Physics XI - Chapter 12: Thermodynamics
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- Thermodynamic System: Quantity of matter or region in space chosen for study.
- Surroundings: Everything external to the system that can exchange energy/matter.
- State Variables: Properties that define the state of a system (P, V, T, U).
- Extensive Properties: Depend on system size (mass, volume, internal energy).
- Intensive Properties: Independent of system size (pressure, temperature, density).
- Zeroth Law: Defines temperature; thermal equilibrium is transitive.
- First Law: ΔU = Q - W (Energy conservation for thermal systems).
- Sign Convention: Q>0 (heat to system), W>0 (work by system).
- Isothermal Process: Constant temperature (ΔT=0); for ideal gas ΔU=0.
- Adiabatic Process: No heat exchange (Q=0); ΔU = -W.
- Isobaric Process: Constant pressure (ΔP=0); W = PΔV.
- Isochoric Process: Constant volume (ΔV=0); W=0, Q=ΔU.
- Cyclic Process: System returns to initial state; ΔU=0, Q_net=W_net.
- Internal Energy (U): For ideal gas, depends only on temperature.
- Work Calculation: W = ∫P dV (area under P-V curve).
- Heat Engine: Converts heat to work; η = W/Q₁ = 1 - Q₂/Q₁.
- Refrigerator: Transfers heat from cold to hot; COP = Q₂/W.
- Carnot Engine: Ideal reversible engine; η_max = 1 - T₂/T₁.
- Second Law (Kelvin-Planck): No engine can convert all heat to work.
- Second Law (Clausius): Heat cannot flow spontaneously from cold to hot.
- Entropy (S): Measure of disorder; ΔS = Q_rev/T.
- Entropy Principle: ΔS_universe ≥ 0 (always increases for irreversible processes).
- Specific Heats: C_v = (dU/dT)_v, C_p = C_v + R (for ideal gas).
- Adiabatic Relations: PV^γ = constant, TV^(γ-1) = constant, P^(1-γ)T^γ = constant.
- γ = C_p/C_v: Monatomic: 5/3, Diatomic: 7/5, Polyatomic: 4/3.
- Reversible Process: Infinitely slow; system in equilibrium throughout.
- Irreversible Process: Finite rate; creates entropy.
- Third Law: Absolute zero (0 K) is unattainable.
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Chapter Summary
Thermodynamics is the magnificent science that governs energy transformations in our universe - from the steam engines that powered the industrial revolution to the biological processes that sustain life itself. This chapter takes us on a journey through the fundamental laws that dictate how energy flows and transforms, revealing why some processes occur spontaneously while others require external intervention.
We begin with the Zeroth Law, which establishes the very concept of temperature through thermal equilibrium. The First Law of Thermodynamics then introduces us to the principle of energy conservation in thermal systems through the elegant equation ΔU = Q - W. This powerful relationship connects internal energy changes with heat transfer and work done, providing the mathematical foundation to analyze diverse thermodynamic processes - isothermal, adiabatic, isobaric, and isochoric - each with its unique characteristics and applications.
The Second Law of Thermodynamics unveils the directional nature of natural processes, explaining why heat flows spontaneously from hot to cold but never the reverse. Through heat engines and refrigerators, we discover the limits of energy conversion efficiency. The Carnot cycle represents the ultimate benchmark - an ideal reversible engine that sets the maximum possible efficiency for any heat engine operating between two temperatures.
The concept of entropy emerges as the quantitative measure of disorder, providing deep insights into why processes occur in specific directions. The entropy principle - that the entropy of the universe always increases - gives us a powerful criterion to distinguish between possible and impossible processes. From the perfect order of crystals at low temperatures to the chaotic molecular motions in gases, thermodynamics provides a unified framework to understand energy transformations across all scales of nature.
This chapter not only equips us with the tools to analyze engines and refrigerators but also provides profound insights into the fundamental principles that govern our physical world, from microscopic molecular interactions to macroscopic energy systems that power our civilization.