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Theme B — The Particulate Nature of Matter

B.4.5 Second Law of Thermodynamics — IB Physics HL Revision Notes & Topical Questions

5 min read 10 parts Lesson 37 of 53 1 quiz

📚 IB Physics HL Hub / Topical Revision
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Curriculum Equation Key Gibbs Free Energy FormulaFast Exam Solution
ΔG = ΔH - TΔS | (ΔG < 0: تلقائي Spontaneous, ΔG > 0: غير تلقائي, ΔG = 0: اتزان)
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Lesson summary

B.4.5 Second Law of Thermodynamics — IB PHYSICS HL notes: State the entropy, Clausius and Kelvin forms of the second law and use entropy changes to decide whether a process is possible.Statement — in every process the total entropy of an isolated system always increases, ΔSuniver…

  • Non-isolated system
  • Impossible process
  • Local decrease
  • Clausius form
  • Entropy form
  • Engine consequence

Part 1 of 8The law in terms of entropy

1Statement
  • Statement in every process the total entropy of an isolated system always increases, ΔSuniverse ≥ 0
2Non-isolated system
  • Non-isolated system the entropy of the Universe must increase
  • Its own entropy may fall if the surroundings gain at least as much
3Impossible process
  • Impossible process one in which the entropy of the universe would fall
4Example
  • Example a refrigerator cools its contents but its compressor rejects heat that raises the room's entropy by more

Part 2 of 8Isolated and non-isolated systems

Isolated and non-isolated systems
Isolated system
  • Isolated system neither matter nor energy can be transferred in or out
Non-isolated system
  • Non-isolated system matter and energy can be transferred in or out
  • It can be treated as part of a larger isolated system
Local decrease
  • Local decrease compensated by an equal or greater increase in the entropy of the surroundings

Part 3 of 8Equivalent statements

1Clausius form
  • Clausius form thermal energy cannot spontaneously transfer from a region of lower temperature to a region of higher temperature
  • Work must be supplied, as in a heat pump or refrigerator
2Kelvin form
  • Kelvin form when extracting energy from a heat reservoir it is impossible to convert it all into work
  • Some heat must be rejected to a colder reservoir
3Entropy form
  • ΔSuniverse ≥ 0 in any process
4Engine consequence
  • Engine consequence an engine that breaks either form cannot exist

Part 4 of 8Why no engine is 100% efficient

Ordered motion
  • Ordered motion converting all the heat to work would turn random molecular motion into ordered piston motion
Entropy
  • Entropy that would decrease the entropy of the universe, which is forbidden
Compensation
  • Compensation the engine must reject heat Qc to a cold reservoir to create enough entropy

Part 5 of 8Reversible and irreversible processes

1Reversible process
  • Reversible process no overall change in entropy when system and surroundings are returned to their original states
  • It is an idealised process with the system always in equilibrium and no friction
2Irreversible process
  • Irreversible process the entropy increases and system and surroundings cannot return to their original states
  • Friction, turbulence, rapid expansion, mixing and heat flow through a temperature difference all produce entropy
3Real isolated systems
  • Real isolated systems processes are almost always irreversible so entropy always increases
4Perpetual motion
  • Perpetual motion machines that convert thermal energy completely into work cannot exist

Part 6 of 8Testing whether a process is possible

1
Calculate ΔS = Q/T for every body that exchanges heat.
2
Add the entropy changes to find ΔSuniverse.
3
A positive total means the process is possible but irreversible.
4
A zero total means it is possible and reversible, the limiting case.
5
A negative total means the process is impossible.
  • Engine case
    • the working fluid returns to its initial state so only the reservoirs count
    • ΔSuniverse = Qc/Tc − Qh/Th ≥ 0

Part 7 of 8Wider significance

Direction of time
  • Direction of time a film of an egg breaking looks natural, played backwards it does not
Life
  • Life organisms build order by taking in low-entropy energy and releasing high-entropy waste heat
Heat death
  • Heat death the idea that entropy reaches a maximum and no more work can be extracted

Part 8 of 8More from the specification

1
Carnot heat engine cycle
  • Carnot heat engine cycle the theoretical Carnot cycle consists of reversible isothermal and adiabatic stages, establishing maximum thermodynamic efficiency limits.
2
Internal energy in ideal gas particles
  • Internal energy in ideal gas particles the internal energy of ideal gas particles depends solely on thermodynamic temperature, independent of volume.
3
Adiabatic versus isothermal expansion
  • Adiabatic versus isothermal expansion during adiabatic expansion at constant entropy, gas particles do work on surroundings, decreasing temperature and internal energy.
4
Second Law of Thermodynamics and Heat Dissipation
  • The second law establishes that thermal energy cannot spontaneously transfer from a cooler body to a hotter body,
  • and no cyclic heat engine can convert absorbed thermal energy entirely into work.
5
The Four-Stage Carnot Cycle
  • An ideal Carnot heat engine operates through four reversible thermodynamic stages: isothermal expansion, adiabatic expansion, isothermal compression,
  • and adiabatic compression.
6
Carnot Thermal Efficiency Limit
  • The maximum theoretical efficiency of a reversible heat engine operating between hot reservoir TH and cold reservoir TC is given by eta = 1 - (TC / TH),
  • where temperatures must be expressed in kelvin.
7
Thermodynamic State Variables on P-V Diagrams
  • In an isothermal process temperature remains constant (delta_U = 0),
  • whereas in an adiabatic process no heat enters or leaves the system (Q = 0), altering gas volume and internal energy.
8
Atmospheric pressure
  • Pressure of the air, about 1.01 × 105 Pa at sea level.
9
Volume
  • The amount of space an object occupies.
10
Constant
  • Not changing.
11
Efficiency
  • Useful output divided by total input.
12
Adiabatic
  • With no thermal energy transfer
13
Isothermal
  • At constant temperature
14
Internal energy
  • Total kinetic and potential energy of the particles in a system
15
Carnot cycle
  • A reversible cycle of two isothermal and two adiabatic processes between two reservoirs.
16
Constant volume
  • The container is rigid and sealed.

Key terms

TermMeaning
Second lawThe total entropy of the universe does not decrease, ΔS ≥ 0.
Kelvin formIt is impossible to convert all heat taken from one reservoir completely into work.
Clausius formHeat cannot spontaneously flow from a colder to a hotter body.
Reversible processAn idealised process that leaves the total entropy unchanged.
Irreversible processA real process that increases the total entropy of the universe.
Isolated systemA system in which neither matter nor energy can be transferred in or out.

Avoid these mistakes

1
Saying the entropy of a system can never decrease — the law concerns the total entropy of the system plus surroundings.
2
Believing the second law forbids refrigerators — it only forbids spontaneous flow from cold to hot without work.
3
Stating that a frictionless heat engine is 100% efficient — even a reversible engine must reject heat.
4
Confusing the first and second laws — the first concerns energy conservation and the second the direction of processes.
5
Treating a reversible process as practically achievable — it is an idealised limit.

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