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(Redirected to High explosive anti-tank warhead article)

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A HEAT round. The copper-lined conical shaped area can be clearly seen in this cutaway.
PARS 3 LR with HEAT warhead of the German Army.

High explosive anti-tank (HEAT) warheads are made of an explosive shaped charge that uses the Neumann effect (a development of the Munroe effect) to create a very high-velocity partial stream of metal in a state of superplasticity that can punch through solid armor.



The stream moves at hypersonic speeds (up to 25 times the speed of sound) in solid material and therefore erodes exclusively in the contact area of jet and armor material. The correct detonation point of the warhead and spacing is critical for optimum penetration, for two reasons:

  1. If the HEAT warhead is detonated too close to the target's surface there is not enough time for the particle stream to fully develop. That is why most modern HEAT warheads have what is called a "standoff", in the form of an extended nose cape or probe in front of the warhead.[notes 1]
  2. The distance is critical because the stream disintegrates and disperses after a relatively short distance, usually well under 2 metres. The stream material is formed by a cone of metal foil lining, usually copper, though tin foil was commonly used during the Second World War.

The key to the effectiveness of a HEAT round is the diameter of the warhead. As the penetration continues through the armor, the width of the hole decreases leading to a characteristic "fist to finger" penetration, where the size of the eventual "finger" is based on the size of the original "fist". In general, HEAT rounds can expect to penetrate armor of 150% to 250% of their diameter, and these numbers were typical of early weapons used during World War II. Since the Second World War, the penetration of HEAT rounds relative to projectile diameters has steadily increased as a result of improved liner material and metal jet performance. Some modern examples claim numbers as high as 700%[1]

HEAT rounds are less effective if they are spinning, the normal method for giving a shell accuracy. The centrifugal force disperses the jet, so the round needs to be fired from smoothbore weapons, or else modified for use with rifled guns. A further problem is that if the warhead is contained inside the barrel, then its diameter is restricted to the caliber of the gun. Cannon-fired HEAT rounds are not as accurate and have a shorter effective range than armor piercing rounds that depend on kinetic energy to penetrate armor. These characteristics are a result of the shape of cannon-fired HEAT rounds, a shape that is optimized for delivery of the round's high velocity metal jet, but less than optimal for long flight and accuracy. The lessening of accuracy increases dramatically with range. A stationary Soviet T-62 tank, firing at a range of 1000 meters against a target moving 19 km/h, was rated to have a first-round hit probability of 70% when firing a kinetic (APFSDS) projectile. Under the same conditions, the T-62 could only expect a first-round hit probability of 25% when firing its HEAT round.[2] These characteristics of HEAT rounds are disadvantageous only on open battlefields with long lines of sight; the same T-62 could expect a 70% first-round hit probability using HEAT rounds on target at 500 meters.

In non-gun applications, when HEAT is used as the warhead for guided missiles, glide bombs, rifle grenades, or spigot mortars, warhead size is not a limiting factor, as these are not contained within the firing weapon's barrel. In these cases HEAT rounds often use seemingly oversized warheads on smaller bodies. Classic examples include the German Panzerfaust and Soviet RPG7, while most modern examples are based on missile bodies. In recent years, with the ending of the cold war, attention has turned to lighter armour, and especially weapons that are useful against bunkers and similar targets. In these cases, weapons originally intended to deal with light armour using HEAT are once again finding use in the field, the Swedish Carl Gustav) has recently re-entered production for use by U.S. forces.

Contrary to a widespread misconception, HEAT rounds do not depend in any way on thermal phenomena for their effectiveness. In particular, the shaped charge jets do not "melt their way" through armor. This confusion has arisen from the acronym HEAT, as well as early descriptions of how the weapons worked, including the first official manuals for the M72 LAW issued to the US Army and USMC at the start of the Vietnam War.


Soviet 125 mm HEAT BK-14

The development of HEAT weapons was spurred by some Swiss inventors who exhibited a "new" weapon before the Second World War. Observers from several countries realised that the principle was not new but an application of the shaped charge.

