Mr. Rogers' AP Physics C: IB Physics Topics

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IB SL Thermo IB HL Thermo IB HL Waves AP Review  

Topic 9: HL Thermal Physics (SL optional) - Equations

1st Law 2nd Law Processes Heat Engines HL Equations
The equations below were part of those printed in the November 2002 Physics Data Booklet, which is provided with the IB Physics Test. Obviously, if an equation is supplied, there's a good chance it will be used in the test. The equations are provided here as a study aide for the HL IB Physics Test and the Energy option for IB SL Physics to familiarize students with them ahead of time.
  Equation Explanation
1) DW = pDV Work ( mechanical energy transfer ): For work to be done by the system, the volume must increase. If the volume decreases, work is being done on the system.
2) DQ = DU + W 1st Law of Thermodynamics: This is a mathematical expression of the law of conservation of energy. Of all the principals in all of science, the 1st law of Thermo is the closest absolute truth. We know of no exceptions.
+DQ  = thermal energy transferred into the system
Heat Transfer: The transfer of thermal energy as opposed to work which is the transfer of mechanical energy.


increase in internal energy of the system
Internal Energy: Is increased though positive heat transfer or negative work (work done on the system).


work done by the system
Work: Negative work would be work done on the system. Negative work results in a decrease in volume.


Qh - Qc
Thermal Efficiency: The % of heat input into a heat engine which is output as work. This equation is based on the 1st Law of Thermo and assumes no energy storage in the system. Wikipedia Article


Thermal Efficiency: While not specifically given in the IB Data Book, in reality this equation is more likely to be used than equation 6 because Qc can be hard to measure. Both equations 6 and 7 are identical in meaning. Work is measured with a dynamometer is measured by knowing the heat of combustion of the heat engine's fuel and its fuel consumption.


Carnot Cycle: A theoretical thermodynamic cycle for a perfect heat engine.


Th - Tc
Carnot Efficiency: The highest possible efficiency a heat engine can achieve assuming no friction. Real heat engines always achieve less than the Carnot efficiency.

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