College Physics 2e

# Section Summary

College Physics 2eSection Summary

## 14.1Heat

• 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 $1.00 kcal=4186 J.1.00 kcal=4186 J.$

## 14.2Temperature 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.

## 14.3Phase 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:
$Q=mL,Q=mL,$

where $LL$ is the latent heat coefficient.

## 14.4Heat Transfer Methods

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

## 14.5Conduction

• 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 $T2−T1T2−T1$ and the contact area $AA$ and inversely proportional to the distance $dd$ between the objects:
$Qt=kAT2−T1d.Qt=kAT2−T1d.$

## 14.6Convection

• 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:
$Qt=σeAT4,Qt=σeAT4,$

where $σ=5.67×10−8J/s⋅m2⋅K4σ=5.67×10−8J/s⋅m2⋅K4$ 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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