The first HEAT warhead was a rifle grenade, the British No. 68 AT grenade. It was followed by more effective combinations of warhead and delivery systems in the US "Bazooka", and the British PIAT spigot mortar. In mid-1940 Germany introduced the first HEAT round to be fired by a gun, the 7.5 cm Gr. 38 fired by the 7.5 cm Kw.K. of the Panzer IV tank and the Stug-III self propelled gun. In mid-1941 the first HEAT rifle-grenade (issued to paratroopers) and the improved 7.5 cm Gr. 38 Hl/A design followed. In 1942 Germany started the production of HEAT rifle-grenades for regular army units and introduced another improved design, the Gr. 38 Hl/B. In 1943 finally the Püppchen, Panzerfaust and Panzerschreck were introduced together with the design of the Gr. 38 HL/C. Other uses of HEAT, by the Germans, was for the neutralisation of the armoured gun turrets of the Belgian Fort Eben-Emael by commando troops, and later as warhead on the "Mistel" weapons. The latter was perhaps the largest HEAT warhead ever used, and was intended for use against heavily armored warships, for example battleships.

The need for a large bore made HEAT rounds relatively ineffective in existing small-caliber anti-tank guns of the era. The Germans were able to capitalize on this, however, introducing a round that was placed over the end on the outside of their otherwise outdated (and basically useless) 37 mm anti-tank guns to produce a medium-range low-velocity weapon. A more convincing system was created by making a much larger tripod-mounted version of the Panzerschreck, producing the 7.5 cm Leichtgeschütz 40, what is today known as a recoilless rifle. The recoilless rifle had the range to stay easily hidden on the battlefield, was light enough to be portable by a small team, but had the performance needed to defeat any tank. The main drawbacks, was a short range and a large back blast that gave the weapon's position away when fired.

Adaptations to existing tank guns were somewhat more difficult, although all major forces had done so by the end of the war. Since velocity has little effect on the armor-piercing capability of the round, which is defined by explosive power, HEAT rounds were particularly useful in long-range combat where the slower terminal velocities were not an issue. The Germans were again the ones to produce the most capable gun-fired HEAT rounds, using a driving band on bearings to allow it to fly unspun from their existing rifled tank guns. HEAT was particularly useful to them because it allowed the low-velocity large-bore guns used on their numerous assault guns to become useful anti-tank weapons as well. Likewise, the Germans, Italians, and Japanese had many obsolescent "infantry guns" in service (short-barreled, low-velocity artillery pieces capable of both direct and indirect fire and intended for infantry support, similar in tactical role to mortars; generally an infantry battalion had a battery of four or six). HEAT rounds for these old infantry guns made them semi-useful anti-tank guns, particularly the German 150 mm guns (the Japanese 70 mm and Italian 65 mm infantry guns also had HEAT rounds available for them by 1944 but they were not very effective).

HEAT rounds caused a revolution in anti-tank warfare when they were first introduced in the later stages of World War II. A single infantryman could effectively destroy any existing tank with a handheld weapon, thereby dramatically altering the nature of mobile operations. After the war HEAT became almost universal as the primary anti-tank weapon. HEAT rounds of varying effectiveness were produced for almost all weapons from infantry weapons like rifle grenades and the M203 grenade launcher, to larger dedicated anti-tank systems like the Carl Gustav recoilless rifle. When combined with the wire-guided missile, infantry weapons were able to operate in the long-range role as well. Anti-tank missiles altered the nature of tank warfare throughout the 1960s and into the 80s, and remain an effective system today.

Helicopter armament

Helicopters have also carried antitank guided missiles (ATGM) tipped with HEAT warheads since the early 1960's. The first example of this was the use of the Nord SS.11 ATGM on the Aérospatiale Alouette II helicopter by the French armed forces. Subsequently, such antitank-capable helicopters were widely adopted by other nations.

