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Thermal Physics · Physics

Turning heat into work has a built-in limit.

Learn how engines and refrigerators move energy between hot and cold reservoirs, and how to quantify performance.

This topic

Heat Engines and Refrigerators

Use energy flow diagrams to define efficiency, coefficient of performance, and the direction of energy transfer.

Device
Heat engines
A heat engine operates in a cycle: it absorbs heat from a hot reservoir, does net work, and rejects heat to a cold reservoir.
  • Hot reservoir → engine → cold reservoir
  • Net work output over a cycle
  • Why some heat must be rejected
Start nano-lesson Cycles are the key idea.
Device
Refrigerators and heat pumps
A refrigerator uses work input to move heat from cold → hot. A heat pump does the same but the useful output is heating.
  • Work input is required
  • Same machine, different “goal”
  • Reservoir language and real examples
Start nano-lesson Cold → hot requires work.
Metric
Efficiency and coefficient of performance
Engines use efficiency (useful work out per heat in). Refrigerators/heat pumps use COP (useful heat moved per work in).
  • Engine efficiency: what counts as “useful”
  • Refrigerator COP vs heat pump COP
  • Interpreting values (why COP can exceed 1)
Start nano-lesson Different devices, different metrics.
Representation
Energy flow diagrams
A clean diagram prevents sign errors and makes it clear what is entering and leaving the device in one cycle.
  • Label Qin, Qout, and W clearly
  • Cycle idea: net ΔU = 0
  • Quick checks for physical consistency
Start nano-lesson Diagrams first, algebra second.
Practice
Practice & Exercises
Practice computing efficiency/COP, drawing correct energy flow diagrams, and identifying what’s “useful” for each device.
  • Engine efficiency computations (cycle data)
  • Refrigerator and heat pump COP problems
  • Energy-flow diagram completion drills
  • Concept checks on limits and direction
  • Exam-style mixed device sets
Open practice Label hot/cold reservoirs carefully.