Hey all,

am having difficulty with this question:

 

Calculate how many conduction electrons pass through a given area per second when a current of 1A flows.

 

I've used these formulae:

Current density, j = i/A (i=current, A=cross sectional area)

Current desntiy, j = nevD (n=no of electrons per unit volume, e=charge on electron, vD=drift velocity)

 

I've taken an element of volume of length vDt and cross-sect area, A. I've surmised then that the total number of electrons in that area will be N=nAvDt.

 

All of those electrons will then cross a plane in 1 second, so i've equated j=i/A=nevD, and substituted n from the earlier (bold) equation - n=N/AvD. However, that produces a result of N=i/e=1/e ... doesn't look right! Help!!

You may be overthinking this.

 

What is the definition of an Ampere?

^ The rate of flow of charge, Q/t. I see what you're saying, I just wondered if perhaps there was more to it... the question was set after a full derivation of Drude's formula for the conductivity making me think it was more complicated

Is it possible it was a precursor to other questions? Once you have the number of electrons, you can then e.g. go on to show that the drift velocity is very small, showing that while electricity is very fast, the charges themselves don't move very fast at all.