Heat engines
A machine that turns heat into work. Efficiency is work out over heat in. Carnot's limit is 1 - Tc/Th.
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Analogy
A waterwheel, where heat flows from hot to cold instead of water from high to low.
Example
e max = 1 - Tc / Th= 1 - 300 / 500= 0.40 = 40%
In practice
An engine between 500 K and 300 K can be at most 40 percent efficient.
Practice
Flashcards
- What is the Carnot efficiency?1 - Tc/Th, using kelvin.
Interview questions
- A heat engine absorbs 1000 J from a hot reservoir at 500 K and rejects 700 J to a cold reservoir at 300 K. Find its efficiency and compare it with the Carnot limit.
- Work = 1000 - 700 = 300 J
- Efficiency 30 percent
- Carnot limit 40 percent
- Real engine is below the limit
Questions people ask
Why can't an engine be 100 percent efficient?
The second law says some heat must always flow to a cold reservoir. If all heat became work, entropy would fall. Real petrol car engines reach about 25 to 35 percent efficiency, with the rest lost as heat out of the exhaust and radiator.
What does a refrigerator do?
A refrigerator is a heat engine run backwards. It uses electric work to move heat from the cold inside to the warmer kitchen. The back of the fridge is warm because it releases both the heat taken from the food and the work put in.