In physics, **temperature** is the physical property of a system which underlies the common notions of "hot" and "cold"; generally the material with the higher temperature is said to be hotter.

Formally, temperature is that property which governs the transfer of thermal energy, or heat, between one system and another. When two systems are at the same temperature, they are in *thermal equilibrium* and no heat transfer will occur. When a temperature difference does exist, heat will tend to move from the *higher* temperature system to the *lower* temperature system, until thermal equilibrium is again established. This heat transfer may occur via conduction, convection or radiation (see heat for additional discussion of the various mechanisms of heat transfer). The formal properties of temperature are studied in thermodynamics. Temperature also plays an important role in almost all fields of science, including physics, chemistry, and biology.

Temperature is related to the amount of thermal energy or heat in a system. As more heat is added the temperature rises, similarly a decrease in temperature corresponds to a loss of heat from the system. On the microscopic scale this heat corresponds to the random motion of atoms and molecules in the system. Thus, an increase in temperature corresponds in an increase in the rate of movement of the atoms in the system.

Many physical properties of materials including the phase (gas, liquid or solid), density, solubility, vapor pressure, and electrical conductivity depend on the temperature. Temperature also plays an important role in determining the rate and extent to which chemical reactions occur. This is one reason why the human body has several elaborate mechanisms for maintaining the temperature at 37 °C, since temperatures only a few degrees higher can result in harmful reactions with serious consequences. Temperature also controls the type and quantity of thermal radiation emitted from a surface. One application of this effect is the incandescent light bulb, in which a tungsten filament is electrically heated to a temperature at which significant quantities of visible light are emitted.

Temperature is an intrinsic property of a system, meaning that it does not depend on the system size or the amount of material in the system. Other intrinsic properties include pressure and density. By contrast, mass and volume are extrinsic properties, and depend on the amount of material in the system.

Table of contents |

2 Theoretical foundation of temperature 3 Heat capacity 4 Negative Temperatures 5 Temperature in gases 6 Temperature Measurement 7 External links |

The basic unit of temperature in the International System of Units is the kelvin (K). One kelvin is formally defined as 1/273.16 of the temperature of the triple point of water (the point at which water, ice and water vapor exist in equilibrium). The temperature 0 K is called absolute zero and corresponds to the point at which the molecules and atoms have the least possible thermal energy. An important unit of temperature in theoretical physics is the Planck temperature (1.4×10^{32} K).

For everyday applications, it is often convenient to use the Celsius (previously centigrade) scale, in which 0 °C corresponds to the temperature at which water freezes and 100 °C corresponds to the boiling point of water at sea level. In this scale a temperature difference of 1 degree is the same as a 1 K temperature difference, so the scale is essentially the same as the kelvin scale, but offset by the temperature at which water freezes (273.15 K). Thus the following equation can be used to convert from Celsius to kelvin.

Now a basis for the definition of temperature can be obtained from the 'zeroth law of Thermodynamics, which states that if two systems, A and B, are in thermal equilibrium and a third system C is in thermal equilibrium with system A then systems B and C will also be in thermal equilibrium. This is an empirical fact, based on observation rather than theory. Since A, B, and C are all in thermal equilibrium, it is reasonable to say each of these systems shares a common value of some property. We call this property temperature.

Generally, it is not convenient to place any two arbitrary systems in thermal contact to see if they are in thermal equilibrium and thus have the same temperature. Therefore, it is useful to establish a temperature scale based on the properties of some reference system. Then, a measuring device can be calibrated based on the properties of the reference system and used to measure the temperature of other systems. One such reference system is a fixed quantity of gas. Boyle's law indicates that the product of the Pressure and volume (P×V) of a gas is directly proportional to the temperature. This can be expressed by the Ideal gas law as:

- (1)

Equation 1 indicates that for a fixed volume of gas, the pressure increases with increasing temperature. Pressure is just a measure of the force applied by the gas on the walls of the container and is related to the energy of the system. Thus, we can see that an increase in temperature corresponds to an increase in the thermal energy of the system. When two systems of differing temperature are placed in thermal contact, the temperature of the hotter system decreases, indicating that heat is leaving that system, while the cooler system is gaining heat and increasing in temperature. Thus heat always moves from a region of high temperature to a region of lower temperature and it is the temperature difference that drives the heat transfer between the two systems.

In the previous section temperature was defined in terms of the Zeroth Law of thermodynamics. It is also possible to define temperature in terms of the second law of thermodynamics, which deals with entropy. Entropy is a measure of the disorder in a system. The second law states that any process will result in either no change or a net increase in the entropy of the universe. This can be understood in terms of probability. Consider a series of coin tosses. A perfectly ordered system would be one in which every coin toss would come up either heads or tails. For any number of coin tosses, there is only one combination of outcomes corresponding to this situation. On the other hand, there are multiple combinations that can result in disordered or mixed systems, where some fraction are heads and the rest tails. As the number of coin tosses increases, the number of combinations corresponding to imperfectly ordered systems increases. For a very large number of coin tosses, the number of combinations corresponding to ~50% heads and ~50% tails dominates and obtaining an outcome significantly different than 50/50 becomes extremely unlikely. Thus the system naturally progresses to a state of maximum disorder or entropy.

Now, we have stated previously that temperature controls the flow of heat between two systems and we have just shown that the universe, and we would expect any natural system, tends to progress so as to maximize entropy. Thus, we would expect there to be some relationship between temperature and entropy. In order to find this relationship let's first consider the relationship between heat, work and temperature. A Heat engine is a device for converting heat into mechanical work and analysis of the Carnot heat engine provides the necessary relationships we seek. The work from a heat engine corresponds to the difference between the heat put into the system at the high temperature, q_{H} and the heat ejected at the low temperature, q_{C}. The efficiency is the work divided by the heat put into the system or:

- (2)

- (3)

- (4)

- (5)

- (6)

- (7)

- (8)

Also see Specific heat capacity.

