Hi. It's Paul Andersen and welcome
to disciplinary core idea PS3B. This is on
conservation of energy and then energy transfer,
especially in the form of heat. And most people
understand that energy is conserved. In other
words the amount of energy going into a system
is always equal to the amount of energy coming
out of a system. And so people understand
that. They also could even site this. That
energy can neither be created nor destroyed.
But when I find, if I start asking kids questions
about this, there's a lot of misconceptions
related to energy. And so you really want
to get this point across to your students
that energy is conserved. And energy is conserved
in the universe for that matter. And a great
place to look at that is when we're looking
at collisions, in energy transfer and collisions.
And so if we're looking at the macroscopic
or the big level, a great example of this
would be in a break. If you're playing pool,
when you hit the cue ball and it breaks the
other ones apart, the amount of energy that
you put into that cue ball is going to be
conserved in all of the things that come from
that. And likewise at the microscopic level,
if we're looking at chemical reactions they
amount of energy in those molecules before
the reaction is going to equal the amount
of energy after. And sometimes that energy
is going to be released as heat or consumed
as well. And so again, energy is conserved.
And heat is something that's important, an
important way to look at that energy transfer.
Heat is going to be energy transfer when there's
is going to be a difference in the object's
temperatures. In other words they're molecular
motion. So if we have a hot object and a cold
object, we're going to see energy transferred
from the hot object to the cold object. How
does that occur? Well it occurs in one of
three ways. First way is something called
conduction. And so if we have a candle and
you want to measure the heat or the transfer
from some of that heat of the candle to an
object, we'll say it's this object right here.
In order for it to be conduction you would
actually have to have those objects touching.
And so conduction is going to be movement
of energy, transfer of energy, when the objects
are in direct contact. So if you touch a burner
on a stove, that would be conduction. What
about convection? Convection is going to be
when the heat is transferred through a fluid
or through another medium. And so in this
case, if the flame is right here, there's
air between the flame and our testing device
right here. And so some of that heat is going
to be transferred. You don't have to necessarily
touch the flame in order to feel some of that
heat. If you've ever played with a flame at
all, if you put the tester above it, as opposed
to on the side, it's going to be much hotter
above it. And that's because we're heating
up the air and hot air is going to rise. And
so lot's of times you'll hear as convection
as like a convection current. And so if we
have a pot of boiling water on a stove it's
conduction. Touching to the stove. But then
the movement of that fluid up is going to
be convection. And then finally we have what
is called radiation. Radiation occurs when
we have our testing device and it is separated
from our heat source using space. And space
remember is just going to be emptiness, or
nothingness. Like a vacuum. And so we also
have energy that can move from here to here
through radiation. We call that electromagnetic
radiation. And so what's an example of that?
Well the sun is a great example of that. So
where does most of the energy on our planet
come from? It comes from the sun. How does
it get here? It gets to us through the emptiness
of space. And so we're not directly touching
the sun. And there's no fluid connecting to
us. But radiation can allow it to get here
as well. One important thing about radiation
is once an object picks up that radiation,
it can give off heat in the form of thermal
energy as well. And so the heat that we have
on our planet was delivered in radiation from
the sun. But then it creates heat on our planet.
Okay. Another thing that's important to understand
in nature is that systems that are unstable
are going to eventually going to become stable
over time. And this is related to energy as
well. And so what do we know about water?
Water is going to run down hill. And why?
That's more stable. In other words it's moving
along this gravitational field. Or if we were
to look at this heat right here. This heat
eventually what's going to happen is that
heat is going to be lost and that heat is
eventually going to go to the surroundings.
It's going to be the same temperature as its
surrounding. But it's transferred some of
that heat into the surroundings. Or is we
look at this train car. What will this train
car look like ten years from now? About the
same. But what about a million years? Or a
billion years in the future? Eventually the
whole thing is going to breakdown. It's going
to be come a more stable system over time.
But we're still going to conserve energy.
And sometimes this takes a huge amount of
time for it to occur. So let me show you what
this graph is. This shows us the number of
protons that we have inside an atom. So that's
going to tell us what type of atom it is.
And this is going to be the sum of the protons.
Excuse me. This is the number of neutrons
on this side. And so this is all of the atoms
on the periodic table. And so the big thing
is as we move up the periodic table we're
getting more protons. And we're getting more
neutrons. So hopefully this graph makes sense.
What do we have over here on the side, is
we're looking at the half life. In other words
how long does it take for half of these unstable
radioactive isotopes to eventually breakdown.
And you can see that the time scale is incredible.
So down here we're dealing with seconds and
fractions of seconds. But as we move up and
up and up and up, that half life becomes really,
really long. And so even though things might
be in an unstable state, in other words it
might be a nuclei that's radioactively unstable,
it can take billions of years sometimes for
that to reach a more stable state. It takes
a lot of time. And so that's pretty deep.
