University of California, San Diego
Physics 1b - Thermal Physics & Electromagnetism

H. E. Smith   Spring 2000

Physics 1B - Tutorial #2

  1. Very cold ice is placed in a closed container. Heat is then applied to the container. Below is a graph of the temperature inside the container plotted against the amount of heat delivered to the system.

  2. Materials A and B have equal densities. But A has twice the specific heat of B. You have two 100 g cubes made of materials A and B.

  3. A thermometer is laid out in direct sunlight. Does it measure the temperature of the air, of the sun, or of something else? The thermometer always measures the temperature of the thermometer. Its temperature is indicative of the temperature of another object only if it is in thermal equilibrium with the object (that's why your Mother makes you hold the thermometer under your tongue for so long - to establish thermal equilibrium. In principle, if the thermometer had perfect thermal radiative properties (called a blackbody) it would reach thermal equilbrium with the sun by radiative processes. Since the thermometer doesn't have perfect radiative properties (for one thing it would melt before reaching 6000K), in this case, its temperature is neither that of the sun nor that of the air.

  4. The inside of an oven is at 400° F. But you can still put your hand in the oven as long as you don't touch anything. But since the air inside the oven is also at 400°F, why isn't your hand burned just the same? What if you leave your hand in the oven for a long time (e.g. 2 hours)?

    Air is a very poor thermal conductor. (thousands of times less than that of iron). Therefore, its heat cannot be effectively transferred to your hand. If you touch the shelves made of iron, heat can be transferred quickly and you will be burned. If you leave your hand in the oven for too long, however, even just the air will transfer enough thermal energy to roast you.

  5. While jogging, an average 65 kg student generates thermal energy at a rate of 300 W (about 0.4 horse power). To maintain a constant body temperature of 37°C, this energy has to be dissipated by perspiration or other mechanisms.

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Gene Smith

Last modified: Thurs., 13 Apr 2000