Assuming that a positive charge Q is uniformly distributed over the surface of a shell, the field at a distane r from the centre of the shell where r = 3R (R being the radius of the shell ), is

Options

A. `(Q)/(epsilon_(0) rpi(2R)^(2))`
B. `(Q)/(epsilon_(0) rpi R^(2))`
C. `(Q)/(epsilon_(0) rpi(3R)^(2))`
D. none of these

Answer

C

Explanation

It is as if the whole charge is concentrated at the centre of the shell.
By Gauss.s theorem,
`E = (Q)/(epsilon_(0)(4pi r^(2))) = (Q)/(epsilon_(0)4pi (3R)^(2)) (because r = 3R)`