Heat

Heat
From Wikipedia, the free encyclopedia

For other uses, see Heat (disambiguation).
Heat from the sun is the driving force of life on Earth. The science of heat and its relation to work is thermodynamics. Heat flow can be created in many ways.

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.
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".

Energy transfer by heat can occur between objects by radiation, conduction and convection. Temperature is used as a measure of the internal energy or enthalpy, that is the level of elementary motion giving rise to heat transfer. Energy can only be transferred by heat between objects - or areas within an object - with different temperatures (as given by the zeroth law of thermodynamics). This transfer happens spontaneously only in the direction of the colder body (as per the second law of thermodynamics). The transfer of energy by heat from one object to another object with an equal or higher temperature can happen only with the aid of a heat pump, which does work

Overview
The first law of thermodynamics states that the energy of a closed system is conserved. Therefore, to change the energy of a system, energy must be transferred to or from the system. Heat and work are the only two mechanisms by which energy can be transferred to or from a control mass. Work performed on a body is, by definition an energy transfer to the body that is due to a change of the external parameters of the body (such as the volume, magnetization, center of mass position in a gravitational field etc.). Heat is the energy transferred to the body in any other way. This definition of heat applies generally, it does not appeal to any notion of thermal equilibrium.

In case of bodies close to thermal equilibrium where notions such as the temperature can be defined, heat transfer can be related to temperature difference between bodies. It is an irreversible process that leads to the bodies coming closer to mutual thermal equilibrium.

The unit for the amount of energy transferred by heat in the International System of Units SI is the joule (J), though the British Thermal Unit and the calorie are still used in the United States. The unit for the rate of heat transfer is the watt (W = J/s).


Heat Q can flow across the boundary of the system and thus change its internal energy U.

Heat transfer is a path function (process quantity), as opposed to a point function (state quantity). Heat flows between systems that are not in thermal equilibrium with each other; it spontaneously flows from the areas of high temperature to areas of low temperature. When two bodies of different temperature come into thermal contact, they will exchange internal energy until their temperatures are equalized; that is, until they reach thermal equilibrium. The adjective hot is used as a relative term to compare the object’s temperature to that of the surroundings (or that of the person using the term). The term heat is used to describe the flow of energy. In the absence of work interactions, the heat that is transferred to an object ends up getting stored in the object in the form of internal energy.


Heat exposure as part of a fire test for firestop products.
Other heat transfer mechanisms
Latent heat: Transfer of heat through a physical change in the medium such as water-to-ice or water-to-steam or steam-to-water or ice-to-water involves significant energy and is exploited in many ways: steam engine, refrigerator etc. (see latent heat of fusion)

Heat pipes: Using latent heat and capillary action to move heat, heat pipes can carry many times as much heat as a similar-sized copper rod. Originally invented for use in satellites, they are starting to have applications in personal computers.

Heat dissipation
In cold climates, houses with their heating systems form dissipative systems. In spite of efforts to insulate such houses to reduce heat losses to their exteriors, considerable heat is lost, or dissipated, from them, which can make their interiors uncomfortably cool or cold. For the comfort of its inhabitants, the interior of a house must be maintained out of thermal equilibrium with its external surroundings. In effect, domestic residences are oases of warmth in a sea of cold and the thermal gradient between the inside and outside is often quite steep. This can lead to problems such as condensation and uncomfortable draughts (drafts) which, if left unaddressed, can cause structural damage to the property. This is why modern insulation techniques are required to reduce heat loss.

In such a house, a thermostat is a device capable of starting the heating system when the house's interior falls below a set temperature, and of stopping that same system when another (higher) set temperature has been achieved. Thus the thermostat controls the flow of energy into the house, that energy eventually being dissipated to the exterior.