Inductive charging differs from non-inductive charging in that it
Answer: C
A particle polarised by the ambient field exchanges charge on contact with another. Because it needs a field to begin with, it can amplify but not initiate the electrification. Something else must produce the first field for it to act on.
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State whether the following is true or false.
Inductive charging can initiate thunderstorm electrification on its own.
Answer: D
Correct answer: False
Inductive charging requires an existing electric field to polarise the particles before a collision can separate charge, so it cannot start the process - it can only amplify a field that already exists. The non-inductive graupel-ice mechanism needs no field and is therefore the one that must initiate electrification. Inductive charging may contribute later, once the field has grown.
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The electric field required for breakdown in dry air at sea level is approximately
Answer: A
Correct answer: 3 million volts per metre
The breakdown strength of dry air at sea level is about 3 MV per metre. Fields within thunderclouds rarely exceed a tenth of that, partly because the air is less dense at altitude and partly because breakdown begins locally at hydrometeors and on sharp points long before the average field reaches the nominal value. How lightning is initiated at these lower fields remains an open question, with runaway breakdown one proposed answer.
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Assertion (A): Measured fields inside thunderclouds are lower than the conventional breakdown value.
Reason (R): Lightning nonetheless initiates, so some other process must lower the effective threshold.
Answer: B
Both are true and the reason gives the puzzle, and runaway electron breakdown and local field enhancement on hydrometeors are among the proposals. The initiation problem is not yet fully settled. Balloon and aircraft measurements continue to be made.
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Consider the following statements about lightning initiation.
1. Fields measured in clouds fall short of conventional breakdown.
2. Local enhancement near hydrometeors may bridge the gap.
3. The initiation mechanism is completely understood.
Answer: C
The first two are correct and describe the current position. The third overstates matters considerably, since initiation remains one of the least settled questions in the subject. Decades of measurement have not closed the gap.
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Arrange the stages of a negative cloud to ground flash in order.
1. attachment completes the channel to the cloud
2. the stepped leader descends in discrete steps
3. the return stroke propagates rapidly upward
4. upward streamers rise from objects on the ground
Answer: D
Correct answer: 2 - 4 - 1 - 3
The stepped leader descends from the cloud in discrete steps of about fifty metres, carrying negative charge and lowering the potential of the air below it; as it approaches, upward streamers rise from trees, buildings and high points; one of them attaches to the leader, completing a conducting path from cloud to ground; and the return stroke then propagates rapidly upward along that path, carrying the large current and producing the visible flash.
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The bright flash of a lightning discharge is chiefly
Answer: A
The return stroke carries the great majority of the current and produces nearly all the light. The leader that preceded it is orders of magnitude fainter and is not normally seen. Almost all the emitted light comes from the return stroke.
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Assertion (A): Lightning appears to strike downward although the bright stroke travels upward.
Reason (R): The eye cannot resolve the sequence, and the channel was established from above beforehand.
Answer: B
Both are true and the reason gives the illusion, and high speed photography resolves the two stages clearly. The channel geometry is set by the descending leader throughout. The upward stroke merely follows the path already made.
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The peak current in a typical negative return stroke is of the order of
Answer: C
Correct answer: 30 kiloamperes
A typical negative first return stroke peaks near 30 kA, rising in a few microseconds and decaying over tens of microseconds. Positive flashes are less frequent and often carry far larger currents and much greater charge transfer. It is this current, in a channel a few centimetres across, that heats the air to some 30,000 kelvin and produces both the light and the shock wave heard as thunder.
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State whether the following is true or false.
A single lightning flash may consist of several return strokes along the same channel.
Answer: D
Correct answer: True
Most negative cloud-to-ground flashes contain three or four return strokes, and some contain more than ten, separated by tens of milliseconds. After the first stroke a dart leader runs down the still-warm channel and initiates the next. The successive strokes are what produce the flickering appearance of a flash, and they matter to protection design, since a struck conductor is subjected to the whole sequence.
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