PHYSICS NOTES 4U / DETAILED TOPIC SERIES
Thermal physics
AQA 7408 | Detailed topic revision
Detailed topic pack / AQA 7408 section 3.6.2
Thermal measurements connect temperature, pressure and volume to microscopic particle
energy. This pack separates internal energy from temperature, empirical gas laws from their
molecular explanation, and ideal models from the limits of real apparatus.
Internal energy, specific heat capacity, latent heat, continuous flow, gas laws, ideal gases,
pressure-work, molecular kinetic theory and its derivation. Includes required practical 8, an
original gas graph, 16 tasks and worked solutions.
Find it Page
Internal energy and temperature 2
Specific heat and electrical heating 3
Latent heat and continuous flow 4
Gas laws and absolute temperature 5
Moles, molecules and ideal gas equations 6
Gas work and energy transfer 7
Kinetic model and pressure derivation 8–9
Molecular speed and average energy 10
Required practical 8 / gas laws 11
Practice 1––16
Solutions 1––16
Checklist and source mapping 16
Independent original notes, not produced or endorsed by AQA. Numerical practice and drawings are original.
Laboratory methods are revision accounts for supervised school work, not unsupervised construction instructions.
Independent revision notes • Edition 1.0 for review • October 2026 1
,PHYSICS NOTES 4U / DETAILED TOPIC SERIES
Internal energy / not temperature
AQA 7408 | Detailed topic revision
Internal energy is the sum of the randomly distributed microscopic kinetic energies and
potential energies of the particles in a system. It excludes the kinetic energy of bulk motion of
the whole system and the gravitational potential energy of the whole body at its height above
the ground.
Energy transfers: heating and work can change internal energy. Heating is transfer
caused by a temperature difference, not a substance stored in an object. Work done on the
system can increase internal energy even with no heating, as in rapid compression.
Temperature and equilibrium
Temperature determines the direction of spontaneous thermal energy transfer. Net energy
transfers by heating from higher to lower temperature until thermal equilibrium. Temperature is
related to average particle kinetic energy in the ideal-gas model, not the total energy of a body.
Change Microscopic interpretation
Temperature rises Average random kinetic energy increases; internal energy generally rises.
Melting / boiling at fixed Arrangement/separation changes and potential energy rises, while average
transition temperature kinetic energy stays unchanged.
Same material, more mass at More particles: greater total internal energy, not a higher average kinetic
same temperature energy merely because there are more particles.
Qualitative first law
With Q positive for heating into the system and Won positive for work on it, ΔU=Q+Won. If Wby
is work done by the system, the same statement is ΔU=Q−Wby. State the convention; never
combine signs from different conventions.
During a phase transition, temperature can stay constant while energy enters. Internal energy need not stay
constant.
Independent revision notes • Edition 1.0 for review • October 2026 2
, PHYSICS NOTES 4U / DETAILED TOPIC SERIES
Specific heat capacity / energy input
AQA 7408 | Detailed topic revision
For temperature change with no change of state: Q=mcΔθ.
c is specific heat capacity in J kg−1 K−1. Temperature differences in kelvin and degrees
Celsius have the same numerical size. Gas laws require kelvin temperatures.
Specific heat capacity is energy per unit mass per unit temperature rise. A larger c means more
energy is needed for given m and Δθ, not necessarily that an object has a higher temperature.
Worked heating
A 0.60 kg sample with c=900 J kg−1 K−1 warms from 18°C to 43°C. Δθ=25 K and
Q=0.60×900×25=13 500 J. If useful heating power is 45 W, time=Q/P=300 s. Assume c
approximately constant and no phase change.
Electrical method
Measure sample mass and temperature. Use a fitted electrical heater with measured voltage V,
current I and time t. Input energy E=VIt; an ideal estimate c=VIt/(mΔθ). Stir liquids for uniform
temperature. For a solid block improve heater/thermometer thermal contact and insulate
appropriately.
Problem Effect or improvement
Energy heats vessel/heater too Assigning all VIt to sample alone can overestimate c. Include apparatus
heat capacity when known.
Energy lost to surroundings Smaller temperature rise: ignoring losses tends to overestimate c.
Insulate and consider cooling corrections.
Temperature nonuniform or Stir and allow sensor response; avoid a local hot spot.
delayed
Use measured V and I, not a label power when accuracy matters. Record uncertainties and assess losses rather
than assuming insulation makes them zero.
