Thermodynamics: Heat, Temperature and Humidification

In continuing the concepts of energy and work, heat is a form of energy, and heat and work are related by the laws of thermodynamics. The first law states that the amount of mechanical work achievable out of a thermodynamic system going through a cyclic process is proportional to the amount of heat put into it, which is intuitive. The second law states that the amount of work gained from the system is always less than the amount of heat put into it; in other words there is some energy wasted, which is also believable. Heating a substance gives its constituent molecules increased kinetic energy, which can be put to mechanical use, for example in driving a turbine. Such an increase in energy either raises its temperature or changes its state. The increasing energy causes increasing velocity of movement of molecules and eventually breaks intermolecular bonds, giving the molecules more freedom of movement. As characteristic temperatures are reached, e.g. melting point or boiling point, the energy increase goes into changing the substance’s state into a more energetic one, e.g. solid to liquid, or liquid to gas, without any associated temperature change. Loss of heat from a substance involves the reverse processes. The gas laws have their origin in this concept of the kinetic theory of gases, and are an inherent part of consideration of the effects of heating and cooling, compression and expansion of gases. Pressure has been defined in Chapter 7, but to understand the concept, one that is taken for granted, it is necessary to start with the idea of molecules of matter undergoing a change in momentum (the physical definition of momentum being mass × velocity, see Chapter 3). It is unnecessary here to deduce the following equation, but the proof can be found in any physics or engineering text [Macintosh et al. 1987; Dorrington 1989; Hill 1976]).