let's
work on this together.

00:00:11.910 --> 00:00:14.550
So we know from the form
that it's written here

00:00:14.550 --> 00:00:18.810
that the modulus of w sub
one is equal to three.

00:00:18.810 --> 00:00:20.620
And we know that the argument

00:00:20.620 --> 00:00:24.683
of w sub one is equal to 330 degrees.

00:00:25.700 --> 00:00:27.830
And by the same line of reasoning, we know

00:00:27.830 --> 00:00:31.610
that the modulus of w
sub two is equal to two.

00:00:31.610 --> 00:00:35.650
And that the argument of w sub two

00:00:35.650 --> 00:00:37.720
is going to be equal to,

00:00:37.720 --> 00:00:42.300
we can see that right
over here, 120 degrees.

00:00:42.300 --> 00:00:44.250
Now, when you multiply complex numbers

00:00:44.250 --> 00:00:46.930
you could view as one
transforming the other.

00:00:46.930 --> 00:00:49.150
We've seen this in multiple examples.

00:00:49.150 --> 00:00:52.600
So let's imagine that we
are transforming w two

00:00:52.600 --> 00:00:55.200
by multiplying it by w one.

00:00:55.200 --> 00:00:56.740
So what is going to happen?

00:00:56.740 --> 00:00:58.200
Well, let me write it here.

00:00:58.200 --> 00:01:03.200
So what's the resulting
modulus of w one times w two?

00:01:03.230 --> 00:01:05.620
Well, we're just going to
scale up w two's modulus

00:01:05.620 --> 00:01:07.410
by w one's modulus.

00:01:07.410 --> 00:01:09.320
Or essentially we're just
going to multiply the two.

00:01:09.320 --> 00:01:12.850
So this is going to be equal
to six, three times two.

00:01:12.850 --> 00:01:17.850
And then the argument of
w sub one times w sub two,

00:01:18.210 --> 00:01:21.800
if we start at w sub two's
argument, 120 degrees

00:01:21.800 --> 00:01:25.120
and then we rotate it
by w sub one's argument,

00:01:25.120 --> 00:01:27.270
well then you're going
to add these two angles,

00:01:27.270 --> 00:01:30.350
that gets you to 450 degrees.

00:01:30.350 --> 00:01:32.380
So this is equal to 450 degrees,

00:01:32.380 --> 00:01:34.320
which is more than a complete rotation.

00:01:34.320 --> 00:01:36.130
And so if we wanted to give it an angle

00:01:36.130 --> 00:01:38.860
between zero and 360 degrees,

00:01:38.860 --> 00:01:41.260
if we just subtract 360 from that,

00:01:41.260 --> 00:01:45.140
that is going to be equal to 90 degrees.

00:01:45.140 --> 00:01:48.060
And so we can rewrite this here,

00:01:48.060 --> 00:01:49.890
or we can rewrite the product

00:01:49.890 --> 00:01:54.200
as w sub one times w sub two is equal

00:01:54.200 --> 00:01:59.200
to its modulus six times cosine
of its argument, 90 degrees.

00:02:01.170 --> 00:02:05.980
Plus i times sine of its argument.

00:02:05.980 --> 00:02:10.210
Now we know what the cosine
and sine of 90 degrees is.

00:02:10.210 --> 00:02:13.830
Cosine of 90 degrees is equal to zero

00:02:13.830 --> 00:02:16.700
and sine of 90 degrees is equal to one.

00:02:16.700 --> 00:02:18.590
So all of the simplifies quite nicely.

00:02:18.590 --> 00:02:21.430
All you're left with is a six times I.

00:02:21.430 --> 00:02:26.123
So this is equal to
six i, and we are done.


Multiplying complex numbers graphically example: -1-i

WEBVTT
Kind: captions
Language: en

00:00:00.150 --> 00:00:02.420
- [Instructor] We are
told, suppose we multiply

00:00:02.420 --> 00:00:06.380
a complex number Z by -1-i.

00:00:07.460 --> 00:00:09.440
So this is Z right over here.

00:00:09.440 --> 00:00:13.740
Which point represents
the product of Z and -1-I?

00:00:16.550 --> 00:00:19.150
Pause this video and see
if you can figure that out.

00:00:20.320 --> 00:00:23.350
All right, now let's work
through this together.

00:00:23.350 --> 00:00:26.490
So the way I think about this is,

00:00:26.490 --> 00:00:30.660
when you multiply by a complex number,

00:00:30.660 --> 00:00:33.680
you are going to rotate by the argument

00:00:33.680 --> 00:00:35.260
of that complex number.

00:00:35.260 --> 00:00:38.640
And you're going to scale the modulus of Z

00:00:38.640 --> 00:00:42.070
by the modulus of this complex number.

00:00:42.070 --> 00:00:44.160
Now, let me just think
about that a little bit.

00:00:44.160 --> 00:00:47.163
So I'm gonna draw another
complex plane here.

00:00:48.240 --> 00:00:50.770
And so this is my real axis,

00:00:50.770 --> 00:00:55.053
this is my imaginary
axis, right over here.

00:00:56.740 --> 00:00:58.790
And -1-I, so that's -1 and then minus 1i.

00:01:03.580 --> 00:01:05.480
So it would go right over there.

00:01:05.480 --> 00:01:07.870
It would be that right over here.

00:01:07.870 --> 00:01:10.390
And so let's think about two things.

00:01:10.390 --> 00:01:12.710
Let's think about what its argument is,

00:01:12.710 --> 00:01:16.090
and let's think about
what it's modulus is.

00:01:16.090 --> 00:01:20.810
So its argument is going to
be this angle right over here.

00:01:20.810 --> 00:01:23.090
And you might already recognize

00:01:23.090 --> 00:01:26.190
that if this has a length of one,

00:01:26.190 --> 00:01:28.340
if this has a length of one,

00:01:28.340 --> 00:01:29.360
or another way of thinking about,

00:01:29.360 --> 00:01:34.360
this has a length of one,
this is a 45, 45, 90 triangle.

00:01:35.290 --> 00:01:37.340
So this is 45 degrees

00:01:37.340 --> 00:01:40.400
but then of course you
have this 180 before that.

00:01:40.400 --> 00:01:45.400
So that's going to be 180 plus
45, is a 225 degree argument.

00:01:47.260 --> 00:01:52.260
So the argument here is going
to be equal to 225 degrees.

00:01:53.740 --> 00:01:55.230
So when you multiply by this,

00:01:55.230 --> 00:01:58.740
you are going to rotate by 225 degrees.

00:01:58.740 --> 00:02:02.380
So let's see this is going
to be rotating by 180 degrees

00:02:02.380 --> 00:02:04.370
and then another 45.

00:02:04.370 --> 00:02:06.467
So if you just rotate it by that,

00:02:06.467 --> 00:02:08.743
you would end up right over here.

00:02:09.890 --> 00:02:12.740
Now we also are going
to scale the modulus.

00:02:12.740 --> 00:02:16.350
And you can see two choices
that scale that modulus.

00:02:16.350 --> 00:02:19.090
And so we know it's going
to be choice A or choice B

00:02:19.090 --> 00:02:22.300
because choices C or D
you'd have to rotate more

00:02:22.300 --> 00:02:23.630
to get over there.

00:02:23.630 --> 00:02:24.960
And so to think about that,

00:02:24.960 --> 00:02:27.270
we have to just think about the modulus

00:02:27.270 --> 00:02:31.040
of -1-i, this point right over here

00:02:31.040 --> 00:02:35.130
and then just scale up this
modulus by that same amount.

00:02:35.130 --> 00:02:36.910
Well, the modulus is just the distance

00:02:36.910 --> 00:02:39.000
from zero in the complex plane.

00:02:39.000 --> 00:02:41.380
So it's going to be this
distance right over here.

00:02:41.380 --> 00:02:43.130
And you could use the Pythagorean theorem

00:02:43.130 --> 00:02:46.370
to know that this squared,
if you call this C,

00:02:46.370 --> 00:02:49.750
C squared is equal to one
squared plus one squared

00:02:49.750 --> 00:02:51.630
or C squared is equal to two

00:02:51.630 --> 00:02:54.540
or C is equal to the square root of two.

00:02:54.540 --> 00:02:56.840
So that's the modulus right over here.

00:02:56.840 --> 00:03:00.670
Modulus is equal to square root of two

00:03:00.670 --> 00:03:03.550
which is approximately, it's
a little bit more than 1.4.

00:03:03.550 --> 00:03:05.550
So let's just call it approximately 1.4.

00:03:06.950 --> 00:03:10.720
So not only going to
rotate by 225 degrees,

00:03:10.720 --> 00:03:12.640
we're going to scale the modulus,

00:03:12.640 --> 00:03:14.440
the distance from the origin by 1.4.

00:03:15.347 --> 00:03:18.630
So it looks like it's three units

00:03:18.630 --> 00:03:20.550
from the origin right over here.

00:03:20.550 --> 00:03:22.480
If you multiply that by 1.4,

00:03:22.480 --> 00:03:27.120
three times 1.4 is about
four, or it is exactly 4.2.

00:03:27.120 --> 00:03:31.030
So 4.2 of these units is
one, two, three, four,

00:03:31.030 --> 00:03:34.000
a little bit further,
you get right over here

00:03:34.000 --> 00:03:36.273
to choice B and we're done.


Multiplying complex numbers graphically example: -3i

WEBVTT
Kind: captions
Language: en

00:00:00.000 --> 00:00:02.460
- [Instructor] Suppose we
multiply a complex number Z

00:00:02.460 --> 00:00:04.130
by negative three i,

00:00:04.130 --> 00:00:06.600
and there shows Z right over here

00:00:06.600 --> 00:00:09.540
Plot the point that
represents the product of Z

00:00:09.540 --> 00:00:11.563
and negative three i.

00:00:11.563 --> 00:00:15.308
I so pause this video and see
if you can work through that.

00:00:15.308 --> 00:00:17.800
All right, now let's do it step-by-step.

00:00:17.800 --> 00:00:20.290
First I wanna think about what would,

00:00:20.290 --> 00:00:22.123
where would three Z be?

00:00:23.340 --> 00:00:28.050
Well, three Z would have
the same angle as Z,

00:00:28.050 --> 00:00:30.010
but it's absolute value

00:00:30.010 --> 00:00:32.920
or it's modulus would
be three times larger.

00:00:32.920 --> 00:00:34.440
So you'd be going in this direction

00:00:34.440 --> 00:00:35.610
but it'd be three times further.

00:00:35.610 --> 00:00:37.860
So that's one times it's modulus.

00:00:37.860 --> 00:00:38.802
That's two times it's modulus.

00:00:38.802 --> 00:00:40.580
That's three times it's modulus

00:00:40.580 --> 00:00:42.800
or it's three times it's absolute value.

00:00:42.800 --> 00:00:45.820
So three Z would be right over here.

00:00:45.820 --> 00:00:50.060
Now what about negative three Z?

00:00:50.060 --> 00:00:51.917
Well, if you multiply it by a negative,

00:00:51.917 --> 00:00:53.385
it's just going to flip it around.

00:00:53.385 --> 00:00:56.920
You can think about it as
flipping it at 180 degrees

00:00:56.920 --> 00:00:59.730
but it's going to have the same modulus.

00:00:59.730 --> 00:01:01.400
So instead of being right over here,

00:01:01.400 --> 00:01:02.540
at three in this direction,

00:01:02.540 --> 00:01:05.190
it's going to be one, two,
three in this direction,

00:01:05.190 --> 00:01:06.023
right over here.

00:01:06.023 --> 00:01:08.860
So that is negative three Z.

00:01:08.860 --> 00:01:11.100
And now perhaps most interestingly,

00:01:11.100 --> 00:01:13.830
what happens when you multiply it by i?

