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Kelvin–Planck statement
Intermediate
No engine converts heat entirely into work.
It is impossible for a cyclic device to take heat from one reservoir and convert it completely to work. Some heat must be rejected to a colder reservoir.
A car engine always wastes heat through the exhaust.
Equivalent to Clausius' statement.
In more detail
The Kelvin-Planck statement says that no cyclic process can convert heat from a single reservoir completely into work. Some heat must be rejected to a colder reservoir.
Real-world uses
- Explains why every power station releases waste heat.
- Limits the design of engines that run on a single heat source.
- A car engine must reject waste heat to the environment, because no engine can turn all its heat into work.
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Frequently asked questions
Can a heat engine use one reservoir?
No. It needs a hot and a cold reservoir to work continuously.
Why is waste heat unavoidable?
Because the second law requires rejection of heat to a colder place.
Is a perfect heat engine impossible?
Yes. Any engine that converted all its heat into work with no waste would violate the second law.
Related topics in Heat & Thermodynamics
Ratio of specific heats · P–V diagrams · Second law of thermodynamics · Clausius statement · Heat engines · Carnot engine