Temperature is related to the amount of thermal energy or heat in a system. As heat is added to the system, the temperature increases by an amount proportional to the amount of heat being added. The constant of proportionality is called the heat capacity and reflects the ability of the material to store heat.

The heat is stored in a variety of modes, corresponding to the various quantum states accessible to the system. As the temperature increases more quantum states become accessible, resulting in an increase in heat capacity. For a monatomic gas at low temperatures, the only accessible modes correspond to the translational motion of the atoms, so all of the energy is due to movement of the atoms (Actually, a small amount of energy, called the Zero Point Energy arises due to the confinement of the gas into a fixed volume, this energy is present even at 0 K). Since the kinetic energy is related to the motion of the atoms, 0 K corresponds to the point at which all atoms are motionless. For such a system, a temperature below 0 K is not possible, since it is not possible for the atoms to move slower than to be motionless.

At higher temperatures, electronic transitions become accessible, further increasing the heat capacity. For most materials these transitions are not important below 10^{4} K, however for a few common molecules, such transitions are important even at room temperature. At extremely high temperatures (>10^{8} K) nuclear transitions become accessible. In addition to translational, electronic, and nuclear modes, polyatomic molecules also have modes associated with rotation and vibrations along the molecular bonds, which are accessible even at low temperatures. In solids most of the stored heat corresponds to atomic vibrations.

The previous section described how heat is stored in the various translational, vibrational, rotational, electronic, and nuclear modes of a system. The macroscopic temperature of a system is related to the total heat stored in all of these modes and in a normal system thermal energy is constantly being exchanged between the various modes. However, for some cases it is possible to isolate one or more of the modes. In practice the isolated modes still exchange energy with the other modes, but the time scale of this exchange is much slower than for the exchanges within the isolated mode. One example is the case of nuclear spins in a strong external magnetic field. In this case energy flows fairly rapidly among the spin states of interacting atoms, but energy transfer between the nuclear spins and other modes is relatively slow. Since the energy flow is predominantly within the spin system, it makes sense to think of a spin temperature that is distinct from the temperature due to other modes.

Based on Equation 7, we can say a positive temperature corresponds to the condition where entropy increases as thermal energy is added to the system. This is the normal condition in the macroscopic world and is always the case for the translational, vibrational, rotational, and non-spin related electronic and nuclear modes. The reason for this is that there are an infinite number of these types of modes and adding more heat to the system increases the number of modes that are energetically accessible, and thus the entropy. However, for the case of electronic and nuclear spin systems there are only a finite number of modes available (often just 2, corresponding to spin up and spin down). In the absence of a magnetic field, these spin states are degenerate, meaning that they correspond to the same energy. When an external magnetic field is applied, the energy levels are split, since those spin states that are aligned with the magnetic field will have a different energy than those that are anti-parallel to it.

In the absence of a magnetic field, one would expect such a two-spin system to have roughly half the atoms in the spin-up state and half in the spin-down state, since this maximizes entropy. Upon application of a magnetic field, some of the atoms will tend to align so as to minimize the energy of the system, thus slightly more atoms should be in the lower-energy state (for the purposes of this example we'll assume the spin-down state is the lower-energy state). It is possible to add energy to the spin system using radio frequency (RF) techniques. This causes atoms to flip from spin-down to spin-up. Since we started with over half the atoms in the spin-down state, initially this drives the system towards a 50/50 mixture, so the entropy is increasing, corresponding to a positive temperature. However, at some point more than half of the spins are in the spin-up position. In this case adding additional energy, reduces the entropy since it moves the system further from a 50/50 mixture. This reduction in entropy with the addition of energy corresponds to a negative temperature. For additional information see [1].

As mentioned previously for a monatomic ideal gas the temperature is related to the translational motion or average speed of the atoms. The Kinetic theory of gases uses Statistical mechanics to relate this motion to the average kinetic energy of atoms and molecules in the system. For this case 11300 degrees Celsius corresponds to an average kinetic energy of one electronvolt; to take room temperature (300 kelvin) as an example, the average energy of air molecules is 300/11300 eV, or 0.0273 electronvolts. This average energy is independent of particle mass, which seems counterintuitive to many people. Although the temperature is related to the *average* kinetic energy of the particles in a gas, each particle has its own energy which may or may not correspond to the average. In a gas the distribution of energy (and thus speeds) of the particles corresponds to the Boltzmann distribution.

An electronvolt is a very small unit of energy, on the order of 1.602e-19 joules.

Other important devices for measuring temperature include:

- Thermocouples
- Thermistors
- Resistance Temperature Detector (RTD)
- Pyrometers
- Other thermometers

See also: color temperature, Timeline of temperature and pressure measurement technology, Planck temperature

Articles about temperature ranges:

- 1 picokelvin
- 1 nanokelvin
- 1 microkelvin
- 1 millikelvin
- 1 kelvin
- 10 kelvin
- 100 kelvin
- 1,000 kelvin
- 10,000 kelvin
- 100,000 kelvin
- 10
^{6}kelvin - 10
^{9}kelvin - 10
^{12}kelvin - 10
^{15}kelvin - 10
^{18}kelvin - 10
^{21}kelvin - 10
^{24}kelvin - 10
^{27}kelvin - 10
^{30}kelvin

- An elementary introduction to temperature aimed at a middle school audience
- National Science Digital Library - Temperature
- How to Convert Temperatures : A middle school lesson plan on converting temperatures.