What is heat? | TOINETSU

Where this question starts

If warm air carries energy, why can that energy not simply be used to generate power? Questions about heat and energy begin at that snag.

Getting further, though, means lining up the words first. “Heat” in everyday speech and “heat” in physics do not point at quite the same thing.

What the reading found

Temperature is a state, heat is a transfer

Temperature is a measure of the thermal state a body is currently in.

Heat is energy that moves from one system to another when there is a temperature difference^[1]. Put something warm against something cool and, in a natural process, energy moves from the warmer side to the cooler one. That transfer, driven by a temperature difference, is what physics calls heat.

What a body holds is internal energy

Inside a body, atoms and molecules move and act on each other. The energy tied to that microscopic motion and interaction is collected under the name internal energy^[2].

Internal energy corresponds to the state a body is in. Heat describes energy in transfer because of a temperature difference. A quantity a body holds and a quantity that moves during a process are treated as different things.

Heat moves in three forms

Heat transfer appears mainly in these forms^[3].

conduction, through the inside of a body or across a contact surface
convection, carried by the movement of a liquid or a gas itself
thermal radiation, carried by electromagnetic waves

Radiation crosses space that holds no matter. Energy reaches the Earth from the Sun in that form.

What is easy to mix up

“This object has a lot of heat in it” is an ordinary thing to say. In the vocabulary of physics, what the object holds is internal energy, and heat is the name for that energy while a temperature difference is moving it.

“Thermal energy” carries the same ambiguity. Depending on the text it can mean internal energy, and it can mean the energy in transfer. It seems less that everyday usage is wrong than that a conversation drifts once the two senses stop being checked.

Something you can notice

Metal and wood left in the same room long enough sit at nearly the same temperature, and the metal still feels colder.

The difference is how quickly energy moves between a hand and the object^[3]. What the sense picks up appears to be the rate at which heat leaves the hand, rather than temperature itself. It also means touch is a poor way to read a temperature.

What is still open

Heat and internal energy are separated here, but temperature — the thing they were compared against — has not been explained.

what quantity temperature actually represents
why heat moves from the warmer side to the cooler one
how heat crosses a vacuum
what is missing before the energy in ordinary air can be turned into usable work

None of these are open in the sense that science has no answer for them. They are open in the sense that this page has not covered them.

This is the only question published in English so far, so the reading continues on the route it belongs to: Energy questions, starting from heat.