On April 13, 1972, Chief Warrant Officer Barry McIntyre, Major Larry McKay, First Lieutenant Steve Shields, and Captain Bill Causey became the first helicopter crews to destroy enemy armour in combat during the Vietnam War. A flight of two Cobra helicopters from Battery F, 79th Artillery, 1st Cavalry Division, U.S. Army, were armed with the newly developed 2.75" HEAT rockets, which were yet untested in combat. The specially modified Huey which was shipped in an emergency destroyed three T-54 tanks that were about to overrun a U.S. command post. McIntyre and McKay engaged first, destroying the lead tank.[3]

Armor developments

Improvements to the armor of main battle tanks have reduced the usefulness of HEAT warheads by making man portable HEAT missiles heavier, although many of the world's armies continue to carry man portable HEAT rocket launchers for use against vehicles and bunkers. In unusual cases, shoulder-launched HEAT rockets are believed to have shot down U.S. helicopters in Iraq.[4] Today HEAT rounds are primarily used in shoulder-launched and in jeep- and helicopter-based missile systems. Tanks mostly use the more effective APFSDS rounds.

The reason for the ineffectiveness of HEAT-munitions against modern main battle tanks can be attributed in part to the use of new types of armor. The jet created by the explosion of the HEAT-round must have a certain distance from the target and must not be deflected. Reactive armor attempts to defeat this with an outward directed explosion under the impact point, causing the jet to deform and so penetration power is greatly reduced. Alternatively, composite armor featuring ceramics erode the liner jet more quickly than rolled homogeneous armor steel, the then-preferred material in the construction of armored fighting vehicles.

Spaced armor and slat armor are also designed to defend against HEAT rounds, protecting the vehicle by causing a premature detonation of the explosive at a relatively safe distance away from the main armor of the vehicle.


A Russian 3BK29 HEAT round

Many HEAT-missiles today have two (or more) separate warheads (known as a tandem charge) to be more effective against reactive or multilayered armor; the first, smaller warhead initiates the reactive armor, while the second (or other), larger warhead penetrates the armor below. This approach requires highly sophisticated fuzing electronics to set off the two warheads the correct time apart, and also special barriers between the warheads to stop unwanted interactions; this makes them rather more expensive to produce.

Some anti-armor weapons incorporate a variant on the shaped charge concept that, depending on the source, can be called a Self Forging Fragment (SFF), Explosively Formed Penetrator (EFP), SElf FOrging Projectile (SEFOP), plate charge, or Misznay Schardin (MS) charge. This warhead type uses the interaction of the detonation wave(s), and to a lesser extent the propulsive effect of the detonation products, to deform a dish/plate of metal (iron, tantalum, etc) into a slug shaped projectile of low length to diameter ratio (L to D) and project this towards the target at around two kilometres per second.

The SFF is relatively unaffected by first generation reactive armor, it can also travel up to, and above 1000 cone diameters (CDs) before its velocity becomes ineffective at penetrating armor due to aerodynamic drag, or hitting the target becomes a problem. The impact of a SFF normally causes a large diameter, but relatively shallow hole (in comparison to a shaped charge) of, at best, a few CDs. If the SFF perforates the armor, extensive behind armor damage (BAD), also called behind armor effect (BAE) occurs. The BAD is mainly caused by the high temperature and velocity armor and slug fragments being injected into the interior space and also overpressure (blast) caused by the impact.

More modern SFF warhead versions, through the use of advanced initiation modes, can also produce rods (stretched slugs), multi-slugs and finned projectiles, and this in addition to the standard short L to D ratio projectile. The stretched slugs able to penetrate a much greater depth of armor, at some loss to BAD, multi-slugs are better at defeating light and/or area targets and the finned projectiles have greatly enhanced accuracy. The use of this warhead type is mainly restricted to lightly armored areas of MBTs (Main Battle Tanks), the top, belly and rear armored areas for example. It is well suited for use in the attack of other less heavily armored AFVs (armored fighting vehicles) and in the breaching of material targets (buildings, bunkers, bridge supports, etc).. The newer rod projectiles may be effective against the more heavily armored areas of MBTs.

