Physics · Year 13

Active learning ideas

Refrigerators and Heat Pumps

Active learning works here because Year 13 students need to visualize thermodynamic processes that contradict everyday intuition. Working with models, simulations, and real measurements lets them trace heat flow and pressure changes step-by-step, turning abstract equations into concrete experience.

• National Curriculum Attainment Targets
• A-Level: Physics - Thermodynamics

• 25–60 min
• Pairs → Whole Class
• 4 activities

Activity 01

• Problem-Based Learning
• 30 min · Pairs

Demo: Syringe Cycle Model

Provide pairs with syringes connected by tubing filled with air or water to represent refrigerant. Students compress the 'refrigerant' to feel temperature rise, then expand it to observe cooling. Record pressure and temperature changes, then sketch the cycle on P-V diagrams.

Explain how a refrigerator transfers heat from a cold reservoir to a hot reservoir.

Facilitation Tip

During the Syringe Cycle Model, circulate with a thermocouple probe to let each pair feel the temperature change at each stage and connect it to the cycle diagram.

What to look for

Present students with a diagram of a refrigerator's vapour-compression cycle. Ask them to label each component (compressor, condenser, expansion valve, evaporator) and briefly describe the state of the refrigerant (gas/liquid, high/low pressure, high/low temperature) at each stage.

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Activity 02

• Simulation Game
• 45 min · Small Groups

Simulation Game: PhET Heat Pump Explorer

In small groups, use the PhET 'Refrigerator' simulation. Adjust compressor work, reservoir temperatures, and observe heat flows. Calculate COP for different settings and plot efficiency against temperature ratio.

Compare the coefficient of performance for refrigerators and heat pumps.

Facilitation Tip

In PhET Heat Pump Explorer, have pairs set the same input conditions and compare mode outputs side-by-side to reinforce the difference between Q_c and Q_h.

What to look for

Pose the question: 'Why is the COP of a heat pump typically greater than 1, while the COP of a refrigerator is also often greater than 1, even though both move heat against its natural flow?' Guide students to discuss the definition of COP for each and the direction of heat transfer.

• Apply
• Analyze
• Evaluate
• Create
• Social Awareness
• Decision-Making

Activity 03

• Collaborative Problem-Solving
• 60 min · Whole Class

Collaborative Problem-Solving: Compare COP Values

Whole class collects data from a school refrigerator and heat pump demonstrator. Measure power input, temperature differences, and estimate heat transfers using thermometers and wattmeters. Compute and discuss COP values.

Analyze the energy efficiency of different refrigeration cycles.

Facilitation Tip

In the Lab, assign roles so one student controls the power supply while the other logs temperatures and pressures, ensuring both collect data simultaneously.

What to look for

Provide students with a simplified problem: A refrigerator removes 1000 J of heat from its cold interior using 200 J of work. Calculate its COP. Then, ask them to write one sentence explaining how this COP would change if the refrigerator were less efficient due to friction.

• Apply
• Analyze
• Evaluate
• Create
• Relationship Skills
• Decision-Making
• Self-Management

Activity 04

• Problem-Based Learning
• 25 min · Pairs

Pairs: Efficiency Trade-Off Cards

Pairs sort scenario cards showing design choices like refrigerant type or insulation. Rank by predicted COP, justify with equations, then debate real-world compromises.

Explain how a refrigerator transfers heat from a cold reservoir to a hot reservoir.

Facilitation Tip

During Efficiency Trade-Off Cards, listen for pairs explaining why a higher pressure ratio raises COP but also increases compressor work.

What to look for

Present students with a diagram of a refrigerator's vapour-compression cycle. Ask them to label each component (compressor, condenser, expansion valve, evaporator) and briefly describe the state of the refrigerant (gas/liquid, high/low pressure, high/low temperature) at each stage.

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

A few notes on teaching this unit

Start with a simple demonstration of a balloon expanding and contracting to introduce the idea of pressure-volume work. Then, use the vapour-compression cycle diagram to map each real-world component to the abstract processes. Avoid overemphasizing formulas before students see the cycle in action; let the model and simulation build intuition first. Research shows that concrete analogies (like using a bicycle pump to represent the compressor) help students anchor abstract concepts before moving to calculations.

Students will confidently trace refrigerant states through each component, correctly calculate COP values for both refrigerators and heat pumps, and explain why heat pumps achieve higher efficiency than refrigerators with the same work input.

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Watch Out for These Misconceptions

• During the Syringe Cycle Model, watch for students describing the syringe as 'making cold air' when they compress the air.

  Use the thermocouple probe to show that compression raises temperature, and relate the cold feeling after expansion to heat removal, not cold creation. Have students annotate their cycle diagrams with heat flow arrows at each stage.

• During the PhET Heat Pump Explorer simulation, listen for students interpreting a COP greater than 1 as 'free energy.'

  Have pairs calculate total heat delivered (Q_h) as the sum of extracted heat (Q_c) and input work (W) using the simulation’s energy bar chart, then write the energy balance equation next to their COP values.

• During the Efficiency Trade-Off Cards activity, watch for students assuming heat pumps and refrigerators have equal COP because they use the same components.

  Ask pairs to toggling the mode in the simulation and calculate both COP_R and COP_HP under identical conditions, then compare the numeric difference using the equations on the cards.

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Methods used in this brief

• Problem-Based Learning
• Simulation Game
• Collaborative Problem-Solving