Q.(a) A comb run through one's dry hair attracts small bits of paper. Why? What happens if the hair is wet or if it is a rainy day? (Remember, a paper does not conduct electricity.)
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Start your 14-day free trial to unlock the full solution →All four parts hinge on electrostatic induction and charge flow — the key idea is that insulators hold static charge, conductors allow charge to move, and grounding provides a path for charge to escape. The answers: (a) dry hair charges by friction, attracting neutral paper via induction; wet hair/rain dissipates charge.
(b) Aircraft tyres must be slightly conducting to drain static charge built up during flight, preventing sparks.
(c) Metallic ropes ground the vehicle, draining static charge that could ignite inflammable vapours.
(d) A bird on a single wire has no potential difference across its body; a man touching the same wire while standing on ground completes a circuit to earth, drawing a lethal current.
(a) Comb attracting paper — the classic induction demo
When you run a comb through dry hair, electrons transfer from hair to comb (or vice versa, depending on materials). The comb becomes charged — say, negatively. Paper is an insulator, so charge cannot flow through it. But the comb's electric field repels electrons in the paper's molecules, pushing them to the far side. The near side of the paper is left with a net positive charge (by induction). Opposite charges attract, so the paper jumps to the comb.
If hair is wet, water (slightly conducting) provides a path for charge to leak away — the comb never builds up enough charge. On a rainy day, the air is humid; water molecules in the air also help discharge the comb. So no attraction.
A common mistake is to think the paper itself becomes charged by contact. It doesn't — paper is an insulator, so charge stays localised. The attraction is purely induced polarisation, not net charge transfer.
(b) Aircraft tyres — why slightly conducting?
Aircraft fly through air, dust, and clouds, accumulating static charge by friction with air particles. If the tyres were perfect insulators (like ordinary rubber), the charge would stay on the aircraft body. When the plane lands and the tyres touch the runway, the charge cannot drain quickly — it builds up. When ground crew approach or refuelling begins, a spark could jump, igniting fuel vapours.
By making tyres slightly conducting (by adding carbon black or other conductive fillers), the charge leaks gradually to the ground through the tyres as soon as the wheels touch the runway. This prevents a sudden discharge.
The key phrase in exam answers: "to prevent accumulation of static charge that could cause sparking."
(c) Metallic ropes on fuel tankers
A tanker carrying petrol or other inflammable liquids builds up static charge as the liquid sloshes inside, rubbing against the tank walls. The tank itself is metal (conductor), so charge spreads over its surface. If the tanker is insulated by rubber tyres, the charge stays on the vehicle. When the tanker stops and a metal nozzle approaches, a spark could jump — igniting the vapour.
The metallic rope dragging on the ground provides a continuous conducting path to earth. Charge flows to the ground, keeping the tanker at earth potential. No spark.
Grounding: (same as earth) → no potential difference → no spark.
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