Weapons using the SEFOP principle have already been used in combat; the smart submunitions in the CBU-97 cluster bomb used by the US Air Force and US Navy in the 2003 Iraq war used this principle, and the US Army is reportedly experimenting with precision-guided artillery shells under Project SADARM (Seek And Destroy ARMor). There are also various other projectile (BONUS, DM 642) and rocket submunitions (Motiv-3M, DM 642) and mines (MIFF, TMRP-6) that use SFF principle.

With the effectiveness of gun-fired single charge HEAT rounds being lessened, or even negated by the increasingly sophisticated armoring techniques, a class of HEAT rounds known as high explosive anti-tank multi-purpose, or HEAT-MP, has become more popular. These are essentially HEAT rounds which are effective against older tanks and other armored vehicles, but have improved fragmentation, blast and fuzing. This gives the projectiles an overall reasonable light armor and anti-personnel/materiel effect so that they can be used in place of conventional high explosive rounds against infantry and other battlefield targets. This reduces the total number of rounds that need to be carried for different roles, which is particularly important for modern tanks like the M1 Abrams, due to the sheer size of 120 mm rounds used. The M1A1 / M1A2 tank can carry only 40 rounds for its 120 mm M256 gun—the M60A3 Patton tank (the Abrams' predecessor), carried 63 rounds for its 105 mm M68 gun. This effect is reduced by the higher first round hit rate of the Abrams with its improved fire control system compared to the M60. The frequent fuel replenishments required for the Abrams' fuel hungry turbine also make simultaneous ordnance replenishment a marginal burden.

High explosive dual purpose

M430A1 HEDP.

Another variation on HEAT warheads is surrounding them with a conventional fragmentation casing, to allow the warhead to be more effectively used for blast and fragmentation attacks on unarmored targets. In some cases this is merely a side effect of the armor piercing design, in other cases a dual role is specifically designed in. Some warheads have been known as HEDP—High Explosive Dual Purpose.

See also


  1. ^ Both the US TOW and the French-German MILAN wire guided antitank missiles almost doubled their maximum penetration by the addition of a standoff probe.


  1. ^ Jane's Ammunition Handbook 1994, pp. 140-141, addresses the reported ~700 mm penetration of the Swedish 106 3A-HEAT-T and Austrian RAT 700 HEAT projectiles for the 106 mm M40A1 recoilless rifle.
  2. ^ Jane's Armour and Artillery 1981-82, p. 55.
  3. ^ Michael P Kelley, Where We Were In Vietnam.
  4. ^ Aviation Week Report.

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 to another due to a difference in temperature.[1] In thermodynamics, the quantity TdS is used as a representative measure of the (inexact) heat differential δQ, which is the absolute temperature of an object multiplied by the differential quantity of a system's entropy measured at the boundary of the object.

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.



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. Heat is the transfer of energy caused by the temperature difference. 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).

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.

to the surrounding environment will be primarily through radiation.]]

Specific heat is defined as the amount of energy that has to be transferred to or from one unit of mass or mole of a substance to change its temperature by one degree. Specific heat is a property, which means that it depends on the substance under consideration and its state as specified by its properties. Fuels, when burned, release much of the energy in the chemical bonds of their molecules. Upon changing from one phase to another, a pure substance releases or absorbs heat without its temperature changing. The amount of heat transfer during a phase change is known as latent heat and depends primarily on the substance and its state.

