# Heat

heat energythermalhotthermal energyheat sourceheat dissipationheatstropical heatwarmthcalor
In thermodynamics, heat is energy in transfer to or from a thermodynamic system, by mechanisms other than thermodynamic work or transfer of matter.wikipedia
989 Related Articles

### Thermodynamics

thermodynamicthermodynamicallyclassical thermodynamics
In thermodynamics, heat is energy in transfer to or from a thermodynamic system, by mechanisms other than thermodynamic work or transfer of matter. In 1856, Rudolf Clausius, referring to closed systems, in which transfers of matter do not occur, defined the second fundamental theorem (the second law of thermodynamics) in the mechanical theory of heat (thermodynamics): "if two transformations which, without necessitating any other permanent change, can mutually replace one another, be called equivalent, then the generations of the quantity of heat Q from work at the temperature T, has the equivalence-value:"
Thermodynamics is the branch of physics that deals with heat and temperature, and their relation to energy, work, radiation, and properties of matter.

### Energy

energy transferenergiestotal energy
In thermodynamics, heat is energy in transfer to or from a thermodynamic system, by mechanisms other than thermodynamic work or transfer of matter.
In physics, energy is the quantitative property that must be transferred to an object in order to perform work on, or to heat, the object.

### Joule heating

resistive heatingohmic heatingJoule's law
The mechanisms include conduction, through direct contact of immobile bodies, or through a wall or barrier that is impermeable to matter; or radiation between separated bodies; or isochoric mechanical work done by the surroundings on the system of interest; or Joule heating by an electric current driven through the system of interest by an external system; or a combination of these.
Joule heating, also known as Ohmic heating and resistive heating, is the process by which the passage of an electric current through a conductor produces heat.

### Work (thermodynamics)

workthermodynamic workpressure-volume work
In thermodynamics, heat is energy in transfer to or from a thermodynamic system, by mechanisms other than thermodynamic work or transfer of matter. The mechanisms include conduction, through direct contact of immobile bodies, or through a wall or barrier that is impermeable to matter; or radiation between separated bodies; or isochoric mechanical work done by the surroundings on the system of interest; or Joule heating by an electric current driven through the system of interest by an external system; or a combination of these. In 1856, Rudolf Clausius, referring to closed systems, in which transfers of matter do not occur, defined the second fundamental theorem (the second law of thermodynamics) in the mechanical theory of heat (thermodynamics): "if two transformations which, without necessitating any other permanent change, can mutually replace one another, be called equivalent, then the generations of the quantity of heat Q from work at the temperature T, has the equivalence-value:"
When it is done isochorically, and no matter is transferred, such an energy transfer is regarded as a heat transfer into the system of interest.

### Internal energy

specific internal energyenergyheat energy
In thermodynamics, energy transferred as heat (a process function) contributes to change in the system's cardinal energy variable of state, for example its internal energy, or for example its enthalpy. For a closed system (a system from which no matter can enter or exit), one version of the first law of thermodynamics states that the change in internal energy
The internal energy of a system can be increased by introduction of matter, by heat, or by doing thermodynamic work on the system.

Another thermodynamic type of heat transfer device is an active heat spreader, which expends work to speed up transfer of energy to colder surroundings from a hotter body, for example a computer component.
A heat spreader transfers energy as heat from a hotter source to a colder heat sink or heat exchanger.

### Enthalpy

enthalpiesspecific enthalpyenthalpic
In thermodynamics, energy transferred as heat (a process function) contributes to change in the system's cardinal energy variable of state, for example its internal energy, or for example its enthalpy.
In a system enclosed so as to prevent matter transfer, at constant pressure, the enthalpy change equals the energy transferred from the environment through heat transfer or work other than expansion work.

### Heat pump

heat pumpsHeat Pumpingheat-pump
Although heat flows spontaneously from a hotter body to a cooler one, it is possible to construct a heat pump which expends work to transfer energy from a colder body to a hotter body.
Heat energy naturally transfers from warmer places to colder spaces.

### British thermal unit

BTUBTUsBritish Thermal Units
However, in many applied fields in engineering the British thermal unit (BTU) and the calorie are often used.
The British thermal unit (Btu or BTU) is a non-SI, traditional unit of heat; it is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit.

### Calorimetry

calorimetriccalorimeterconstant pressure calorimetry
The quantification of heat via the temperature change of a body is called calorimetry.
Scottish physician and scientist Joseph Black, who was the first to recognize the distinction between heat and temperature, is said to be the founder of the science of calorimetry.

### Process function

process quantitiespath function
In thermodynamics, energy transferred as heat (a process function) contributes to change in the system's cardinal energy variable of state, for example its internal energy, or for example its enthalpy.
As an example, mechanical work and heat are process functions because they describe quantitatively the transition between equilibrium states of a thermodynamic system.

### Rudolf Clausius

ClausiusRudolf Julius Emanuel ClausiusClausius, Rudolf
Use of the symbol Q for the total amount of energy transferred as heat is due to Rudolf Clausius in 1850: In 1856, Rudolf Clausius, referring to closed systems, in which transfers of matter do not occur, defined the second fundamental theorem (the second law of thermodynamics) in the mechanical theory of heat (thermodynamics): "if two transformations which, without necessitating any other permanent change, can mutually replace one another, be called equivalent, then the generations of the quantity of heat Q from work at the temperature T, has the equivalence-value:"
In 1870 he introduced the virial theorem which applied to heat.

