B.1
Thermal Energy Transfers
Molecular structure of phases, density, temperature scales, internal energy, specific heat capacity, latent heat, conduction, convection, radiation, black body radiation, the Stefan-Boltzmann law, Wien's displacement law, and luminosity.
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Key Concepts, Thermal Energy Transfers
Molecular Structure of Solids, Liquids and Gases
In a solid, particles are closely packed in a fixed, regular arrangement. They vibrate about fixed positions and have strong bonds holding them together. In a liquid, particles are still closely packed but no longer in a fixed arrangement. They can move past each other freely, which is why liquids flow. In a gas, particles are widely spaced, move rapidly in random directions, and the forces between them are negligible. Phase changes (melting, freezing, evaporating, boiling, condensing) involve changes in intermolecular potential energy, not kinetic energy, which is why temperature stays constant during a phase change even as energy is added or removed.
Density
Density is defined by the equation ρ = m/V, where ρ (rho) is density in kg/m³, m is mass in kg, and V is volume in m³. Unusually, the definition of density is the equation itself. Because particles in a solid are more closely packed than in a gas, solids have a much higher density. Gases are easy to compress precisely because of the large spaces between particles. Always check your units before substituting: density units are determined by whatever units you use for mass and volume.
Kelvin and Celsius Temperature Scales
The Celsius scale uses the freezing point of water as 0°C. The Kelvin scale starts at absolute zero (the lowest possible temperature, where particles have minimum energy) and increases at the same intervals as Celsius. The conversion is T(K) = T(°C) + 273. In IB Physics, almost all calculations require temperature in Kelvin. Forgetting to convert is one of the most common ways students drop marks, particularly in gas law problems and any calculation involving the Boltzmann equation.
Internal Energy and Kinetic Theory
The internal energy of a substance is the total potential energy and total random kinetic energy of all its molecules. The random kinetic energy of molecules has three components: translational (motion in a straight line), rotational (spinning) and vibrational. For an ideal gas, only translational kinetic energy matters. The average translational kinetic energy of a gas molecule is directly proportional to the temperature of the gas in Kelvin: E_k ∝ T. This is a key relationship expressed through the Boltzmann equation: E_k = (3/2)kT, where k is the Boltzmann constant.
Phase Changes and Why Temperature Stays Constant
When a substance changes state (e.g. melting ice or boiling water), energy is supplied but the temperature does not rise. This is because all the energy being added goes into breaking intermolecular bonds, not into increasing the kinetic energy of the molecules. Since kinetic energy is directly proportional to temperature, and kinetic energy is not changing during the phase change, temperature stays constant. Once all bonds have been broken and the change of state is complete, further energy input raises the kinetic energy and therefore the temperature again.
Specific Heat Capacity and Specific Latent Heat
Specific heat capacity (c) is the energy needed to raise the temperature of 1 kg of a substance by 1 K (or 1°C). The equation is Q = mcΔT, where Q is energy in joules, m is mass in kg, c is specific heat capacity in J/kg/K, and ΔT is the temperature change. Specific latent heat (L) is the energy needed to change the state of 1 kg of a substance at constant temperature. Q = mL. Specific latent heat of fusion applies to solid-liquid changes. Specific latent heat of vaporisation applies to liquid-gas changes. Vaporisation requires significantly more energy than fusion because more bonds must be completely broken.
Conduction, Convection and Radiation
Conduction is heat transfer through a material by the vibration of neighbouring particles and, in metals, by the movement of free electrons. Metals are good conductors because of their free electrons. Liquids, gases and vacuums are poor conductors. Convection occurs in fluids (liquids and gases) when warmer, less dense regions rise and cooler, denser regions sink, creating convection currents. Convection cannot occur in solids or vacuums. Radiation is the transfer of energy as electromagnetic waves (primarily infrared). Unlike conduction and convection, radiation does not require a medium and can travel through a vacuum. This is how energy reaches Earth from the Sun.
Black Body Radiation
A black body is a theoretical perfect absorber and emitter of radiation. It absorbs 100% of incident radiation and emits radiation across a continuous spectrum. The intensity and peak wavelength of the emitted radiation depend only on the body's temperature. Stars are good approximations of black bodies. The radiation emitted by a black body is called black body radiation, and its spectrum produces a characteristic curve that peaks at a specific wavelength and shifts to shorter wavelengths as temperature increases.
Wien's Displacement Law and the Stefan-Boltzmann Law
Wien's displacement law states that the peak wavelength of radiation emitted by a black body is inversely proportional to its temperature: λ_max × T = 2.90 × 10⁻³ m K. A hotter star emits radiation that peaks at a shorter (bluer) wavelength. A cooler star peaks at a longer (redder) wavelength. The Stefan-Boltzmann law gives the total power radiated per unit surface area: P = σAT⁴, where σ = 5.67 × 10⁻⁸ W/m²/K⁴ is the Stefan-Boltzmann constant, A is the surface area, and T is the temperature in Kelvin. Power increases with the fourth power of temperature, so even small temperature increases produce very large increases in radiated power.
Luminosity and Apparent Brightness
Luminosity (L) is the total power output of a star, i.e. the total energy radiated per second, measured in Watts. For a perfect black body, L = σAT⁴. Apparent brightness (b) is the power received per unit area at the location of the observer, and depends on both the luminosity of the star and how far away it is. The relationship is b = L / (4πd²), where d is the distance to the star. Apparent brightness decreases with the square of distance because the same total power is spread over an increasingly large sphere.
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Frequently Asked Questions, IB Physics Thermal Energy Transfers
What is Thermal Energy Transfers in IB Physics? ↓
Molecular structure of phases, density, temperature scales, internal energy, specific heat capacity, latent heat, conduction, convection, radiation, black body radiation, the Stefan-Boltzmann law, Wien's displacement law, and luminosity. This topic is part of Theme B (The Particulate Nature of Matter) in the current IB Physics syllabus.
Is Thermal Energy Transfers SL or HL in IB Physics? ↓
Thermal Energy Transfers is covered by both SL and HL students in the current IB Physics syllabus. HL students study additional depth and extension content beyond the SL core.
What equations do I need for IB Physics Thermal Energy Transfers? ↓
The key equations for Thermal Energy Transfers are covered in the concept tutorial above. For a structured set of notes with all equations, conditions and worked examples, the GradePod Exam Pack includes a revision note template for every topic.
What are common exam mistakes in IB Physics Thermal Energy Transfers? ↓
Common mistakes are covered in detail in the exam technique video above. The GradePod Exam Pack also includes exam-style questions with mark schemes so you can see exactly how marks are awarded and where students typically drop them.
How do I revise Thermal Energy Transfers for the IB Physics exam? ↓
Follow the GradePod three-step method. First, watch the concept tutorial and tick off each learning objective on the checklist above as you go. Second, watch the exam technique video to see how IB-style questions are answered under exam conditions. Third, use the Exam Pack to practise independently with knowledge questions, exam questions and mark schemes. That's it. It works. I promise.