Reversed Heat Engines (AQA A Level Physics)

Heat Pumps & Refrigerators

• A reversed heat engine is one that transfers heat energy from a cold space (sink, at a low temperature) to a hot space (source, at a higher temperature) by inputting work

  • Work needs to be done because heat energy naturally flows from a warmer to a colder space (like in heat engines)

• Reversed heat engines are used for:

  • Refrigerators or air-conditioning

  • Heat pumps (an engine used to heat a building)

• The source-sink diagram looks like this:

Source-sink diagram for a reversed heat engine, notice the direction of all arrows are reversed from the normal source-sink diagram

• Where:

  • Q_C = energy extracted from the cold space (J)

  • Q_H = energy delivered to the hot space (J)

  • W = work inputted (J)

• Q_H is sometimes referred to as Q_out and Q_C as Q_in

• Q_H will always be greater than Q_C for this to work

• Applying the First Law of Thermodynamics means that:

• Efficiency is not calculated the same way as a heat engine, because its effectiveness depends on the device and its function

• A refrigerator extracts as much energy as possible from the cold space per joule of work done

  • A refrigerator wants to stay cold, so it is taking the heat out of the system

  • The inside of the fridge is the cold space and the outside is the hot space

• A heat pump (e.g. to heat a house) provides as much energy as possible to the hot space per joule of work done

  • A heat pump wants to make a room warm, so it puts heat into the system

  • The outside of the house is the cold space and the inside is the hot space

• They are both identical in principle, and it is possible to use one to fulfil the function of the other, but it wouldn't work as well

  • This is because a domestic refrigerator keeps the contents inside it cool, but simultaneously acts as a heater, as it warms up the room it is placed in