In physics, heat is energy that spontaneously passes between a system and its surroundings in some way other than through work or the transfer of matter. When a suitable physical pathway exists, heat flows spontaneously from a hotter to a colder body. The transfer can be by contact between the source and the destination body, as in conduction; or by radiation between remote bodies; or by conduction and radiation through a thick solid wall; or by way of an intermediate fluid body, as in convective circulation; or by a combination of these.
Because heat refers to a quantity of energy transferred between two bodies, it is not a state function of either of the bodies, in contrast to temperature and internal energy. Instead, according to the first law of thermodynamics heat exchanged during some process contributes to the change in the internal energy, and the amount of heat can be quantified by the equivalent amount of work that would bring about the same change.
While heat flows spontaneously from hot to cold, it is possible to construct a heat pump or refrigeration system that does work to increase the difference in temperature between two systems. Conversely, a heat engine reduces an existing temperature difference to do work on another system.
Microscopic view of heat
In the kinetic theory, heat is explained in terms of the microscopic motions and interactions of constituent particles, such as electrons, atoms, and molecules. The immediate meaning of the kinetic energy of the constituent particles is not as heat. It is as a component of internal energy. In microscopic terms, heat is a transfer quantity, and is described by a transport theory, not as steadily localized kinetic energy of particles. Heat transfer arises from temperature gradients or differences, through the diffuse exchange of microscopic kinetic and potential particle energy, by particle collisions and other interactions. An early and vague expression of this was made by Francis Bacon Precise and detailed versions of it were developed in the nineteenth century.
In statistical mechanics, for a closed system (no transfer of matter), heat is the energy transfer associated with a disordered, microscopic action on the system, associated with jumps in occupation numbers of the energy levels of the system, without change in the values of the energy levels themselves. It is possible for macroscopic thermodynamic work to alter the occupation numbers without change in the values of the system energy levels themselves, but what distinguishes transfer as heat is that the transfer is entirely due to disordered, microscopic action, including radiative transfer. A mathematical definition can be formulated for small increments of quasi-static adiabatic work in terms of the statistical distribution of an ensemble of microstates.
Does the heat travel faster through the iron or through the glass?
What really happens down among the molecules when the heat travels along the rods is that the molecules
near the flame are made to move more quickly; they joggle their neighbors and make them move faster; these
joggle the ones next to them, and so on down the line. Heat that travels through things in this way is called
conducted heat. Anything like iron, that lets the heat travel through it quickly, is called a good conductor of
heat. Anything like glass, that allows the heat to travel through it only with difficulty, is called a poor
conductor of heat, or an insulator of heat.
A silver spoon used for stirring anything that is cooking gets so hot all the way up the handle that you can
hardly hold it, while the handle of a wooden spoon never gets hot. Pancake turners usually have wooden
handles. Metals are good conductors of heat; wood is a poor conductor.
An even more obvious example of the conducting of heat is seen in a stove lid; your fire is under it, yet the top
gets so hot that you can cook on it.
When anything feels hot to the touch, it is because heat is being conducted to and through your skin to the
sensitive little nerve ends just inside. But when anything feels cold, it is because heat is being conducted away
from your skin into the cold object.
AIR CARRIES HEAT BY CONVECTION. One of the poorest conductors of heat is air; that is, one particle
of air can hardly give any of its heat to the next particle. But particles of air move around very easily and carry
their heat with them; and they can give the heat they carry with them to any solid thing they bump into. So
when air can move around, the part that is next to the stove, for instance, becomes hot; this hot air is pushed
up and away by cold air, and carries its heat with it. When it comes over to you in another part of the room,
some of its heat is conducted to your body. When air currents–or water currents, which work the same
way–carry heat from one place to another like this, we say that the heat has traveled by convection.