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.

 

[rwisz]

Heat energy is considered unusable because it cannot be converted into other forms of energy with 100% efficiency. This is due to the second law of thermodynamics, which states that in any energy conversion, some energy will always be lost as heat. This means that even if we try to convert heat energy into another form, such as mechanical or electrical energy, there will always be some amount of heat that is lost in the process.

In terms of cells, the statement that the temperature in cells is fairly even throughout means that there is not a significant difference in temperature between different parts of the cell. This is important because in order to harness energy, there needs to be a temperature difference that can be used to drive a heat engine, which converts heat energy into mechanical energy. In cells, the temperature difference is not significant enough to drive a heat engine, making the heat energy unusable.

Furthermore, even if we were able to harness heat energy from cells, the efficiency of the conversion would be very low due to the second law of thermodynamics. This is why cells and other living organisms have evolved to use other forms of energy, such as chemical energy, which can be converted into other forms with higher efficiency.

In summary, heat energy is considered unusable because it cannot be converted into other forms with 100% efficiency and in cells, the temperature is not significant enough to drive a heat engine. This is why other forms of energy are used instead.