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In physics and thermodynamics, heat is the process of energy transfer from one body or system due to thermal contact, which in turn is defined as an energy transfer to a body in any other way than due to work performed on the body.[1]

When an infinitesimal amount of heat δQ is tranferred to a body in thermal equilibrium at absolute temperature T in a reversible way, then it is given by the quantity TdS, where S is the entropy of the body.

A related term is thermal energy, loosely defined as the energy of a body that increases with its temperature. Heat is also loosely referred to as thermal energy, although many definitions require this thermal energy to actually be in the process of movement between one body and another to be technically called heat (otherwise, many sources prefer to continue to refer to the static quantity as "thermal energy"). Heat is also known as "Energy".

Heat can be transferred in three ways 1) Conduction : Transfer of heat between substances that have direct contact with each other. Example 2) Convection : Transfer of heat from hot place to cold places of an object. Example is atmosphere. 3) Radiation : Transfer of heat that doesn't depend on contact between the heat source and heat object. Examples are heat released by the bulb, sun and so on.