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College Physics 2e

Section Summary

College Physics 2eSection Summary

14.1 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 14.5ºC14.5ºC and 15.5ºC15.5ºC.
  • The mechanical equivalent of this heat transfer is 1.00 kcal=4186 J.1.00 kcal=4186 J.

14.2 Temperature Change and Heat Capacity

  • The transfer of heat QQ that leads to a change ΔTΔT in the temperature of a body with mass mm is Q=mcΔTQ=mcΔT, where cc 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 LL is the latent heat coefficient.

14.4 Heat Transfer Methods

  • Heat is transferred by three different methods: conduction, convection, and radiation.

14.5 Conduction

  • Heat conduction is the transfer of heat between two objects in direct contact with each other.
  • The rate of heat transfer Q/tQ/t (energy per unit time) is proportional to the temperature difference T2T1T2T1 and the contact area AA and inversely proportional to the distance dd between the objects:

14.6 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.

14.7 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:

    where σ=5.67×108J/sm2K4σ=5.67×108J/sm2K4 is the Stefan-Boltzmann constant and ee is the emissivity of the body. For a black body, e=1e=1 whereas a shiny white or perfect reflector has e=0e=0, with real objects having values of ee between 1 and 0. The net rate of heat transfer by radiation is

    Q net t = σ e A T 2 4 T 1 4 Q net t = σ e A T 2 4 T 1 4

    where T1T1 is the temperature of an object surrounded by an environment with uniform temperature T2T2 and ee is the emissivity of the object.

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