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
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Famous quotes containing the word heat:
“Two wooden tubs of blue hydrangeas stand at the foot of the stone steps.
The sky is a blue gum streaked with rose. The trees are black.
The grackles crack their throats of bone in the smooth air.
Moisture and heat have swollen the garden into a slum of bloom.
Pardie! Summer is like a fat beast, sleepy in mildew....”
—Wallace Stevens (18791955)
“I have a blood bolt
and I have made it mine.
With this man I take in hand
his destiny and with this gun
I take in hand the newspapers and
with my heat I will take him.”
—Anne Sexton (19281974)