Reason for charge buildup on moving vehicles

When a vehicle moves, especially one carrying inflammable fluids like petrol or other fuels, friction with air can cause static electric charge to accumulate on the body of the vehicle.

Why this is dangerous

If excess charge remains on the vehicle, a potential difference can develop. Then a sudden discharge may occur in the form of a spark.

Since the vehicle carries inflammable fluid, even a small spark can be hazardous and may ignite the fuel vapour.

Role of the metallic chain

The metallic chain touches the ground continuously, so it provides a conducting path for the accumulated charge to flow into the earth.

This is called earthing. It prevents charge buildup and reduces the risk of sparking.

Checking the options

• A: To protect tyres from catching dirt from ground
  This has nothing to do with the metallic chain. Incorrect.

• B: It is a custom
  This is not a scientific reason. Incorrect.

• C: To alert other vehicles
  The chain is not meant for signaling. Incorrect.

• D: To conduct excess charge due to air friction to ground and prevent sparking
  This correctly explains the purpose of the metallic chain. Correct.

Final Answer

The correct option is: $\boxed{\text{D}}$