Calculate Area of Long Conducting Cylinder

[Quadruple Bypass]

Homework Statement

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

 

[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 $\pi (b^2- a^2)$.

 

[Quadruple Bypass]

thank you kind sir