Heat

In thermodynamics, heat is (symbol, unit Joule, Calorie earlier ) the energy that is transferred between two systems due to temperature differences. The heat always flows from this place of high temperature to low temperature place. The heat transfer can be accomplished by thermal conduction, thermal radiation or convection.

The heat is in contrast to the mechanical work without changing external parameters such as the volume taken up or released. Instead, the heat causes a change in the entropy of the system concerned. Since no change in the thermal interaction, the external parameters, the energy levels remain unchanged. Thus, the heat causes only a change in the distribution of the ensemble on these energy levels.

In common parlance, the term of the heat is often confused with the thermal energy of a system. Wherein the thermal energy as part of the internal energy, however, it is a state variable. Their value is thus (eg the temperature ) is determined by the quantity of the system (mass or molar ) and the system state. The heat transfer is, however, dependent on the conditions under which the process runs. Heat is thus a process variable. It is in their meaning therefore comparable with the work, with those available in a mechanical way for the transfer of energy. Both - heat and work - together determine the change in internal energy of a system.

Derived quantities

Taking into account the heat transfer to the time, so to get to the heat flow (or more precisely, the heat current)

With units of Watts.

The heat flux density ( at a given time ) is the quotient of the differential heat flow and the differential area dA of the surface system through which it is transmitted:

The process variable heat, it is also useful in certain cases, by reference to the system ground, or in stationary permeable systems to the mass flow to define a specific heat (not to be confused with the specific heat capacity )

With the unit J / kg ( Joules per kilogram).

Historical definition

This was once one imagines that every material body has a feature, called caloric ( phlogiston ), and that this heat material when you bring together the body at different temperatures, from which a body flows into the other. It was further assumed that the massless, in every body more or less contained phlogiston is flammable. Without phlogiston, the body can not burn. On combustion, phlogiston leaves the body and the glow of the body, it evaporates, leaving behind the ashes. This phlogiston theory was the dominant concept in the science of the 18th century.

Today we know that there is no heat fabric. The expression amount of heat and thermal energy are remnants of the old caloric theory. As part of the heat energy change is referred to, part of the thermal interaction. Heat is thus not analogous to the substance associated energy but work. The meaning of the term ' heat energy ' today still commonly used will only become clear from the context, because depending on use may allow the state variable internal energy or the process variable heat be meant.

Heat transfer and the first law ( conservation of energy )

As long as a temperature difference between two thermally coupled systems is, therefore these do not yet exist in thermal equilibrium, flows a heat flow:

Here, and the temperatures of the systems involved. At the system boundary of the heat transfer is described by the heat transfer coefficient.

Often this heat flow causes the temperatures of the two systems converge to each other. But there are also systems in which heat is input to the phase conversion and does not lead to increase in temperature, for example, upon evaporation of liquids.

Thermodynamic determines the heat along with the work done on the basis of the first law of thermodynamics on the increase in internal energy of a closed system:

The signs of heat and work are as defined by convention that they are positive when they are fed to the system. They are negative if the system emits heat or work. For example, the system can absorb heat and leave work, so to speak, to convert heat into work (the principle of the heat engine ).

The internal energy is an (extensive ) size, which only depends on the state of the system, such as its temperature and mass.

Heat supply and the 2nd law ( law of entropy )

The heat associated with the absolute temperature of the state variable of the entropy

It is meant that the heat supplied to a reversible way. This means that the processes involved therein (at least in infinitesimally small increments ) can be reversed at any time without permanent changes remain. In non-reversible processes (such as the occurrence of frictional loss ) is considered instead of the above equation, the more general relationship

This is the Second Law of Thermodynamics. The equal sign applies only to reversible processes. For irreversible processes there is also the power dissipated in the system work.

Entropy is often regarded as a measure of the inherent system disorder. So In liquefaction of a crystal are increasing.

Application

You can convert heat into work in accordance with the first law. This happens in the so-called heat engines. However, it is equally the second law must be observed. This gives as a general principle that the waste heat can be all the less, the lower their temperature level. Conversely, so-called heat pumps at a low temperature heat from a "reservoir" (eg the ground ) and place under workload (eg electrical energy ) re-emit at a higher temperature for heating about. Now the temperature difference should be as small as possible in order to minimize the administrative burden small can.