Thermal energy

Template:See main Thermal energy is a term often confused with that of heat. Loosely speaking, when heat is added to a thermodynamic system its thermal energy increases and when heat is withdrawn its thermal energy decreases. In this point of view, objects that are hot are referred to as being in possession of a large amount of thermal energy, whereas cold objects possess little thermal energy. Thermal energy then is often mistakenly defined as being synonym for the word heat. This, however, is not the case: an object cannot possess heat, but only energy. The term "thermal energy" when used in conversation is often not used in a strictly correct sense, but is more likely to be only used as a descriptive word. In physics and thermodynamics, the words “heat”, “internal energy”, “work”, "enthalpy" (heat content), "entropy", "external forces", etc., which can be defined exactly, i.e. without recourse to internal atomic motions and vibrations, tend to be preferred and used more often than the term "thermal energy", which is difficult to define.


In the history of science, the history of heat traces its origins from the first hominids to make fire and to speculate on its operation and meaning to modern day particle physicists who study the sub-atomic nature of heat. In short, the phenomenon of heat and its definition evolved from mythological theories of fire, to heat, to terra pinguis, phlogiston, to fire air, to caloric, to the theory of heat, to the mechanical equivalent of heat, to thermo-dynamics (sometimes called energetics) to thermodynamics. Most of the history of heat, then, is a precursor to the history of thermodynamics.


The total amount of energy transferred through heat transfer is conventionally abbreviated as Q. The conventional sign convention is that when a body releases heat into its surroundings, Q < 0 (-); when a body absorbs heat from its surroundings, Q > 0 (+). Heat transfer rate, or heat flow per unit time, is denoted by:

\dot{Q} = {dQ\over dt} \,\!.

It is measured in watts. Heat flux is defined as rate of heat transfer per unit cross-sectional area, and is denoted q, resulting in units of watts per square metre, though slightly different notation conventions can be used.


In 1854, German physicist Rudolf Clausius 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:"[2][3]

{} \frac {Q}{T}

In 1865, he came to define this ratio as entropy symbolized by S, such that, for a closed, stationary system:

\Delta S = \frac {Q}{T}

and thus, by reduction, quantities of heat δQ (an inexact differential) are defined as quantities of TdS (an exact differential):

\delta Q = T dS \,

In other words, the entropy function S facilitates the quantification and measurement of heat flow through a thermodynamic boundary.


In modern terms, heat is concisely defined as energy in transit. Scottish physicist James Clerk Maxwell, in his 1871 classic Theory of Heat, was one of the first to enunciate a modern definition of “heat”. In short, Maxwell outlined four stipulations on the definition of heat. One, it is “something which may be transferred from one body to another”, as per the second law of thermodynamics. Two, it can be spoken of as a “measurable quantity”, and thus treated mathematically like other measurable quantities. Three, it “can not be treated as a substance”; for it may be transformed into something which is not a substance, e.g. mechanical work. Lastly, it is “one of the forms of energy”. Similar such modern, succinct definitions of heat are as follows:

  • In a thermodynamic sense, heat is never regarded as being stored within a body. Like work, it exists only as energy in transit from one body to another; in thermodynamic terminology, between a system and its surroundings. When energy in the form of heat is added to a system, it is stored not as heat, but as kinetic and potential energy of the atoms and molecules making up the system.[4]
  • The noun heat is defined only during the process of energy transfer by conduction or radiation.[5]
  • Heat is defined as any spontaneous flow of energy from one object to another, caused by a difference in temperature between the objects.[1]
  • Heat may be defined as energy in transit from a high-temperature object to a lower-temperature object.[6]
  • Heat as an interaction between two closed systems without exchange of work is a pure heat interaction when the two systems, initially isolated and in a stable equilibrium, are placed in contact. The energy exchanged between the two systems is then called heat.[7]
  • Heat is a form of energy possessed by a substance by virtue of the vibrational movement, i.e. kinetic energy, of its molecules or atoms.[8] The kinetic energy and heat may formally be equivalent, but they are not identical.
  • Heat is the transfer of energy between substances of different temperatures.


