Concept Summary

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Topic | Discussion | Learning Aids
| 13.1 CONVECTION Convection is the process in which heat is carried from place to place by the bulk movement of a fluid. |
Natural convection | During natural convection, the warmer, less dense part of a fluid is pushed upward by the buoyant force provided by the surrounding cooler and denser part. |
Forced convection | Forced convection occurs when an external device, such as a fan or a pump, causes the fluid to move. |
| 13.2 CONDUCTION Conduction is the process whereby heat is transferred directly through a material, with any bulk motion of the material playing no role in the transfer. |
Thermal conductors and thermal insulators | Materials that conduct heat well, such as most metals, are known as thermal conductors. Materials that conduct heat poorly, such as wood, glass, and most plastics, are referred to as thermal insulators. |
Conduction of heat through a material | The heat Q conducted during a time t through a bar of length L and cross-sectional area A is

| (13.1)

where is the temperature difference between the ends of the bar and k is the thermal conductivity of the material.
| 13.3 RADIATION Radiation is the process in which energy is transferred by means of electromagnetic waves. |
Absorbers and emitters | All objects, regardless of their temperature, simultaneously absorb and emit electromagnetic waves. Objects that are good absorbers of radiant energy are also good emitters, and objects that are poor absorbers are also poor emitters. |
A perfect blackbody | An object that absorbs all the radiation incident upon it is called a perfect blackbody. A perfect blackbody, being a perfect absorber, is also a perfect emitter. |
Stefan–Boltzmann law of radiation | The radiant energy Q emitted during a time t by an object whose surface area is A and whose Kelvin temperature is T is given by the Stefan–Boltzmann law of radiation:

| (13.2)

Emissivity | where is the Stefan–Boltzmann constant and e is the emissivity, a dimensionless number characterizing the surface of the object. The emissivity lies between 0 and 1, being zero for a nonemitting surface and one for a perfect blackbody. |
Net radiant power | The net radiant power is the power an object emits minus the power it absorbs. The net radiant power Pnet emitted by an object of temperature T located in an environment of temperature T0 is

| (13.3)

Use Self-Assessment Test 13.1 to evaluate your understanding of Sections 13.1, 13.2 and cutnell3158c13-sec1-0003.

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