Electric current is the movement of mobile charge carriers. It is measured in amperes — coulombs per second. With positive and negative charge carriers we have to take into consideration the total charge going in each direction. So, if we have a conductor with:
then for positive charge we have:
coulombs of charge passing a point in one second. Similarly for negative charge we have:
This gives us in total:
Q++Q- = (qpnpvp + -qnnn-vn)ΔtA
Now, current is the rate of charge flow, so:
I = (Q++Q-)/Δt = (qpnpvp + -qnnn-vn)A
Note that in a metal, we only have one type of mobile charge carrier, the electron, so:
I = Q-/Δt = -qnnn-vnA
The current divided by the area (I/A in our notation) gives us what is known as the current density.
Fischer-Cripps. A.C., The Electronics Companion. Institute of Physics, 2005.
Copyright © 2014 Barry Watson. All rights reserved.