00:01:13.830 --> 00:01:17.940
So if we have negative three i times Z,

00:01:17.940 --> 00:01:21.010
now which is exactly what
they want us to figure out.

00:01:21.010 --> 00:01:23.000
Well, let's think about
what happens if you had one

00:01:23.000 --> 00:01:24.830
and if you multiply that by i.

00:01:24.830 --> 00:01:26.900
So one times i becomes one i.

00:01:26.900 --> 00:01:28.930
So it goes over there.

00:01:28.930 --> 00:01:32.060
What if you then took one
i and multiplied it by i?

00:01:32.060 --> 00:01:33.750
Well, then you have negative one.

00:01:33.750 --> 00:01:35.625
What if you took negative one
and you multiplied it by i?

00:01:35.625 --> 00:01:40.360
Well, then now you have negative one i.

00:01:40.360 --> 00:01:42.790
So notice every time we multiply by i,

00:01:42.790 --> 00:01:45.550
we are rotating by 90 degrees.

00:01:45.550 --> 00:01:47.669
So over here, if we take negative three Z

00:01:47.669 --> 00:01:52.390
and multiply it by i, you're
just going to rotate 90 degrees

00:01:52.390 --> 00:01:55.040
and you're going to get right over there.

00:01:55.040 --> 00:01:58.930
So this is negative three i times Z,

00:01:58.930 --> 00:02:01.373
which is exactly what we were looking for.


Converting a complex number from polar to rectangular form

WEBVTT
Kind: captions
Language: en

00:00:00.200 --> 00:00:03.840
- [Instructor] We are told,
consider the complex number Z,

00:00:03.840 --> 00:00:06.900
which is equal to the square root of 17

00:00:06.900 --> 00:00:11.900
times cosine of 346 degrees
plus I sine of 346 degrees.

00:00:13.530 --> 00:00:17.290
And they ask us to plot Z
in the complex plane below.

00:00:17.290 --> 00:00:20.520
If necessary round the points coordinates

00:00:20.520 --> 00:00:22.290
to the nearest integer.

00:00:22.290 --> 00:00:23.830
So, I encourage you to pause this video

00:00:23.830 --> 00:00:24.663
and at least think about

00:00:24.663 --> 00:00:29.130
where we would likely
plot this complex number.

00:00:29.130 --> 00:00:29.963
All right.

00:00:29.963 --> 00:00:31.410
Now let's work through it together.

00:00:31.410 --> 00:00:32.900
So when you look at it like this,

00:00:32.900 --> 00:00:34.910
you can see that what's being attempted

00:00:34.910 --> 00:00:39.910
is a conversion from polar
form to rectangular form.

00:00:40.240 --> 00:00:42.820
And if we're thinking about polar form,

00:00:42.820 --> 00:00:47.100
we can think about the angle
of this complex number,

00:00:47.100 --> 00:00:49.840
which is clearly 346 degrees.

00:00:49.840 --> 00:00:51.870
And so, 346 degrees

00:00:52.850 --> 00:00:57.180
is about 14 degrees
short of a full circle.

00:00:57.180 --> 00:01:01.880
So, it would get us probably
something around there.

00:01:01.880 --> 00:01:06.580
And then we also see what the magnitude

00:01:06.580 --> 00:01:09.500
or the modulus of the complex
number is right over here.

00:01:09.500 --> 00:01:11.000
Square root of 17.

00:01:11.000 --> 00:01:14.470
Square root of 17 is a
little bit more than four

00:01:14.470 --> 00:01:16.250
'cause four squared is 16.

00:01:16.250 --> 00:01:17.630
So if we go in this direction,

00:01:17.630 --> 00:01:22.043
let's see, that's gonna be
about one, two, three, four.

00:01:23.060 --> 00:01:24.630
We're gonna go right about there.

00:01:24.630 --> 00:01:26.880
So, if I were to just guess

00:01:26.880 --> 00:01:28.580
where this is going to put us,

00:01:28.580 --> 00:01:30.920
it's going to put us right around here,

00:01:30.920 --> 00:01:34.450
right around four minus one I.

00:01:34.450 --> 00:01:36.040
But let's actually (indistinct)
get a calculator out

00:01:36.040 --> 00:01:40.830
and see if this evaluates
to roughly four minus one I.

00:01:40.830 --> 00:01:45.830
So for the real part,
let's go 346 degrees.

00:01:46.250 --> 00:01:49.200
And we're gonna take the cosine of it.

00:01:49.200 --> 00:01:50.650
And then we're gonna multiply that

00:01:50.650 --> 00:01:52.940
times the square root of 17.

00:01:52.940 --> 00:01:57.940
So times 17 square root,
a little over four,

00:01:58.020 --> 00:01:59.340
which is equal to that.

00:01:59.340 --> 00:02:00.173
Actually, yes.

00:02:00.173 --> 00:02:02.390
The real part does look
almost exactly four.

00:02:02.390 --> 00:02:05.050
Especially, if we are rounding
to the nearest integer.

00:02:05.050 --> 00:02:06.620
It's a little bit more than four.

00:02:06.620 --> 00:02:09.200
And now let's do the imaginary part.

00:02:09.200 --> 00:02:13.100
So we have 346 degrees.

00:02:13.100 --> 00:02:15.400
And we're gonna take the sine of it.

00:02:15.400 --> 00:02:18.800
And we're going to multiply
that times the square root of 17

00:02:18.800 --> 00:02:23.800
times 17 square root,
which is equal to, yep.

00:02:24.090 --> 00:02:25.840
If we were round to the nearest integer,

00:02:25.840 --> 00:02:27.960
it's about negative one.

00:02:27.960 --> 00:02:30.820
So, we get to this point right over here,

00:02:30.820 --> 00:02:35.820
which is approximately four minus I.

00:02:36.330 --> 00:02:37.963
And we are done.


Energy Conservation

WEBVTT
Kind: captions
Language: en

00:00:00.290 --> 00:00:01.123
- [Narrator] In this video,

00:00:01.123 --> 00:00:04.360
we're going to talk
about energy conservation

00:00:04.360 --> 00:00:09.310
or trying to save or lower the
amount of energy that we use.

00:00:09.310 --> 00:00:11.350
Now, a lot of y'all might
already have a sense

00:00:11.350 --> 00:00:12.320
that that is a good thing

00:00:12.320 --> 00:00:13.950
while others of you might say,

00:00:13.950 --> 00:00:17.710
hey why can't I just use
as much energy as possible?

00:00:17.710 --> 00:00:20.060
Why should I try to use less energy?

00:00:20.060 --> 00:00:22.690
And there's several
answers to that question.

00:00:22.690 --> 00:00:26.410
The number one reason is
energy has a huge impact

00:00:26.410 --> 00:00:27.730
on the environment.

00:00:27.730 --> 00:00:29.510
Most of the energy we produce.

00:00:29.510 --> 00:00:31.270
And we'll talk about that in a little bit

00:00:31.270 --> 00:00:35.270
has the by-product of
emitting greenhouse gases

00:00:35.270 --> 00:00:38.270
which are contributing to global warming.

00:00:38.270 --> 00:00:41.300
And then on top of that, it
has implications to society

00:00:41.300 --> 00:00:43.140
the infrastructure around us.

00:00:43.140 --> 00:00:45.380
And it has frankly impacts on your

00:00:45.380 --> 00:00:48.760
and your family's pocketbooks
because energy costs money.

00:00:48.760 --> 00:00:51.830
So let's first think about the household.

00:00:51.830 --> 00:00:53.600
So this chart right over here,

00:00:53.600 --> 00:00:57.600
which is from the U.S. Energy
Information Administration

00:00:57.600 --> 00:01:02.230
says residential site electricity
consumption by end use.

00:01:02.230 --> 00:01:03.780
And I want to stress that it says

00:01:03.780 --> 00:01:06.710
electricity consumption
because electricity consumption

00:01:06.710 --> 00:01:10.870
is not the only energy
consumption in a household.

00:01:10.870 --> 00:01:12.880
Depending on where you
are it might be around

00:01:12.880 --> 00:01:15.330
50 or 60% of consumption

00:01:15.330 --> 00:01:18.190
but natural gas is another major source.

00:01:18.190 --> 00:01:20.400
And in some cases even petroleum.

00:01:20.400 --> 00:01:21.620
But when you look over here,

00:01:21.620 --> 00:01:26.220
the major uses of energy in a
household are air conditioning

00:01:26.220 --> 00:01:27.980
and that's going to be
especially pronounced

00:01:27.980 --> 00:01:31.290
if you live in a hot and
humid part of the country

00:01:31.290 --> 00:01:32.650
or part of the world.

00:01:32.650 --> 00:01:33.730
You have space heating

00:01:33.730 --> 00:01:35.970
which is what most of us
associate with heaters.

00:01:35.970 --> 00:01:37.460
And then you have water heating.

00:01:37.460 --> 00:01:39.200
Many of y'all probably don't appreciate

00:01:39.200 --> 00:01:42.080
when you take those long hot showers

00:01:42.080 --> 00:01:44.810
that it took energy to warm up that water.

00:01:44.810 --> 00:01:45.880
And then you have lighting.

00:01:45.880 --> 00:01:48.300
You have appliances after that.

00:01:48.300 --> 00:01:51.430
So as you as an individual
wanted to conserve energy

00:01:51.430 --> 00:01:55.920
it makes a lot of sense to
look at things like this.

00:01:55.920 --> 00:01:58.210
And so if you wanna
conserve energy at home

00:01:58.210 --> 00:02:01.070
use less air conditioning if you can.

00:02:01.070 --> 00:02:03.560
Use less heating if you can.

00:02:03.560 --> 00:02:06.720
Take shorter showers or
maybe not as hot showers.

00:02:06.720 --> 00:02:09.550
And showers not only have
the energy consumption

00:02:09.550 --> 00:02:10.970
from heating the water

00:02:10.970 --> 00:02:12.990
but it also has the energy
consumption that's happening

00:02:12.990 --> 00:02:15.740
at the water treatment
plant to clean your water

00:02:15.740 --> 00:02:18.150
and to process your water
that also takes energy.

00:02:18.150 --> 00:02:20.480
And right over here is a refrigerator.

00:02:20.480 --> 00:02:22.680
And if you have an older refrigerator

00:02:22.680 --> 00:02:24.600
or a less efficient refrigerator

00:02:24.600 --> 00:02:27.410
that's going to use a lot more
energy to do the same work.

00:02:27.410 --> 00:02:29.880
And once again, it's not
just impact on the climate.

00:02:29.880 --> 00:02:32.140
It's going to save you
and your family money

00:02:32.140 --> 00:02:34.460
by using less energy.

00:02:34.460 --> 00:02:35.780
But as we'll see,

00:02:35.780 --> 00:02:38.066
energy consumption is
not just a phenomenon

00:02:38.066 --> 00:02:40.270
inside of the house.

00:02:40.270 --> 00:02:43.360
What we see here is
U.S. energy consumption

00:02:43.360 --> 00:02:46.570
by source and sector in 2020.

00:02:46.570 --> 00:02:49.100
And this is a little
bit of a complex diagram

00:02:49.100 --> 00:02:51.690
but it's telling us a lot of information

00:02:51.690 --> 00:02:55.040
Here on the left it tells
us our sources of energy.

00:02:55.040 --> 00:02:59.100
So 35% of the energy in the United States

00:02:59.100 --> 00:03:00.720
comes from petroleum.

00:03:00.720 --> 00:03:03.800
Then 34% from natural gas.

00:03:03.800 --> 00:03:05.720
Then 12% from renewable energy.

00:03:05.720 --> 00:03:08.780
That'd be things like
wind power or solar power.

00:03:08.780 --> 00:03:13.130
Then you have 10% from
coal and 9% from nuclear.

00:03:13.130 --> 00:03:14.880
And then not only does this tell us where

00:03:14.880 --> 00:03:16.340
the energy is coming from

00:03:16.340 --> 00:03:18.800
it's telling us how it is being used.

