AP® Physics 2: Algebra-Based · Unit 9 · Topic 9.4
The Four Thermodynamic Processes
The four thermodynamic processes describe what happens to heat, work, and internal energy when one variable of a gas is held fixed: pressure, volume, temperature, or heat flow.
Every one of these processes is just the first law of thermodynamics, dU = Q - W, with one term locked down. Isobaric means constant pressure: the gas can expand and do work while heat flows in or out, and both Q and W are usually nonzero. Isochoric means constant volume: the gas cannot push on anything, so W = 0, and every joule of heat added goes straight into internal energy. Isothermal means constant temperature: for an ideal gas, internal energy depends only on temperature, so if T doesn't change, dU = 0, and any heat added must leave exactly as work done by the gas. Adiabatic means no heat exchange at all, Q = 0, so any change in internal energy comes entirely from work done on or by the gas.
The bike pump getting hot is an adiabatic story happening fast: you shove the piston in quickly, so there's no time for heat to escape, and all the work you do on the gas becomes a jump in internal energy, which shows up as higher temperature. An isothermal expansion is the opposite kind of slow: the gas pushes a piston outward, doing positive work, but it absorbs heat from its surroundings at exactly the same rate, keeping temperature and internal energy fixed. These four processes are special cases drawn on a pressure-volume diagram, and together they let you track energy through any cycle, which is exactly what matters next when engines and efficiency enter the picture.
Key terms
- First law of thermodynamics
- The energy conservation rule dU = Q - W relating internal energy change, heat added, and work done by a gas.
- Isochoric process
- A process at constant volume where the gas does zero work.
- Isothermal process
- A process at constant temperature where an ideal gas's internal energy stays constant.
- Adiabatic process
- A process with zero heat transfer, so any energy change comes entirely from work.
- Isobaric process
- A process at constant pressure where both heat and work are typically nonzero.
Watch it explained
- How it worksHow does an isothermal expansion hold temperature fixed while the gas keeps doing work?
- How it worksWalk me through what heat, work, and internal energy each do during isochoric heating.
- Worked exampleFind the heat added to a gas held at constant volume whose internal energy climbs by 350 J.
- Going furtherWhat changes about an adiabatic compression if you do it very slowly instead of very fast?
Check yourself
End of topic.