A metre stick has its two ends held at 100 C and 0 C, giving a uniform temperature gradient along it. The end of a separate rod is kept at 25 C. At what point along the metre stick should the other end of this rod touch so that, in steady state, no heat flows through the rod?

15 cm from the cold end
25 cm from the cold end
15 cm from the hot end
25 cm from the hot end

Correct answer: 25 cm from the cold end

Solution

Along the stick the temperature varies linearly from 0 C (cold end) to 100 C; the point at 25 C is 25 cm from the cold end, so touching there keeps both rod ends equal and no heat flows.

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