Thermodynamics – Laws & Entropy
System & surroundings
- Universe: the system plus its surroundings
- Energy transfer: happens as heat (Q) or work (W)
First law
- First law: dU = δQ − δW (energy is conserved)
- Closed system: ΔU = Q − W
- Sign convention: Q > 0 into the system, W > 0 done by the system
Second law & entropy
- Entropy (S): a measure of disorder, or the number of microstates
- Isolated system: ΔS ≥ 0 always
- Reversible process: ΔS = ∫ δQrev / T
- Irreversible process: ΔS is strictly greater than ∫ δQ / T
Ideal gas
- Internal energy: U = nCvT, so U depends only on temperature
- Ideal gas law: PV = nRT relates pressure, volume and temperature
- Mayer's relation: Cp − Cv = R, so Cp > Cv
- Isothermal process: ΔU = 0, therefore Q = W