Why Is Heat Energy Considered Unusable?

[Qube]

Homework Statement

Why is heat energy unusable?

Homework Equations

Heat = total kinetic energy of an object

The Attempt at a Solution

I've come across this statement so many times - whether it be in Physics, Environmental Science, Chemistry, or Biology - "thermal energy (heat) is unusable."

What makes heat energy unusable? Why can't it be harnessed? I read that heat is unusable in cells because the temperature in cells are fairly even throughout. Can someone please explain what that means, and also explain why exactly heat is considered "unusable"?

 

[Antiphon]

To do work you need to flow heat from higher to lower temperature.

If everything is the same temperature then thermal energy cannot flow and perform useful work.

Refer to something called Maxwell's Demon for the full explanation.

 

[elegysix]

Check out the carnot heat engine.

 

[kjohnson]

Heat energy is not unusable, it is just that it needs to be converted by a heat engine to perform mechanical work (like a turbine / piston) which is usually desired.