Heat Engines
Engines convert heat into work by cycling a gas.
About this topic
A heat engine takes in heat from a hot source, converts part of it to work and rejects the rest to a cold sink. Its efficiency is W/Q_hot = 1 − Q_cold/Q_hot, and the Carnot limit is 1 − T_cold/T_hot with temperatures in kelvin.
Car engines, power stations and steam turbines are heat engines, and so are jet engines. Typical petrol engines are only 25 to 30% efficient, and power stations about 35 to 45%, with the rest leaving as waste heat through exhaust and cooling. Refrigerators and heat pumps run the cycle in reverse, using work to move heat from cold to hot.
Worked example: an engine that takes in 1000 J of heat and rejects 700 J does 300 J of work, an efficiency of 30%. The Carnot limit for a source at 600 K and a sink at 300 K is 1 − 300/600 = 50%, and no real engine can beat that. Raising the source temperature improves the limit.
Sadi Carnot worked this out in 1824, before the second law was fully formed. A misconception is that a better design can reach 100% efficiency; the second law forbids it. Another is to use Celsius in the Carnot formula, which gives wrong answers.
Key ideas
- Efficiency = W/Q_in
- Carnot efficiency = 1 − Tc/Th
- Waste heat is unavoidable
Quick check
Use cases: where Heat Engines helps
- Homework: review the key idea behind Heat Engines before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Heat Engines without notes.
- Visual learning: play with the interactive lab to see how Heat Engines changes when you adjust its inputs.
- Teaching a friend: explain Heat Engines in your own words, then compare your version with the key ideas.
The same laws describe car crashes, sports, satellites and everyday machines.
Frequently asked questions
What is Heat Engines?
Engines convert heat into work by cycling a gas.
What will I learn in Heat Engines?
Efficiency = W/Q_in. Carnot efficiency = 1 − Tc/Th. Waste heat is unavoidable.
Is Heat Engines part of Physics?
Yes. It is one of 150 Physics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Heat Engines lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Heat Engines?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.