Section Summary

Systems are in thermal equilibrium when they have the same temperature.
Thermal equilibrium occurs when two bodies are in contact with each other and can freely exchange energy.
The zeroth law of thermodynamics states that when two systems, A and B, are in thermal equilibrium with each other, and B is in thermal equilibrium with a third system, C, then A is also in thermal equilibrium with C.

Pressure is the force per unit area over which the force is applied perpendicular to the area.
Thermal expansion is the increase, or decrease, of the size (length, area, or volume) of a body due to a change in temperature.
The ideal gas law relates the pressure and volume of a gas to the number of gas particles (atoms or molecules) and the absolute temperature of the gas.
Heat and work are the two distinct methods of energy transfer.
Heat is energy transferred solely due to a temperature difference.
The first law of thermodynamics is given as $Δ U = Q - W Δ U = Q - W$ , where $Δ U Δ U$ is the change in internal energy of a system, Q is the net energy transfer into the system by heat (the sum of all transfers by heat into and out of the system), and W is the net work done by the system (the sum of all energy transfers by work out of or into the system).
Both Q and W represent energy in transit; only $Δ U Δ U$ represents an independent quantity of energy capable of being stored.
The internal energy U of a system depends only on the state of the system, and not how it reached that state.

Entropy is a measure of a system's disorder: the greater the disorder, the larger the entropy.
Entropy is also the reduced availability of energy to do work.
The second law of thermodynamics states that, for any spontaneous process, the total entropy of a system either increases or remains constant; it never decreases.
Heat transfers energy spontaneously from higher- to lower-temperature bodies, but never spontaneously in the reverse direction.

Heat engines use the heat transfer of energy to do work.
Cyclical processes are processes that return to their original state at the end of every cycle.
The thermal efficiency of a heat engine is the ratio of work output divided by the amount of energy input.
The amount of work a heat engine can do is determined by the net heat transfer of energy during a cycle; more waste heat leads to less work output.
Heat pumps draw energy by heat from cold outside air and use it to heat an interior room.
A refrigerator is a type of heat pump; it takes energy from the warm air from the inside compartment and transfers it to warmer exterior air.