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Cloud Electrification and Lightning Questions and Answers

71.

Assertion (A): A thunderstorm can be regarded as an electrical generator driven by convection.
Reason (R): The updraught does mechanical work against the electrostatic attraction between charges.

Answer: B

Both are true and the reason gives the energetics, and the ultimate source is latent heat released by condensation. The electrical energy is a small fraction of the total energy the storm processes. Latent heat dwarfs the electrical output entirely.

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72.

The energy dissipated in a typical lightning flash is of the order of

Answer: C

Correct answer: one gigajoule

A typical flash transfers a few tens of coulombs through a potential difference of tens or hundreds of megavolts, giving an energy of the order of a gigajoule - roughly the chemical energy in thirty litres of petrol. Most of it goes into heating and expanding the channel, into light and into radio emission, and the fraction available at the strike point is small. It is far too diffuse and too brief to be worth capturing.

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73.

State whether the following is true or false.
The electrical energy in lightning is a small fraction of a thunderstorm's total energy budget.

Answer: D

Latent heat release dwarfs the electrical output by many orders of magnitude. Lightning is a conspicuous but energetically minor product of the storm. Visibility and energy are quite unrelated here. The storm's heat output exceeds its electrical output vastly.

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74.

Volcanic lightning is thought to arise chiefly from

Answer: A

Correct answer: frictional charging of ash particles in the plume

Volcanic plumes generate lightning at heights and temperatures where no ice exists, so the graupel mechanism cannot apply. Charging is attributed instead to fracto-emission as the magma shatters and to triboelectric charging as ash particles collide in the turbulent plume. Where the plume rises above the freezing level the ice mechanism may then contribute as well, giving a second lightning maximum higher up.

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75.

Consider the following statements about volcanic lightning.
1. Frictional charging of ash contributes near the vent.
2. Ice charging can operate higher in a tall plume.
3. It requires no particles in the plume at all.

Answer: B

The first two are correct and describe the two regimes. The third is nonsensical, since charge separation requires particles to collide and separate in the first place. No particles means no charge separation of any kind.

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76.

Lightning strikes to aircraft

Answer: C

Airliners are struck roughly once a year on average and are designed to conduct the current over the skin. Bonding and static discharge wicks are part of the standard design. Certification requires demonstrated lightning tolerance.

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77.

Assertion (A): An aircraft usually suffers little damage from a lightning strike.
Reason (R): The conducting skin carries the current around the occupants and the systems.

Answer: D

Both are true and the reason gives the protection, and this is an application of the Faraday cage principle. Composite airframes require conducting mesh to achieve the same effect. Metal skins provide the shielding naturally.

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78.

All of the following are elements of aircraft lightning protection EXCEPT

Answer: A

Correct answer: restricting flight to below the freezing level

Flying below the freezing level would neither avoid lightning - strikes occur there, and aircraft frequently trigger their own - nor be operationally possible. Protection instead accepts the strike and manages it: a continuous conducting path across the airframe so the current passes over the skin and out, conducting mesh in composite panels which are not naturally conductive, and static wicks to bleed off accumulated charge.

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79.

A Faraday cage protects its contents by

Answer: B

Charge redistributes on the conducting surface and the interior field remains near zero. A car body acts in the same way, which is why a vehicle is a reasonably safe place in a storm. The shielding comes from the shell, not the tyres.

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80.

State whether the following is true or false.
A car offers reasonable protection from lightning because of its metal body rather than its rubber tyres.

Answer: C

The tyres are irrelevant to a discharge that has already crossed kilometres of air. The conducting shell carries the current around the occupants. Rubber cannot insulate against such a discharge. The tyres play no part in the protection at all.

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