Estimate the potential of the cloud relative to the ground

1. Apr 30, 2005

Clari

Hello! here is one question which I still donno how to solve:

A thunder cloud with a large flat square base measuring 3000m x 3000m passes over an area on level ground. Assuming the cloud carries a total charge of 40C which is spread evenly over its base and is at a height of 0.5 km above the ground, estimate the potential of the cloud relative to the ground.

My solution: by V = 1/(4*pi*epsilon) Q/r^2
Potential of the cloud relative to the ground = (9x10^9) 40/ 0.5x10^3 = 7.2 x10^8 V