00:03:18.800 --> 00:03:21.670
So we could see 36% in
the industrial sector.

00:03:21.670 --> 00:03:23.180
So that's all of the factories

00:03:23.180 --> 00:03:25.550
the manufacturing that
produces all of the goods

00:03:25.550 --> 00:03:28.910
and services and raw materials
that we have in society.

00:03:28.910 --> 00:03:32.100
35% of our energy in the
United States is used

00:03:32.100 --> 00:03:34.350
for transportation, moving things around.

00:03:34.350 --> 00:03:37.770
Moving ourselves around, but
also moving stuff around.

00:03:37.770 --> 00:03:40.410
And then 17% is residential.

00:03:40.410 --> 00:03:41.740
12% is commercial.

00:03:41.740 --> 00:03:43.420
So this would be things like

00:03:43.420 --> 00:03:45.570
the energy that the
shopping mall all is using

00:03:45.570 --> 00:03:48.630
or the energy that's being
used in an office building.

00:03:48.630 --> 00:03:51.370
This little gray and black
box down here is interesting

00:03:51.370 --> 00:03:54.520
because it shows the role of electricity

00:03:54.520 --> 00:03:56.180
in this whole scheme.

00:03:56.180 --> 00:03:58.050
Sometimes something like petroleum

00:03:58.050 --> 00:04:02.490
might be directly used by
say an industrial user,

00:04:02.490 --> 00:04:04.529
but sometimes that petroleum is then used

00:04:04.529 --> 00:04:07.350
for electricity generation

00:04:07.350 --> 00:04:09.550
which can then be used
by these various sectors.

00:04:09.550 --> 00:04:13.570
Similarly, something like
coal could be used directly

00:04:13.570 --> 00:04:16.360
or it could be used to
produce electricity.

00:04:16.360 --> 00:04:18.770
Now one of the eye-popping
things that I didn't appreciate

00:04:18.770 --> 00:04:20.550
until I saw this diagram

00:04:20.550 --> 00:04:23.740
are the electrical system energy losses,

00:04:23.740 --> 00:04:27.090
roughly 65% of the energy is lost.

00:04:27.090 --> 00:04:29.440
If you have a more
efficient electrical grid

00:04:29.440 --> 00:04:31.550
or we know when to
produce the electricity.

00:04:31.550 --> 00:04:34.030
So it more matches up with the demand.

00:04:34.030 --> 00:04:37.350
Then we can once again
conserve energy as a society.

00:04:37.350 --> 00:04:39.030
And so that's why it's
important to realize

00:04:39.030 --> 00:04:42.010
some people think, hey,
if I'm using electricity

00:04:42.010 --> 00:04:43.860
or if I'm using an electric car

00:04:43.860 --> 00:04:46.770
that maybe has less
impact on the environment.

00:04:46.770 --> 00:04:49.170
Well it depends where that
electricity is coming from.

00:04:49.170 --> 00:04:51.400
An electric car actually
gives us the option

00:04:51.400 --> 00:04:53.660
of not necessarily using petroleum.

00:04:53.660 --> 00:04:56.304
It gives us the option of
potentially using renewable energy

00:04:56.304 --> 00:04:58.100
or nuclear energy.

00:04:58.100 --> 00:04:59.391
But that electricity could be coming from

00:04:59.391 --> 00:05:02.810
things that significantly
impact the environment.

00:05:02.810 --> 00:05:04.490
But when you generally look at this,

00:05:04.490 --> 00:05:06.560
it tells us that as a society

00:05:06.560 --> 00:05:09.020
we have to think about things like

00:05:09.020 --> 00:05:10.912
investing in public transportation

00:05:10.912 --> 00:05:13.600
so that we conserve energy there.

00:05:13.600 --> 00:05:15.690
This right over here is a cement plant.

00:05:15.690 --> 00:05:17.470
Are there ways to produce these things

00:05:17.470 --> 00:05:20.190
that are more efficient
that use less energy.

00:05:20.190 --> 00:05:22.750
And then there's regulations that might

00:05:22.750 --> 00:05:25.100
motivate us as individuals to

00:05:25.100 --> 00:05:27.970
say carpool or drive electric cars

00:05:27.970 --> 00:05:30.820
which once again, aren't
necessarily going to be clean.

00:05:30.820 --> 00:05:33.000
It depends where that
electricity comes from.

00:05:33.000 --> 00:05:36.740
But it gives us the option
of using renewable sources.

00:05:36.740 --> 00:05:37.840
So I'll leave you there.

00:05:37.840 --> 00:05:40.480
Energy conservation is a complex topic

00:05:40.480 --> 00:05:42.420
but a very very very important one.

00:05:42.420 --> 00:05:43.938
But hopefully this gives you a start

00:05:43.938 --> 00:05:47.460
on how you can look at how
energy is being used in the world

00:05:47.460 --> 00:05:50.110
where it comes from and
how you can make a change

00:05:50.110 --> 00:05:53.030
both at the personal
level, right over here.

00:05:53.030 --> 00:05:56.670
And as a member of our
democracy at the societal level


Reduction of Air Pollutants

WEBVTT
Kind: captions
Language: en

00:00:00.820 --> 00:00:01.950
- [Instructor] Hey there, friends.

00:00:01.950 --> 00:00:03.960
Today, we're gonna learn
about air pollution.

00:00:03.960 --> 00:00:06.340
And to start off, we're going back in time

00:00:06.340 --> 00:00:08.990
to the small town of Donora, Pennsylvania,

00:00:08.990 --> 00:00:11.447
in October of 1948.

00:00:11.447 --> 00:00:13.930
(pensive harp music)

00:00:13.930 --> 00:00:16.480
Walking into this small industrial town,

00:00:16.480 --> 00:00:19.800
you can immediately sense
that something is wrong.

00:00:19.800 --> 00:00:20.900
It's the middle of the day,

00:00:20.900 --> 00:00:24.130
but there's a thick
yellowish smog everywhere,

00:00:24.130 --> 00:00:27.350
enveloping everything and
even blocking out the sun.

00:00:27.350 --> 00:00:31.400
It's so dark that streetlights
are on during the daytime.

00:00:31.400 --> 00:00:35.820
It stings your eyes and it's
hard, even painful to breathe.

00:00:35.820 --> 00:00:38.890
What we're experiencing
is the Donora Death Fog,

00:00:38.890 --> 00:00:41.100
one of the worst air pollution disasters

00:00:41.100 --> 00:00:42.860
in the United States.

00:00:42.860 --> 00:00:47.300
Donora was an industrial town
full of steel plants and mills

00:00:47.300 --> 00:00:49.000
which released toxic emissions,

00:00:49.000 --> 00:00:52.010
such as hydrogen fluoride
and sulfur dioxide

00:00:52.010 --> 00:00:54.850
when processing steel and other metals.

00:00:54.850 --> 00:00:56.700
Normally, these poisonous gases

00:00:56.700 --> 00:00:58.960
would disperse into the atmosphere.

00:00:58.960 --> 00:01:02.250
But this time, there was
a temperature inversion,

00:01:02.250 --> 00:01:03.940
which caused a blanket of warm air

00:01:03.940 --> 00:01:06.700
to cover a layer of colder
air near the surface

00:01:06.700 --> 00:01:08.453
and ride over Donora.

00:01:09.630 --> 00:01:11.400
Consequently, the toxic emissions

00:01:11.400 --> 00:01:14.400
were essentially trapped
under the warm air.

00:01:14.400 --> 00:01:15.870
Over the course of several days

00:01:15.870 --> 00:01:19.360
from October 26th to October 31st,

00:01:19.360 --> 00:01:22.210
these toxic emissions
had accumulated so much

00:01:22.210 --> 00:01:25.760
that half of the 14,000
people living in Donora

00:01:25.760 --> 00:01:29.490
suffered from respiratory
problems and 20 people died.

00:01:29.490 --> 00:01:32.320
Relief only came when the
steel mills were shut down

00:01:32.320 --> 00:01:34.683
and a rainstorm alleviated the smog.

00:01:36.230 --> 00:01:38.950
But following the deadly Donora smog,

00:01:38.950 --> 00:01:41.360
the public began to
realize just how dangerous

00:01:41.360 --> 00:01:43.770
and life-threatening
air pollution could be,

00:01:43.770 --> 00:01:46.380
and citizens demanded change.

00:01:46.380 --> 00:01:48.850
Donora became a turning
point in US history

00:01:48.850 --> 00:01:51.780
and was a start of the clean air movement.

00:01:51.780 --> 00:01:54.810
The Air Pollution Control Act of 1955

00:01:54.810 --> 00:01:57.940
was the first piece of
US federal legislation

00:01:57.940 --> 00:02:00.790
involving air pollution and provided funds

00:02:00.790 --> 00:02:03.810
for research about air pollution.

00:02:03.810 --> 00:02:07.640
Then, in 1963, the Clean
Air Act was passed,

00:02:07.640 --> 00:02:11.240
the first federal legislation
to control air pollution,

00:02:11.240 --> 00:02:15.440
and later expanded in 1970,
which resulted in the creation

00:02:15.440 --> 00:02:19.300
of the US Environmental
Protection Agency, the EPA,

00:02:19.300 --> 00:02:21.160
to develop and enforce regulations

00:02:21.160 --> 00:02:22.780
to protect the general public

00:02:22.780 --> 00:02:25.810
from exposure to major
outdoor air pollutants.

00:02:25.810 --> 00:02:30.810
The Clean Air Act was expanded
in 1977 and again in 1990.

00:02:30.890 --> 00:02:33.550
And throughout its nearly-60-year history,

00:02:33.550 --> 00:02:36.570
our air quality has drastically improved

00:02:36.570 --> 00:02:38.743
and pollutants have dropped sharply.

00:02:39.950 --> 00:02:44.300
Since 1990, major air pollutants
such as carbon monoxide,

00:02:44.300 --> 00:02:47.240
nitrogen oxides, sulfur dioxide,

00:02:47.240 --> 00:02:50.360
and volatile organic compounds
have greatly decreased,

00:02:50.360 --> 00:02:52.700
and that's just since 1990.

00:02:52.700 --> 00:02:55.550
These four main air
pollutants that I highlighted

00:02:55.550 --> 00:02:59.160
are largely released as emissions
from burning fossil fuels,

00:02:59.160 --> 00:03:01.220
which comes from driving vehicles

00:03:01.220 --> 00:03:03.400
and operating coal-fired power plants

00:03:03.400 --> 00:03:05.590
and other industrial facilities.

00:03:05.590 --> 00:03:09.020
So, as we've started to
drive more efficient vehicles

00:03:09.020 --> 00:03:12.670
and obtain more energy from
clean renewable sources,

00:03:12.670 --> 00:03:15.560
we've decreased the amount
of fossil fuels that we use.

00:03:15.560 --> 00:03:18.090
And in turn, we've reduced emissions

00:03:18.090 --> 00:03:21.263
from fossil fuels and
associated air pollutants.

00:03:22.670 --> 00:03:25.710
But how does the Clean
Air Act work exactly?

00:03:25.710 --> 00:03:27.140
How do we clean the air

00:03:27.140 --> 00:03:30.310
and limit emissions of harmful pollutants?

00:03:30.310 --> 00:03:33.190
Clean Air Act regulations
implemented by the EPA

00:03:33.190 --> 00:03:36.140
have led to new technologies
that help to limit emissions

00:03:36.140 --> 00:03:38.290
and remove pollutants from the air.

00:03:38.290 --> 00:03:40.240
In particular, many of these technologies

00:03:40.240 --> 00:03:41.620
help to reduce air pollution

00:03:41.620 --> 00:03:45.020
from coal-burning power
plants and vehicles.

