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Second law of thermodynamics
Intermediate
Heat does not flow spontaneously from cold to hot.
Heat doesn't flow spontaneously from a colder to a hotter body, and no cyclic engine can turn all heat into work. Entropy of an isolated system never decreases.
An ice cube melts in warm water but never regrows spontaneously.
Doesn't forbid local entropy decrease with external work.
In more detail
The second law of thermodynamics states that heat cannot spontaneously flow from a colder body to a hotter body, and no engine can convert heat completely into work. Entropy of an isolated system tends to increase.
Real-world uses
- Power plants are limited by the second law to efficiencies below 100%.
- Refrigerators need work input to move heat from cold to hot.
- A refrigerator needs electricity because heat would not flow spontaneously from its cold interior to the warm kitchen.
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Frequently asked questions
Why does the second law matter?
It sets limits on efficiency and explains the direction of natural processes.
Does it contradict the first law?
No. The first law conserves energy, while the second law restricts the direction and efficiency of conversion.
Does the second law forbid cooling a room?
No. A fridge does cool its contents, but only by using work, and the total entropy of the surroundings increases.
Related topics in Heat & Thermodynamics
Heat capacities C_p and C_v · Ratio of specific heats · P–V diagrams · Kelvin–Planck statement · Clausius statement · Heat engines