QUICK REVIEW
Quick Review — Heat Engines, Refrigerators, and Heat Pumps — Algebra-Based
Refresh the essential ideas and relationships for Heat Engines, Refrigerators, and Heat Pumps in just a few minutes.
TIME
5–10 minutes
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A rapid refresh
FINISH WITH
Key ideas refreshed
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Course Alignment
This Physics Sensei Unit Review is an independent learning resource. Use it to reinforce key concepts, prepare for homework, or review before a quiz or exam.
RESOURCE: Physics Sensei Unit Review | UNIT ID: THM-U10 | TOPIC: Heat Engines, Refrigerators, and Heat Pumps | COURSE LEVEL: Algebra-Based
BEST USED ✓ After learning the unit ✓ Before starting homework ✓ Before a quiz or exam
Your Review Plan
Complete these four stages to quickly refresh the essential ideas and confirm you're ready to continue.
4 Stages • Approximately 5–10 minutes.
Quick Recall
Let's quickly refresh what you already know. These short recall activities will help you bring the most important ideas back to mind before reviewing them.
QUICK RECALL
Recall Activity
Complete the three statements from memory before revealing the answer.
Name the three energy flows in a heat engine, refrigerator, and heat pump. For each device, draw arrows for heat entering, heat leaving, and work; circle the useful output.
Reveal Answers
Engine: QH = Wout + QC. Refrigerator/heat pump: QH = QC + Win. The useful quantity changes with the job.
Why it works: Each device obeys conservation of energy; identify whether work leaves or enters.
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Essential Idea
Take one last look at the most important concept from this unit. If you remember this idea, the rest will come back much more easily.
ESSENTIAL IDEA
One cycle, three useful outputs
Using positive heat-transfer magnitudes, a cyclic heat engine satisfies QH = Wout + QC and η = Wout/QH = 1 − QC/QH. The heat rejected to the cold reservoir is part of the energy balance. A refrigerator or heat pump requires work input: QH = QC + Win. COPR = QC/Win measures cooling; COPHP = QH/Win measures heating. Consequently COPHP = COPR + 1 for the same device. For reservoirs at TH and TC in kelvin, an ideal reversible limit is ηCarnot = 1 − TC/TH. An actual engine has lower efficiency. A coefficient of performance may exceed 1 because heat is moved as well as supplied by work.
Engine: QH = Wout + QC. Reversed cycle: QH = QC + Win.
Example: A refrigerator removes 300 J from a cold compartment using 100 J of work. It delivers 400 J to the room; COPR = 300/100 = 3 and COPHP = 400/100 = 4.
Sensei note: Do not call a COP above 1 an efficiency above 100%; the device moves heat.
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Confidence Check
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CONFIDENCE CHECK
Track all three energy flows
Answer all three without notes.
Use the energy balance before a performance ratio. A refrigerator removes 300 J from a cold space using 100 J of work. How much heat reaches the room, and what are its cooling and heating COP values?
Reveal Answers
A refrigerator removes 300 J from a cold compartment using 100 J of work. It delivers 400 J to the room; COPR = 300/100 = 3 and COPHP = 400/100 = 4.
Why it works: The room receives both the removed heat and the electrical work. Cooling and heating divide different useful heat transfers by the same work input.
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