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Second Law & Engines

Physics: AQA A Level, topic 11.2.5

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.

Source and sink

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.
Efficiency

Efficiency

  • Efficiency =WQH=QH−QCQH= \frac{W}{Q_H} = \frac{Q_H - Q_C}{Q_H}
Maximising efficiency

Maximising efficiency

  • Maximum efficiency =TH−TCTH= \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.

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