# GCSE Additional physics - NEED HELP =D

## Homework Statement

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?

## Homework Equations

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

## 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:

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Hey olliebellamy, welcome to PF.

First off, I think you are over complicating things.

Kurdt
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How is the ampere defined?

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

Kurdt
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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.

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 -.-

Kurdt
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Gold Member
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?

4000? or 0.004?

Kurdt
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Check what milli means again.

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

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

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

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

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

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

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

Kurdt
Staff Emeritus
Gold Member
can i not just have an answer to thie question from somebody? possibly with an explanation of how they came around with this answer?
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 $Q = I t$.

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,