# Homework Help: Current and Avogadro's Number

1. Sep 18, 2008

### Ithryndil

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

The electron beam emerging from a certain high-energy electron accelerator has a circular cross section of radius 1.20 mm.
(a) The beam current is 7.75 µA. Find the current density in the beam assuming it is uniform throughout.
correct check mark A/m2

(b) The speed of the electrons is so close to the speed of light that their speed can be taken as 300 Mm/s with negligible error. Find the electron density in the beam.
correct check mark m-3

(c) Over what time interval does Avogadro's number of electrons emerge from the accelerator?
s

2. Relevant equations

$$J=I/A$$
[tex]I_{avg} = nqv_{d}A

3. The attempt at a solution

Part a and b are straight forward.

For part a I have: 1.71 A/m^2

For part b I have: 3.565 x 10^10 m^-3

I am having issues with part c. I know I need to figure out how many electrons are leaving the wire per second and then from there it should be a straight division problem using the 6.022 x 10^23 for Avogradro's number.

2. Sep 18, 2008

### LowlyPion

Think of it as a bucket. How long to fill'er up.

So what's the definition of an ampere?

3. Sep 18, 2008

### Ithryndil

Right...an ampere is a coulomb per second. So we take the current which is a coulomb per second and divide it by the elementary charge to get the number of electrons per second. Afterwards it's a simple division of avogadro's number by the aforementioned number...far easier than I anticipated. Sometimes your mind can just be clouded.