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A heat source S is placed near a hand held at three different positions, with the dominant mode of heat transfer at the three positions labelled a, b and c. The hand in contact transfers heat one way, the hand held above receives rising hot air, and the hand held to the side receives heat across the gap. Which matching is correct?

1. a - conduction; b - convection; c - radiation
2. b - conduction; a - convection; c - radiation
3. a - conduction; c - convection; b - radiation
4. c - conduction; b - radiation; a - convection

Correct answer: a - conduction; b - convection; c - radiation

Solution

The three principal modes of heat transfer are distinguished by their mechanism. Position a (direct contact with the source) transfers heat by conduction. Position b (above the source) is warmed by hot air rising from the source, which is convection. Position c (to the side, across a gap) is warmed mainly by infrared radiation, which needs no medium and travels in all directions.

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