Second Law & Engines

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Second Law and Engines

It is impossible for engines to work only using the first law of thermodynamics. The second law must also be obeyed.

Illustrative background for Source and sinkIllustrative background for Source and sink ?? "content

Source and sink

  • In order to obey the second law of thermodynamics, all engines need a source and a sink of heat.
  • Because the sink cannot be at absolute zero, no engine can be 100% efficient.
Illustrative background for EfficiencyIllustrative background for Efficiency ?? "content


  • Efficiency =WQH=QHQCQH= \frac{W}{Q_H} = \frac{Q_H - Q_C}{Q_H}
Illustrative background for Maximising efficiencyIllustrative background for Maximising efficiency ?? "content

Maximising efficiency

  • Maximum efficiency =THTCTH= \frac{T_H - T_C}{T_H}
  • To maximise efficiency, (TH - TC) must be large, but TC is usually limited by the temperature of the environment.
    • TH is limited by the material characteristics of the engine.
  • In modern systems, the use of QH and W is maximised, whilst practical uses for QC are sought.

Jump to other topics

1Measurements & Errors

2Particles & Radiation


4Mechanics & Materials


6Further Mechanics & Thermal Physics (A2 only)

7Fields & Their Consequences (A2 only)

8Nuclear Physics (A2 only)

9Option: Astrophysics (A2 only)

10Option: Medical Physics (A2 only)

11Option: Engineering Physics (A2 only)

12Option: Turning Points in Physics (A2 only)

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