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A copper wire of fixed volume V is stretched out to a length l. If a constant force F is applied to this wire and it elongates by Δl, which of the following plots would be a straight line?

1. Δl against 1/l
2. Δl against l²
3. Δl against 1/l²
4. Δl against l

Correct answer: Δl against l²

Solution

With fixed volume V, cross-section A = V/l, so Delta-l = F*l/(A*Y) = F*l^2/(V*Y). Hence Delta-l is proportional to l^2, giving a straight line for Delta-l vs l^2.

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