If a temperature difference exists between two points heat will flow from the point at the higher temperature to the point at the lower temperature. This flow will continue until equilibrium has been reached. The transmission of heat may be due to three different processes.
- Conduction – This is the process by which heat flows from the hotter region of a substance to the colder region without there being any net flow of the material itself.
- Convection – The circulatory motion that occurs in a fluid at a non-uniform temperature owing to the variation of its density and the action of gravity. The heat is transferred from the solid or liquid by the surrounding gas. If the flow of gas away from the object starts by itself it is called natural convection. However, if the gas is forced past the object it is called forced convection. Note: convection may also occur within a fluid carrying heat away from a solid.
- Radiation – This is the electromagnetic radiation emitted by a body as a function of its temperature. The idealised body is a black body, this being one which absorbs all radiation that impinges on it. Stefan’s law states that the total energy radiated per unit time per unit surface area of a black body is proportional to the fourth power of the temperature (kelvins) of the body. E = σT4 where σ = Stefan’s constant = 5.67×10-8Wm-2K-4
Gases – are usually transparent to radiation, which can occur along with either conduction or convection but not both. In gases, radiation is usually significant compared to conduction, but negligible compared to convection.
Solids – In opaque solids, heat transfer is only by conduction. In semitransparent solids, heat transfer is by conduction and radiation.
Fluids – In a still fluid, heat transfer is by conduction; in a flowing fluid, heat transfer is by convection. Radiation may also occur in fluids, usually with a strong absorption factor.
Vacuum – In a vacuum, heat transfer is by radiation only.
Heat transfer and work are interactions between a system and its surroundings. Both are recognized as they cross the boundaries of a system. Heat and work are transfer phenomena, not properties. They are associated with a process, not a state. Both are path functions, meaning that their magnitudes depend on the path taken as well as the end states.
Thermodynamics is the branch of physics that looks at the relationships between heat and other forms of energy. The thermodynamic relationships are key to our understanding of so many areas of engineering, if not all areas.