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Theme B — The Particulate Nature of Matter

B.4.1 Thermodynamic Systems — IB Physics HL Revision Notes & Topical Questions

5 min read 9 parts Lesson 33 of 53 1 quiz

📚 IB Physics HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

B.4.1 Thermodynamic Systems — IB PHYSICS HL notes: Describe thermodynamic systems and state variables, relate internal energy to temperature and calculate the work done by a gas from W = pΔV and p–V graphs.System — the chosen part of the universe whose energy transfers are studie…

  • Open system
  • Closed system
  • Isolated system
  • Thermal equilibrium
  • p–V diagram
  • Definition

Section 1 of 9System and surroundings

System and surroundings
System
  • System the chosen part of the universe whose energy transfers are studied
  • Everything else is the surroundings
  • The boundary may be real, like a cylinder wall, or imaginary
Open system
  • Open system exchanges both matter and energy with the surroundings
  • Examplea boiling pan without a lid
Closed system
  • Closed system exchanges energy but not matter
  • Examplea sealed gas cylinder with a movable piston
Isolated system
  • Isolated system exchanges neither matter nor energy
  • Examplean ideal insulated flask

Section 2 of 9State variables and thermal equilibrium

State variables
  • State variables pressure p, volume V, temperature T, amount n and internal energy U
  • They depend only on the present state, not on how it was reached
Thermal equilibrium
  • Thermal equilibrium no net heat flows between two systems because they are at the same temperature
  • If two systems are each in equilibrium with a third they are in equilibrium with each other
Ideal gas
  • Ideal gas in equilibrium it obeys pV = nRT
  • For fixed n only two of p, V and T are independent
p–V diagram
  • p–V diagram each equilibrium state is one point and a slow change is a line
  • Only slow processes through a series of equilibrium states can be drawn

Section 3 of 9Internal energy of an ideal gas

Definition
  • Definition the total of the random kinetic energies and intermolecular potential energies of all particles
Ideal gas
  • Ideal gas there are no intermolecular forces so there is no potential energy
  • The internal energy is the translational kinetic energy only
Equations
  • U = (3/2)NkBT = (3/2)nRT = (3/2)pV
  • ChangeΔU = (3/2)NkBΔT = (3/2)nRΔT
Temperature link
  • ΔU is proportional to ΔT in kelvin
  • A larger kinetic energy means faster molecules
Solids and liquids
  • Solids and liquids particles have both kinetic and potential energy because intermolecular forces bind them
  • Melting or boiling at constant temperature raises U by increasing potential energy

Section 4 of 9Work done by a gas

1Expansion at constant pressure
  • Expansion at constant pressure the gas pushes the piston with force F = pA through a distance s
  • W = Fs = pAs = pΔV
    What the symbols mean: W = work done = energy transferred (J) · F = force (N) · d, s or x = distance moved in the direction of the force (m)
2Symbols
  • W in J, p in Pa, ΔV = Vfinal − Vinitial in m3
  • The surrounding pressure is assumed constant, as for atmospheric pressure
3Sign
  • Sign expansion gives ΔV > 0 and positive work done by the gas
  • Compression gives ΔV < 0 and negative work, meaning work is done on the gas
4Constant volume
  • Constant volume no work is done
  • Free expansion into a vacuum also does no work

Section 5 of 9Work from a p–V graph

1Varying pressure
  • Varying pressure the work done equals the area under the curve
  • Convert volumes to m3 before calculating
2Constant pressure
  • Constant pressure appears as a horizontal line on the p–V graph
3Constant volume
  • Constant volume appears as a vertical line, with zero area under it
4Cycle
  • Cycle the net work equals the area enclosed by the loop
  • A clockwise loop gives positive net work

Section 6 of 9Heat, work and sign convention

1Heat Q
  • Heat Q energy transferred because of a temperature difference by conduction, convection or radiation
2Work W
  • Work W energy transferred by a force moving its point of application
3Process quantities
  • Process quantities heat and work describe a process and are not state variables
4IB convention
  • Q is positive when heat enters the system and W is positive when the system does work on its surroundings
  • The first law is then Q = ΔU + W

Section 7 of 9More from the specification

1
Four Stages of the Carnot Cycle
  • An ideal Carnot heat engine operates through four reversible thermodynamic processes between a hot reservoir at TH and a cold reservoir at TC: isothermal expansion, adiabatic expansion, isothermal compression,
  • and adiabatic compression.
2
Maximum Thermal Efficiency of a Heat Engine
  • The theoretical maximum efficiency of a Carnot cycle engine is determined solely by reservoir temperatures: eta_Carnot = 1 - (TC / TH);
    • efficiency increases as temperature difference between reservoirs increases.
3
Second Law of Thermodynamics and Entropy
  • In any spontaneous irreversible thermodynamic process within an isolated system, total entropy increases;
    • (Delta S >= 0)
  • while local entropy of a working fluid decreases during cooling, overall reservoir-system entropy increases.
4
Adiabatic vs Isothermal Expansions
  • During an isothermal expansion, temperature remains constant and absorbed thermal energy converts entirely into external work (Delta U = 0),
  • whereas in an adiabatic expansion, zero heat enters or leaves (Q = 0) so the internal energy and temperature decrease as work is done.
5
Entropy
  • A measure of the number of possible arrangements of particles and energy, S = kB ln Ω.
6
Efficiency
  • Useful output divided by total input.
7
Reservoir
  • Artificial lake that stores water.
8
Adiabatic
  • With no thermal energy transfer
9
Isothermal
  • At constant temperature
10
Carnot cycle
  • A reversible cycle of two isothermal and two adiabatic processes between two reservoirs.

Section 8 of 9Key terms

TermMeaning
SystemThe part of the universe being studied.
Closed systemExchanges energy but not matter with its surroundings.
Isolated systemExchanges neither energy nor matter.
State variableA property such as p, V, T or U that depends only on the present state.
Internal energyThe total random kinetic and potential energy of the particles of a system.
Thermal equilibriumNo net heat transfer because the temperatures are equal.
p–V diagramA graph of pressure against volume in which the area under the curve is the work done by the gas.

Section 9 of 9Exam technique

1
Mixing up the sign of W — positive W means work done by the gas.
2
Using volumes in dm3 or cm3 in W = pΔV — convert to m3 first.
3
Treating heat as something a system contains — heat is energy in transit while internal energy is the stored quantity.
4
Expecting work to be done at constant volume — no work is done when the volume does not change.

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