In physics and chemistry, heat is energy transferred from one body to another by thermal interactions. The transfer of energy can occur in a variety of ways, among them conduction, radiation, and convection. Heat is not a property of a system or body, but instead is always associated with a process of some kind, and is synonymous with heat flow and heat transfer.

Heat flow from high to low temperature occurs spontaneously, and is always accompanied by an increase in entropy. This flow of energy can be harnessed and partly converted into useful work by means of a heat engine. The second law of thermodynamics prohibits heat flow directly from low to high temperature, but with the aid of a heat pump external work can be used to transport internal energy indirectly from a low to a high temperature body.

Heat is a characteristic of macroscopic processes and is described by thermodynamics, but its origin and properties can be understood in terms of microscopic constituents using statistical mechanics. For instance, heat flow can occur when the rapidly vibrating molecules in a high temperature body transfer some of their energy (by direct contact, radiation exchange, or other mechanisms) to the more slowly vibrating molecules in a lower temperature body.

The SI unit of heat is the joule. Heat can be measured by calorimetry, or determined indirectly by calculations based on other quantities, relying for instance on the first law of thermodynamics. In physics, especially in calorimetry, and in meteorology, the concepts of latent heat and of sensible heat are used. Latent heat produces changes of state without temperature change, while sensible heat produces temperature change.

Read more about Heat:  Overview, Microscopic Origin of Heat, History, Notation and Units, Estimation of Quantity of Heat, Internal Energy and Enthalpy, Latent and Sensible Heat, Specific Heat, Entropy, Heat Transfer in Engineering, Practical Applications, Usage of Words

Famous quotes containing the word heat:

    When the heat of the summer
    Made drowsy the land,
    A dragon-fly came
    And sat on my hand;
    Eleanor Farjeon (1881–1965)

    I remember my youth and the feeling that will never come back any more—the feeling that I could last for ever, outlast the sea, the earth, and all men; the deceitful feeling that lures us on to joys, to perils, to love, to vain effort—to death; the triumphant conviction of strength, the heat of life in the handful of dust, the glow in the heart that with every year grows dim, grows cold, grows small, and expires—and expires, too soon, too soon—before life itself.
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    She knows the heat of a luxurious bed.
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    William Shakespeare (1564–1616)