Internal energy

Heat is related to the internal energy U of the system and work W done by the system by the first law of thermodynamics:

\Delta U = Q + W \

which means that the energy of the system can change either via work or via heat flows across the boundary of the thermodynamic system. In more detail, Internal energy is the sum of all microscopic forms of energy of a system. It is related to the molecular structure and the degree of molecular activity and may be viewed as the sum of kinetic and potential energies of the molecules; it comprises the following types of energies:[9]

Type Composition of Internal Energy (U)
Sensible energy the portion of the internal energy of a system associated with kinetic energies (molecular translation, rotation, and vibration; electron translation and spin; and nuclear spin) of the molecules.
Latent energy the internal energy associated with the phase of a system.
Chemical energy the internal energy associated with the atomic bonds in a molecule.
Nuclear energy the tremendous amount of energy associated with the strong bonds within the nucleus of the atom itself.
Energy interactions those types of energies not stored in the system (e.g. heat transfer, mass transfer, and work), but which are recognized at the system boundary as they cross it, which represent gains or losses by a system during a process.
Thermal energy the sum of sensible and latent forms of internal energy.

The transfer of heat to an ideal gas at constant pressure increases the internal energy and performs boundary work (i.e. allows a control volume of gas to become larger or smaller), provided the volume is not constrained. Returning to the first law equation and separating the work term into two types, "boundary work" and "other" (e.g. shaft work performed by a compressor fan), yields the following:

\Delta U + W_{boundary} = Q + W_{other}\

This combined quantity \Delta U + W_{boundary} is enthalpy, H, one of the thermodynamic potentials. Both enthalpy, H , and internal energy, U are state functions. State functions return to their initial values upon completion of each cycle in cyclic processes such as that of a heat engine. In contrast, neither Q nor W are properties of a system and need not sum to zero over the steps of a cycle. The infinitesimal expression for heat, \delta Q , forms an inexact differential for processes involving work. However, for processes involving no change in volume, applied magnetic field, or other external parameters, \delta Q , forms an exact differential. Likewise, for adiabatic processes (no heat transfer), the expression for work forms an exact differential, but for processes involving transfer of heat it forms an inexact differential.

Heat capacity

For a simple compressible system such as an ideal gas inside a piston, the changes in enthalpy and internal energy can be related to the heat capacity at constant pressure and volume, respectively. Constrained to have constant volume, the heat, Q, required to change its temperature from an initial temperature, T0, to a final temperature, Tf is given by:

Q = \int_{T_0}^{T_f}C_v\,dT = \Delta U\,\!

Removing the volume constraint and allowing the system to expand or contract at constant pressure:

Q = \ \Delta U + \int_{V_0}^{V_f}P\,dV = \ \Delta H = \int_{T_0}^{T_f}C_p\,dT \,\!

For incompressible substances, such as solids and liquids, the distinction between the two types of heat capacity disappears, as no work is performed. Heat capacity is an extensive quantity and as such is dependent on the number of molecules in the system. It can be represented as the product of mass, m , and specific heat capacity, c_s \,\! according to:

C_p = mc_s \,\!

or is dependent on the number of moles and the molar heat capacity, c_n \,\! according to:

C_p = nc_n \,\!

The molar and specific heat capacities are dependent upon the internal degrees of freedom of the system and not on any external properties such as volume and number of molecules.

The specific heats of monatomic gases (e.g., helium) are nearly constant with temperature. Diatomic gases such as hydrogen display some temperature dependence, and triatomic gases (e.g., carbon dioxide) still more.

In liquids at sufficiently low temperatures, quantum effects become significant. An example is the behavior of bosons such as helium-4. For such substances, the behavior of heat capacity with temperature is discontinuous at the Bose-Einstein condensation point.

The quantum behavior of solids is adequately characterized by the Debye model. At temperatures well below the characteristic Debye temperature of a solid lattice, its specific heat will be proportional to the cube of absolute temperature. For low-temperature metals, a second term is needed to account for the behavior of the conduction electrons, an example of Fermi-Dirac statistics.

