The Sun and Earth form an ongoing example of a heating process. Some of the Sun's thermal radiation strikes and heats the Earth. Compared to the Sun, Earth has a much lower temperature and so sends far less thermal radiation back to the Sun. The heat of this process can be quantified by the net amount, and direction (Sun to Earth), of energy it transferred in a given period of time.

In thermodynamics, heat is energy in transfer to or from a thermodynamic system, by mechanisms other than thermodynamic work or transfer of matter.[1][2][3][4][5][6][7] 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. When there is a suitable path between two systems with different temperatures, heat transfer occurs necessarily, immediately, and spontaneously from the hotter to the colder system. Thermal conduction occurs by the stochastic (random) motion of microscopic particles (such as atoms or molecules). 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. The definition of heat transfer does not require that the process be in any sense smooth. For example, a bolt of lightning may transfer heat to a body.

Convective circulation allows one body to heat another, through an intermediate circulating fluid that carries energy from a boundary of one to a boundary of the other; the actual heat transfer is by conduction and radiation between the fluid and the respective bodies.[8][9][10] Though spontaneous, convective circulation does not necessarily and immediately occur merely because of temperature difference; for it to occur in a given arrangement of systems, there is a threshold temperature difference that needs to be exceeded.

Like thermodynamic work, heat transfer is a process involving two systems, not a property of any one system. 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. This is to be distinguished from the ordinary language conception of heat as a property of the system.

Although heat flows from a hotter body to a cooler one, it is possible to construct a heat pump or refrigeration system that does work to increase the difference in temperature between two systems. In contrast, a heat engine reduces an existing temperature difference to do work on another system.

The amount of heat transferred in any process can be defined as the total amount of transferred energy excluding any macroscopic work that was done and any energy contained in matter transferred. For the precise definition of heat, it is necessary that it occur by a path that does not include transfer of matter.[11] As an amount of energy (being transferred), the SI unit of heat is the joule (J). The conventional symbol used to represent the amount of heat transferred in a thermodynamic process is Q. Heat is measured by its effect on the states of interacting bodies, for example, by the amount of ice melted or a change in temperature.[12] The quantification of heat via the temperature change of a body is called calorimetry.

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 one joule per second.

Use of the symbol Q for the total amount of energy transferred as heat is due to Rudolf Clausius in 1850:

"Let the amount of heat which must be imparted during the transition of the gas in a definite manner from any given state to another, in which its volume is v and its temperature t, be called Q"[13]

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 . This should not be confused with a time derivative of a function of state (which can also be written with the dot notation) since heat is not a function of state.[14] Heat flux is defined as rate of heat transfer per unit cross-sectional area (units watts per square metre).

Other Languages
አማርኛ: ሙቀት
Ænglisc: Hāt
العربية: حرارة
armãneashti: Câldurâ
asturianu: Calor
Aymar aru: Lupi
azərbaycanca: İstilik
تۆرکجه: ایستی‌لیک
বাংলা: তাপ
Bân-lâm-gú: Jia̍t
български: Топлина
Boarisch: Wärme
bosanski: Toplota
català: Calor
Cebuano: Kainit
čeština: Teplo
chiShona: Madziya
Cymraeg: Gwres
dansk: Varme
Deutsch: Wärme
eesti: Soojus
Ελληνικά: Θερμότητα
español: Calor
Esperanto: Varmo
euskara: Bero
فارسی: گرما
Gaeilge: Teas
Gaelg: Çhiass
galego: Calor
ગુજરાતી: ઉષ્મા
հայերեն: Ջերմաքանակ
हिन्दी: ऊष्मा
hrvatski: Toplina
Ido: Kaloro
Bahasa Indonesia: Panas
íslenska: Varmi
italiano: Calore
Basa Jawa: Kalor
Kabɩyɛ: Fefeku
ಕನ್ನಡ: ಉಷ್ಣತೆ
ქართული: სითბო
қазақша: Жылулық
Kiswahili: Joto
Kreyòl ayisyen: Chalè
Latina: Calor
latviešu: Siltums
lietuvių: Šiluma
Limburgs: Wermte
lingála: Eyángala
मैथिली: ताप
македонски: Топлина
മലയാളം: താപം
مصرى: حرارة
Bahasa Melayu: Haba
монгол: Дулаан
မြန်မာဘာသာ: အပူ
Nederlands: Warmte
नेपाली: ताप
Nordfriisk: Waremk
norsk: Varme
norsk nynorsk: Varme
ਪੰਜਾਬੀ: ਗਰਮੀ
Patois: Iit
polski: Ciepło
português: Calor
română: Căldură
русский: Теплота
Scots: Heat
sicilianu: Caluri
Simple English: Heat
slovenčina: Teplo
slovenščina: Toplota
Soomaaliga: Kul
کوردی: گەرمی
српски / srpski: Топлота
srpskohrvatski / српскохрватски: Toplina
Basa Sunda: Panas
suomi: Lämpö
svenska: Värme
తెలుగు: ఉష్ణము
Türkçe: Isı
українська: Кількість теплоти
اردو: حرارت
Tiếng Việt: Nhiệt
Võro: Lämmüs
吴语: 热量
ייִדיש: היץ
中文: 熱量