When thunderclouds form, the base of the cloud is generally about 2 km above the surface of the Earth, while the top of the cloud may extend to about 8 km above the Earth. The charge structure is quite complicated. The top of the cloud has a strong positive charge; the middle of the cloud, a strong negative charge; and the bottom of the cloud, a weaker positive charge. Meanwhile the cloud induces a positive charge on the surface of the Earth. (See figure.) The resulting potential difference between the bottom of the cloud and the ground is around 108 volts.
When lightning strikes, some of the negative charge of the cloud neutralizes the positive charge of the Earth. Approximately 4 Coulombs of negative charge pass from the ground to the cloud, forming a current of 20 kamps. This results in a huge release of energy in the form of dissociation, ionization, and excitation of molecules in air, the heating and expanding of gas, and electromagnetic radiation.
In places especially prone to lightning, it is helpful to install lightning rods. A lightning rod is a long piece of metal with one end embedded in the ground and the other end extending up higher than the surrounding buildings. The end in the air comes to a sharp point. The purpose of such a piece of metal is to conduct electrons from the Earth into the air and reduce the charge imbalance, thus reducing the probability of lightning. If lightning strikes anyway, it is more likely to strike the lightning rod than the buildings.
Which of the following is most likely to result in the light associated with lightning?