A short chain is usually attached to the rear side of petro tanker trailing behind it to ensure that the

Step 1: Analyze the question The question asks why a short chain hangs from the back of a petrol tanker and drags along the ground.

Step 2: The concept When a tanker moves, friction occurs between the tyres and the road, and between the petrol and the tank walls as it sloshes around. This friction produces static electric charge. The charge builds up on the body of the tanker.

If enough charge accumulates, it can discharge as a spark to the ground or to a nearby object. Petrol vapour is highly flammable, so a spark could cause a fire or an explosion.

Step 3: How the chain helps The chain is a metal conductor that touches the ground. It provides a path for the accumulated charge to flow safely to the earth (earthing), so no dangerous charge builds up.

Step 4: Check each option

• A. Charges generated by friction are conducted to the earth. This is correct, and it is the purpose of the chain.
• B. Resonance with the engine has nothing to do with it, and resonance would not be a desirable effect anyway.
• C. The chain is there to deal with electric charge, not heat. Heat from the engine is not removed through a chain.
• D. A short chain does nothing to balance the tanker.
• E. The chain does not limit speed.

Answer: A

Exam tip: Whenever you see "chain", "petrol tanker" or "lightning conductor" in a question, think earthing. The aim is always to let charge flow safely to the ground and prevent sparks.