- 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 and .
- The mechanical equivalent of this heat transfer is
14.2 Temperature Change and Heat Capacity
- The transfer of heat that leads to a change in the temperature of a body with mass is , where is the specific heat of the material. This relationship can also be considered as the definition of specific heat.
14.3 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:
where is the latent heat coefficient.
14.4 Heat Transfer Methods
- Heat is transferred by three different methods: conduction, convection, and radiation.
- Heat conduction is the transfer of heat between two objects in direct contact with each other.
- The rate of heat transfer (energy per unit time) is proportional to the temperature difference and the contact area and inversely proportional to the distance between the objects:
- 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.
- 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:
where is the Stefan-Boltzmann constant and is the emissivity of the body. For a black body, whereas a shiny white or perfect reflector has , with real objects having values of between 1 and 0. The net rate of heat transfer by radiation is
where is the temperature of an object surrounded by an environment with uniform temperature and is the emissivity of the object.