heat

Heat is a form of energy transfer between objects or systems that have different temperatures. It flows from the hotter body to the cooler one until thermal equilibrium is achieved. In physics, heat is energy in transit, while temperature is a property of matter that reflects the average kinetic energy of its microscopic constituents.

Heat is measured in joules in the International System of Units, with calories still in common use in some contexts. The amount of heat transferred depends on the temperature difference, the material properties, and the path of transfer. Specific heat capacity describes how much heat is needed to raise the temperature of a given mass by one degree.

Heat can be transferred by conduction, convection, or radiation. Conduction occurs mainly in solids through lattice vibrations and electron interactions. Convection involves the bulk movement of fluids, and radiation transfers energy by electromagnetic waves and does not require a medium.

In thermodynamics, the first law states that heat added to a system changes its internal energy and may do work on or by the surroundings. The second law introduces entropy and dictates that, in an isolated system, natural processes increase the total entropy and that heat flows spontaneously from hot to cold. Temperature scales include Kelvin, Celsius, and Fahrenheit.

Practical considerations involve phase changes and latent heat, engines and heat exchangers, and everyday heating and cooling of buildings and foods. The concept of heat thus encompasses both a fundamental physical quantity and a common everyday sense of warmth.