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Heat
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Heat

Commonly, heat is estrus, a period of increased sexual drive in female mammals. For the National Basketball Association team, see Miami Heat. For the movie, see Heat (movie).

Heat (abbreviated Q, also called heat change) is the transfer of thermal energy between two bodies which are at different temperatures. The SI unit for heat is the joule.

The relationship between heat and energy is similar to that between work and energy. Heat flows between regions that are not in thermal equilibirum; in particular, it flows from areas of high temperature to areas of low temperature. All objects (matter) have a certain amount of internal energy that is related to the random motion of their atoms or molecules. This internal energy is directly proportional to the temperature of the object. When two bodies of different temperature come in to thermal contact, they will exchange internal energy until the temperature is equalized. The amount of energy transferred is the amount of heat exchanged. It is a common misconception to confuse heat with internal energy, but there is a difference: heat is related with the change in internal energy and the work performed by the system. Understanding this difference is a necessary part of understanding the first law of thermodynamics.

Table of contents
1 Notation
2 Changes of temperature
3 Changes of phase
4 How heat moves
5 Other important heat transfer mechanisms
6 Heat dissipation
7 See also
8 External links

Notation

When heat is released then the symbol is -Q. When heat is absorbed, the symbol is +Q.

Total heat, heat transfer rate, and heat flux are all notated with different permutations of the letter Q. They are often confusingly switched in different contexts.

Total heat is notated as Q, and is measured in joules.

Heat transfer rate, or heat flow per unit time, is labeled

to indicate a change per unit time. In unicode, this is , though it may not display correctly in all browsers. It is often shown as ˙Q , .Q, , or as a Q with no dot, where it is not easy to produce a dotted Q. Some form of dotted Q, such as .Q, is preferable, since undotted Q is used for total heat. It is measured in joules per second.

Heat flux is defined as amount of heat per unit time per unit cross-sectional area, and is abbreviated q, and is measured in joules per second per meter squared. It is also sometimes notated as Q'' or q'' or

Changes of temperature

The amount of heat energy, , required to change the temperature of a material from an initial temperature, T0, to a final temperature, Tf depends on the
heat capacity of that material according to the relationship:

The heat capacity is dependent on both the amount of material that is exchanging heat and its properties. The heat capacity can be broken up in several different ways. First of all, it can be represented as a product of mass and specific heat capacity (more commonly called specific heat):

Cp = m cs

or the number of moless and the molar heat capacity:

Cp = n cmolar

Both the molar and specific heat capacities only depend upon the physical properties of the substance being heated, not on any specific properties of the sample. The above definitions of heat capacity only work approximately for solids and liquids, but for gases they don't work at all most of the time. The molar heat capacity can be "patched up" if the changes of temperature occur at either a constant volume or constant pressure. Otherwise, it's generally easiest to use the first law of thermodynamics in combination with an equation relating the internal energy of the gas to its temperature.

Changes of phase

A boiling pot of water, at sea level and normal atmospheric pressure, will always be at 100oC 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 a Latin word for hidden). Latent heat is heat per unit mass necessary to change the state of a given substance. Thus:

dQ/dm = L (should this be a partial derivative or a full one?)

and:

where Mo is the amount of mass initially in the new phase, and M is the amount of mass that ends up in the new phase.

L generally doesn't depend on the amount of mass that changes phase, so the equation can normally be written:

Q = L Δm

Sometimes L can be time-dependent if pressure and volume are time-varying, so that the integral can be handled:

Q = ∫L (dm/dt) dt

someone check the above, please, to see if the latent heat really depends on where on the (P, V, T) curve the transition is taking place.

How heat moves

As mentioned previously, heat tends to move from a high temperature region to a low temperature region. This heat transfer may occur by any of three mechanisms, conduction, convection, and radiation.

Conduction is the most common 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.

Convection is usually the dominant form of heat transfer in liquids and gases. In convection, heat transfer occurs by the movement of hot or cold portions of the fluid. For example, when water is heated on a stove, hot water from the bottom of the pan rises, heating the water at the top of the pan. 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.

Radiation is the final means of heat transfer. Radiative heat transfer is the only form of heat transfer that can occur in the absence of any form of medium and as such is the only means of heat transfer 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.

Other important heat transfer mechanisms

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 would makes their interiors uncomfortably cool or cold. The house is an open system inasmuch as it is incapable of preventing heat from escaping. Furthermore, the interior of the house must be maintained out of thermal equilibrium with its exterior for the sake of its inhabitants.

In such a house, a thermostat is a device capable of starting the heating system when the house's interior falls to a set temperature, and of stopping that same system when another set temperature has been achieved. Thus the thermostat controls the flow of energy into the house, that energy eventually being dissipated to the exterior.

See also

External links