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# Thermodynamics Rapid Review for AP Physics B

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For a more thorough review, refer to these concepts:

### Rapid Review

• Heat is a form of energy that can be transferred from one body to another. The internal energy of an object is the sum of the kinetic energies of all the molecules that make up that object. An object's temperature is directly related to average kinetic energy per molecule of the object.
• Two bodies with the same temperature do not necessarily have the same internal energy, and vice versa.
• When objects are heated, they expand. The linear expansion of an object is proportional to the initial length of the object and the temperature by which it is heated.
• The pressure and volume of an ideal gas are related by the ideal gas law. The law can either be written as PV = nRT, or PV = NkBT.
• The first law of thermodynamics states that the change in a gas's internal energy equals the heat added to the gas plus the work done on the gas.
• PV diagrams illustrate the relationship between a gas's pressure and volume as it undergoes a process. Four types of processes a gas can undergo are (1) isothermal [temperature stays constant]; (2) adiabatic [no heat is transferred to or from the gas]; (3) isobaric [pressure stays constant]; and (4) isochoric [volume stays constant].
• The second law of thermodynamics says that heat flows naturally from a hot object to a cold object.
• A heat engine works by letting heat flow from hot stuff to cold stuff; some of the flowing heat is used to do work.
• The efficiency of a heat engine is always less than 1. The real efficiency of a heat engine equals the work done by the engine divided by the amount of input heat. The ideal efficiency of a heat engine depends only on the temperatures of the hot and cold reservoirs.
• A refrigerator is like a heat engine, except in a refrigerator, work is done on a system to make cold stuff hotter.
• Entropy is a measure of disorder. The second law of thermodynamics says that the entropy of a system cannot decrease unless work is done on that system.

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