In physics, heat is energy as it spontaneously passes between a system and its surroundings, other than as work or with the transfer of matter. In thermodynamics, finer detail of the process of transfer is in principle unspecified. When there is a suitable physical pathway, heat transfer occurs from a hotter to a colder body. The transfer can be direct, by contact, from the source to the destination body, as in conduction combined with radiation, or remote, as in radiation without conduction; or indirect, by conduction and radiation through a thick solid wall, or by way of an intermediate fluid body, as in convective circulation.

Originally, quantity of heat transferred was measured by how much it changed the states of participating bodies, for example, as amount of ice melted, or change in temperature, without work or matter transfer. This is possible because many bodies, over most temperature ranges, expand reversibly on being heated. This is called thermal expansion.

Historically, when the first and second laws of thermodynamics had been established, it came to be regarded by physicists as more rational to define quantity of heat transferred in terms of equivalent work. Thus, for the sake of logical development, the concept of temperature was reserved for definition in terms of the second law, segregated from the statement of the first law.

Kinetic theory explains heat as a macroscopic manifestation of the motions and interactions of microscopic constituents such as molecules and photons.

In calorimetry, sensible heat is defined with respect to a specific chosen state variable of the system, such as pressure or volume. Sensible heat transfer causes change of temperature of the system while leaving the chosen state variable unchanged. Heat transfer that occurs with the system at constant temperature and that does change that particular state variable is called latent heat with respect to that variable. For infinitesimal changes, the total incremental heat transfer is then the sum of the latent and sensible heat increments. This is a basic paradigm for thermodynamics, and was important in the historical development of the subject.

The quantity of energy transferred as heat is a scalar expressed in an energy unit such as the joule (J) (SI), with a sign that is customarily positive when a transfer adds to the energy of a system.

### Watermelon He Cooling System

Joe HF

Aya complained about the heat while she was away on business, so I drew her a watermelon shirt...

"Cherish the certainty of now, it kills you a bit at a time."
Faith No More
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