# Conducting cylinder

1. Dec 7, 2006

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

a long conducting cylinder with inner radius a and outer radius b carries a current along its central axis. blah blah blah, i found the current density.

now how do i calculate the area of the cylinder with a hole in it?

is it A = pi (b-a)^2 OR is it A = pi ( (a+b)/2)^2

2. Dec 7, 2006

### HallsofIvy

Neither one! The area of the base is the area of the larger circle, $\pi b^2$, minus the area of the inner circle, $\pi a^2$, or $\pi (b^2- a^2)$.

3. Dec 7, 2006