# 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:

## Answers and Replies

Ed Aboud
Hey olliebellamy, welcome to PF.

First off, I think you are over complicating things.
Read the question more carefully.

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 $Q = I t$.