UniversityPhysicsVolume2-Ch01.  Morgan State University PHYS 205L

University Physics Volume II
Unit 1: Thermodynamics
Chapter 1: Temperature and Heat
Conceptual Questions
1. What does it mean to say that two systems are in thermal equilibrium?
Solution
They are at the same temperature, and if they are placed in contact, no net heat flows between
them.
2. Give an example in which A has some kind of non-thermal equilibrium relationship with B,
and B has the same relationship with C, but A does not have that relationship with C.
Solution
One possibility is that in some game, A and C are both equally matched with B, but C
consistently beats A, maybe because C’s style is hard for A to cope with or C has some
psychological advantage. Another is that if two people of different weights lean at the same angle
against a wall, they are each in equilibrium with the wall, but they would not be in equilibrium
with each other. Other answers are possible.
3. If a thermometer is allowed to come to equilibrium with the air, and a glass of water is not in
equilibrium with the air, what will happen to the thermometer reading when it is placed in the
water?
Solution
The reading will change.
4. Give an example of a physical property that varies with temperature and describe how it is
used to measure temperature.
Solution
Volume—as the volume of liquid in an ordinary thermometer increases, the liquid’s interface
with air rises along a scale. Electrical resistance—the resistance of a piece of silicon in a
thermistor decreases with increasing temperature and is read electronically. Color—certain
chemical reactions cause color changes at certain temperatures, so a set of spots containing such
chemicals with slightly different color-change temperatures can indicate temperature. Radiated
power and frequency spectrum—a bolometer can be pointed at an object, especially one that is
too hot or too distant to put a thermometer in contact with, and the radiated power or peak
frequency can be measured and converted into a temperature. Other answers are possible.
5. Pouring cold water into hot glass or ceramic cookware can easily break it. What causes the
breaking? Explain why Pyrex®, a glass with a small coefficient of linear expansion, is less
susceptible.
Solution
The cold water cools part of the inner surface, making it contract, while the rest remains
expanded. The strain is too great for the strength of the material. Pyrex contracts less, so it
experiences less strain.
6. One method of getting a tight fit, say of a metal peg in a hole in a metal block, is to
manufacture the peg slightly larger than the hole. The peg is then inserted when at a different
temperature than the block. Should the block be hotter or colder than the peg during insertion?
Explain your answer.
Solution
The block should be hotter so the hole will be bigger.
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