A refrigerator is plugged into the mains electricity supply. It keeps food cold by transferring thermal energy from inside the refrigerator to the room. Explain why the room temperature increases over time when a refrigerator is running, even though the refrigerator is removing heat from inside it.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A refrigerator has a power rating of 150 W. It is left running in a kitchen with all windows and doors closed.
Model answer (4 marks)
The refrigerator takes electrical energy from the mains and converts it into work that moves heat from inside the fridge to the room.
1. The electrical energy supplied is not all converted into useful work; the compressor and other components are only about 60–70 % efficient.
2. The waste energy appears as heat in the room.
3. The heat removed from the food plus the waste heat is greater than the heat taken from the fridge interior.
4. By conservation of energy the total energy in the kitchen increases, raising the room temperature.
1. The electrical energy supplied is not all converted into useful work; the compressor and other components are only about 60–70 % efficient.
2. The waste energy appears as heat in the room.
3. The heat removed from the food plus the waste heat is greater than the heat taken from the fridge interior.
4. By conservation of energy the total energy in the kitchen increases, raising the room temperature.
Examiner tips
- Use the word ‘transfer’ and ‘waste’ to show understanding of energy conversion.
- Show the energy flow: mains → fridge → room (useful + waste).
- Explain that the room is a closed system so energy added raises temperature.
Common mistakes
- Confusing the refrigerator’s internal temperature with the room temperature.
- Saying the fridge ‘removes heat’ from the room instead of ‘adds heat’ to it.
- Ignoring the efficiency factor and saying all electrical energy becomes useful work.
Mark scheme (4 marks)
- The refrigerator transfers electrical energy from the mains supply into the system
- Not all energy transferred is useful — some is wasted / the refrigerator is not 100% efficient
- The total energy released into the room (heat removed from food + waste energy) is greater than the heat removed from inside
- By conservation of energy, the total energy in the closed kitchen increases, so room temperature rises
Related
- All Eduqas GCSE Physics revision notes →
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- Decode the mark scheme abbreviations →
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