00:03:45.020 --> 00:03:47.250
Each of these
pollution-control technologies

00:03:47.250 --> 00:03:50.260
functions to remove harmful
components out of emissions

00:03:50.260 --> 00:03:53.000
and release a less harmful substance.

00:03:53.000 --> 00:03:55.870
In the last decade or so,
you've also probably noticed

00:03:55.870 --> 00:03:59.120
more and more electric
vehicles on the road.

00:03:59.120 --> 00:04:01.100
Improving the fuel economy of vehicles

00:04:01.100 --> 00:04:04.010
and even using battery-powered
electric vehicles

00:04:04.010 --> 00:04:06.670
can reduce the need to
burn as much gasoline,

00:04:06.670 --> 00:04:10.010
thereby reducing emissions
and giving us cleaner air.

00:04:10.010 --> 00:04:11.950
A good example is the growing demand

00:04:11.950 --> 00:04:14.730
for hybrid and purely electric vehicles.

00:04:14.730 --> 00:04:16.390
Here we have a simplified figure

00:04:16.390 --> 00:04:18.280
that explains the sources of energy

00:04:18.280 --> 00:04:19.890
for different types of vehicles

00:04:19.890 --> 00:04:21.800
and their respective emissions.

00:04:21.800 --> 00:04:24.220
On the left, we have
conventional vehicles,

00:04:24.220 --> 00:04:27.680
which rely on fossil fuels
such as gasoline or diesel

00:04:27.680 --> 00:04:30.030
and, when driven,
produce lots of emissions

00:04:30.030 --> 00:04:33.080
like carbon dioxide and air pollutants.

00:04:33.080 --> 00:04:35.240
Hybrid and plug-in hybrid vehicles

00:04:35.240 --> 00:04:37.930
are similar to conventional vehicles

00:04:37.930 --> 00:04:40.500
in that they have an
internal combustion engine,

00:04:40.500 --> 00:04:42.670
but they also have an electric motor

00:04:42.670 --> 00:04:45.080
which uses energy stored in batteries.

00:04:45.080 --> 00:04:48.410
These batteries can be charged
by regenerative braking

00:04:48.410 --> 00:04:50.580
or, in the case of plug-in hybrids,

00:04:50.580 --> 00:04:53.940
just by using a wall outlet
or other charging equipment.

00:04:53.940 --> 00:04:56.110
In turn, because these hybrid cars

00:04:56.110 --> 00:04:58.240
are partially fueled by batteries,

00:04:58.240 --> 00:05:01.620
they produce fewer emissions
than a conventional car.

00:05:01.620 --> 00:05:04.780
Finally, vehicles that
rely solely on electricity,

00:05:04.780 --> 00:05:08.510
known as battery electric
vehicles or BEVs,

00:05:08.510 --> 00:05:10.530
can use an alternative electricity source

00:05:10.530 --> 00:05:12.050
so that there's no emissions

00:05:12.050 --> 00:05:14.530
at the source of the electricity.

00:05:14.530 --> 00:05:17.090
What else can we do to ensure cleaner air?

00:05:17.090 --> 00:05:19.360
We can reduce our reliance on fossil fuels

00:05:19.360 --> 00:05:22.220
and instead invest in
cleaner renewable resources

00:05:22.220 --> 00:05:26.890
to generate electricity such
as geothermal, wind, and solar.

00:05:26.890 --> 00:05:29.440
And we can make decisions
in our day-to-day lives

00:05:29.440 --> 00:05:31.160
to reduce or prevent air pollution

00:05:31.160 --> 00:05:34.460
by using less energy
and alternative fuels.

00:05:34.460 --> 00:05:37.880
For example, walking, biking,
or using mass transportation

00:05:37.880 --> 00:05:40.550
can reduce the need to burn fossil fuels.

00:05:40.550 --> 00:05:42.190
And there are plenty of other ways

00:05:42.190 --> 00:05:44.010
to reduce our electrical needs;

00:05:44.010 --> 00:05:47.540
in particular, using more
energy-efficient appliances.

00:05:47.540 --> 00:05:49.830
For example, think of the LED bulb,

00:05:49.830 --> 00:05:53.870
which uses 75% less energy
than incandescent lighting.

00:05:53.870 --> 00:05:55.750
So, even switching out bulbs

00:05:55.750 --> 00:05:57.560
in the lights around
your house or apartment

00:05:57.560 --> 00:05:59.363
can make a huge difference.

00:06:01.070 --> 00:06:04.790
But there's still many other
places out there like Donora,

00:06:04.790 --> 00:06:07.040
and oftentimes folks living in cities

00:06:07.040 --> 00:06:10.810
with heavy air pollution
literally can't afford to leave.

00:06:10.810 --> 00:06:13.120
There's still much work to be done.

00:06:13.120 --> 00:06:16.010
In Donora, though, there's
the Donora Smog Museum,

00:06:16.010 --> 00:06:19.640
which has the tagline
"Clean air started here."

00:06:19.640 --> 00:06:22.160
The terrible incident
suffered by Donora's community

00:06:22.160 --> 00:06:24.290
played a huge and pivotal role

00:06:24.290 --> 00:06:25.860
in opening the eyes of Americans

00:06:25.860 --> 00:06:29.100
to the hazards of air pollution
and spurred political action

00:06:29.100 --> 00:06:31.090
that's carried forth through today

00:06:31.090 --> 00:06:33.650
and will continue into the future.

00:06:33.650 --> 00:06:36.373
Let's all take a deep breath
and be glad that we can.


Nuclear Power Generation

WEBVTT
Kind: captions
Language: en

00:00:01.300 --> 00:00:02.610
- [Professor] Hey there, friends.

00:00:02.610 --> 00:00:05.520
Today, we're going to
learn about nuclear power,

00:00:05.520 --> 00:00:08.800
and to do so, we're gonna
visit my home state.

00:00:08.800 --> 00:00:10.710
Idaho? That's right.

00:00:10.710 --> 00:00:14.390
Land of the potatoes
and also nuclear power?

00:00:14.390 --> 00:00:15.860
If you've driven through Idaho,

00:00:15.860 --> 00:00:17.350
there's a good chance that you passed

00:00:17.350 --> 00:00:19.960
by a quaint small town called Arco

00:00:19.960 --> 00:00:22.260
where you'll find the
restaurant Pickle's Place,

00:00:22.260 --> 00:00:24.520
home to the Atomic Burger.

00:00:24.520 --> 00:00:29.520
Wait, a radioactive burger?
Sounds a little disturbing.

00:00:30.320 --> 00:00:32.610
Actually, Arco became the
first city in the world

00:00:32.610 --> 00:00:35.090
to be powered by nuclear energy.

00:00:35.090 --> 00:00:37.430
And of course, Arco became the first city

00:00:37.430 --> 00:00:40.620
to serve Atomic Burgers,
grilled and seared

00:00:40.620 --> 00:00:43.053
to perfection using nuclear energy.

00:00:43.890 --> 00:00:46.470
But what's going on under that grill?

00:00:46.470 --> 00:00:48.440
Are they using glowing green rocks

00:00:48.440 --> 00:00:50.880
to make those delicious Atomic Burgers?

00:00:50.880 --> 00:00:51.913
Let's find out.

00:00:54.270 --> 00:00:56.750
Nuclear power plants often look ominous

00:00:56.750 --> 00:00:58.470
and a little bit scary,

00:00:58.470 --> 00:01:00.330
but they produce power the same way

00:01:00.330 --> 00:01:02.550
most other power plants do.

00:01:02.550 --> 00:01:05.720
Simply put, they boil
water to create steam,

00:01:05.720 --> 00:01:08.710
which spins turbines to produce energy.

00:01:08.710 --> 00:01:12.100
Most nuclear power plants
use light water reactors

00:01:12.100 --> 00:01:13.930
to generate electricity,

00:01:13.930 --> 00:01:16.513
which are made up of five basic parts.

00:01:17.480 --> 00:01:20.120
First off, we have the core of the reactor

00:01:20.120 --> 00:01:22.650
where fuel rods are inserted.

00:01:22.650 --> 00:01:24.780
Next up, we have the containment shell

00:01:24.780 --> 00:01:28.610
that encases the reactor
and the spent fuel rods.

00:01:28.610 --> 00:01:30.670
Within there, we have supply of water,

00:01:30.670 --> 00:01:32.900
which is boiled to reduce steam.

00:01:32.900 --> 00:01:35.720
That steam then rotates a turbine attached

00:01:35.720 --> 00:01:38.890
to a generator which produces electricity.

00:01:38.890 --> 00:01:41.480
This act of turning an electric generator

00:01:41.480 --> 00:01:45.060
is actually the same process
that's used for coal,

00:01:45.060 --> 00:01:49.030
gas, geothermal, hydro
power, and wind power.

00:01:49.030 --> 00:01:52.170
No matter how complex the
electricity generation system,

00:01:52.170 --> 00:01:54.660
that all boils down to the same idea,

00:01:54.660 --> 00:01:56.410
basically turning a wheel,

00:01:56.410 --> 00:02:00.120
one of the oldest
agricultural-era human inventions,

00:02:00.120 --> 00:02:02.500
and that's what makes electricity.

00:02:02.500 --> 00:02:06.090
Finally, we have excess
steam or water vapor,

00:02:06.090 --> 00:02:07.510
which is the only direct emission

00:02:07.510 --> 00:02:09.470
from nuclear power generation.

00:02:09.470 --> 00:02:13.393
Easy as pie, right? Well,
it's actually pretty complex.

00:02:14.650 --> 00:02:18.050
So how is the water heated exactly?

00:02:18.050 --> 00:02:21.140
Nuclear energy isn't as
easy as lighting up a grill,

00:02:21.140 --> 00:02:23.120
and it requires us to go down

00:02:23.120 --> 00:02:26.490
to the smallest unit of matter, the atom.

00:02:26.490 --> 00:02:28.960
Here, we get our energy
at the atomic level,

00:02:28.960 --> 00:02:31.360
but it's not from the atom alone.

00:02:31.360 --> 00:02:34.720
No, to gain energy, we
need to split the atom.

00:02:34.720 --> 00:02:37.130
This process is called
fission, which occurs

00:02:37.130 --> 00:02:39.680
when neutrons are fired at an atom,

00:02:39.680 --> 00:02:41.560
causing it to split into separate atoms

00:02:41.560 --> 00:02:43.610
of other smaller elements.

00:02:43.610 --> 00:02:46.230
This split produces a
huge amount of energy,

00:02:46.230 --> 00:02:48.400
which is largely converted to heat,

00:02:48.400 --> 00:02:51.310
which boils the water and produces steam.

00:02:51.310 --> 00:02:53.160
However, we need a special kind

00:02:53.160 --> 00:02:55.160
of atom for fission to happen,

00:02:55.160 --> 00:02:58.623
and most nuclear reactors use uranium-235.

00:03:00.290 --> 00:03:03.200
Wait, why uranium-235 though?

00:03:03.200 --> 00:03:06.470
Well, first, uranium-235 is big,

00:03:06.470 --> 00:03:10.770
not a triple 1/4 pounder big
but big on the atomic scale.

00:03:10.770 --> 00:03:13.830
In the atomic world, this
is known as being heavy.

00:03:13.830 --> 00:03:18.600
Secondly, uranium-235 is unstable
because it's not only big,

00:03:18.600 --> 00:03:21.700
but it's also an isotope,
meaning it has a different number

00:03:21.700 --> 00:03:24.620
of neutrons than the more
common form of uranium,

00:03:24.620 --> 00:03:28.710
which is uranium-238, which
has three more neutrons.

00:03:28.710 --> 00:03:33.530
This makes uranium-235 unstable
or fissile like fission,

00:03:33.530 --> 00:03:36.120
which means it can be split by a neutron,

00:03:36.120 --> 00:03:40.350
thereby producing other elements,
energy, and more neutrons.

