Temperature is a physical quantity that is a measure of hotness and coldness on a numerical scale. It is a measure of the local thermal energy of matter or radiation; it is measured by a thermometer, which may be calibrated in any of various temperature scales, Celsius, Fahrenheit, Kelvin, etc., etc.

Empirically it is found that an isolated system, one that exchanges no energy or material with its environment, tends to a spatially uniform temperature as time passes. When a path permeable only to heat is open between two bodies, energy always flows spontaneously as heat from a hotter body to a colder one. The flow rate increases with the temperature difference and with a property of the path called the thermal conductance. Between two bodies with the same temperature, connected by a path permeable only to heat, no energy will flow. These bodies are said to be “in the relation of thermal equilibrium”. In a body in its personal internal thermal equilibrium, the temperature is spatially uniform.

In kinetic theory and in statistical mechanics, temperature is explained by the motions and interactions of microscopic constituents such as molecules and photons.her or schoolteacher is a person who provides education for pupils and students.

Temperature

Temperature

Temperature

Temperature

Temperature

Generally, when substances are heated, their particles begin to move faster. In solids, molecules vibrate together; heat makes them vibrate harder, and when the particles get hot enough, they vibrate together hard enough to break away from the molecule structure — this is called melting, like when ice melts into water.

In liquids, molecules tend to slide around each other; when they are heated, they start to bounce off of each other, and the liquid mass expands. If they bump off of each other hard enough, they too can break away from their molecule structure, and float about independently as a gas molecule — that’s called evaporation, like when you heat water and steam is produced.

Gas molecules tend to bounce off of each other; when they are heated, the gas mass expands because they bounce off of each other harder.

Much of the world uses the Celsius scale (°C) for most temperature measurements. It has the same incremental scaling as the Kelvin scale used by scientists, but fixes its null point, at 0°C = 273.15K, approximately the freezing point of water (at one atmosphere of pressure).[note 1] The United States uses the Fahrenheit scale for common purposes, a scale on which water freezes at 32 °F and boils at 212 °F (at one atmosphere of pressure).

For practical purposes of scientific temperature measurement, the International System of Units (SI) defines a scale and unit for the thermodynamic temperature by using the easily reproducible temperature of the triple point of water as a second reference point. The reason for this choice is that, unlike the freezing and boiling point temperatures, the temperature at the triple point is independent of pressure (since the triple point is a fixed point on a two-dimensional plot of pressure vs. temperature). For historical reasons, the triple point temperature of water is fixed at 273.16 units of the measurement increment, which has been named the kelvin in honor of the Scottish physicist who first defined the scale. The unit symbol of the kelvin is K.

Absolute zero is defined as a temperature of precisely 0 kelvins, which is equal to −273.15 °C or −459.67 °F.

It is framed in terms of an idealized device called a Carnot engine, imagined to define a continuous cycle of states of its working body. The cycle is imagined to run so slowly that at each point of the cycle the working body is in a state of thermodynamic equilibrium. There are four limbs in such a Carnot cycle. The engine consists of four bodies. The main one is called the working body. Two of them are called heat reservoirs, so large that their respective non-deformation variables are not changed by transfer of energy as heat through a wall permeable only to heat to the working body. The fourth body is able to exchange energy with the working body only through adiabatic work; it may be called the work reservoir. The substances and states of the two heat reservoirs should be chosen so that they are not in thermal equilibrium with one another. This means that they must be at different fixed temperatures, one, labeled here with the number 1, hotter than the other, labeled here with the number 2. This can be tested by connecting the heat reservoirs successively to an auxiliary thermometric body, which is required to show changes in opposite senses to its non-deformation variable, and which is composed of a material that has a strictly monotonic relation to the amount of work done on it in an isochoric adiabatic process. Typically, such a material expands as the surrounds do isochoric work on it. In order to settle the structure and sense of operation of the Carnot cycle, it is convenient to use such a material also for the working body; because most materials are of this kind, this is hardly a restriction of the generality of this definition. The Carnot cycle is considered to start from an initial condition of the working body that was reached by the completion of a reversible adiabatic compression. From there, the working body is initially connected by a wall permeable only to heat to the heat reservoir number 1, so that during the first limb of the cycle it expands and does work on the work reservoir. The second limb of the cycle sees the working body expand adiabatically and reversibly, with no energy exchanged as heat, but more energy being transferred as work to the work reservoir. The third limb of the cycle sees the working body connected, through a wall permeable only to heat, to the heat reservoir 2, contracting and accepting energy as work from the work reservoir. The cycle is closed by reversible adiabatic compression of the working body, with no energy transferred as heat, but energy being transferred to it as work from the work reservoir.

With this set-up, the four limbs of the reversible Carnot cycle are characterized by amounts of energy transferred, as work from the working body to the work reservoir, and as heat from the heat reservoirs to the working body. The amounts of energy transferred as heat from the heat reservoirs are measured through the changes in the non-deformation variable of the working body, with reference to the previously known properties of that body, the amounts of work done on the work reservoir, and the first law of thermodynamics. The amounts of energy transferred as heat respectively from reservoir 1 and from reservoir 2 may then be denoted respectively Q1 and Q2. Then the absolute or thermodynamic temperatures, T1 and T1, of the reservoirs are defined so that to be such that,