Air is usually a good insulator. However, a spark can jump through dry air when the electric field strength is greater than about 40,000 V cm⁻¹. This is called electrical breakdown. The spark shows that electrical charge is passing through the air–there is a current. (Do not confuse this with a chemical spark such as you might see when watching fireworks; in that case, small particles of a chemical substance are burning quickly.)
a. A Van de Graaff generator (Figure 21.13) is able to make sparks jump across a 4 cm gap. Estimate the voltage produced by the generator?
b. The highest voltage reached by the live wire of a conventional mains supply is 325 V. In theory (but DO NOT try this), how close would you have to get to a live wire to get a shock from it?
c. Estimate the voltage of a thundercloud from which lightning strikes the ground 100 m below.