AP Night Lecture #7: Thermodynamics

Online Video Lessons:

For animated lessons with narration and problems, visit HippoCampus's AP Physics B II website and scroll down to the "Temperature and Heat" and "Kinetic Theory and Thermodynamics" topics.

  1. Temperature and Heat
    1. Definition: avg. K of molecules
    2. Scales: Kelvin, Celsius, etc.
    3. Heat is defined as flow of energy Q from substance to environment due to temperature difference
  2. Thermal expansion
    1. Linear expansion ΔL = L0αΔT
    2. Volume expansion ΔV = V0ΒΔT
  3. First Law of Thermodynamics
    1. ΔU = Qadded + Wdone ON system
    2. Consider a piston that sinks to compress a gas or rises to expand it. The first law says that the gas gets hotter (+ΔU) if heat flows into the system (Qadded is positive) or if work is done on the system by pushing down the piston (Wdone ON system is positive). The gas gets colder (-ΔU) if heat flows from the system (Qadded is negative), or if work is done by the system (Wdone ON system is negative).
  4. Adiabatic, Isothermal, Isochoric, Isobaric Processes
    1. Adiabatic processes have Q=0 (piston in a thermos) so by 1st law, ΔU = Wdone ON system. Thus the gas either gets hotter as the piston sinks or colder as it rises.
    2. Isothermal processes have constant temperature, so ΔU=0. Thus Qadded = -Wdone ON system. So if any heat flows to the piston, it must rise so the gas doesn't get hotter. If any heat flows from the system, the piston must sink so the gas doesn't get colder.
    3. Isochoric processes have constant volume, so the piston is motionless and Wdone ON system=0. Thus ΔU = Qadded and any heat flowing into or from the system causes the gas to heat or cool.
    4. Isobaric processes have constant pressure, and the work done on the system will then be pΔV.
  5. pV diagrams and the above processes
    1. Each process traces a distinctive shape on the pV diagram.
    2. Any cyclic process eventually returns to the same pV it started at, thus the same temperature (by pV=nRT).
    3. The work done during a given process is the area under the pV curve.
  6. Second Law of Thermodynamics
    1. In all heat flow processes, heat flows from a source to a "sink" with some extracted off as useful work. The second law says all of the heat flow canNOT be turned into useful work; there is always some waste.
    2. Thus no machine is 100% efficient. Efficiency is the useful work output divided by the total heat input flowing from the source.

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