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Heat

In physics, the relationship between heat and energy is similar to that
between work and energy. Heat is said to flow from areas of high temperature
to areas of low temperature. All objects have a certain amount of energy
within them that is related to the random motion of their atoms. 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, and understanding the difference is a necessary part of
understanding the first law of thermodynamics.

Changes of Temperature

The amount of heat energy, ΔQ, 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:

[\Delta Q = \int_{T_0}^{T_f}C_p\,dT]

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 moles 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 State

A boiling pot of water, at atmospheric pressure, will always be at 100oC no
matter how much heat is added. The heat in circumstances such as this is
said to be "hidden," and thus it is called latent heat (from a Latin word
for hidden). Latent heat is the rate of 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:

[Q = \int_{M_0}^{M} L\,dm]

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

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

Latent Heat

Transfer of heat through a physical change in the medium such as water/ice
or water/steam involves significant enrgy and is exploited in many ways
steam engine, refrigerator etc.

Heat Pipe

A heat transfer mechanism using latent heat and capiliiary action to move
heat. A heat pipe can carry many times as much heat as a similar sixed coppr
rod and is starting to have applications in laptop personal computers.

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Heat is a popular term for estrus, a period of increased sexual drive in
female mammals.


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