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Unit 1 · Thermodynamics Flashcards Cheat Sheet Essentials Visual Review MC Practice FRQ Practice

AP Physics 2 Unit 1 Essentials

The must-know terms and big ideas for Unit 1: Thermodynamics. Every vocabulary word and concept you need to master.

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Big Idea 1
Macroscopic properties are statistical descriptions of microscopic motion
Temperature and pressure feel like simple, single numbers, but they're really statistical summaries of billions of particles moving and colliding. Temperature is the average translational kinetic energy of those particles; pressure is the average force per area from their collisions with a container's walls. Kinetic theory is the bridge between the invisible motion of individual molecules and the measurable quantities you read off a thermometer or gauge.
Kinetic Theory Temperature Pressure
Big Idea 2
The first law of thermodynamics is energy conservation, full stop
ΔU = Q − W looks like a new equation, but it's the same energy conservation principle from mechanics applied to thermal systems. Every joule of heat added to a gas either raises its internal energy or gets converted into work pushing against its surroundings — none of it disappears. Reading PV diagrams (area under the curve = work) is really just reading an energy ledger.
First Law PV Diagrams Energy Conservation
Big Idea 3
Entropy gives thermodynamic processes a direction
Unlike most of mechanics, where time could in principle run backward and the physics would look the same, thermodynamics has a built-in arrow of time: the second law says entropy of an isolated system never decreases. That's why heat flows from hot to cold and not the reverse, and why no heat engine — no matter how cleverly designed — can convert 100% of input heat into useful work.
Entropy Second Law Heat Engines
Big Idea 4
The same gas law describes every ideal-gas process — only the path changes
Isothermal, isobaric, isochoric, and adiabatic processes all obey PV = nRT at every instant. What differs is which variable is held fixed (or which quantity, like heat, is set to zero), which changes the shape of the path on a PV diagram and how much work and heat flow during the process. Recognizing the type of process from a graph — flat line, vertical line, curve — is one of the most-tested skills in this unit.
Ideal Gas Law Thermodynamic Processes PV Diagrams
Kinetic theory of gases
The model explaining macroscopic gas behavior (temperature, pressure) in terms of the motion and collisions of individual atoms or molecules.
Kinetic Theory
Temperature
A measure of the average translational kinetic energy of the particles in a substance. Must be measured in kelvin for gas-law calculations.
Kinetic Theory
Pressure
Force per unit area exerted on a surface, caused at the molecular level by countless particle collisions with the container walls.
Kinetic Theory
Ideal gas
A theoretical gas whose particles have negligible volume, exert no forces on each other except during collisions, and collide elastically. Real gases approximate this at low pressure and high temperature.
Ideal Gas Law
Ideal gas law
PV = nRT = Nk_BT — relates pressure, volume, amount of gas (moles or molecules), and temperature for an ideal gas.
Ideal Gas Law
Boltzmann's constant (k_B)
A fundamental constant relating the energy of individual particles to temperature; used in the molecular form of the ideal gas law.
Ideal Gas Law
Thermal equilibrium
The condition reached when two objects in contact have the same temperature and no net heat flows between them.
Heat Transfer
Zeroth law of thermodynamics
If A is in thermal equilibrium with B, and B with C, then A is in thermal equilibrium with C. This makes temperature a consistent, transitive property.
Heat Transfer
Conduction
Heat transfer through direct contact, as faster-moving particles collide with and transfer energy to slower ones.
Heat Transfer
Convection
Heat transfer via the bulk movement of a fluid (liquid or gas), carrying thermal energy from one place to another.
Heat Transfer
Radiation (thermal)
Heat transfer via electromagnetic waves; the only mechanism of heat transfer that requires no medium (e.g., sunlight warming the Earth through the vacuum of space).
Heat Transfer
Internal energy (U)
The total kinetic (and for real gases, potential) energy of all the particles in a system. For an ideal gas, it depends only on temperature.
First Law
First law of thermodynamics
ΔU = Q − W. The change in internal energy of a system equals heat added to the system minus work done by the system — a statement of energy conservation.
First Law
PV diagram
A graph of pressure (y-axis) versus volume (x-axis) used to track a gas through a thermodynamic process. The area under the curve equals work.
First Law
Isothermal process
A process occurring at constant temperature; on a PV diagram, follows a curve where PV is constant.
First Law
Isobaric process
A process occurring at constant pressure; appears as a horizontal line on a PV diagram. Work = PΔV.
First Law
Isochoric process
A process occurring at constant volume; appears as a vertical line on a PV diagram. No work is done (ΔV = 0).
First Law
Adiabatic process
A process in which no heat is exchanged with the surroundings (Q = 0); any internal energy change comes entirely from work.
First Law
Specific heat capacity
The amount of heat required to raise the temperature of one unit mass of a substance by one degree. Used in Q = mcΔT.
Heat & Calorimetry
Calorimetry
The technique of measuring heat transfer between substances by tracking temperature changes until thermal equilibrium is reached.
Heat & Calorimetry
Thermal conductivity
A material property describing how readily heat conducts through it; metals have high thermal conductivity, insulators have low thermal conductivity.
Heat & Calorimetry
Entropy
A measure of the disorder, or number of possible microscopic arrangements, of a system. Higher entropy corresponds to more disorder.
Entropy
Second law of thermodynamics
The total entropy of an isolated system never decreases — it increases for real (irreversible) processes and stays constant for ideal reversible ones.
Entropy
Heat engine
A device that converts thermal energy into mechanical work by moving heat from a hot reservoir to a cold one, represented as a clockwise loop on a PV diagram.
Entropy
Engine efficiency
The ratio of net work output to heat input (W_net / Q_in) for a heat engine. Always less than 100% because of the second law.
Entropy