I was learning about the laws of thermo dynamics. And that entropy in a system always increases. And there is one thing i don't understand. So if a planet or universe or something had a tempreture high enough to make a metal glow and the light can be used to do useful work, would this decrease entropy? Am i misunderstanding how entropy works? Or would the metal simply not glow at equilibrium?

  • The planet would glow, but how would you use it to do useful work? – BowlOfRed May 1 at 2:07
  • Related; discusses how we can obtain work from a glowing object and what happens to the entropy. – Chemomechanics May 1 at 3:07
  • That could be the answer, not sure if you could. Thought you could capture the light with something like our pannels, or like plants do it. – Mariano_Stanco May 1 at 4:57
up vote 1 down vote accepted

The difficulty is that an object being a source of heat/light does not make it a source of useful energy. There has to be a temperature gradient to allow the energy to be extracted.

Assuming you had some magical materials that did not melt or degrade, you couldn't turn it into something that did photosynthesis or photovoltaic work while it was at the same temperature as the incoming radiation. Here on earth, our machines can extract work as high-energy photons are absorbed and heat is dumped into the cold environment.

On your glowing planet, with nowhere to dump the heat, the machine would run backward just as often as it ran forward. It would spend as much time emitting photons as it did absorbing them.

So the only way to do useful work is to dump heat off the planet, and in the process increase entropy.

And that entropy in a system always increases.

The entropy of an isolated system never decreases, but the entropy of a non-isolated system can decrease. Air conditioners are used to cool buildings, for example.

The second law of thermodynamics does not preclude air conditioners from working, nor does it precluding a hot burner on a stove from cooling and losing entropy after being turned off. The hot burner disperses energy to its environment as it cools. While the burner loses entropy, the surrounding environment gains entropy, and does so in a manner such that the entropy of the burner + environment system increases.

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