Theme B — The Particulate Nature of Matter
B.1 Thermal Energy Transfers — IB Physics HL Revision Notes & Topical Questions
5 min read 10 parts Lesson 2 of 53 1 quiz
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.
Lesson summary
B.1 Thermal Energy Transfers — IB PHYSICS HL notes: Use the molecular model of matter, specific heat capacity and latent heat, and describe conduction, convection and thermal radiation including black-body radiation.PropertySolidLiquidGasParticle arrangementFixed lattice, closely…
- Cube conversions
- Absolute zero
- For an ideal gas the average kinetic energy of a molecule is Ek
- Internal energy
- Thermal equilibrium
- A high c means a slow temperature change
Part 1 of 8States of matter and density
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Particle arrangement | Fixed lattice, closely packed | Random, closely packed | Random, far apart |
| Particle movement | Vibrate about fixed positions | Flow past each other | Move freely in all directions |
| Particle energy | Low | Medium | High |
| Shape | Fixed | Not fixed | Not fixed |
| Volume | Fixed | Fixed | Not fixed |
| Density | High | Medium | Low |
Kinetic theory models particles as small spheres.
Density
Density mass per unit volume, in kg m-3 (or g cm-3).
- Gas particles are about 10 times further apart and densities are about 1000 times smaller.
Cube conversions
- 1 cm3 = 1 × 10-6 m3, 1 mm3 = 1 × 10-9 m3.
Part 2 of 8Temperature and internal energy
1Absolute zero
0 K, the temperature at which molecules have zero kinetic energy.
- T (K) = θ (°C) + 273; a change of 1 K equals a change of 1 °C.
2For an ideal gas the average kinetic energy of a molecule is Ek
(3/2)kBT, with kB = 1.38 × 10-23 J K-1.
- Absolute temperature is proportional to average kinetic energy.
3Internal energy
Internal energy the sum of the total kinetic energy and total intermolecular potential energy of the particles.
- A rise in temperature needs an increase in average kinetic energy.
- Changes in potential energy alone do not change temperature — this is the case in all changes of state.
4Thermal equilibrium
Thermal equilibrium two bodies in contact no longer exchange thermal energy because they have the same temperature.
- Energy always flows from the hotter to the cooler region.
Part 3 of 8Specific heat capacity
Specific heat capacity
Q
Q mcΔT — m is mass (kg), c specific heat capacity (J kg-1 K-1), ΔT temperature change (K or °C).
Specific heat capacity
Specific heat capacity the energy needed to change the temperature of 1 kg of a substance by 1 K.
A high c means a slow temperature change
- A high c means a slow temperature change water (4200) heats more slowly than copper (390).
- Ice 2200, aluminium 900, gold 130 J kg-1 K-1.
Mixing hot and cold bodies, with no loss
- Mixing hot and cold bodies, with no loss energy lost by one = energy gained by the other.
A change of temperature in °C equals a change in kelvin, so no conversion is needed for ΔT.
Part 4 of 8Changes of state and latent heat
Changes of state: melting, freezing, vaporisation (boiling) and condensation.
- Thermal energy changes only the potential energy, so temperature stays constant.
Q
Q mL — L is the specific latent heat (J kg-1).
What the symbols mean: E or Q = energy to change state without a change of temperature (J) · m = mass (kg) · L = specific latent heat (J kg−1)
Specific latent heat
Specific latent heat the energy to change the state of 1 kg of a substance without changing its temperature.
- Fusion (solid to liquid) and vaporisation (liquid to gas) have separate values Lf and Lv.
Lv is greater than Lf
- Lv is greater than Lf vaporisation completely overcomes intermolecular forces while melting only partly does.
- WaterLf = 4.0 × 105 J kg-1 and Lv = 1.1 × 107 J kg-1 (data table value for water vapour 2.3 × 106).
Heating and cooling curves: flat sections show a change of state; sloping sections show temperature change.
- Reading from a graph of temperature against energy, c comes from the sloping part and L from the flat part.
Part 5 of 8Conduction and convection
Conduction
Conduction the main way thermal energy moves through solids.
- Atoms at the hot end vibrate more and collide with neighbours, passing on energy.
- Metals conduct best because of delocalised free electrons;
- liquids and gases conduct poorly because atoms are far apart.
