Two otherwise identical metal cans contain equal masses of water initially at 78 °C. One surface is dull black and the other shiny silver. After 300 s in the same room the water temperatures are 59 °C and 67 °C respectively. (a) Calculate each temperature drop and compare the mean cooling rates. (b) Explain the difference. (c) Identify two controls and explain why the result does not isolate radiation perfectly.

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Accepted Answer

Drops: 19 °C and 11 °C; mean rates 0.06333 and 0.03667 °C/s; dull black cools faster. Subtract final temperature from initial temperature. The dull can loses 78 − 59 = 19 °C, while the shiny can loses 78 − 67 = 11 °C over the same interval. Mean cooling rates are 19/300 = 0.06333 °C/s and 11/300 = 0.03667 °C/s. These are interval averages; the instantaneous rate need not remain constant as the water cools. A dull black surface is a better emitter of infrared radiation than a shiny surface at the same temperature. This supports the observed greater cooling, provided other relevant features of the two arrangements are kept equivalent. Control water mass and starting temperature, and also use matching can area, lids and surroundings. Conduction through supports, convection to air and evaporation may also remove energy, so the measured cooling is not solely radiation. Repeat and swap positions to test environmental bias.

Testing thermal radiation 4

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