Phase Changes

The boiling point of water, at sea level and normal atmospheric pressure and temperature, will always be at nearly 100 °C, no matter how much heat is added. The extra heat changes the phase of the water from liquid into water vapor. The heat added to change the phase of a substance in this way is said to be "hidden" and thus it is called latent heat (from the Latin latere meaning "to lie hidden"). Latent heat is the heat per unit mass necessary to change the state of a given substance, or:

L = \frac{Q}{\Delta m} \,\!


Q = \int_{M_0}^{M} L\,dm.

Note that, as pressure increases, the L rises slightly. Here, M_o is the amount of mass initially in the new phase, and M is the amount of mass that ends up in the new phase. Also, L generally does not depend on the amount of mass that changes phase, so the equation can normally be written:

Q = L\Delta m.

Sometimes L can be time-dependent if pressure and volume are changing with time, so that the integral can be written as:

Q = \int L\frac{dm}{dt}dt.

Heat transfer mechanisms

Heat tends to move from a high-temperature region to a low-temperature region. This heat transfer may occur by the mechanisms of conduction and radiation. In engineering, the term convective heat transfer is used to describe the combined effects of conduction and fluid flow and is regarded as a third mechanism of heat transfer.


Conduction is the most significant means of heat transfer in a solid. On a microscopic scale, conduction occurs as hot, rapidly moving or vibrating atoms and molecules interact with neighboring atoms and molecules, transferring some of their energy (heat) to these neighboring atoms. In insulators the heat flux is carried almost entirely by phonon vibrations.

used to test the heat transfer through firestops and penetrants used in construction listing and approval use and compliance.]]

The "electron fluid" of a conductive metallic solid conducts nearly all of the heat flux through the solid. Phonon flux is still present, but carries less than 1% of the energy. Electrons also conduct electric current through conductive solids, and the thermal and electrical conductivities of most metals have about the same ratio. A good electrical conductor, such as copper, usually also conducts heat well. The Peltier-Seebeck effect exhibits the propensity of electrons to conduct heat through an electrically conductive solid. Thermoelectricity is caused by the relationship between electrons, heat fluxes and electrical currents.


Convection is usually the dominant form of heat transfer in liquids and gases. This is a term used to characterise the combined effects of conduction and fluid flow. In convection, enthalpy transfer occurs by the movement of hot or cold portions of the fluid together with heat transfer by conduction. Commonly an increase in temperature produces a reduction in density. Hence, when water is heated on a stove, hot water from the bottom of the pan rises, displacing the colder denser liquid which falls. Mixing and conduction result eventually in a nearly homogeneous density and even temperature. Two types of convection are commonly distinguished, free convection, in which gravity and buoyancy forces drive the fluid movement, and forced convection, where a fan, stirrer, or other means is used to move the fluid. Buoyant convection is due to the effects of gravity, and hence does not occur in microgravity environments.


Radiation is the only form of heat transfer that can occur in the absence of any form of medium (i.e., through a vacuum). Thermal radiation is a direct result of the movements of atoms and molecules in a material. Since these atoms and molecules are composed of charged particles (protons and electrons), their movements result in the emission of electromagnetic radiation, which carries energy away from the surface. At the same time, the surface is constantly bombarded by radiation from the surroundings, resulting in the transfer of energy to the surface. Since the amount of emitted radiation increases with increasing temperature, a net transfer of energy from higher temperatures to lower temperatures results.

The power that a black body emits at various frequencies is described by Planck's law. For any given temperature, there is a frequency fmax at which the power emitted is a maximum. Wien's displacement law, and the fact that the frequency of light is inversely proportional to its wavelength in vacuum, mean that the peak frequency fmax is proportional to the absolute temperature T of the black body. The photosphere of the Sun, at a temperature of approximately 6000 K, emits radiation principally in the visible portion of the spectrum. The Earth's atmosphere is partly transparent to visible light, and the light reaching the Earth's surface is absorbed or reflected. The Earth's surface emits the absorbed radiation, approximating the behavior of a black body at 300 K with spectral peak at fmax. At these lower frequencies, the atmosphere is largely opaque and radiation from the Earth's surface is absorbed or scattered by the atmosphere. Though some radiation escapes into space, it is absorbed and subsequently re-emitted by atmospheric gases. It is this spectral selectivity of the atmosphere that is responsible for the planetary greenhouse effect.