### Entropy

entropicentropicallyspecific entropy
In 1865, he came to define the entropy symbolized by S, such that, due to the supply of the amount of heat Q at temperature T the entropy of the system is increased by
Building on this work, in 1824 Lazare's son Sadi Carnot published Reflections on the Motive Power of Fire which posited that in all heat-engines, whenever "caloric" (what is now known as heat) falls through a temperature difference, work or motive power can be produced from the actions of its fall from a hot to cold body.

### First law of thermodynamics

firstenergy balancechange in temperature
For a closed system (a system from which no matter can enter or exit), one version of the first law of thermodynamics states that the change in internal energy A calculation of quantity of heat transferred can rely on a hypothetical quantity of energy transferred as adiabatic work and on the first law of thermodynamics.
The first law of thermodynamics is a version of the law of conservation of energy, adapted for thermodynamic processes, distinguishing two kinds of transfer of energy, as heat and as thermodynamic work.

### International System of Units

SISI unitsSI unit
As an amount of energy (being transferred), the SI unit of heat is the joule (J).

### Caloric theory

caloriccaloric fluidcaloric,
Speculation on thermal energy or "heat" as a separate form of matter has a long history, see caloric theory, phlogiston and fire (classical element).
The caloric theory is an obsolete scientific theory that heat consists of a self-repellent fluid called caloric that flows from hotter bodies to colder bodies.

### Calorie

calorieskcalkilocalorie
However, in many applied fields in engineering the British thermal unit (BTU) and the calorie are often used.
The calorie was first introduced by Nicolas Clément, as a unit of heat energy, in lectures during the years 1819–1824.

### Thermal energy

thermalheatenergy
As a common noun, English heat or warmth (just as French chaleur, German Wärme, Latin calor, Greek θάλπος, etc.) refers to (the human perception of) either thermal energy or temperature.
Thermal energy refers to several distinct thermodynamic quantities, such as the internal energy of a system; heat or sensible heat, which are defined as types of energy transfer (as is work); or for the characteristic energy of a degree of freedom in a thermal system kT, where T is temperature and k is the Boltzmann constant.

### Joule

JkJMJ
As an amount of energy (being transferred), the SI unit of heat is the joule (J).
Wilhelm Siemens, in his inauguration speech as chairman of the British Association for the Advancement of Science (23 August 1882) first proposed the Joule as unit of heat, to be derived from the electromagnetic units Ampere and Ohm, in cgs units equivalent to erg.

### Inexact differential

(an inexact differential) is analyzed as a quantity
It is primarily used in calculations involving heat and work because they are path functions, not state functions.

### State function

function of statestate variablesstate functions
In thermodynamics, energy transferred as heat (a process function) contributes to change in the system's cardinal energy variable of state, for example its internal energy, or for example its enthalpy. In contrast, thermodynamic work is defined by mechanisms that act macroscopically and directly on the system's whole-body state variables; for example, change of the system's volume through a piston's motion with externally measurable force; or change of the system's internal electric polarization through an externally measurable change in electric field.
In contrast, mechanical work and heat are process quantities or path functions, because their values depend on the specific transition (or path) between two equilibrium states.

### Mechanical equivalent of heat

Circa 1797, Count Rumford (born Benjamin Thompson)heat generated by mechanical workmechanical equivalence of heat
The modern understanding of thermal energy originates with Thompson's 1798 mechanical theory of heat ([[An Experimental Enquiry Concerning the Source of the Heat which is Excited by Friction]]), postulating a mechanical equivalent of heat.
In the history of science, the mechanical equivalent of heat states that motion and heat are mutually interchangeable and that in every case, a given amount of work would generate the same amount of heat, provided the work done is totally converted to heat energy.

### Theory of heat

mechanical theory of heatheat theory
The modern understanding of thermal energy originates with Thompson's 1798 mechanical theory of heat ([[An Experimental Enquiry Concerning the Source of the Heat which is Excited by Friction]]), postulating a mechanical equivalent of heat. In 1856, Rudolf Clausius, referring to closed systems, in which transfers of matter do not occur, defined the second fundamental theorem (the second law of thermodynamics) in the mechanical theory of heat (thermodynamics): "if two transformations which, without necessitating any other permanent change, can mutually replace one another, be called equivalent, then the generations of the quantity of heat Q from work at the temperature T, has the equivalence-value:"
In the history of science, the theory of heat or mechanical theory of heat was a theory, introduced in 1798 by Sir Benjamin Thompson (better known as 'Count Rumford') and developed more thoroughly in 1824 by the French physicist Sadi Carnot, that heat and mechanical work are equivalent.

### Benjamin Thompson

Count RumfordBenjamin Thompson, Count RumfordRumford
The modern understanding of thermal energy originates with Thompson's 1798 mechanical theory of heat ([[An Experimental Enquiry Concerning the Source of the Heat which is Excited by Friction]]), postulating a mechanical equivalent of heat.
While recuperating in Woburn in 1769 from an injury, Thompson conducted experiments concerning the nature of heat and began to correspond with Loammi Baldwin and others about them.