College Physics

# Section Summary

College PhysicsSection 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 $QQ size 12{Q} {}$ that leads to a change $ΔTΔT size 12{ΔT} {}$ in the temperature of a body with mass $mm size 12{m} {}$ is $Q=mcΔTQ=mcΔT size 12{Q= ital "mc"ΔT} {}$, where $cc size 12{c} {}$ 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:
$Q=mL,Q=mL, size 12{Q= ital "mL"} {}$

where $LL size 12{L} {}$ 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 $Q/tQ/t size 12{Q/t} {}$ (energy per unit time) is proportional to the temperature difference $T2−T1T2−T1 size 12{T rSub { size 8{2} } - T rSub { size 8{1} } } {}$ and the contact area $AA size 12{A} {}$ and inversely proportional to the distance $dd size 12{d} {}$ between the objects:
$Qt=kAT2−T1d.Qt=kAT2−T1d. size 12{ { {Q} over {t} } = { { ital "kA" left (T rSub { size 8{2} } - T rSub { size 8{1} } right )} over {d} } } {}$

### 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:
$Qt=σeAT4,Qt=σeAT4, size 12{ { {Q} over {t} } =σeAT rSup { size 8{4} } } {}$

where $σ=5.67×10−8J/s⋅m2⋅K4σ=5.67×10−8J/s⋅m2⋅K4$ is the Stefan-Boltzmann constant and $ee size 12{e} {}$ 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

$Q net t = σ e A T 2 4 − T 1 4 Q net t = σ e A T 2 4 − T 1 4 size 12{ { {Q rSub { size 8{"net"} } } over {t} } =σeA` left (T rSub { size 8{2} } rSup { size 8{4} } - T rSub { size 8{1} } rSup { size 8{4} } right )} {}$

where $T1T1 size 12{T rSub { size 8{1} } } {}$ is the temperature of an object surrounded by an environment with uniform temperature $T2T2 size 12{T rSub { size 8{2} } } {}$ and $ee size 12{e} {}$ is the emissivity of the object.

Do you know how you learn best?