Chapter 14 Learning ObjectivesConcepts and Skills to Review
Energy conservation (Section 6.7)
Absolute temperature and ideal gas law (Section 13.5)
Kinetic theory of the ideal gas (Section 13.6)
Summary
The internal energy of a system is the total energy of all of the molecules in the system except for the macroscopic kinetic energy (kinetic energy associated with macroscopic translation or rotation) and the external potential energy (energy due to external interactions).
Heat is a flow of energy that occurs due to a temperature difference.
The joule is the SI unit for all forms of energy, for heat, and for work. An alternate unit often used for heat and internal energy is the calorie:
1 cal = 4.186 J
(14-1)
The ratio of heat flow into a system to the temperature change of the system is the heat capacity of the system:
The molar heat capacity, C, is the heat capacity per mole:
Q = nCVΔT
(14-8)
At room temperature, the molar heat capacity at constant volume for a monatomic ideal gas is approximately CV = 3/2 R; for a diatomic ideal gas it is approximately CV= 5/2 R.
Phase transitions occur at constant temperature. The heat per unit mass that must flow to melt a solid or to freeze a liquid is the latent heat of fusion Lf . The latent heat of vaporization Lv is the heat per unit mass that must flow to change the phase from liquid to gas or from gas to liquid.
Sublimation occurs when a solid changes directly to a gas without going into a liquid form.
A phase diagram is a graph of pressure vs. temperature that indicates solid, liquid, and vapor regions for a substance. The sublimation, fusion, and vapor pressure curves separate the three phases. Crossing one of these curves represents a phase transition.
Heat flows by three processes: conduction, convection, and radiation.
Conduction is due to atomic (or molecular) collisions within a substance or from one object to another when they are in contact. The rate of heat flow within a substance is:
where Icd = Q/t is the rate of heat flow (or power delivered), κ is the thermal conductivity of the material, A is the cross-sectional area, d is the thickness (or length) of the material, and ΔT is the temperature difference between one side and the other.
Convection involves fluid currents that carry heat from one place to another. In convection, the material itself moves from one place to another.
Radiation does not have to pass through a material medium. The energy is carried by electromagnetic waves that travel at the speed of light. All bodies emit energy through electromagnetic radiation. An idealized body that absorbs all the radiation incident upon it is called a blackbody. A blackbody emits more radiant power per unit surface area than any real object at the same temperature. Stefan's law of radiation is
where the emissivity e ranges from 0 to 1, A is the surface area, T is the surface temperature of the blackbody in kelvins, and Stefan's constant is σ = 5.670 × 10-8 W/(m2·K4). The wavelength of maximum power emission is inversely proportional to the absolute temperature:
λmaxT = 2.898 × 10-3 m·K
(14-20)
The difference between the power emitted by the body and that absorbed by the body from its surroundings is the net power emitted:
