State the second law of thermodynamics in its Kelvin-Planck and Clausius forms. Explain how a refrigerator works as a heat engine run backwards.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Kelvin-Planck statement. No system can take heat from one source and convert all of it into work.
2 · Clausius statement. Heat cannot pass from a colder to a hotter body as the only result, without external work.
3 · Draw the block diagram. Cold box (inside, T₂) at the bottom and room (T₁) at the top; arrows H₂ up into the device, W in from the side, H₁ up to the room.
4 · Explain the working. The refrigerant takes heat H₂ from the food; the compressor does work W; heat H₁ = H₂ + W leaves through the back coils.
5 · Link to Clausius. Heat goes from cold to hot only because the compressor does external work.
6 · Real life. The coils at the back of a fridge feel warm: that is H₁ going into the kitchen.
7 · Close in one line. No perfect engine, and no refrigerator without work.
Scoring tip: Draw the refrigerator's arrows the right way: H₂ from the cold box, H₁ into the hot room. Reversed arrows lose the diagram mark.