00:03:40.350 --> 00:03:44.140
Those produced neutrons
crash into other U-235 atoms,

00:03:44.140 --> 00:03:47.060
splitting them and
causing a chain reaction,

00:03:47.060 --> 00:03:49.163
which is what makes nuclear energy work.

00:03:50.070 --> 00:03:52.850
This chain reaction is
really important to note

00:03:52.850 --> 00:03:56.010
because it's what makes a
nuclear power plant so different

00:03:56.010 --> 00:04:00.240
from its, well, more destructive
cousin, the atomic bomb.

00:04:00.240 --> 00:04:04.380
In atomic bombs, the same process
of nuclear fission is used

00:04:04.380 --> 00:04:07.450
except that it's a fast
destructive, runaway,

00:04:07.450 --> 00:04:09.490
and uncontrolled reaction that results

00:04:09.490 --> 00:04:11.910
in massively powerful explosions,

00:04:11.910 --> 00:04:13.510
not something that we would want

00:04:13.510 --> 00:04:15.523
to happen in a nuclear power plant.

00:04:16.490 --> 00:04:18.330
Now a little goes a long way

00:04:18.330 --> 00:04:20.400
when it comes to nuclear fission.

00:04:20.400 --> 00:04:23.400
The fuel is actually
composed of tiny pellets

00:04:23.400 --> 00:04:28.070
of uranium-235, each the
size of a pencil eraser,

00:04:28.070 --> 00:04:32.160
but each also has the equivalent
energy of a ton of coal.

00:04:32.160 --> 00:04:34.440
Yes, a literal ton.

00:04:34.440 --> 00:04:37.190
These pellets are packed
together to form fuel rods,

00:04:37.190 --> 00:04:39.580
which are bunched into fuel assemblies

00:04:39.580 --> 00:04:42.200
and then placed in the nuclear reactor.

00:04:42.200 --> 00:04:44.810
Nuclear fusion is
therefore really powerful

00:04:44.810 --> 00:04:48.810
and can generate a lot of heat
from very little material.

00:04:48.810 --> 00:04:51.390
But to keep temperatures
from getting too hot,

00:04:51.390 --> 00:04:53.380
which would cause a nuclear meltdown,

00:04:53.380 --> 00:04:57.210
and no, I'm not talking about
melty cheese, unfortunately,

00:04:57.210 --> 00:05:00.320
the reactor is therefore
cooled with water.

00:05:00.320 --> 00:05:03.170
When more heat is generated
by the nuclear reactor

00:05:03.170 --> 00:05:05.410
than can be removed by the cooling system,

00:05:05.410 --> 00:05:08.120
or water in the case of nuclear reactors,

00:05:08.120 --> 00:05:11.340
the fuel rods can get so hot
that they could start to melt

00:05:11.340 --> 00:05:13.090
and fall to the bottom of the reactor

00:05:13.090 --> 00:05:15.100
and potentially melt through and escape

00:05:15.100 --> 00:05:16.940
into the surrounding environment.

00:05:16.940 --> 00:05:18.950
That's called a nuclear meltdown,

00:05:18.950 --> 00:05:21.980
and that's also why in part,
the reactor is surrounded

00:05:21.980 --> 00:05:24.480
by a containment shell of
thick steel and concrete,

00:05:24.480 --> 00:05:27.550
which keeps radioactive
materials from escaping.

00:05:27.550 --> 00:05:29.833
We don't wanna have any
radioactive burgers.

00:05:30.750 --> 00:05:33.550
But fuel rods don't last forever.

00:05:33.550 --> 00:05:35.660
After three to six years in a reactor,

00:05:35.660 --> 00:05:38.730
fuel rods can't sustain the
fission reaction effectively

00:05:38.730 --> 00:05:41.510
anymore and become highly radioactive.

00:05:41.510 --> 00:05:44.450
In turn, they need to be
carefully removed and stored.

00:05:44.450 --> 00:05:46.950
But what to do with nuclear waste?

00:05:46.950 --> 00:05:49.130
The problem with spent nuclear fuel

00:05:49.130 --> 00:05:51.170
is that it's really radioactive.

00:05:51.170 --> 00:05:54.640
These leftover radioactive
materials can persist in the air,

00:05:54.640 --> 00:05:57.930
soil, and water for thousands
and thousands of years

00:05:57.930 --> 00:06:00.140
and damage the DNA of living organisms,

00:06:00.140 --> 00:06:02.990
causing cancer and
other health conditions.

00:06:02.990 --> 00:06:05.550
For a while, actually quite a long time

00:06:05.550 --> 00:06:08.980
from 1946 to 1993, to be exact,

00:06:08.980 --> 00:06:12.290
many countries just dumped
radioactive nuclear waste

00:06:12.290 --> 00:06:13.640
into the ocean.

00:06:13.640 --> 00:06:17.380
This was consequently banned,
and you can imagine why.

00:06:17.380 --> 00:06:20.030
Instead, nuclear waste can be buried,

00:06:20.030 --> 00:06:22.280
but there's problems with that, too.

00:06:22.280 --> 00:06:24.400
Nuclear waste can still leak into soil

00:06:24.400 --> 00:06:27.060
and water if it isn't properly contained.

00:06:27.060 --> 00:06:30.930
So where do we safely bury
it? Well, nowhere really.

00:06:30.930 --> 00:06:33.110
Radioactive spent fuel is stored all

00:06:33.110 --> 00:06:35.600
over the world in various
containment systems,

00:06:35.600 --> 00:06:38.120
but none of them are truly longterm.

00:06:38.120 --> 00:06:41.210
Alternatively, spent fuel
rods can also be recycled

00:06:41.210 --> 00:06:43.810
and reprocessed where unused uranium

00:06:43.810 --> 00:06:46.480
is separated from spent nuclear fuel.

00:06:46.480 --> 00:06:51.110
However, this reprocessing is
quite expensive and dangerous.

00:06:51.110 --> 00:06:53.660
Reprocessing is often much more expensive

00:06:53.660 --> 00:06:56.730
than storing or disposing
of spent nuclear fuel,

00:06:56.730 --> 00:06:59.070
and it still results
in a substantial amount

00:06:59.070 --> 00:07:01.060
of leftover radioactive materials

00:07:01.060 --> 00:07:02.993
that still need to be disposed of.

00:07:04.440 --> 00:07:06.230
There's no perfect solution when it comes

00:07:06.230 --> 00:07:08.050
to energy production, though.

00:07:08.050 --> 00:07:10.030
Any kind of electricity production

00:07:10.030 --> 00:07:12.780
has its own benefits and drawbacks.

00:07:12.780 --> 00:07:16.280
But Pickle Place's Atomic
Burger is quite perfect,

00:07:16.280 --> 00:07:17.743
and I think I'll eat one now.


Student tips for using course mastery on Khan Academy

WEBVTT
Kind: captions
Language: en

00:00:03.200 --> 00:00:06.050
- Hi, I'm Shannon from Khan Academy,

00:00:06.050 --> 00:00:08.240
and I want to show you
how to make the most

00:00:08.240 --> 00:00:09.483
of your learning time.

00:00:10.490 --> 00:00:12.570
First, make sure you're logged in

00:00:12.570 --> 00:00:14.410
to your Khan Academy account

00:00:14.410 --> 00:00:17.793
by checking for your name in
the upper right hand corner.

00:00:19.550 --> 00:00:21.410
Now on the left side,

00:00:21.410 --> 00:00:22.860
you should see your classes

00:00:22.860 --> 00:00:25.640
where your teacher has
given you a mastery goal,

00:00:25.640 --> 00:00:27.253
or assignment to work on.

00:00:28.520 --> 00:00:32.150
Click on the tab that says Course mastery

00:00:32.150 --> 00:00:34.543
to view the goal your teacher set for you.

00:00:35.520 --> 00:00:37.450
Now that you're viewing your goal,

00:00:37.450 --> 00:00:40.360
let's talk about the
top five things to know

00:00:40.360 --> 00:00:43.913
about working towards Course
mastery on Khan Academy.

00:00:45.020 --> 00:00:48.180
Number one, your course mastery placement

00:00:48.180 --> 00:00:51.010
is a big goal from your teacher.

00:00:51.010 --> 00:00:52.860
On your learner home page,

00:00:52.860 --> 00:00:55.170
you can see the progress
you've already made

00:00:55.170 --> 00:00:57.110
towards your mastery goal,

00:00:57.110 --> 00:00:59.720
and the due date your teacher set.

00:00:59.720 --> 00:01:01.710
But you can always work ahead,

00:01:01.710 --> 00:01:03.393
or view past goals.

00:01:05.180 --> 00:01:06.670
Click on the mastery goal

00:01:06.670 --> 00:01:09.113
to be taken to the course homepage.

00:01:10.940 --> 00:01:14.010
Here, you'll see the units
that make up the course,

00:01:14.010 --> 00:01:17.453
as well as your progress
towards mastery on each unit.

00:01:18.530 --> 00:01:21.010
As you make progress on each unit,

00:01:21.010 --> 00:01:24.373
you'll see the purple bar
fill from left to right.

00:01:25.940 --> 00:01:28.900
It's helpful to regularly
check your unit progress,

00:01:28.900 --> 00:01:31.350
so you know if you're on
track to meet your goal.

00:01:33.090 --> 00:01:36.280
Click into a unit to view
your current mastery level

00:01:36.280 --> 00:01:39.313
for each skill on the left-hand side.

00:01:40.430 --> 00:01:41.890
With every skill,

00:01:41.890 --> 00:01:43.300
try to get the crown,

00:01:43.300 --> 00:01:46.003
and move your mastery level to Mastered.

00:01:46.890 --> 00:01:51.343
Number two, mastery
takes time and practice.

00:01:52.600 --> 00:01:55.840
As you practice skills
and answer questions,

00:01:55.840 --> 00:01:59.260
your mastery level for
each skill will go up,

00:01:59.260 --> 00:02:01.110
if you answered correctly,

00:02:01.110 --> 00:02:03.453
or down, if you miss questions.

00:02:04.550 --> 00:02:07.040
If you want to make progress more quickly,

00:02:07.040 --> 00:02:08.803
try a Mastery challenge.

00:02:09.660 --> 00:02:12.760
Mastery challenges allow
you to strengthen the skills

00:02:12.760 --> 00:02:16.343
you've already practiced
in just six questions.

00:02:17.650 --> 00:02:20.000
Mastery is not easily earned,

00:02:20.000 --> 00:02:21.870
and that's intentional!

00:02:21.870 --> 00:02:24.710
But putting in the work to achieve mastery

00:02:24.710 --> 00:02:27.323
will prove you've earned and learned it.

00:02:28.400 --> 00:02:31.360
Remember, you can retry an exercise

00:02:31.360 --> 00:02:33.170
as many times as you'd like

00:02:33.170 --> 00:02:35.782
until you earn a score you're happy with.

00:02:35.782 --> 00:02:36.615
(stars twinkling)

00:02:36.615 --> 00:02:39.629
Struggles and mistakes are
what helps your brain grow.

00:02:39.629 --> 00:02:41.920
(congratulatory sound)

00:02:41.920 --> 00:02:45.023
Number three, follow the blue button.

00:02:46.490 --> 00:02:49.590
Your big goal has a lot
of skills you can practice

00:02:49.590 --> 00:02:50.973
at your own pace.

00:02:51.870 --> 00:02:54.250
To make sure you stay on track,

00:02:54.250 --> 00:02:57.053
look for the blue buttons
throughout the course.

00:02:58.140 --> 00:03:00.110
The blue buttons will always guide you

00:03:00.110 --> 00:03:01.993
to what you should work on next.

00:03:03.350 --> 00:03:06.863
Number four, if you're stuck, take a hint.

00:03:08.030 --> 00:03:11.320
It's normal to feel stuck when
you're learning new skills.

00:03:11.320 --> 00:03:13.503
The important thing is you don't give up.

