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# 3: Temperature

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During our studies of heat and thermodynamics, we shall come across a number of simple, easy-tounderstand terms such as entropy, enthalpy, Gibbs free energy, chemical potential and fugacity, and we expect to have no difficulty with these. There is, however, one concept that is really quite difficult to grasp, and that is temperature. We shall do our best to understand it in this chapter.

• 3.1: Zeroth Law of Thermodynamics
Perhaps the simplest concept of temperature is to regard it as a potential function whose gradient determines the direction and rate of flow of heat. If heat flows from one body to another, the first is at a higher temperature than the second. If there is no net flow of heat from one body to another, the two bodies are in thermal equilibrium, and their temperatures are equal.
• 3.2: Temperature Scales I
In everyday practice, we use either the Celsius or the Fahrenheit temperature scales, depending on what we are used to, or the fashion of the day, or what our Government tells us we should be using.
• 3.3: Temperature Scales II
We now know – by definition – the temperatures at the two fixed points on the Celsius and Kelvin scales. But what about temperatures between the fixed points? We could say that the temperature halfway between the melting point of ice and the boiling point of water is 50 ºC, or we could divide the temperature between the two fixed points into 100 equal intervals. But: What do we mean by “halfway” or by “equal intervals” in such a proposal?

3: Temperature is shared under a CC BY-NC license and was authored, remixed, and/or curated by Jeremy Tatum.