Physical change
Start with the big picture
The First Law relates a system’s change in internal energy to heat and work: ΔU = q + w. With the IUPAC convention, heat absorbed by the system and work done on it are positive. Internal energy and enthalpy are state functions, whereas heat and work depend on the path. The topic connects these ideas to PV work and to cyclic, adiabatic, and isochoric processes. It introduces enthalpy, heat capacities, and the ideal-gas relationship between Cₚ and Cᵥ, then applies energy bookkeeping to phase changes at constant temperature and pressure. The Joule experiment illustrates that an ideal gas’s internal energy depends on temperature. Finally, the lesson distinguishes energy conservation from the direction of spontaneous heat flow: the First Law accounts for energy but does not, by itself, set that direction.
What you'll learn
- Apply the First Law equation and IUPAC sign convention to physical changes.
- Distinguish state functions from path functions in thermodynamic descriptions.
- Relate PV work to expansion, compression, and specified process conditions.
- Connect enthalpy and heat capacities to constant-pressure and constant-volume processes.
- Describe energy changes during phase transitions without confusing conservation with heat-flow direction.
Continue your study
Work through the complete notes and reinforce the topic with the study tools available in the full lesson.
