Question

Charge is distributed uniformly on the surface of a large flat plate the electric field 2 cm from the plate is 30 N/C .what is the electric field 4 cm from the plate?

Answer 1

The electric field of uniformly charged large flat plate equals to: $E = \dfrac{\sigma}{2\varepsilon_0},$ where $\sigma$ is charge per unit area, $\varepsilon_0$ is electric constant (see http://farside.ph.utexas.edu/teaching/302l/lectures/node27.html).

Thus, the electric field does not depend on the distance to the plate as long as this distance is much less then plate sizes. This means, that electric field 2 cm from the plate is equal to the electric field 4 cm from the plate and equal to 30 N/C.

Answer. 30 N/C