A definition of temperature in terms of concepts that are independently defined or accepted as primitive is difficult to give. However, it is possible to arrive at an objective understanding of equality of temperature by using the fact that when the temperature of a body changes, other properties also change.

To illustrate this, consider two copper blocks, and suppose that our senses tell us that one is warmer than the other. If the blocks were brought into contact and isolated from their surroundings, they would interact in a way that can be described as a thermal (heat) interaction.

During this interaction, it would be observed that the volume of the warmer block decreases somewhat with time, while the volume of the colder block increases with time. Eventually, no further changes in volume would be observed, and the blocks would feel equally warm.

Similarly, we would be able to observe that the electrical resistance of the warmer block decreases with time, and that of the colder block increases with time; eventually the electrical resistances would become constant also. When all changes in such observable properties cease, the interaction is at an end.

The two blocks are then in thermal equilibrium. Considerations such as these lead us to infer that the blocks have a physical property that determines whether they will be in thermal equilibrium. This property is called temperature, and we may postulate that when the two blocks are in thermal equilibrium, their temperatures are equal.

The rate at which the blocks approach thermal equilibrium with one another can be slowed by separating them with a thick layer of polystyrene foam, rock wool, cork, or other insulating material. Although the rate at which equilibrium is approached can be reduced, no actual material can prevent the blocks from interacting until they attain the same temperature.

However, by extrapolating from experience, an ideal insulator can be imagined that would preclude them from interacting thermally. An ideal insulator is called an adiabatic wall. When a system undergoes a process while enclosed by an adiabatic wall, it experiences no thermal interaction with its surroundings.

Such a process is called an adiabatic process. A process that occurs at constant temperature is an isothermal process. An adiabatic process is not necessarily an isothermal process, nor is an isothermal process necessarily adiabatic.

It is a matter of experience that when two bodies are in thermal equilibrium with a third body, they are in thermal equilibrium with one another. This statement, which is sometimes called the zeroth law of thermodynamics, is tacitly assumed in every measurement of temperature.

Thus, if we want to know if two bodies are at the same temperature, it is not necessary to bring them into contact and see whether their observable properties change with time, as described previously. It is necessary only to see if they are individually in thermal equilibrium with a third body.

The third body is usually a thermometer.

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1 comment:

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