物理学专业英语 仲海洋版9.2节(5)

ical properties change. Most solids and liquids expand when they are heated. A gas, if its pressure is kept constant, will also expand when it is heated, or, if its volume is kept constant, its pressure will rise. If an electrical conductor is heated, its electrical resistance changes. A physical property that changes with temperature is called a thermometric property. A change in a thermometric property indicates a change in the temperature of the object.
Suppose that we place a warm copper bar in close contact with a cold iron bar so that the copper bar cools and the iron bar warms. We say that the two bars are in thermal contact. The copper bar contracts slightly as it cools, and the iron bar expands slightly as it warms. Eventually this process stops and the lengths of the bars remain constant. The two bars are then in thermal equilibrium with each other.
Suppose instead that we place the warm copper bar in a cool running stream. The bar cools until it stops contracting, at the point at which the bar and the water are in thermal equilibrium. Next we place a cold iron bar in the stream on the side opposite the copper bar. The iron bar will warm until it and the water are also in thermal equilibrium. If we remove the bars and place them in thermal contact with each other, we find that their lengths do not change. They are in thermal equilibrium with each other. Though it is common sense, there is no logical way to deduce this fact, which is called the zeroth law of thermodynamics (Fig. 9-1):
Fig. 9-1 If objects ?A? and ?B? are separately in thermal equilibrium with a third object, ?C?, then ?A? and ?B? are in thermal equilibrium with each other
?If two objects are in thermal equilibrium with a third, then they are in thermal equilibrium with each other?.
Two objects are defined to have the same temperature if they are in thermal equilibrium with each other.
9.2.2 The Ideal-Gas Law（理想气体定律）
The properties of gases at low densities allow the definition of the ideal-gas temperature scale. If we compress such a gas while keeping its temperature constant, the pressure increases. Similarly, if a gas expands at constant temperature, its pressure decreases. To a good approximation, the product of the pressure and volume of a low-density gas is constant at a constant temperature. This result was discovered experimentally by Robert Boyle (1627-1691), and is known as Boyle's law:??PV=?constant?(constant temperature)?? A more general law exists that reproduces Boyle's law as a special case. As we know, the absolute temperature of a low-density gas is proportional to its pressure at constant volume. In addition-a result discovered experimentally by Jacques Charles (1746-1823) and Joseph Gay-Lussac(1778-1850)-the absolute temperature of a low-de
nsity gas is proportional to its volume at constant pressure. We can combine these two results by stating ??PV=CT(9-1)??where ?C? is a constant

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