1. You decide to have a cup of coffee with cream. When first poured, the coffee is too hot to drink. Should you add the cream immediately or after several minutes, if you wish to cool the coffee down as quickly as possible? Explain your conclusion, using the equation for heat transfer via conduction.
2. If you were to walk briefly in space without a spacesuit while far from the Sun (as an astronaut does in the movie 2001 ), you would feel the cold of space: while you radiated energy, you would absorb almost none from your environment. (a) At what rate would you lose energy, assuming that the T of space is 2.7 K? (b) How much energy would you lose in 30 s? Assume that your emissivity is 0.90 and estimate other data needed in the calculations.
3. The following solid objects, made of the same material, are maintained at a temperature of 300 K in an environment whose temperature is 350 K: a cube of edge length r, a sphere of radius r, and a hemisphere of radius r. Rank the objects according to the net rate at which thermal radiation is exchanged with the environment, greatest first.
Answers
Chapter T3:
S7: -68 C
S6: 483 K
2. Answers will vary, but on order of 1000W, 28 kJ
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Last updated January 24, 2005