A thundercloud has the charge distribution illustrated below left. Treat this distribution as two point charges, a negative charge -30 C at a height of 2 km above ground and a positive charge +30 C at a height of 3 km. The presence of these charges induces charges in the ground. Assuming the ground is a conductor, it can be shown that the induced charges can be treated as a charge of +30 C at a depth of 2 km below ground and a charge of -30 C at a depth of 3 km, as shown below right. Consider point P_{1}, which is just above the ground directly below the thundercloud and point P_{2}, which is 1 km horizontally away from P_{1}. |