# Surface Charge Problem

1. Sep 19, 2004

### pulser45

Hi, I need help with a surface charge problem:

The cylindrical drum of a copy machine is 40 cm long and has a radius of 5 cm. If charge is uniformly distributed over the curved surface of the drum, and the electric field near the surface is 2 x 10^5 N/C, estimate the amount of charge on the drum.

I know you set the electric flux to Q/E0. I calculated the Q = o*2pi*r*L. Since electric flux E(2*pi*r*L) should cancel out 2pi*r*L when setting them both equally, leaving E=o/E0. However, i still keep getting the wrong answer. Thanks for the help.

2. Sep 19, 2004

### Galileo

It looks ok to me. You probably used your Gaussian surface (cilinder) having the same radius as the cilinder itself, because you know the field very near the surface. This way you can cancel the r's, giving o=E/E0.
So multipy the charge density by the surface area to get the total charge.

Alternatively, you could've used E(2*pi*r*L)=Q/E0
and plug in the numbers immediately to find Q.

3. Sep 19, 2004

### pulser45

Wouldn't o=E*E0 ? So i would just multiply o by 2*pi*r*L and I would be fine?

My professor adopted this online homework website from ilrn. IMHO, its horrible, because it will only accept the exact answer that was entered. No partial credit given at all because you don't even show your work. Thanks.