00:03:14.410 --> 00:03:17.600
Take a hint to get
step-by-step instructions

00:03:17.600 --> 00:03:20.680
to the specific question
you're working on.

00:03:20.680 --> 00:03:23.280
And write them down so you
can reference them later.

00:03:24.940 --> 00:03:27.000
You can also try watching a video,

00:03:27.000 --> 00:03:29.133
or reading an article on the skill.

00:03:30.240 --> 00:03:31.410
Once you've done that,

00:03:31.410 --> 00:03:33.743
you're ready to retry the exercise.

00:03:34.970 --> 00:03:36.810
And if you're still stuck,

00:03:36.810 --> 00:03:39.750
reach out to your classmate, a teacher,

00:03:39.750 --> 00:03:41.605
or a family member for support.

00:03:41.605 --> 00:03:43.900
(stars twinkling)

00:03:43.900 --> 00:03:46.813
Number five, the sky's the limit!

00:03:48.000 --> 00:03:51.810
Remember, you have the
potential to succeed.

00:03:51.810 --> 00:03:54.510
Keep trying, keep making mistakes,

00:03:54.510 --> 00:03:57.140
and keep asking for help when you need it.

00:03:57.140 --> 00:03:59.393
There is no limit to what you can learn.


Student tips for completing assignments on Khan Academy

WEBVTT
Kind: captions
Language: en

00:00:03.200 --> 00:00:06.050
- Hi, I'm Shannon from Khan Academy,

00:00:06.050 --> 00:00:08.240
and I wanna show you how to make the most

00:00:08.240 --> 00:00:10.000
of your learning time.

00:00:10.000 --> 00:00:11.910
First, make sure you're logged into

00:00:11.910 --> 00:00:14.850
your Khan Academy account
by checking for your name

00:00:14.850 --> 00:00:17.500
in the upper right-hand corner.

00:00:17.500 --> 00:00:19.270
If you are not logged in,

00:00:19.270 --> 00:00:21.200
you won't be able to view your assignments

00:00:21.200 --> 00:00:22.500
and any progress you make

00:00:22.500 --> 00:00:24.520
won't be counted towards your classes.

00:00:24.520 --> 00:00:26.440
Now, on the left-hand side,

00:00:26.440 --> 00:00:27.870
you should see your classes

00:00:27.870 --> 00:00:30.700
where your teacher has
given you a mastery goal

00:00:30.700 --> 00:00:32.363
or an assignment to work on.

00:00:33.500 --> 00:00:36.430
Click on the tab that says assignments

00:00:36.430 --> 00:00:38.373
to view assignments from your teacher.

00:00:40.050 --> 00:00:41.730
On the assignments tab,

00:00:41.730 --> 00:00:46.470
you can see all your upcoming
and past due assignments.

00:00:46.470 --> 00:00:48.180
Assignments with the nearest due date

00:00:48.180 --> 00:00:51.121
will appear at the very top of the list.

00:00:51.121 --> 00:00:53.480
Now that you're ready to go,

00:00:53.480 --> 00:00:56.280
let's talk about the
top five things to know

00:00:56.280 --> 00:00:58.720
about assignments on Khan Academy.

00:00:58.720 --> 00:01:01.943
Number one, types of assignments.

00:01:03.180 --> 00:01:05.480
There are two main types of assignments

00:01:05.480 --> 00:01:08.230
you can receive on Khan Academy.

00:01:08.230 --> 00:01:12.163
The first type is practice,
noted with the star symbol.

00:01:13.240 --> 00:01:18.120
Practice can be an exercise,
quiz, or unit test.

00:01:18.120 --> 00:01:20.770
When you complete
practice on Khan Academy,

00:01:20.770 --> 00:01:23.693
you receive instant
feedback after every answer.

00:01:24.730 --> 00:01:27.460
Answer all questions to
complete the assignment

00:01:27.460 --> 00:01:28.593
and receive a score.

00:01:31.330 --> 00:01:36.110
The second type is instruction,
noted with the play symbol

00:01:36.110 --> 00:01:37.483
and the paper symbol.

00:01:38.550 --> 00:01:42.253
Instruction can be in the
form of videos or articles.

00:01:43.300 --> 00:01:46.360
Videos and articles can
help you learn new skills

00:01:46.360 --> 00:01:48.373
or review things you learned in class.

00:01:49.810 --> 00:01:51.590
All videos have subtitles,

00:01:51.590 --> 00:01:54.080
so you can easily follow along.

00:01:54.080 --> 00:01:57.950
Number two, if you're stuck, take a hint.

00:01:57.950 --> 00:02:00.780
It's normal to feel stuck when
you're learning new skills,

00:02:00.780 --> 00:02:03.700
but the important thing
is you don't give up.

00:02:03.700 --> 00:02:06.520
Take a hint to get
step-by-step instructions

00:02:06.520 --> 00:02:08.833
to the specific question
you're working on.

00:02:09.700 --> 00:02:12.060
We recommend that you
write these hints down

00:02:12.060 --> 00:02:13.713
so you can reference them later.

00:02:15.230 --> 00:02:17.180
You can also try watching a video

00:02:17.180 --> 00:02:19.420
or reading an article on this skill,

00:02:19.420 --> 00:02:22.120
even if it hasn't been assigned to you.

00:02:22.120 --> 00:02:24.140
You'll find the videos and articles

00:02:24.140 --> 00:02:26.460
related to each skill you're practicing

00:02:26.460 --> 00:02:30.560
by clicking this stuck or get help link.

00:02:30.560 --> 00:02:31.660
Once you've done that,

00:02:31.660 --> 00:02:34.373
you're ready to retry
the practice exercise.

00:02:35.920 --> 00:02:37.460
And if you're still stuck,

00:02:37.460 --> 00:02:41.080
reach out to a classmate,
teacher, or a family member

00:02:41.080 --> 00:02:41.913
for support.

00:02:44.060 --> 00:02:47.830
Number three, be patient and persistent.

00:02:47.830 --> 00:02:49.710
Not happy with the score you received

00:02:49.710 --> 00:02:51.660
on a practice exercise?

00:02:51.660 --> 00:02:54.070
Trust me, we've all been there.

00:02:54.070 --> 00:02:56.290
But remember, on Khan Academy,

00:02:56.290 --> 00:02:58.200
you can always retry assignments

00:02:58.200 --> 00:03:00.890
until you earn a that you are proud of.

00:03:00.890 --> 00:03:03.103
Click try again to retry.

00:03:08.690 --> 00:03:10.480
You can also return to assignments

00:03:10.480 --> 00:03:11.880
where the due dates have passed

00:03:11.880 --> 00:03:13.433
to try to improve your score.

00:03:14.540 --> 00:03:17.430
Struggles and mistakes are
what helps your brain grow,

00:03:17.430 --> 00:03:18.380
so keep persisting.

00:03:19.860 --> 00:03:23.120
Number four, check your progress.

00:03:23.120 --> 00:03:24.810
On the left-hand side,

00:03:24.810 --> 00:03:28.520
select progress under my account.

00:03:28.520 --> 00:03:30.520
From here, you can review

00:03:30.520 --> 00:03:33.780
all of the activity on Khan Academy.

00:03:33.780 --> 00:03:38.130
This includes exercises you
completed, videos you watched,

00:03:38.130 --> 00:03:39.783
or articles you read.

00:03:41.210 --> 00:03:43.260
Depending on your learning goals,

00:03:43.260 --> 00:03:44.270
you may wanna filter

00:03:44.270 --> 00:03:47.823
to view a certain type
of content or activity.

00:03:49.180 --> 00:03:51.330
And for each practice you completed,

00:03:51.330 --> 00:03:53.960
you can view your current mastery level,

00:03:53.960 --> 00:03:56.910
the number of questions
you answered correctly,

00:03:56.910 --> 00:03:59.533
and how much time you
spent on the activity.

00:04:01.120 --> 00:04:03.620
It's helpful to regularly
check your progress,

00:04:03.620 --> 00:04:06.790
so you know if you're on
track to meet your goals.

00:04:06.790 --> 00:04:11.070
Number five, make Khan Academy your own.

00:04:11.070 --> 00:04:13.640
And finally, adjust Khan Academy

00:04:13.640 --> 00:04:16.630
to work for your unique learning style.

00:04:16.630 --> 00:04:21.190
You can slow down, speed
up, or rewind the videos

00:04:21.190 --> 00:04:22.203
when you need to.

00:04:23.400 --> 00:04:26.930
You can learn in the language
you're most comfortable with.

00:04:26.930 --> 00:04:29.830
Khan Academy is available
in over 50 languages

00:04:29.830 --> 00:04:32.100
and you can easily
switch between languages

00:04:32.100 --> 00:04:33.313
in your settings.

00:04:34.810 --> 00:04:36.160
And if you prefer,

00:04:36.160 --> 00:04:38.870
you can complete
assignments on a smartphone.

00:04:38.870 --> 00:04:43.163
Just download the Khan
Academy app on Android or iOS.

00:04:44.930 --> 00:04:48.870
And remember, you have
the potential to succeed.

00:04:48.870 --> 00:04:51.730
Keep trying, keep making mistakes,

00:04:51.730 --> 00:04:54.320
and keep asking for help when you need it.

00:04:54.320 --> 00:04:56.533
There is no limit to what you can learn.


Impacts of Urbanization

WEBVTT
Kind: captions
Language: en

00:00:00.150 --> 00:00:01.000
- [Instructor] In this video,

00:00:01.000 --> 00:00:03.160
we're going to talk about cities.

00:00:03.160 --> 00:00:06.410
Today, more than 50% of
the world's population

00:00:06.410 --> 00:00:09.900
lives in a city, and this
percentage grows every year

00:00:09.900 --> 00:00:12.360
because of something called urbanization.

00:00:12.360 --> 00:00:15.460
Urbanization is the creation
and growth of cities,

00:00:15.460 --> 00:00:18.010
human settlements with
higher population densities

00:00:18.010 --> 00:00:19.440
than rural areas.

00:00:19.440 --> 00:00:21.750
The high population density in cities

00:00:21.750 --> 00:00:23.560
has a lot of advantages.

00:00:23.560 --> 00:00:25.320
Because people are closer together,

00:00:25.320 --> 00:00:28.530
it's easier to coordinate
community efforts like recycling,

00:00:28.530 --> 00:00:31.793
public education, and
public transportation.

00:00:33.320 --> 00:00:35.500
Urban residents also tend
to have better access

00:00:35.500 --> 00:00:38.573
to things like medical
care and family planning.

00:00:39.710 --> 00:00:42.450
However, the high
population density of cities

00:00:42.450 --> 00:00:45.530
also can have a negative
impact on the environment.

00:00:45.530 --> 00:00:47.190
You can imagine, with so many people

00:00:47.190 --> 00:00:49.110
living close together in one space,

00:00:49.110 --> 00:00:52.170
it can cause a strain on the
natural resources of the area

00:00:52.170 --> 00:00:54.760
and generate more air and water pollution.

00:00:54.760 --> 00:00:56.520
Cities can use sustainable planning

00:00:56.520 --> 00:00:58.130
to counter these effects.

00:00:58.130 --> 00:01:00.240
Urban planners often talk about the ratio

00:01:00.240 --> 00:01:03.400
between green space and
gray space in a city,

00:01:03.400 --> 00:01:07.570
where the green spaces are
vegetation and the gray spaces

00:01:07.570 --> 00:01:10.790
are things like buildings
and paved surfaces,

00:01:10.790 --> 00:01:13.720
like roads and sidewalks and parking lots.

00:01:13.720 --> 00:01:15.940
These gray spaces may be necessary,

00:01:15.940 --> 00:01:17.900
but cities should be planned thoughtfully

00:01:17.900 --> 00:01:19.730
to add some green spaces too.

