Chapter Summary
12.1 Heat
- Heat and work are the two distinct methods of energy transfer.
- Heat is energy transferred solely due to a temperature difference.
- Any energy unit can be used for heat transfer, and the most common are kilocalorie (kcal) and joule (J).
- Kilocalorie is defined to be the energy needed to change the temperature of 1.00 kg of water between [latex]\text{14} . 5ºC[/latex] and [latex]\text{15} . 5ºC[/latex].
- The mechanical equivalent of this heat transfer is [latex]1 .\text{00 kcal} = \text{4186 J}.[/latex]
12.2 Temperature Change and Heat Capacity
- The transfer of heat [latex]Q[/latex] that leads to a change [latex]\Delta T[/latex] in the temperature of a body with mass [latex]m[/latex] is [latex]Q = \text{mc} \Delta T[/latex], where [latex]c[/latex] is the specific heat of the material. This relationship can also be considered as the definition of specific heat.
12.3 Phase Change and Latent Heat
- Most substances can exist either in solid, liquid, and gas forms, which are referred to as “phases.”
- Phase changes occur at fixed temperatures for a given substance at a given pressure, and these temperatures are called boiling and freezing (or melting) points.
- During phase changes, heat absorbed or released is given by:
[latex]Q = \text{mL},[/latex]
where [latex]L[/latex] is the latent heat coefficient.
12.4 Heat Transfer Methods
- Heat is transferred by three different methods: conduction, convection, and radiation.
12.5 Conduction
- Heat conduction is the transfer of heat between two objects in direct contact with each other.
- The rate of heat transfer [latex]Q / t[/latex] (energy per unit time) is proportional to the temperature difference [latex]T_{2} - T_{1}[/latex] and the contact area [latex]A[/latex] and inversely proportional to the distance [latex]d[/latex] between the objects:
[latex]\frac{Q}{t} = \frac{\text{kA} \left(T_{2} - T_{1}\right)}{d} .[/latex]
12.6 Convection
- Convection is heat transfer by the macroscopic movement of mass. Convection can be natural or forced and generally transfers thermal energy faster than conduction. Table 14.4 gives wind-chill factors, indicating that moving air has the same chilling effect of much colder stationary air. Convection that occurs along with a phase change can transfer energy from cold regions to warm ones.
12.7 Radiation
- Radiation is the rate of heat transfer through the emission or absorption of electromagnetic waves.
- The rate of heat transfer depends on the surface area and the fourth power of the absolute temperature:
[latex]\frac{Q}{t} = \sigma e A T^{4} ,[/latex]
where [latex]\sigma = 5 .\text{67} \times \text{10}^{- 8} \text{J}/\text{s} \cdot \text{m}^{2} \cdot \text{K}^{4}[/latex] is the Stefan-Boltzmann constant and [latex]e[/latex] is the emissivity of the body. For a black body, [latex]e = 1[/latex] whereas a shiny white or perfect reflector has [latex]e = 0[/latex], with real objects having values of [latex]e[/latex] between 1 and 0. The net rate of heat transfer by radiation is
[latex]\frac{Q_{\text{net}}}{t} = \sigma e A \left(T_{2}^{4} - T_{1}^{4}\right)[/latex]where [latex]T_{1}[/latex] is the temperature of an object surrounded by an environment with uniform temperature [latex]T_{2}[/latex] and [latex]e[/latex] is the emissivity of the object.
“Section Summary” from College Physics 2e by OpenStax is licensed under a Creative Commons Attribution 4.0 International License.
