The Second Law of Thermodynamics (AQA A Level Physics): Revision Note

Second law of thermodynamics

• A heat engine is a system that converts heat to usable energy which is then used to do mechanical work

• The second law of thermodynamics states that:

A heat engine requires a source and a sink to operate

• A source is a high-temperature reservoir

  • It has a high temperature $T_H$ and the heat energy from it is $Q_H$ 

• A sink is a low-temperature reservoir

  • It has a low temperature $T_C$ and the heat energy going into it is $Q_C$

• Other ways of stating the second law are:

Thermal energy cannot spontaneously transfer from a region of lower temperature to a region of higher temperature

OR

When extracting energy from a heat reservoir, it is impossible to convert it all into work

• If the engine reached the temperature of the source, no heat would flow as they would have reached thermal equilibrium

  • Therefore, no work would be done

• This means it is impossible for a heat engine to work solely on the first law of thermodynamics

• If a heat engine only obeyed the first law (there is no friction), the source-sink diagram would look like:

• It is assumed that $T_H$ remains at a constant temperature

• This engine is 100% efficient however, it is not possible to make this type of engine, due to frictional losses in real life

• Therefore, all engines obey the second law of thermodynamics, and a source-sink diagram for an actual heat engine is

• Heat energy ($Q_H$) is transferred from the source at temperature $T_H$

• Some of this energy is transferred into work, $W$

• The remaining energy ($Q_C$), is transferred to the sink at temperature $T_C$