Heat Theories |
CONDUCTION is the transfer of heat, from molecule to molecule, throughout a solid material. The molecules inside the material which are nearest to a heat source gain kinetic energy.
They vibrate vigorously, and their movement affects the molecules immediately next to them. They pass on some of their energy, spreading heat through the material.
Conduction is chiefly associated with solids, because of the closely packed molecular structure of a solid is most suited to it.
Metals are very good conductors of heat.
Conduction is a point-by-point process of heat transfer. If one part of a body is heated by direct contact with a source of heat, the neighboring parts become heated successively.
Thus, as shown in the diagram, if a metal rod is placed in a burner, heat travels along the rod by conduction. This may be explained by the kinetic theory of matter.
The molecules of the rod increase their energy of motion.
This violent motion is passed along the rod from molecule to molecule. In considering the flow of heat by conduction,
it is sometimes helpful to compare the flow of heat to the flow of electricity.
The temperature difference can be thought of as the pressure, or voltage, in an electrical circuit.
The ability of a substance to transfer heat (its thermal conductivity) can be compared to electrical conductivity.
When the temperature difference (or voltage) between two points is great,
the driving force to move heat (or current) is high. The quantity of heat (or current) transferred will depend upon the temperature difference (or voltage difference) and the resistance to the flow of heat (or current) offered by the conductor.
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REFLECTION occurs when light rays hit a surface and bounces off changing direction.
Mirrors are usually used to demonstrate the reflection of light because of their shiny surfaces reflect light more than dull rough surfaces.
This is important when applied to heat because heat as well is transferred in a light wave. Reflection plays a role in reflecting the light waves and thus returning the waves, which also employ heat.
Therefore if a substrate is exposed to the sun and its enormous amount of light as well as radiated energy a large portion of the energy is then transferred into the substrate.
However if the substrate employs a white and shiny surface a large portion of energy transferred, is reflected back into the atmosphere.
This theory also works for radiated energy that can be reflected back to a substrate if the energy is transmitted from within.
For example, if a pipe is covered with a shiny surface the reflected energy is then transmitted back to the pipe.
If the surface was black the energy would simply be radiated to the atmosphere.
Thus reflection plays an important role when considering how energy is either lost or gained.
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RADIATION - This process begins when the internal energy of a system is converted into radiant energy at a source such as a heater.
This energy is transmitted by waves through space, just as the sun radiates heat outwards through the solar system. Finally the radiant energy strikes a body where it is absorbed and converted to internal energy.
It then appears as heat. An electric heater produces radiant energy in this way (see diagram). It may be absorbed, reflected, or transmitted by a body in its path.
When the radiant energy is absorbed, the internal energy of the body increases and its temperature rises.
All bodies, whether hot or cold, radiate energy. The hotter a body is, the more energy it radiates.
Furthermore all bodies receive radiation from other bodies. The exchange of radiant energy goes on continuously.
Thus a body at constant temperature has not stopped radiating. It is receiving energy at the same rate that it is radiating energy.
There is no change in internal energy or temperature.
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| Heat transfer by radiation is not proportional to the difference in temperature between the hot and cold objects as it is in the case of heat transfer by conduction and convection.
It is proportional to the difference between the fourth powers of the absolute temperatures of the two objects. Thus heat transfer by radiation is enormously more effective at high temperatures than at low temperatures.
Radiation transfer depends also upon the shape of the radiating object.
As radiational heat is understood the following new terms of emmisivity, transmittance,
and absorptance describe how radiational heat is transferred from one medium to the next. The following terms are described below.
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EMISSIVITY is the ratio of its power radiated per unit surface area to the power radiated per unit surface area of a black body at the same temperature.
Materials with high emittance radiate more heat than materials with low emittance. For example, black surfaces have an emittance of 0.98 and a polished aluminum surface has an emittance of 0.04. Aluminum tends to block radiant heat transfer while black surfaces tend to emit significant heat.
E = POWER1(AREA1)/POWER2(AREA2)
power1 = radiative power of unit
power2 = radiative power of a perfect black body
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ABSORPTIVITY and is defined as the fraction of the total incident radiation absorbed by the surface Therefore, if the temperature of the surface is constant and energy is conserved, the emissivity is equal to the absorptivity.
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TRANSMITTANCE is the amount of energy that is transferred to a substrate. A low transmittance is desired for thermal insulators. This prevents heat transfer through the insulator by radiation.
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