# GCSE Additional physics - NEED HELP =D

olliebellamy

## 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:

Ed Aboud
Hey olliebellamy, welcome to PF.

First off, I think you are over complicating things.

Staff Emeritus
Gold Member
How is the ampere defined?

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

Staff Emeritus
Gold Member
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.

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 -.-

Staff Emeritus
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?

olliebellamy
4000? or 0.004?

Staff Emeritus
Gold Member
Check what milli means again.

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

lukas86
If the current is smaller (4mA < 1A), what does that say about what the charge passing? Would it be larger or smaller?

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

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

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?

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?

olliebellamy
no sense what so ever...i'm going to 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 don't have to work out a coulomb in the exam...cos i really can't do it

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