Heat is an energy transfer from one thermodynamic system to another. Another definition states that heat is the energy transfer between a system and its environment due the temperature difference that exist between them. Heat is positive when the energy is transferred from the environment to a system's thermal energy and negative when the reverse occurs. Heat transfer can occur through conduction, convection, and radiation. The transfer of heat energy will spontaneously flow from a higher temperature system to a lower temperature system, but the reverse is not true. The term 'heat' is sometimes incorrectly used to refer to the internal energy of a system which is properly called thermal energy. Because heat is a form of energy, measurements are typically made using the International System of Units which include joules, watts, and calories.
The work of Maxwell and Hertz in the late nineteenth century showed categorically that the radiation of heat, light and radio waves were all electromagnetic waves, differing only in frequency and wavelength.
The Heat capacity (symbol C) of a system is the ratio of the amount of heat energy added to or withdraw from the system to the resultant change in the temperature of the system.
In the International System of Units, heat capacity has the unit joules per kelvin.
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