In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is also often described as one of the fundamental processes of energy transfer between physical entities. In this description, it is an energy transfer to a body in any other way than due to thermodynamic work, which is a concept of work more broad than mechanical work.
In engineering, the discipline of heat transfer classifies energy transfer in or between systems resulting in the change of thermal energy of a system as either thermal conduction, first described scientifically by Joseph Fourier, by fluid convection, which is the mixing of hot and cold fluid regions due to pressure differentials, by mass transfer, and by thermal radiation, the transmission of electromagnetic radiation described by black body theory.
Thermodynamically, energy can only be transferred as heat between objects, or regions therein, with different temperatures, as described by the zeroth law of thermodynamics. This transfer happens spontaneously only in the direction to 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 via mechanical work, or by some other similar process in which entropy is increased in the universe in manner that compensates for the decrease of entropy in the cooled object, due to the removal of the heat from it. For example, heat may be removed against a temperature gradient by spontaneous evaporation of a liquid.
A related term is thermal energy, loosely defined as the energy of a body that increases with its temperature. Thermal energy is also sometimes referred to as heat, although the strict definition of heat requires it to be in transfer between two systems.
Overview
Heat flows spontaneously only from systems of higher temperature to systems of lower temperature. When two systems come into thermal contact, they always exchange thermal energy due to the microscopic interactions of their particles. When the systems are at different temperatures, the net flow of thermal energy is not zero and is directed from the hotter region to the cooler region, until their temperatures are equal and the flow of heat ceases. At this point they obtain a state of thermal equilibrium, exchanging thermal energy at an equal rate in both directions.
The first law of thermodynamics states that the energy of an isolated system is conserved. Therefore, to change the energy of a system, energy must be transferred to or from the system. For a closed system, heat and work are the only two mechanisms by which energy can be transferred. Work performed on a system is, by definition , an energy transfer to the system that is due to a change to external parameters of the system, such as the volume, magnetization, center of mass in a gravitational field. Heat is the energy transferred to the system in any other way.
In the case of systems close to thermal equilibrium where notions such as the temperature can be defined, heat transfer can be related to temperature difference between systems. It is an irreversible process that leads to the systems coming closer to mutual thermal equilibrium.
Human notions such as hot and cold are relative terms and are generally used to compare one system’s temperature to another or its surroundings.
Definitions
Scottish physicist James Clerk Maxwell, in his 1871 classic Theory of Heat, was one of the first to enunciate a modern definition of heat. Maxwell outlined four stipulations for the definition of heat:
- It is something which may be transferred from one body to another, according to the second law of thermodynamics.
- It is a measurable quantity, and thus treated mathematically.
- It cannot be treated as a substance, because it may be transformed into something that is not a substance, e.g., mechanical work.
- Heat is one of the forms of energy.
Several modern definitions of heat are as follows:
- The energy transferred from a high-temperature system to a lower-temperature system is called heat.
- Any spontaneous flow of energy from one system to another caused by a difference in temperature between the systems is called heat.
In a thermodynamic sense, heat is never regarded as being stored within a system. Like work, it exists only as energy in transit from one system to another or between a system and its surroundings. When energy in the form of heat is added to a system, it is stored as kinetic and potential energy of the atoms and molecules in the system.
Notation and units
As a form of energy heat has the unit joule (J) in the International System of Units (SI). However, in many applied fields in engineering the British Thermal Unit (BTU) and the calorie are often used. The standard unit for the rate of heat transferred is the watt (W), defined as joules per second.
The total amount of energy transferred as heat is conventionally written as Q for algebraic purposes. Heat released by a system into its surroundings is by convention a negative quantity (Q < 0); when a system absorbs heat from its surroundings, it is positive (Q > 0). Heat transfer rate, or heat flow per unit time, is denoted by
.
Heat flux is defined as rate of heat transfer per unit cross-sectional area, resulting in the unit watts per square metre.
