# This is - number of charges trough

1. Jul 23, 2006

This is urgent - number of charges trough...

hello

I can't figure out why the next formula tells us the number of charges that move trough cross-sectional area of the conductor.

If a segment of wire has N charges moving with average velocity v, then in time [delta]t the number of charges is

N * A * v * [delta]t

A is the cross-sectional area of the conductor
N is number of charged particles per unit volume

thank you

2. Jul 23, 2006

### Gokul43201

Staff Emeritus
The distance traveled by a charge in time $\delta t$ is, on average, $v\delta t[/tex]. Construct a plane that cuts through the wire normally. We want to count the number of electrons passing through this plane in some time [itex]\delta t$. Now, if all the electrons have velocity v, then for an electron to pass through the plane at some time t, it must have been no farther back than $v\delta t[/tex] at a time [itex]t-\delta t$. In other words, only those electrons contained within the cylinder (for a wire of circular cross-section) of length $v\delta t[/tex] and area A, at a time [itex]t-\delta t$ will be able to pass through the wall in the ensuing time [itex]\delta t[/tex]. This number is simply the product of the volume of this cylinder, [itex]Av\delta t[/tex] and the number density, N.