# GCSE Additional physics - NEED HELP =D

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1. Jun 10, 2008

### olliebellamy

1. The problem statement, all variables and given/known data

A beam of electrons leaves an electrom gun. The current carried by the beam is 4mA. a) How many coulombs of charge pass through a certain point in the beam per second. b) How many electrons pass this point per second?

2. Relevant equations

KE(j) = Charge of electrons (e) X Accelerating voltage (V)

3. The attempt at a solution

I need to find coulombs, and when only given '4mA' i do not see how i can work this out. I have my exam tomorrow afternoon and need to try to get this cleared up, please help =S

Last edited: Jun 10, 2008
2. Jun 10, 2008

### Ed Aboud

Hey olliebellamy, welcome to PF.

First off, I think you are over complicating things.

3. Jun 10, 2008

### Kurdt

Staff Emeritus
How is the ampere defined?

4. Jun 10, 2008

### olliebellamy

Blooming forums went down, sorry for not replying.

ok, i've re-looked at the question. i have no idea how to work out the answer. where do i start =P

5. Jun 10, 2008

### Kurdt

Staff Emeritus
The forum is undergoing a software upgrade so its the same for everyone at the minute.

Like I mentioned before, what is the definition of an ampere, or equally as good the definition of a coulomb? If you find that out it will help you with the question in hand.

6. Jun 10, 2008

### olliebellamy

in the revision guide i am using, there isn't any reference to the ampere with the Elecrtron Beam section. Just this question with no other information, except that :

the charge on an electron is -1.6x10^-19C

the book i am using is appauling -.-

7. Jun 10, 2008

### Kurdt

Staff Emeritus
OK well 1 coulomb of charge is the amount of charge that passes a point in a second when a current of 1 ampere is present. So if a current of 4mA is present, how much charge passes a point in a second?

8. Jun 10, 2008

### olliebellamy

4000? or 0.004?

9. Jun 10, 2008

### Kurdt

Staff Emeritus
Check what milli means again.

10. Jun 10, 2008

### olliebellamy

milli is 1/1000 - still i'm confused =[

11. Jun 10, 2008

### lukas86

If the current is smaller (4mA < 1A), what does that say about what the charge passing? Would it be larger or smaller?

12. Jun 10, 2008

### Kurdt

Staff Emeritus
Sorry, I posted before you edited. The latter is correct, and remember your units if this is for an exam.

13. Jun 10, 2008

### olliebellamy

OK - 4mA would be smaller than 1A. In that case i guess the charge would be greater

14. Jun 10, 2008

### olliebellamy

can i not just have an answer to thie question from somebody? possibly with an explanation of how they came around with this answer?

15. Jun 10, 2008

### lukas86

50/50 chance there, Try again :P. If you have a smaller current flowing, less electrons (therefore: charge) would be flowing. Making sense... somewhat?

16. Jun 10, 2008

### olliebellamy

no sense what so ever....i'm gonna skip this section right now and move onto Work, power and energy......at least i can follow some simple formulae for this subject

i'll just hope that i dont have to work out a coulomb in the exam....cos i realy can't do it

17. Jun 10, 2008

### Kurdt

Staff Emeritus
We don't give out answers on this forum as you'll have read when you agreed to the rules when signing up. You had the answer before. 0.004 Coulombs.

The formula you were using effectively was $Q = I t$.

18. Jun 10, 2008

### olliebellamy

thanks so much, if that formula could have just been displayed for my knowledge in my revision guide, i would have had nothing to worry about.

now i know the formula, i shalln't forget it,