00:01:19.730 --> 00:01:23.100
When gray spaces dominate
a cityscape too much,

00:01:23.100 --> 00:01:24.760
the paved surfaces prevent water

00:01:24.760 --> 00:01:27.520
from seeping into the
ground, so when it rains,

00:01:27.520 --> 00:01:29.730
it creates large amounts of runoff.

00:01:29.730 --> 00:01:30.900
Having nowhere to go,

00:01:30.900 --> 00:01:33.690
the water travels across paved surfaces,

00:01:33.690 --> 00:01:35.570
picking up pollutants along the way.

00:01:35.570 --> 00:01:38.220
All this runoff can
lead to flash flooding.

00:01:38.220 --> 00:01:39.420
Adding more green spaces

00:01:39.420 --> 00:01:41.010
can help prevent this from happening

00:01:41.010 --> 00:01:42.960
because plants soak up water.

00:01:42.960 --> 00:01:45.260
There are many creative
ways that urban planners

00:01:45.260 --> 00:01:47.580
can add green spaces to our environment.

00:01:47.580 --> 00:01:50.500
Some buildings in cities
grow plants on their roofs.

00:01:50.500 --> 00:01:52.260
They're called green roofs.

00:01:52.260 --> 00:01:55.960
The plants soak up the rain and
snow melt that hits the roof

00:01:55.960 --> 00:01:58.040
and prevents it from
going into the street.

00:01:58.040 --> 00:02:01.150
Also, plants create shade
and help cool things down,

00:02:01.150 --> 00:02:02.670
so buildings with green roofs

00:02:02.670 --> 00:02:05.340
wouldn't have to spend as much
energy on air conditioning.

00:02:05.340 --> 00:02:07.400
Green spaces can also help the environment

00:02:07.400 --> 00:02:09.730
by allowing for groundwater recharge,

00:02:09.730 --> 00:02:12.580
which is allowing more water
to sink back into the ground

00:02:12.580 --> 00:02:14.710
and replenish the aquifer.

00:02:14.710 --> 00:02:17.300
Some cities do this by
building rain gardens,

00:02:17.300 --> 00:02:19.420
which are gardens specifically designed

00:02:19.420 --> 00:02:21.610
to filter water back into the ground,

00:02:21.610 --> 00:02:23.070
the plants and the rain gardens

00:02:23.070 --> 00:02:24.970
help filter pollutants in the runoff

00:02:24.970 --> 00:02:25.860
and prevent the pollutants

00:02:25.860 --> 00:02:28.230
from contaminating water ecosystems.

00:02:28.230 --> 00:02:30.280
Cities can also help
with groundwater recharge

00:02:30.280 --> 00:02:33.250
by building permeable
surfaces in their cities.

00:02:33.250 --> 00:02:34.970
Parking lots can be constructed

00:02:34.970 --> 00:02:37.920
with permeable paving to reduce runoff.

00:02:37.920 --> 00:02:39.850
Groundwater recharge can
help prevent something

00:02:39.850 --> 00:02:43.223
that happens in coastal cities
called saltwater intrusion.

00:02:44.950 --> 00:02:49.050
Let's say that this city right
here is a city on the coast,

00:02:49.050 --> 00:02:51.300
so it's really close to the ocean.

00:02:51.300 --> 00:02:54.180
And let's say this city decides
to build a pump right here

00:02:54.180 --> 00:02:57.820
to access the freshwater
in the aquifer below it.

00:02:57.820 --> 00:02:59.660
But the city has a lot of people in it

00:02:59.660 --> 00:03:01.000
and it needs a lot of water.

00:03:01.000 --> 00:03:03.820
The city might end up taking
more and more freshwater

00:03:03.820 --> 00:03:05.890
at a rate too fast for the freshwater

00:03:05.890 --> 00:03:07.820
in the aquifer to replenish itself.

00:03:07.820 --> 00:03:09.080
Pulling up so much fresh water

00:03:09.080 --> 00:03:12.580
can create a space for the
saltwater in the nearby ocean

00:03:12.580 --> 00:03:14.230
to come and take its place.

00:03:14.230 --> 00:03:15.600
The saltwater from the ocean

00:03:15.600 --> 00:03:17.730
would contaminate the freshwater supply,

00:03:17.730 --> 00:03:19.540
making it unpotable.

00:03:19.540 --> 00:03:21.710
But if this city decided to increase

00:03:21.710 --> 00:03:24.740
the amount of permeable
surfaces and green spaces,

00:03:24.740 --> 00:03:27.220
then when it rains, the water will be able

00:03:27.220 --> 00:03:29.430
to seep back into the ground

00:03:29.430 --> 00:03:31.130
and it would refill the empty space

00:03:31.130 --> 00:03:33.550
the pump has created with freshwater.

00:03:33.550 --> 00:03:34.760
This would mean that freshwater

00:03:34.760 --> 00:03:36.570
would continually be replacing the water

00:03:36.570 --> 00:03:38.140
that was taken out by the pump,

00:03:38.140 --> 00:03:40.790
so saltwater intrusion couldn't happen.

00:03:40.790 --> 00:03:44.490
Adding more green spaces could
also help with air pollution

00:03:44.490 --> 00:03:48.630
because plants absorb carbon
dioxide and produce oxygen.

00:03:48.630 --> 00:03:51.730
Cities produce most of
the world's air pollution.

00:03:51.730 --> 00:03:54.350
Part of this is because of urban sprawl.

00:03:54.350 --> 00:03:56.130
As cities become more populated,

00:03:56.130 --> 00:03:59.800
they expand into suburban
areas and even exurban areas,

00:03:59.800 --> 00:04:01.830
the areas past the suburbs.

00:04:01.830 --> 00:04:03.360
All of that expansion leads people

00:04:03.360 --> 00:04:05.250
to live farther away from their jobs.

00:04:05.250 --> 00:04:08.700
This causes people to spend
a lot more time in their cars

00:04:08.700 --> 00:04:10.610
on their long commutes to work.

00:04:10.610 --> 00:04:14.380
And these long lines of cars
of people traveling to work

00:04:14.380 --> 00:04:17.330
emit a lot of carbon dioxide.

00:04:17.330 --> 00:04:19.820
Carbon dioxide is a greenhouse gas,

00:04:19.820 --> 00:04:22.470
which means it retains heat from the sun,

00:04:22.470 --> 00:04:24.660
and it contributes to climate change.

00:04:24.660 --> 00:04:26.930
But carbon dioxide isn't the only kind

00:04:26.930 --> 00:04:29.370
of air pollution that cities produce.

00:04:29.370 --> 00:04:30.890
Emissions from motor vehicles

00:04:30.890 --> 00:04:33.130
and industrial facilities in cities

00:04:33.130 --> 00:04:36.370
can create something
called photochemical smog.

00:04:36.370 --> 00:04:39.280
This is the kind of hazy, smoky pollution

00:04:39.280 --> 00:04:42.310
that you can sometimes see
in the horizon line of cities

00:04:42.310 --> 00:04:45.680
and it can be really
harmful to eyes and lungs.

00:04:45.680 --> 00:04:48.480
So, how could urban
planners solve this problem?

00:04:48.480 --> 00:04:50.320
Cities could cluster grocery stores,

00:04:50.320 --> 00:04:52.500
offices, and homes closer together,

00:04:52.500 --> 00:04:54.300
building up rather than out,

00:04:54.300 --> 00:04:57.080
making buildings taller rather than wider.

00:04:57.080 --> 00:04:59.570
This would mean that people
wouldn't have to travel as far

00:04:59.570 --> 00:05:00.610
to get where they need,

00:05:00.610 --> 00:05:03.030
so people could rely on walking or biking

00:05:03.030 --> 00:05:05.310
rather than
fossil-fuel-consuming vehicles.

00:05:05.310 --> 00:05:07.390
Cities can reduce vehicle emissions

00:05:07.390 --> 00:05:08.980
by making public transportation

00:05:08.980 --> 00:05:10.900
more convenient and affordable.

00:05:10.900 --> 00:05:12.200
This would mean that fewer people

00:05:12.200 --> 00:05:14.890
would need to use their
own cars to get around

00:05:14.890 --> 00:05:17.440
and it would cut down on traffic.

00:05:17.440 --> 00:05:19.600
Also, cities could build more paths,

00:05:19.600 --> 00:05:21.480
sidewalks, and bike lanes

00:05:21.480 --> 00:05:22.920
so people wouldn't have to rely

00:05:22.920 --> 00:05:25.810
on fossil-fuel-consuming
vehicles to get around.

00:05:25.810 --> 00:05:27.510
Encouraging walking and biking

00:05:27.510 --> 00:05:30.220
could also improve the
health of city residents.

00:05:30.220 --> 00:05:32.640
Cities can use clean forms of energy,

00:05:32.640 --> 00:05:36.320
like solar or wind power,
to reduce air pollution.

00:05:36.320 --> 00:05:39.130
They could also use fossil
fuels more effectively

00:05:39.130 --> 00:05:41.640
through energy co-generation,

00:05:41.640 --> 00:05:43.270
which is a way of using the heat

00:05:43.270 --> 00:05:45.770
given off by the burning of fossil fuels,

00:05:45.770 --> 00:05:47.850
not just the electricity.

00:05:47.850 --> 00:05:50.600
By creating goals and sustainable plans,

00:05:50.600 --> 00:05:53.350
cities can help reduce
air and water pollution

00:05:53.350 --> 00:05:56.270
and increase the wellbeing
of their residents.

00:05:56.270 --> 00:05:59.060
The city of Espoo in Finland, for example,

00:05:59.060 --> 00:06:03.240
was recognized as a world
leader in energy sustainability

00:06:03.240 --> 00:06:07.400
for its goal to become carbon
neutral by the year 2030.

00:06:07.400 --> 00:06:10.620
Cities are centers for economic
development, innovation,

00:06:10.620 --> 00:06:13.540
social and cultural diversity, and jobs.

00:06:13.540 --> 00:06:14.940
With some urban planning,

00:06:14.940 --> 00:06:17.913
cities can become centers
for sustainability too.


Impacts of Agricultural Practices

WEBVTT
Kind: captions
Language: en

00:00:00.310 --> 00:00:01.470
- [Voiceover] Hey there.

00:00:01.470 --> 00:00:05.180
Today I'm gonna cover the impacts
of agricultural practices.

00:00:05.180 --> 00:00:08.800
And to do so, I'm gonna take
you through my morning ritual.

00:00:08.800 --> 00:00:12.260
It sounds weird, but my
bowl of multigrain Cheerios,

00:00:12.260 --> 00:00:16.180
and rice milk, and relaxing
in my super comfy pajamas,

00:00:16.180 --> 00:00:19.610
they're all connected to
intensive agricultural practices,

00:00:19.610 --> 00:00:23.770
in particular tilling,
slash and burn farming,

00:00:23.770 --> 00:00:25.123
and fertilizers.

00:00:27.960 --> 00:00:30.710
So let's take a closer look
at the Cheerios in my bowl.

00:00:32.350 --> 00:00:35.430
Most of my cereal here is
actually composed of grains

00:00:35.430 --> 00:00:39.280
like oats, corn and
barley, and growing grains

00:00:39.280 --> 00:00:41.700
typically starts with
tilling in good quality soil

00:00:41.700 --> 00:00:43.053
with lots of nutrients.

00:00:44.310 --> 00:00:45.970
But what is tilling?

00:00:45.970 --> 00:00:49.310
In short, tilling is the
process of turning soil,

00:00:49.310 --> 00:00:51.980
and it's really useful
because loosening the soil

00:00:51.980 --> 00:00:54.470
allows farmers to easily control weeds

00:00:54.470 --> 00:00:56.730
and other pests at the
surface of the soil,

00:00:56.730 --> 00:00:59.730
and it helps them to prepare
the soil for seeding.

00:00:59.730 --> 00:01:02.890
Sometimes in areas that
have been heavily farmed,

00:01:02.890 --> 00:01:05.540
the soil can become compacted over time.

