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SESK: Stuff Everyone Should Know · sesk.info/eng-second-law-heat-direction

What heat flow cannot occur spontaneously under the second law?

Heat cannot spontaneously flow from cold to hot.

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  1. From hot to cold
  2. Across a metal wall
  3. From cold to hot ✔

Why: Heat naturally moves from higher to lower temperature. Moving it the other way requires an external effect, such as work supplied to a refrigerator.

Huh, didn't know that: This Clausius statement is equivalent to other formulations of the second law involving heat engines and entropy.

The dad joke
Heat takes the downhill route unless work gives it a lift.
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References

Answer: From cold to hot. Heat naturally moves from higher to lower temperature. Moving it the other way requires an external effect, such as work supplied to a refrigerator.

  1. OpenStax University Physics Volume 2, Reversible and Irreversible Processes openstax.org/books/university-physics-volume-2/pages/4-1-reversible-and-irreversible-processes
    Heat never flows spontaneously from a colder object to a hotter object.
    Checked 2026-10-10.

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