Unit 2: Thermodynamics

Executive Summary

Thermodynamics tracks heat, temperature, and internal energy — with the ideal gas law, the laws of thermodynamics, and entropy as its pillars.

Temperature and heat

Temperature measures average molecular kinetic energy (Kelvin = Celsius + 273). Heat is energy transfer by temperature difference; specific heat $Q = mcDelta T$ and latent heat $Q = mL$ govern phase changes.

Ideal gases

$PV = nRT$ — pressure, volume, and temperature interlock. Kinetic theory: K_{avg} = rac{3}{2}k_B T. At constant T, P and V trade (Boyle); at constant V, P grows with T.

The laws of thermodynamics

First law: $Delta U = Q - W$ (heat in minus work done). Isothermal, isobaric, isovolumetric, and adiabatic processes each trace a path on a PV diagram — work is the area under the curve. Second law: entropy of the universe never decreases; heat engines cannot be 100% efficient ($e = 1 - T_C/T_H$ for Carnot).

PV diagrams and cycles

Clockwise cycles are engines (net work out); counterclockwise cycles are refrigerators (work in). Area enclosed = net work.

Exam traps

Temperature is NOT heat. Work on a PV diagram is the area under the path, not the area enclosed (that's net work per cycle). The first law's sign conventions: Q positive in, W positive when the gas expands. The second law forbids heat flowing spontaneously from cold to hot.