Heat as a substance?

Problem

«Heat is a form of energy.» Explain in five lines why this statement, taken literally, is misleading. What is the correct formulation?

Solution

Why it is misleading. The statement suggests that a body «contains» heat as it contains a substance. This is not the case: heat is not stored anywhere. Heat $Q$ is a way of transferring energy across the boundary of a system, through disordered collisions between molecules at different temperatures. What a body possesses and stores is internal energy $U$.

Heat and work are flows. Both heat $Q$ and work $L$ are flows of energy crossing the boundary, not contents of the system. It makes no sense to ask «how much heat is in a body»: it only makes sense to ask how much heat was exchanged in a process.

Correct formulation. The first law relates the change in internal energy to the two flows: $\Delta U = Q - L$

$\ev{\text{Heat is a transfer of energy, not a stored substance}}$

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Topics: Thermodynamics Concepts: First law of thermodynamics · Internal energy