Thermal conductivity k
Thermal conductivity k the ability of a substance to conduct heat, in W m-1 K-1.
- Air 0.024, rubber 0.13, water 0.6, ice 1.6, iron 80, copper 400, silver 429, diamond 1600.
Rate of conduction
- ΔQ/Δt = kAΔT/Δx.
- A is cross-sectional area, ΔT temperature difference and Δx thickness.
- In a series of rods of equal area the rate is the same through each.
Convection
Convection the movement of groups of particles in a fluid heated from below.
- The heated fluid expands, becomes less dense, rises and is replaced by cooler fluid, forming a convection current.
- It cannot happen in solids.
Part 6 of 8Thermal radiation and black bodies
1Thermal radiation
Thermal radiation heat transfer by electromagnetic waves, normally infrared.
- All bodies above absolute zero emit it — hotter objects emit more.
- It is the only method that works through a vacuum.
2Dark, dull surfaces emit and absorb best
- Dark, dull surfaces emit and absorb best light, shiny surfaces worst; surface area also matters.
3Perfect black body
Perfect black body a body that absorbs all radiation incident on it, and is also the best emitter.
4
Black-body curves of intensity against wavelength.
- As temperature rises the peak moves to a shorter wavelength and the intensity at every wavelength increases.
- Room temperature peaks in the infrared;
- about 1000 °C is red;
- about 6000 °C is white or blue;
- hotter emits UV.
Part 7 of 8Luminosity, brightness and the radiation laws
Luminosity L
Luminosity L the total power output of a star (W).
Apparent brightness b
Apparent brightness b the intensity received on Earth (W m-2).
- Inverse square law — b = L/(4πd2),
- which assumes uniform spreading and no absorption.What the symbols mean: b or F = apparent brightness (W m−2) · L = luminosity (W) · d = distance to the star (m)
- which assumes uniform spreading and no absorption.
- For stars of equal luminosity the brighter appears closer.
Stefan–Boltzmann law
- P = σAT4; for a star L = 4πr2σT4.What the symbols mean: L = luminosity, the total power emitted (W) · σ = 5.67 × 10−8 W m−2 K−4 · r = radius of the star (m) · A = surface area (m2) · T = surface temperature (K)
- σ = 5.67 × 10-8 W m-2 K-4; stars are treated as black bodies.
Wien's law
- λmaxT = 2.9 × 10-3 m K.
- Hotter objects peak at shorter wavelengths.
- Blue stars are above 33 000 K; white 7500–10 000 K; yellow 5000–6000 K; orange 3500–5000 K; red below 3500 K.
- Rigel peaks at 263 nm (about 11 000 K) and Betelgeuse at 828 nm (about 3500 K).
Part 8 of 8More from the specification
1Wien's displacement law
- λmax T = 2.9 × 10-3 m K, so hotter bodies emit peak intensity at shorter wavelengths.
2Specific heat capacity
- Q = mcΔT is used to determine energy needed for a temperature change.
3Phase change
- During melting or boiling the temperature is constant while energy changes the potential energy of particles.
4Latent heat
- Q = mL, where L is the specific latent heat of fusion or vaporisation.
5Equation of power
- Stefan-Boltzmann law, P = σAT4, relates emitted power to temperature and area.
6Phase change
- Change of matter from one state to another
Key terms
| Term | Meaning |
|---|---|
| Internal energy | Sum of kinetic and intermolecular potential energy of the particles. |
| Specific heat capacity | Energy to raise 1 kg of a substance by 1 K. |
| Specific latent heat | Energy to change the state of 1 kg without temperature change. |
| Thermal conductivity | Ability of a material to conduct heat, W m-1 K-1. |
| Black body | Perfect absorber and emitter of radiation. |
| Luminosity | Total power output of a star. |
| Apparent brightness | Intensity of radiation received from a star. |
Avoid these mistakes
1
Thinking a change of state changes temperature — only potential energy changes.
2
Using Celsius in Wien's law or the Stefan–Boltzmann law — use kelvin.
3
Assuming convection occurs in solids or a vacuum.
4
Confusing luminosity with apparent brightness.
5
Equating internal energy with temperature — internal energy includes potential energy.
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