So what should we be teaching our students?
Step one, if we're in the lower elementary
grades we should simply tell them that the
sunlight warms the earth. In other words,
something is being transferred from the sun
to the earth. And if it weren't for the sun,
the earth would get incredibly cold. As we
move into the upper levels of elementary,
we should talk about where energy is present.
And we know that energy is going to be present
if we ever have motion, heat, light and sound.
And we can dig in a little bit more deeply
in elementary. We could talk about energy
present in motion. And so the pool analogy
is great. And so as we break in pool, the
motion of that cue ball is going to be converted
to, or the energy of that cue ball is going
to be converted to other energies. And so
what are some other energies? It's going to
be motion of the other balls in that break.
But it's also going to make sound. You're
going to hear that crack of the breaking.
And you're going to also generate heat inside
there. And what you want your students to
understand is the energy we had before is
equal to the energy we had at the end. We
didn't somehow lose that energy. It's going
to be conserved the whole time. And likewise
if we look at light, that light and the energy
present in the light as it's delivered to
our planet keeps us alive. Keeps the earth
warm. And it's used, that energy is used by
plants to do photosynthesis. And it's also
going to generate a certain amount of heat.
As we're in the elementary grades we should
also talk about energy present in electrical
currents and electricity. And so it's the
electricity that's found you know, as you
plug in a fan that's going to spin that fan.
So we're converting that electrical energy
into motion of the fan. And likewise we could
have motion of a fan. So if we're making wind
power, that could generate electrical currents.
Electrical energy as well. As we move into
middle school we want to start talking about
energy transfer. Energy moving from one object
to another. And so if I slide a box across
the table it's eventually going to come to
a stop. And why is that? Well there's a force
that's opposing that motion. That's frictional
forces that are opposing it. And so what happened
to the energy of the box? Well it didn't go
away. It was converted. It was converted into
heat on that table as the box slid across
it. It was converted into sound energy. And
eventually that energy is going to move off
to the environment and be lost as heat. And
all energy is eventually going to be that
form of heat. You should also start specifically
talking about heat. Heat again this transfer
of energy from a hot object to a cooler object.
And so what determines the rate of that transfer?
From an object to its environment? Well really
only three things. The first thing is what
type of object that is. And so if it's a metal
it's going to convert more or that. Or it's
going to lose more of that energy quickly.
And so if you grab on to the metal on your
desk right now it's going to feel cold. And
why is that? You're feeling the flow of energy
from your hand quickly into that metal. And
vice versa. If it was really hot it would
be converted really quickly to you. What else
is going to determine that rate is going to
be how big it is. The more size we have in
an object, the more heat can be transferred.
And then finally what that environment is
made up of as well. So those three things
are going to determine the amount of energy
and how quickly that energy flows. But how
does the energy actually flow? Again it's
through those three ways. It's going to be
conduction, which is through touch. Convection,
which is movement through the fluids or through
some kind of a matter. And then finally radiation
which is going to go at the speed of light.
Finally as we get into high school we want
to emphasis the importance of the conservation
of energy. In other words the amount of energy
that goes into a system is equal to the amount
of energy that comes out of a system. And
I'm using that right now. So where does the
energy of my motion and talking and thinking
and all of that come from? It came form the
food that I ate. The breakfast that I had.
And where did that breakfast energy come from?
It came from the plants. And before that it
came from the sun. And the amount of energy
is going to be conserved. It's not going to
be destroyed. It's going to be conserved over
time. And then finally we can kind of start
to apply this in different levels. And so
if we have a pendulum right here, a pendulum
is going to have a certain amount of potential
energy up here. And as we let it go, it's
going to convert some of that energy into
kinetic energy. And then it's going to go
back to potential to kinetic to potential
to kinetic to potential. Now this is a magical
pendulum which doesn't slow down. And so it
will keep going forever. But if we were to
really play around with a pendulum in the
classroom it's going to slow down. And why
is it going to slow down It's because we're
converting some of that energy into heat.
And so we're going to eventually lose some
of that energy to the environment. But we're
going to conserve the amount of energy that
we have total. And then the last thing that
you want to mention to your students is this
idea that things and energy tend to be unstable
and reach a more stable state over time. And
if we're talking about energy, that eventually
ends up as heat. And so if we were to look
at this train car, this train car is eventually,
all of that order inside of it is eventually
going to become unordered. But the key thing
is that students need to understand that sometimes
this takes a long amount of time for this
to occur. I mean this is going to look a lot
like a train car for a long period of time.
But that doesn't mean that it won't eventually
become a more stable environment when we remove
energy from the system. And so that's energy
transfer. It's important. And it's most important
that your students understand the conservation
of energy. And I hope that was helpful.
