Question

Electric field between 200 m high and 250 m thick cloud and the surface of earth is 120 v/m directed vertically. At the top of the cloud is +q and the bottom of cloud is –q making the cloud neutral. Calculate the magnitude of the external electrical force on the cloud.

Answer 1

The electric field is only between the lower surface of cloud and earth surface. Thus the upper surface charge of the cloud will not experience any force. The net external force on the cloud is equal to the electric force on the lower surface charge.

Now we know, electric force acting on a charge placed in the region containing electric field is given by F = qE but here the charge is distributed over the cloud surface and is not a point charge so this formula is not applicable directly. However if we think more carefully we find that if we assume this distributed charge to be comprised of infinite point charges and the force on each of them will add up to the value

F = - q*120 N