Chapter 9, Problem 18TE

You can comfortably hold your fingers close beside a candle flame, but not very close above the flame. Why?

To determine

The reason behind the fact that you can comfortably hold your fingers close beside a candle flame, but not very close above the flame.

Answer to Problem 18TE

The reason behind the fact that you can comfortably hold your fingers close beside a candle flame, but not very close above the flame, is that when heated air rises upwards and it results in heating your hand.

Explanation of Solution

In case of a lightened candle, the heat transfer is via convection. When you hold your hand above the flame, then it is via radiation, the heat is transferred when you place your hand beside the flame.

As the heat increases, the density of air decreases, and hence it moves upwards and starts burning your hand. Since there is no rise in heat in the sidewise direction, the air acts as an insulator there and prevents your hand from getting heated.

Conclusion:

Therefore, the reason behind the fact that you can comfortably hold your fingers close beside a candle flame, but not very close above the flame, is that when heated air rises upwards and it results in heating your hand.