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thermodynamics · how energy flows

The rules of change, seen through heat and energy

From temperature and pressure to entropy and cycles, see the paths heat and work travel.

14 lessons

01A State Is a Set of Properties

Fix the scope with a system and boundary, and see a state as a bundle of properties, one point on the P-V plane

#balloon#thermos#pressure cooker

02Pressure and Temperature Measure Molecular Motion

Collisions make pressure, average molecular speed makes temperature; thermal equilibrium, kelvin, and the zeroth law

#tire pressure#thermometer#weather pressure

03Pressure and Temperature Decide a Substance's Phase

The P-T phase diagram splits solid, liquid, gas; plus the P-v saturation dome, triple and critical points

#melting ice#boiling water#dry ice

04Boundary Work Is the Area Under the P-V Curve

Expansion is work; W = ∫P dV is the area under the curve, with path dependence and sign convention

#engine piston#syringe#bicycle pump

05Energy Is Conserved: dU = Q - W

Internal energy changes by heat in minus work out; U is a state property, with adiabatic, isochoric, and cycles

#hand warmer#insulation#thermos

06Enthalpy H = U + PV Is Natural at Constant Pressure

H = U + PV. Heat at constant pressure is ΔH, flow work is PV; why H is natural for flow and constant pressure

#boiler#heater#steam pipe

07Ideal Gas PV = nRT, and Each Process Traces Differently

PV=nRT ties P, V, T together. Isothermal is a hyperbola, isobaric and isochoric are lines, adiabatic a steeper PV^γ curve

#balloon#scuba tank#AC refrigerant

08Heat Engine: A Cycle That Turns Heat into Work

Take Q_H, deliver work W, dump Q_C. A cycle gives Q_H = W + Q_C, efficiency η = W/Q_H = 1 - Q_C/Q_H

#car engine#jet engine#power plant

09The Second Law: Heat Has a Direction

Heat flows hot to cold on its own. Kelvin-Planck (η<1) and Clausius (a fridge needs W) are one law; real processes are irreversible

#cooling coffee#refrigerator#melting ice

10The Carnot Cycle: The Most Efficient Engine

Two isotherms + two adiabats. η = 1 - T_C/T_H is the ceiling, set by the temperature ratio alone; underneath, Q_H/T_H = Q_C/T_C

#car engine#refrigerator#power plant

11Entropy: A Measure of Spreading and the Arrow of Time

A state function dS = dQ_rev/T. The universe obeys ΔS_total ≥ 0 (arrow of time), equality only when reversible; statistically S = k ln Ω

#ink spreading#melting ice#broken cup

12The Rankine Cycle: The Steam Power Plant

Four parts: boiler, turbine, condenser, pump. The phase change trades heat near constant T with tiny pump work. η = W_net/Q_H

#power plant#nuclear plant#steam turbine

13The Otto and Diesel Cycles: Gasoline and Diesel Engines

Gas cycles, no phase change. Otto burns at constant V, Diesel at constant P. η = 1 − 1/r^(γ−1); diesel wins with a higher r

#gasoline car#diesel car#truck engine

14Refrigeration: Running a Cycle Backward to Pump Heat Uphill

An engine reversed: work W moves heat from cold to hot. Q_H = Q_C + W, rated by COP = Q_C/W, Carnot COP = T_C/(T_H−T_C)

#refrigerator#air conditioner#heat pump

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