Flashcards · 9630 · A Level Topic 11

Thermal physics (A Level) flashcards

23 cards on A Level Topic 11, subtopics 3.11.1 to 3.11.4. Pick a subtopic or study them all.

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Internal energy: The sum of the randomly distributed kinetic energies and potential energies of the particles in a body.
First law of thermodynamics: ΔU = Q + W, where Q is the energy transferred to the system by heating and W the work done on the system.
Specific heat capacity and specific latent heat: c: energy to raise the temperature of 1 kg by 1 K, Q = mcΔθ (J kg⁻¹ K⁻¹). l: energy to change the state of 1 kg at constant temperature, Q = ml (J kg⁻¹).
During a change of state, why does the temperature stay constant?: The energy supplied increases the potential energy of the particles as they separate; their average kinetic energy, and so the temperature, does not change.
Heating a continuously flowing fluid: P = (Δm/Δt) c Δθ, where Δm/Δt is the mass flow rate in kg s⁻¹.
Required practical: specific heat capacity by an electrical method. Equation and main error: VIt = mcΔθ, so c = VIt/(mΔθ). Energy lost to the surroundings makes Δθ too small and c too large; reduce it by lagging the block.

Rate of energy transfer by conduction: Rate = kAΔθ/L, where k is the thermal conductivity (W m⁻¹ K⁻¹), A the area, Δθ the temperature difference and L the thickness. Applies in a steady state.
U-value: U = k/L, unit W m⁻² K⁻¹. Rate of energy transfer = UAΔθ. A lower U-value means better insulation.
Why are metals good thermal conductors?: Free electrons carry energy through the metal as well as lattice vibrations.
Thermal conductivity k vs U-value: k is a property of the material. U belongs to a particular thickness of that material (U = k/L).
Roof of area 12 m², U = 0.25 W m⁻² K⁻¹, temperature difference 14 K. Rate of loss?: Rate = UAΔθ = 0.25 × 12 × 14 = 42 W.

Boyle's law, Charles's law and the pressure law: For a fixed mass of gas: Boyle: pV = constant at constant T. Charles: V/T = constant at constant p. Pressure law: p/T = constant at constant V. T in kelvin.
Ideal gas equation in moles and in molecules: pV = nRT, R = 8.31 J mol⁻¹ K⁻¹. pV = NkT, k = 1.38 × 10⁻²³ J K⁻¹. R = N_A k.
Absolute zero: 0 K, about −273 °C: the temperature at which an ideal gas would, by extrapolation, have zero volume at constant pressure (or zero pressure at constant volume).
Work done by a gas expanding at constant pressure: W = pΔV, the area under a p–V graph.
Required practical: verifying Boyle's law: Trap air in a syringe attached to a pressure gauge. Change V slowly, wait for thermal equilibrium and record p. A graph of p against 1/V is a straight line through the origin.
Converting Celsius to kelvin: T/K = θ/°C + 273. Always use kelvin in gas-law calculations.

Kinetic theory equation for pressure: pV = ⅓Nm(c_rms)², where m is the mass of one molecule. Equivalently p = ⅓ρ(c_rms)².
Assumptions of the kinetic theory model: A large number of molecules in random motion; molecular volume negligible compared with the container; no intermolecular forces except during collisions; elastic collisions; collision time negligible compared with time between collisions.
Average kinetic energy of a gas molecule: ½m(c_rms)² = (3/2)kT = 3RT/(2N_A).
Brownian motion: what is seen and what it shows: Smoke or pollen particles jitter randomly because unequal numbers of fast-moving air or water molecules hit them from different sides. It is evidence that molecules exist and move randomly.
Internal energy of an ideal gas: The total kinetic energy of its molecules (no potential energy): U = (3/2)nRT = (3/2)NkT.
Root mean square speed c_rms: The square root of the mean of the squares of the molecular speeds.