The dome of a machine is given a negative charge. A student slowly brings an earthed metal sphere towards the dome. When the gap is 3 cm, a spark jumps across the gap.
Explain, in terms of the electric field, why the spark jumps when the gap reaches 3 cm.
The machine is switched off so that no more charge is added to the dome. The student repeats the test.
Suggest why the spark now jumps only when the gap is smaller than 3 cm.