00:01:05.540 --> 00:01:07.900
Here, tilling can help
to break down the soil

00:01:07.900 --> 00:01:11.120
into smaller pieces,
called soil aggregates,

00:01:11.120 --> 00:01:13.373
and allow for easier crop planting.

00:01:15.110 --> 00:01:18.260
Now, tillage has been done
for thousands of years,

00:01:18.260 --> 00:01:20.180
but it's changed a lot from the past

00:01:20.180 --> 00:01:23.050
when we used human labor
and big draft animals

00:01:23.050 --> 00:01:26.370
to till small fields,
usually once per year.

00:01:26.370 --> 00:01:29.550
Nowadays, in the era of
industrialized agriculture,

00:01:29.550 --> 00:01:32.390
large-scale farmers use
heavy mechanized equipment

00:01:32.390 --> 00:01:35.893
that can till thousands of
acres multiple times per year.

00:01:37.470 --> 00:01:42.210
Okay, we're growing more so
we're tilling more, a lot more.

00:01:42.210 --> 00:01:46.640
And while soil tillage can
help to loosen and aerate soil,

00:01:46.640 --> 00:01:49.070
what do you think happens
when heavy machinery

00:01:49.070 --> 00:01:51.420
passes over the land
and tears up the surface

00:01:51.420 --> 00:01:53.143
multiple times per year?

00:01:54.610 --> 00:01:57.800
This practice of repeated
intensive tilling

00:01:57.800 --> 00:02:00.300
compacts the lower layers of the soil

00:02:00.300 --> 00:02:02.650
and loosens the top soil to the point that

00:02:02.650 --> 00:02:07.490
it loses the ability to hold
water and nutrients in place.

00:02:07.490 --> 00:02:10.570
Tillage also reduces any
leftover crop residue

00:02:10.570 --> 00:02:11.750
like plant stocks.

00:02:11.750 --> 00:02:14.410
So, in turn, the exposed soil surface

00:02:14.410 --> 00:02:17.320
becomes really vulnerable
to wind and rain,

00:02:17.320 --> 00:02:19.900
because nothing is really
holding the soil down

00:02:19.900 --> 00:02:21.770
or providing cover.

00:02:21.770 --> 00:02:23.513
So, what happens next?

00:02:24.950 --> 00:02:27.980
Loose soil can start to
collect in surface runoff

00:02:27.980 --> 00:02:30.520
and become displaced through erosion.

00:02:30.520 --> 00:02:33.300
When this happens, soil, organic matter,

00:02:33.300 --> 00:02:36.673
and nutrients are literally
washed or blown away.

00:02:37.720 --> 00:02:39.440
Who would have thought that these Cheerios

00:02:39.440 --> 00:02:41.940
were doing so much damage?

00:02:41.940 --> 00:02:43.790
But, there's some good news here too.

00:02:45.150 --> 00:02:47.730
Low till or no till farming alternatives

00:02:47.730 --> 00:02:50.640
can alleviate some of
these problems thankfully.

00:02:50.640 --> 00:02:54.620
In the Palouse, a huge
agricultural area in the Western US

00:02:54.620 --> 00:02:57.290
where a lot of grains, just
like the ones in my cereal,

00:02:57.290 --> 00:02:58.160
are grown.

00:02:58.160 --> 00:03:02.140
No till farming is really
important because the fields are,

00:03:02.140 --> 00:03:04.590
well, they're really steep and hilly

00:03:04.590 --> 00:03:07.390
and wind and rain can
cause a lot of erosion

00:03:07.390 --> 00:03:10.020
when the soil is heavily tilled.

00:03:10.020 --> 00:03:13.340
By not tilling the fields,
farmers can prevent soil erosion,

00:03:13.340 --> 00:03:16.480
and, more importantly,
from my perspective,

00:03:16.480 --> 00:03:20.320
make sure that they can grow
lots of grains for my Cheerios

00:03:20.320 --> 00:03:23.183
But wait, cereals aren't
complete without milk.

00:03:25.240 --> 00:03:27.140
Now, I personally like rice milk

00:03:27.140 --> 00:03:29.020
because I'm lactose sensitive,

00:03:29.020 --> 00:03:31.360
and rice is commonly grown in temperate

00:03:31.360 --> 00:03:34.200
and tropical regions,
oftentimes in areas where

00:03:34.200 --> 00:03:37.963
soil quality isn't the best
and nutrients are lacking.

00:03:38.820 --> 00:03:42.500
So, how do farmers get
nutrients back into the soil?

00:03:42.500 --> 00:03:45.090
And, more importantly,
how do they grow rice

00:03:45.090 --> 00:03:47.300
for my rice milk at breakfast?

00:03:47.300 --> 00:03:50.530
Well, they often use
slash and burn farming.

00:03:50.530 --> 00:03:54.330
So, like the name, forest
plots are slashed or cut,

00:03:54.330 --> 00:03:56.700
left to dry and then burned.

00:03:56.700 --> 00:03:59.440
The ash left over from the
burning fertilizes the soil.

00:03:59.440 --> 00:04:01.733
But, it's only a temporary benefit.

00:04:02.920 --> 00:04:05.030
After about three to five years,

00:04:05.030 --> 00:04:06.900
the productivity of
slashed and burned plots

00:04:06.900 --> 00:04:09.780
goes down really quickly
due to the loss of nutrients

00:04:09.780 --> 00:04:12.170
and as weeds start to grow again.

00:04:12.170 --> 00:04:14.840
When this happens, farmers
simply abandon the field,

00:04:14.840 --> 00:04:18.090
move over to a new area
and repeat the process.

00:04:18.090 --> 00:04:20.970
But it can take decades
for these plots to recover

00:04:20.970 --> 00:04:23.370
once they've been slashed,
burned, and farmed,

00:04:23.370 --> 00:04:25.913
and this practice can
become a vicious cycle.

00:04:26.770 --> 00:04:29.770
In the Amazon, for example,
people in rural areas

00:04:29.770 --> 00:04:32.210
rely on slash-and-burn so
that they can make money

00:04:32.210 --> 00:04:35.040
selling the crops they grow
or create open pastures

00:04:35.040 --> 00:04:36.810
for animals to graze.

00:04:36.810 --> 00:04:40.480
In the process, thousands of
acres are burned each year.

00:04:40.480 --> 00:04:43.110
And as these trees and plants burn,

00:04:43.110 --> 00:04:45.360
enormous amounts of greenhouse gases,

00:04:45.360 --> 00:04:47.850
mainly carbon dioxide are produced,

00:04:47.850 --> 00:04:49.870
which contribute to climate change.

00:04:49.870 --> 00:04:53.260
And, sometimes too, fires
may not be well-managed

00:04:53.260 --> 00:04:57.823
and can burn out of control
causing huge, costly wildfires.

00:04:59.180 --> 00:05:00.840
But it's not all doom and gloom

00:05:00.840 --> 00:05:02.960
when it comes to my tasty rice milk.

00:05:02.960 --> 00:05:04.890
There are alternatives to slash and burn

00:05:04.890 --> 00:05:08.180
which include applying
animal fertilizer like manure

00:05:08.180 --> 00:05:10.770
on used plots to add
nutrients back to the soil

00:05:10.770 --> 00:05:14.270
or using alley cropping in
which trees or other vegetation

00:05:14.270 --> 00:05:16.530
are planted between crops
to help keep nutrients

00:05:16.530 --> 00:05:18.023
and moisture in the soil.

00:05:18.990 --> 00:05:21.380
All right, by now I've eaten my breakfast

00:05:21.380 --> 00:05:25.140
and I'm relaxing in my
super comfy cotton pajamas,

00:05:25.140 --> 00:05:27.640
but turns out that cotton is actually

00:05:27.640 --> 00:05:29.410
a really finicky crop to grow

00:05:29.410 --> 00:05:32.240
and it requires a lot of fertilizers.

00:05:32.240 --> 00:05:34.780
And fertilizers help plants grow,

00:05:34.780 --> 00:05:36.863
but that's not a bad thing, right?

00:05:37.730 --> 00:05:38.740
And it's not.

00:05:38.740 --> 00:05:41.860
In fact, we've been using
fertilizers for a millennia.

00:05:41.860 --> 00:05:45.030
For thousands of years, people
have used natural fertilizers

00:05:45.030 --> 00:05:47.780
to replenish or increase
nutrients in the soil

00:05:47.780 --> 00:05:49.420
and promote plant growth.

00:05:49.420 --> 00:05:51.960
Natural fertilizers meant
that farmers use things

00:05:51.960 --> 00:05:55.740
like leftover crops, manure,
wood ash, ground bones,

00:05:55.740 --> 00:05:58.933
fish or fish parts, and bird and bat poop.

00:06:00.170 --> 00:06:03.080
In the early 1800s though,
scientists discovered

00:06:03.080 --> 00:06:05.430
that nitrogen, phosphorus, and potassium

00:06:05.430 --> 00:06:07.130
were key to plant growth.

00:06:07.130 --> 00:06:09.620
And in time, many farmers began to switch

00:06:09.620 --> 00:06:12.430
from natural fertilizers
to artificial fertilizers

00:06:12.430 --> 00:06:14.830
with higher concentrations
of these nutrients

00:06:14.830 --> 00:06:17.130
which greatly increased crop yields.

00:06:17.130 --> 00:06:18.850
In other words, farmers could grow more,

00:06:18.850 --> 00:06:21.913
and that means more cotton
and more comfy pajamas.

00:06:22.990 --> 00:06:25.130
But, there's a catch.

00:06:25.130 --> 00:06:27.050
Now that farmers are growing more crops

00:06:27.050 --> 00:06:29.250
than any time in history, we've learned

00:06:29.250 --> 00:06:31.360
that there are impacts
to using large amounts

00:06:31.360 --> 00:06:33.800
of very concentrated fertilizers.

00:06:33.800 --> 00:06:36.730
Applying too much fertilizer
can pollute runoff water

00:06:36.730 --> 00:06:40.870
with excess fertilizer and
pollute local surface waters.

00:06:40.870 --> 00:06:43.270
As nutrient rich materials like fertilizer

00:06:43.270 --> 00:06:46.100
make their way into nearby
rivers, lakes, and oceans,

00:06:46.100 --> 00:06:49.210
they can cause major problems
in the balance of nutrients

00:06:49.210 --> 00:06:51.660
in marine ecosystems.

00:06:51.660 --> 00:06:53.860
When too many nutrients from fertilizers

00:06:53.860 --> 00:06:57.320
saturate a body of water,
called eutrophication,

00:06:57.320 --> 00:07:00.270
these nutrients feed the
rapid growth of algae.

00:07:00.270 --> 00:07:02.410
In turn, these massive algae blooms

00:07:02.410 --> 00:07:04.870
can suck up all the
oxygen in bodies of water

00:07:04.870 --> 00:07:08.200
and lead to enormous fish
die-offs called dead zones.

00:07:08.200 --> 00:07:11.160
Quite a morbid situation
and very different

00:07:11.160 --> 00:07:13.403
from the happy unicorns of my pajamas.

00:07:14.250 --> 00:07:17.050
But, there's ways to reduce
the amount of fertilizers

00:07:17.050 --> 00:07:18.720
released into waterways.

00:07:18.720 --> 00:07:22.100
Farmers can limit the amount
of fertilizer that they apply

00:07:22.100 --> 00:07:25.500
or use compost, which is
decomposed organic material,

00:07:25.500 --> 00:07:28.640
as a fertilizer which tends
to have lower and safer levels

00:07:28.640 --> 00:07:30.840
of nitrates and phosphates.

00:07:30.840 --> 00:07:32.370
And, there you have it.

00:07:32.370 --> 00:07:35.060
Common agricultural
practices and their impacts

00:07:35.060 --> 00:07:38.003
in a nutshell, or
really, a bowl of cereal.