The Transfer of Heat

[Jessica]

Convection is the process in which heat is carried from place to place by the bulk movement of a fluid.


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.

1. Q is proportional to the time t during which conduction takes place . More heat flows in longer time periods.
2. Q is proportional to the temperature difference ΔT between the ends of the bar . A larger difference causes more heat to flow. No heat flows when both ends have the same temperature and .
3. Q is proportional to the cross-sectional area A of the bar . Figure 1 helps to explain this fact by showing two identical bars (insulated sides not shown) placed between the warmer and cooler bodies. Clearly, twice as much heat flows through two bars as through one, because the cross-sectional area has been doubled.
4. Q is inversely proportional to the length L of the bar . Greater lengths of material conduct less heat. To experience this effect, put two insulated mittens (the pot holders that cooks keep near the stove) on the same hand. Then, touch a hot pot and notice that it feels cooler than when you wear only one mitten, signifying that less heat passes through the greater thickness (“length”) of material.

Figure 1



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

where ΔT is the temperature difference between the ends of the bar (the higher temperature minus the lower temperature) and k is the thermal conductivity of the material.
SI Unit:

[Jessica]

Radiation is the process in which energy is transferred by means of electromagnetic waves.

The Stefan–Boltzmann Law of Radiation
The radiant energy Q, emitted in a time t by an object that has a Kelvin temperature T, a surface area A, and an emissivity e, is given by

where is the Stefan–Boltzmann constant.