14: Heat and Heat Transfer Methods
Energy can exist in many forms and heat is one of the most intriguing. Heat is often hidden, as it only exists when in transit, and is transferred by a number of distinctly different methods. Heat transfer touches every aspect of our lives and helps us understand how the universe functions. It explains the chill we feel on a clear breezy night, or why Earth’s core has yet to cool. This chapter defines and explores heat transfer, its effects, and the methods by which heat is transferred. These topics are fundamental, as well as practical, and will often be referred to in the chapters ahead.
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- 14.1: Heat
- If two objects at different temperatures are brought in contact with each other, energy is transferred from the hotter to the colder object until equilibrium is reached and the bodies reach thermal equilibrium (i.e., they are at the same temperature). No work is done by either object, because no force acts through a distance. The transfer of energy is caused by the temperature difference, and ceases once the temperatures are equal. Heat is the spontaneous transfer of energy due to a temperature
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- 14.2: Temperature Change and Heat Capacity
- One of the major effects of heat transfer is temperature change: heating increases the temperature while cooling decreases it. We assume that there is no phase change and that no work is done on or by the system. Experiments show that the transferred heat depends on three factors—the change in temperature, the mass of the system, and the substance and phase of the substance.
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- 14.3: Phase Change and Latent Heat
- Energy is required to melt a solid because the cohesive bonds between the molecules in the solid must be broken apart such that, in the liquid, the molecules can move around at comparable kinetic energies; thus, there is no rise in temperature. Similarly, energy is needed to vaporize a liquid, because molecules in a liquid interact with each other via attractive forces. There is no temperature change until a phase change is complete.
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- 14.4: Heat Transfer Methods
- Whenever there is a temperature difference, heat transfer occurs. Heat transfer may occur rapidly, such as through a cooking pan, or slowly, such as through the walls of a picnic ice chest. Every process involving heat transfer takes place by only three methods: Heat is transferred by three different methods: conduction, convection, and radiation.
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- 14.5: Conduction
- Heat conduction is the transfer of heat between two objects in direct contact with each other. The rate of heat transfer \(Q/t\) (energy per unit time) is proportional to the temperature difference \(T_2 - T_1\) and the contact area \(A\) and inversely proportional to the distance between the objects: \[\dfrac{Q}{t} = \dfrac{kA(T_2 - T_1)}{d}.\]
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- 14.6: Convection
- Convection is heat transfer by the macroscopic movement of mass. Convection can be natural or forced and generally transfers thermal energy faster than conduction. Table gives wind-chill factors, indicating that moving air has the same chilling effect of much colder stationary air. Convection that occurs along with a phase change can transfer energy from cold regions to warm ones.
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- 14.7: Radiation
- Heat is transferred by radiation. That is, the hot body emits electromagnetic waves that are absorbed by our skin: no medium is required for electromagnetic waves to propagate. Different names are used for electromagnetic waves of different wavelengths: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays.
Thumbnail: Different flame types of a Bunsen burner depend on oxygen supply. On the left a rich fuel with no premixed oxygen produces a yellow sooty diffusion flame; on the right a lean fully oxygen premixed flame produces no soot. (GNU Free Documentation License, Version 1.2; Jan Fijałkowski).