A rod of length l and cross-sectional area A conducts heat to melt ice. The rod is then cut into two equal halves which are placed side by side (in parallel) for the same conduction setup. The time taken to melt the same amount of ice now becomes:

Half
One-fourth
Twice
Four times

Correct answer: One-fourth

Solution

Each half has length l/2 and the two are in parallel (area 2A effective), so the conductance increases by factor 4, and time (inversely proportional to rate) becomes one-fourth.

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