Section Summary

 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.

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.

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:Q=mL,𝑄=mL,where L𝐿 is the latent heat coefficient.

Heat Transfer Methods

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

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 T2−T1𝑇2−𝑇1 and the contact area A𝐴 and inversely proportional to the distance d𝑑 between the objects:Qt=kA(T2−T1)d.𝑄𝑡=kA𝑇2−𝑇1𝑑.

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.

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: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 isQnett=σeA(T42−T41)𝑄net𝑡=𝜎𝑒𝐴𝑇24−𝑇14where T1𝑇1 is the temperature of an object surrounded by an environment with uniform temperature T2𝑇2 and e𝑒 is the emissivity of the object.