Rucete ✏ AP Physics 2 In a Nutshell
7. Thermodynamics
This chapter explores the principles of temperature, heat, the behavior of ideal gases, the laws of thermodynamics, kinetic-molecular theory, heat engines, and heat transfer methods. It connects macroscopic observations with microscopic models to explain thermal phenomena.
Temperature and Its Measurement
• Temperature measures the average kinetic energy of particles in a substance.
• Celsius scale: freezing at 0°C, boiling at 100°C.
• Kelvin scale: absolute zero at 0 K; no negative temperatures.
• T(K) = T(°C) + 273.
• Absolute zero (−273°C) is where all molecular motion ceases (third law of thermodynamics).
Molar Quantities and Ideal Gases
• Mole: 6.02 × 10²³ particles (Avogadro’s number).
• Molar mass: mass of 1 mole of a substance (e.g., O₂ = 32 g/mol).
• 1 mole of ideal gas occupies 22.4 L at STP (0°C, 1 atm).
• Ideal gas assumptions: pointlike molecules, elastic collisions, negligible volume, no intermolecular forces.
The Ideal Gas Law Equation of State
• PV = nRT (R = 8.31 J/mol·K)
• PV/T = constant for a fixed amount of gas.
• Boyle’s law: PV = constant (constant temperature).
• Charles’s law: V/T = constant (constant pressure).
• Pressure-temperature law: P/T = constant (constant volume).
Kinetic-Molecular Theory
• Internal (thermal) energy: kinetic + potential energy of molecules.
• For ideal gases, internal energy = kinetic energy only.
• Average kinetic energy per molecule: KE = (3/2)kT
• Total internal energy: U = (3/2)NkT
• Root-mean-square velocity: relates to temperature and molecular mass.
Work Done by Expanding Gases
• Work done by a gas expanding or contracting at constant pressure: W = PΔV
• Positive work: gas expands (does work on surroundings).
• Negative work: gas is compressed (work done on gas).
• On a PV graph, work = area under the curve.
The First Law of Thermodynamics
• Energy conservation applied to thermal systems:
ΔU = Q – W
• ΔU: change in internal energy.
• Q: heat added to the system (positive if heat flows into system).
• W: work done by the system (positive if gas expands).
• Special cases:
– Isothermal (ΔU = 0): Q = W.
– Adiabatic (Q = 0): ΔU = –W.
– Isobaric (constant pressure): W = PΔV.
– Isochoric (constant volume, ΔV = 0): W = 0 → ΔU = Q.
The Second Law of Thermodynamics and Heat Engines
• Heat flows spontaneously from hot to cold bodies, not the reverse.
• Entropy (S): measure of disorder; entropy of an isolated system never decreases.
Heat Engines
• Convert thermal energy to mechanical work.
• Efficiency: e = W/Qh = 1 – (Qc/Qh)
• Qh: heat absorbed from hot reservoir, Qc: heat rejected to cold reservoir.
• Carnot engine: ideal maximum efficiency depends only on temperatures:
eCarnot = 1 – (Tc/Th)
Heat Transfer
Conduction
• Heat transfer through direct contact; depends on material conductivity.
Convection
• Heat transfer by fluid movement (liquids or gases).
Radiation
• Heat transfer by electromagnetic waves (e.g., infrared radiation).
• All objects emit radiation depending on temperature (Stefan-Boltzmann law).
In a Nutshell
Thermodynamics governs how heat, work, and energy interact in physical systems. The first law ensures energy conservation, while the second law introduces entropy and the direction of heat flow. Heat engines, refrigeration, and everyday thermal phenomena are explained by understanding work, energy transfer, and the molecular behavior of matter.