Independent revision notes • Edition 1.0 for review • October 2026 3
Thermal physics
AQA 7408 | Detailed topic revision
Detailed topic pack / AQA 7408 section 3.6.2
Thermal measurements connect temperature, pressure and volume to microscopic particle
energy. This pack separates internal energy from temperature, empirical gas laws from their
molecular explanation, and ideal models from the limits of real apparatus.
Internal energy, specific heat capacity, latent heat, continuous flow, gas laws, ideal gases,
pressure-work, molecular kinetic theory and its derivation. Includes required practical 8, an
original gas graph, 16 tasks and worked solutions.
Find it Page
Internal energy and temperature 2
Specific heat and electrical heating 3
Latent heat and continuous flow 4
Gas laws and absolute temperature 5
Moles, molecules and ideal gas equations 6
Gas work and energy transfer 7
Kinetic model and pressure derivation 8–9
Molecular speed and average energy 10
Required practical 8 / gas laws 11
Practice 1––16
Solutions 1––16
Checklist and source mapping 16
Independent original notes, not produced or endorsed by AQA. Numerical practice and drawings are original.
Laboratory methods are revision accounts for supervised school work, not unsupervised construction instructions.
Independent revision notes • Edition 1.0 for review • October 2026 1
,PHYSICS NOTES 4U / DETAILED TOPIC SERIES
Internal energy / not temperature
AQA 7408 | Detailed topic revision
Internal energy is the sum of the randomly distributed microscopic kinetic energies and
potential energies of the particles in a system. It excludes the kinetic energy of bulk motion of
the whole system and the gravitational potential energy of the whole body at its height above
the ground.
Energy transfers: heating and work can change internal energy. Heating is transfer
caused by a temperature difference, not a substance stored in an object. Work done on the
system can increase internal energy even with no heating, as in rapid compression.
Temperature and equilibrium
Temperature determines the direction of spontaneous thermal energy transfer. Net energy
transfers by heating from higher to lower temperature until thermal equilibrium. Temperature is
related to average particle kinetic energy in the ideal-gas model, not the total energy of a body.
Change Microscopic interpretation
Temperature rises Average random kinetic energy increases; internal energy generally rises.
Melting / boiling at fixed Arrangement/separation changes and potential energy rises, while average
transition temperature kinetic energy stays unchanged.
Same material, more mass at More particles: greater total internal energy, not a higher average kinetic
same temperature energy merely because there are more particles.
Qualitative first law
With Q positive for heating into the system and Won positive for work on it, ΔU=Q+Won. If Wby
is work done by the system, the same statement is ΔU=Q−Wby. State the convention; never
combine signs from different conventions.
During a phase transition, temperature can stay constant while energy enters. Internal energy need not stay
constant.
Independent revision notes • Edition 1.0 for review • October 2026 2
, PHYSICS NOTES 4U / DETAILED TOPIC SERIES
Specific heat capacity / energy input
AQA 7408 | Detailed topic revision
For temperature change with no change of state: Q=mcΔθ.
c is specific heat capacity in J kg−1 K−1. Temperature differences in kelvin and degrees
Celsius have the same numerical size. Gas laws require kelvin temperatures.
Specific heat capacity is energy per unit mass per unit temperature rise. A larger c means more
energy is needed for given m and Δθ, not necessarily that an object has a higher temperature.
Worked heating
A 0.60 kg sample with c=900 J kg−1 K−1 warms from 18°C to 43°C. Δθ=25 K and
Q=0.60×900×25=13 500 J. If useful heating power is 45 W, time=Q/P=300 s. Assume c
approximately constant and no phase change.
Electrical method
Measure sample mass and temperature. Use a fitted electrical heater with measured voltage V,
current I and time t. Input energy E=VIt; an ideal estimate c=VIt/(mΔθ). Stir liquids for uniform
temperature. For a solid block improve heater/thermometer thermal contact and insulate
appropriately.
Problem Effect or improvement
Energy heats vessel/heater too Assigning all VIt to sample alone can overestimate c. Include apparatus
heat capacity when known.
Energy lost to surroundings Smaller temperature rise: ignoring losses tends to overestimate c.
Insulate and consider cooling corrections.
Temperature nonuniform or Stir and allow sensor response; avoid a local hot spot.
delayed
Use measured V and I, not a label power when accuracy matters. Record uncertainties and assess losses rather
than assuming insulation makes them zero.
Independent revision notes • Edition 1.0 for review • October 2026 3