The common household lightbulb has a spectrum overlapping the blackbody spectra of the sun and the earth. A portion of the photons emitted by a tungsten light bulb filament at 3000K are in the visible spectrum. However, most of the energy is associated with photons of longer wavelengths; these will not help a person see, but will still transfer heat to the environment, as can be deduced empirically by observing a household incandescent lightbulb. Whenever EM radiation is emitted and then absorbed, heat is transferred. This principle is used in microwave ovens, laser cutting, and RF hair removal.

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 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.


  1. 1.0 1.1 Schroeder, Daniel V. (2000). An introduction to thermal physics. San Francisco, California: Addison-Wesley. p. p. 18. ISBN 0-321-27779-1. "Heat is defined as any spontaneous flow of energy from one object to another, caused by a difference in temperature between the objects." 
  2. Published in Poggendoff’s Annalen, Dec. 1854, vol. xciii. p. 481; translated in the Journal de Mathematiques, vol. xx. Paris, 1855, and in the Philosophical Magazine, August 1856, s. 4. vol. xii, p. 81
  3. Clausius, R. (1865). The Mechanical Theory of Heat] – with its Applications to the Steam Engine and to Physical Properties of Bodies. London: John van Voorst, 1 Paternoster Row. MDCCCLXVII.
  4. Smith, J.M., Van Ness, H.C., Abbot, M.M. (2005). Introduction to Chemical Engineering Thermodynamics. McGraw-Hill. ISBN 0073104450. 
  5. Baierlein, Ralph (2003). Thermal Physics. Cambridge University Press. ISBN 0521658381. 
  6. Discourse on Heat and Work - Department of Physics and Astronomy, Georgia State University: Hyperphysics (online)
  7. Perrot, Pierre (1998). A to Z of Thermodynamics. Oxford University Press. ISBN 0198565526. 
  8. Clark, John, O.E. (2004). The Essential Dictionary of Science. Barnes & Noble Books. ISBN 0760746168. 
  9. Cengel, Yungus, A.; Boles, Michael (2002). Thermodynamics - An Engineering Approach, 4th ed.. McGraw-Hill. pp. 17-18. ISBN 0-07-238332-1. 

See also

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Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary



See also heat






HEAT (uncountable)

  1. (military) High Explosive Anti-Tank — antitank munition using a high explosive shaped charge to breach armour.

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Simple English

.]] Heat is a form of energy. In thermodynamics, heat means energy which is moved between two things when one of them is hotter than the other.

Heat is not the same as temperature. The temperature of an object is a measure of the average speed of the moving particles it is made from and the energy of the particles is called the internal energy. When an object is heated, its internal energy can increase to make the object hotter. The first law of thermodynamics says that the increase in internal energy is equal to the heat added minus the work done on the surroundings.


Properties of Heat

Heat is a form of energy and not a physical substance. Heat has no mass.

Heat can move from one place to another in different ways:

The measure of how much heat is needed to cause some change in temperature for a material is the specific heat capacity of the material. If the particles in the material are hard to move, then more energy is needed to make them move quickly, so a lot of heat will cause a small change in temperature. A different particle that is easier to move will need less heat for the same change in temperature.

Specific heat capacities can be looked up in a table, like this one.

Unless some work is done, heat moves only from hot things to cold things.

Measuring Heat

Heat can be measured. That is, the amount of heat given out or taken in can be given a value. One of the units of measurement for heat is the joule.

Heat is usually measured with a calorimeter, where the energy in a material is allowed to flow into nearby water, which has a known specific heat capacity. The temperature of the water is then measured before and after, and heat can be found using a formula.

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