What it covers: The physics of heat and gases — kinetic theory, the ideal gas law, thermal energy transfer, and the first and second laws of thermodynamics.
Exam weight: About 15–18% of the AP Physics 2 exam — tied for the highest weighting with Units 2 and 3.
The big question: How does the microscopic motion of gas particles produce the macroscopic properties (pressure, temperature, volume) we can measure and predict?
In the College Board CED: Unit 9: Thermodynamics (Topics 9.1–9.6) — the revised CED numbers Physics 2 units 9–15.
Key topics at a glance
Kinetic Theory
Temperature = average kinetic energy of particles. Pressure comes from the cumulative force of particle collisions on the container walls divided by area.
The Ideal Gas Law
PV = nRT = Nk_BT. Connects pressure, volume, moles (or molecule count), and temperature for an idealized gas with no intermolecular forces.
Thermal Equilibrium
Objects in contact exchange heat until they reach the same temperature. The zeroth law makes temperature transitive and well-defined.
Heat Transfer Mechanisms
Conduction (contact), convection (fluid motion), and radiation (EM waves, no medium needed) — the three ways thermal energy moves.
First Law of Thermodynamics
ΔU = Q − W. Energy conservation: internal energy change equals heat added minus work done BY the gas. Work = area under a PV curve.
PV Diagrams & Processes
Isothermal (constant T), isobaric (constant P), isochoric (constant V, no work), and adiabatic (Q = 0) — each traces a distinctive path on a PV diagram.
Specific Heat & Conductivity
Q = mcΔT for heat absorbed via temperature change. Thermal conductivity describes how readily a material conducts heat.
Entropy & the Second Law
Entropy of an isolated system never decreases. This is why heat always flows hot → cold and why no heat engine is 100% efficient.
The key terms you must know
Kinetic theory of gases — explains macroscopic temperature and pressure using the motion and collisions of particles.
Ideal gas law (PV = nRT) — the single equation relating pressure, volume, moles, and temperature for an ideal gas.
Thermal equilibrium / zeroth law — objects in contact reach the same temperature; makes temperature a transitive, measurable property.
First law of thermodynamics (ΔU = Q − W) — energy conservation applied to heat and work.
Specific heat capacity / thermal conductivity — how much heat a material absorbs per degree, and how fast it conducts heat.
Entropy / second law of thermodynamics — disorder of an isolated system never decreases; limits the efficiency of any heat engine.
PV diagram — a graph of pressure vs. volume; area under the curve gives work, area of a closed loop gives net work in a cycle.
Key themes to remember
Macroscopic properties emerge from microscopic motion. Temperature and pressure are statistical descriptions of countless particle collisions.
Energy is always conserved — the first law just tracks where it goes. Heat in, work out, internal energy changes by the difference.
The area under a PV curve is always work. Whether it's a straight isobaric line or a curved isotherm, the geometry tells you the energy.
Entropy gives time its direction. Heat flows hot to cold, not the reverse, because that's the direction that increases total entropy.
No engine beats the second law. Some heat must always be exhausted to a cold reservoir — perfect efficiency is physically impossible.
Common exam traps
Always use kelvin, not Celsius, in PV = nRT. Plugging in Celsius gives wildly wrong answers — convert with T(K) = T(°C) + 273.
Work done BY the gas vs. work done ON the gas have opposite signs. Expansion is positive work by the gas (and negative work on it); compression is the reverse.
An isochoric process does zero work — even though heat can still be added or removed, changing internal energy directly.
Adiabatic doesn't mean no temperature change. Q = 0, but ΔU can still be nonzero because of work — temperature changes due to compression or expansion alone.
Entropy can decrease locally — just not for the whole isolated system. A refrigerator decreases entropy inside it, but increases it more outside (in the room).
Specific heat (c) is a property of the material, not the object's size. Don't confuse it with heat capacity (which depends on mass too).