Classical thermodynamics, heat transfer Definition

Heat

Builds on Thermodynamic system Internal energy Thermodynamic work

Definition

Let $\Sigma$ be a closed thermodynamic system with internal energy $U$ . Let a process take $\Sigma$ from the equilibrium state $A$ to the equilibrium state $B$ . Let $W$ be the work done on $\Sigma$ in this process. The heat absorbed by $\Sigma$ in the process is $Q = U(B) - U(A) - W.$

Remarks

Heat a gas in a rigid closed container, so that no work is done and $W = 0$ . Then the heat absorbed is $Q = U(B) - U(A)$ , the rise of internal energy. Stirring a liquid in an insulated vessel raises its internal energy, but the heat absorbed is $0$ . All of the energy then enters as work, so $W = U(B) - U(A)$ . With this definition, $\Delta U = Q + W$ is the first law of thermodynamics. Many engineering texts use the work $W' = -W$ done by the system and write $\Delta U = Q - W'$ .

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Internal energy concept
Thermodynamic system concept
Thermodynamic work concept
Heat definition

Tag	`VQPG7YF5AJ`
URI	`m2://VQPG7YF5AJ`
Kind	definition
Writing	W0 stub
MSC	80A05, 80A10
Defines	Heat
Uses	Thermodynamic system, Internal energy, Thermodynamic work
Changed	3 November 2026

Heat

Definition

Remarks

Reading path

Further reading

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