Total kinetic and potential energy of the particles in a substance.
Temperature
A measure of the average kinetic energy of the particles. T(K) = θ(°C) + 273.
Specific heat capacity (c)
Energy needed to raise the temperature of 1 kg by 1 K, in J kg⁻¹ K⁻¹.
Specific latent heat (L)
Energy needed to change the phase of 1 kg at constant temperature, in J kg⁻¹.
Black body
An ideal object that absorbs all radiation falling on it and emits the maximum possible at each temperature.
Luminosity (L)
Total power radiated by a body, in W.
During a phase change the temperature stays constant: energy goes into breaking bonds (potential energy), not into kinetic energy.
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: Temperature changes are the same in K and °C, but the Stefan–Boltzmann and gas laws need absolute temperature in kelvin.
B.2Greenhouse effect
Key terms
Emissivity (e)
Power radiated by a surface ÷ power radiated by a black body at the same temperature (0 to 1).
Albedo
Fraction of incident radiation that is reflected or scattered (0 to 1).
Solar constant (S)
Intensity of the Sun's radiation at the top of Earth's atmosphere, about 1360 W m⁻².
Greenhouse gas
A gas that absorbs and re-emits infrared radiation, such as H₂O, CO₂, CH₄ and N₂O.
Enhanced greenhouse effect
Extra warming caused by human increases in greenhouse gas concentrations.
Earth receives \(S\) on a disc of area \(\pi R^{2}\) but radiates from a sphere of area \(4\pi R^{2}\), so the mean incoming intensity is \(S/4\approx340\text{ W m}^{-2}\).
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: Explain the greenhouse effect as a chain: short-wave in, long-wave out, absorption by resonance, re-emission in all directions, extra warming of the surface.
B.3Gas laws
Key terms
Pressure
Force per unit area, p = F/A, in Pa.
Mole
Amount of substance containing N_A = 6.02 × 10²³ particles.
Ideal gas
A gas that obeys pV = nRT exactly: point particles, elastic collisions, no forces except during collisions.
Absolute zero
0 K, the temperature at which the particles of an ideal gas would have minimum kinetic energy.
Boyle's law: \(pV\) constant at constant \(T\). Charles's law: \(V\propto T\) at constant \(p\). Gay-Lussac's law: \(p\propto T\) at constant \(V\). Temperatures must be in kelvin.
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: Convert litres to m³ (1 L = 10⁻³ m³) and °C to K before substituting into pV = nRT.
B.4Thermodynamics HL
Key terms
First law
Q = ΔU + W: energy supplied to a gas raises its internal energy and/or lets it do work.
Isothermal
Temperature constant, so ΔU = 0 for an ideal gas.
Adiabatic
No energy transferred as heat: Q = 0.
Isobaric / isovolumetric
Constant pressure / constant volume (W = 0).
Entropy (S)
A measure of disorder: the number of ways the energy of a system can be arranged.
Second law
The total entropy of an isolated system never decreases.
Sign convention: \(W\) is work done by the gas (positive when it expands); \(Q\) is energy supplied to the gas.
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: For any cycle ΔU = 0, so the net energy supplied as heat equals the net work done.
B.5Current and circuits
Key terms
Current (I)
Rate of flow of charge, I = Δq/Δt, in A.
Potential difference (V)
Work done per unit charge moving between two points, in V (J C⁻¹).
Resistance (R)
R = V/I, in Ω.
Resistivity (ρ)
Property of a material: ρ = RA/L, in Ω m.
Electromotive force (ε)
Energy supplied per unit charge by a source, in V.
Internal resistance (r)
Resistance inside a cell, which wastes energy as heat.
Series: same current everywhere; potential differences add. Parallel: same potential difference across each branch; currents add (Kirchhoff's laws).
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: Redraw complicated circuits so every parallel branch is clear, then combine resistors step by step from the inside out.