38. The electric field of Earth always points toward Earth. The magnitude of the field strength varies locally from as low as 100 N/C in fair weather to 20 000 N/C in a thunderstorm. A field mill measures the local electric field strength. In this device, the lower plate, parallel to the ground, is connected to Earth through an ammeter. The upper plate can be moved horizon- tally, and it, too, is connected to Earth. (a) When the mill is arranged as in Figure 6(a), what kind of charge is on the surface of Earth and on each plate? (Hint: Examine the field lines.) (b) What will the ammeter show when you move the upper plate rapidly over the lower plate, as in Figure 6(b)? Explain your answer. (c) What will the ammeter show when the upper plate is quickly pushed away from the lower plate? Explain your answer.

38. The electric field of Earth always points toward Earth. The magnitude of the field strength varies locally from as low as 100 N/C in fair weather to 20 000 N/C in a thunderstorm. A field mill measures the local electric field strength. In this device, the lower plate, parallel to the ground, is connected to Earth through an ammeter. The upper plate can be moved horizon- tally, and it, too, is connected to Earth. (a) When the mill is arranged as in Figure 6(a), what kind of charge is on the surface of Earth and on each plate? (Hint: Examine the field lines.) (b) What will the ammeter show when you move the upper plate rapidly over the lower plate, as in Figure 6(b)? Explain your answer. (c) What will the ammeter show when the upper plate is quickly pushed away from the lower plate? Explain your answer.