heat

In thermodynamics, heat is defined as energy in transfer between a body and its surroundings, other than through thermodynamic work or through transfer of matter.
Rather than by mechanical or other effects in the surroundings, thermodynamic work is defined through changes in the system's macroscopic state variables, in conjugate pairs such as pressure and volume, or magnetisation and magnetic field strength. The thermodynamic definition of heat is by exclusion of other defined modes of transfer in order to ensure a strict logical distinction.
In the process of transfer, heat is not necessarily conserved, but can be generated (though not destroyed) by friction. In thermodynamics, temperature is defined in macroscopic terms, through the concepts of heat and work. Heat is not a state variable or state function; it is a process quantity. A thermodynamic system does not contain heat.
Heat can enter or leave a body of matter by thermal conduction, by electromagnetic radiation, or through friction due to macroscopic mechanical movement. When heat is generated by friction, it may end up partly in each of the two engaged bodies. When energy enters or leaves a body through transfer of matter, it cannot be resolved uniquely into work and heat.
By assuming conservation of heat in a transfer, calorimetry measures it by its effect on the states of interacting bodies, for example, by the amount of ice melted or by change in temperature of a body.
In the International System of Units (SI), the unit of measurement for heat is the joule (J).

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