A-Level Physics — 13) Thermodynamic Systems (IP-Friendly Guide)

TL;DR
Thermodynamics is the energy balance sheet of the universe. Nail the distinctions between U, Q, W and you will unlock marks in gas laws, kinetic theory and every practical calorimetry task.

1 Internal energy \((U)\)

The macroscopic state of a system fixes its internal energy: the sum of random microscopic kinetic \((E_k)\) and potential \((E_p)\) energies. No single thermometer can reveal \(U\) directly — you must track how energy enters or leaves.

1.1 Parent takeaway

A student who writes “heat stored in the gas” is forfeiting method marks. Heat is a transfer, not a store.

1.2 Mini-drill

State whether each contributes to \(U\):

2 Thermodynamic temperature \((T)\)

Absolute temperature in kelvin is directly proportional to the mean kinetic energy per particle. Doubling \(T\) doubles \(\langle E_k \rangle\). Always quote exam answers to 2-3 s.f. unless otherwise stated.

3 Heating \((Q)\) vs work \((W)\)

Sign hack: In SEAB mark schemes, work done by the gas is negative.

4 Zeroth law — the thermometer principle

If A is in thermal equilibrium with B, and B with C, then A is in equilibrium with C. This justifies using a third body (a thermometer) to compare temperatures.

5 First law — the energy ledger

\[ \Delta U = Q + W. \]

1. Positive \(Q\): heating adds energy.
2. Positive \(W\): surroundings compress system.
3. For an ideal gas in free expansion, \(Q = 0\) and \(W = 0\), hence \(\Delta U = 0\): temperature unchanged.

6 Specific heat capacity \((c)\)

Defined as the heat required per unit mass per kelvin rise:

\[ Q = mc \Delta T. \]

6.1 Quick-check

Water has \(c \approx 4.18 \space \pu{kJ.kg-1.K-1}\) — roughly 30x that of copper. That is why water is a coolant.

7 Specific latent heat \((L)\)

Energy per unit mass for a phase change at constant \(T\):

\[ Q = mL. \]

Common values:

8 WA & Paper 4 timing rules

1. 1 mark ≈ 1.5 min — budget your section time.
2. Copy units first when using data-book tables.
3. Show full working; SEAB awards method marks even if arithmetic slips.

9 Further reading

• SEAB H2 Physics 9478 syllabus (2026)
• Internal Energy — Wikipedia
• Kinetic Theory & Temperature — LibreTexts
• Pressure-Volume Work — Khan Academy
• Zeroth Law of Thermodynamics — Wikipedia
• First Law of Thermodynamics — BCcampus
• Specific Heat Capacity — Wikipedia
• Specific Latent Heat Explained — FuseSchool
• Reversible vs Irreversible Work — LibreTexts

10 Call-to-action

Parents: schedule a 1-hour Thermodynamic Systems clinic two weeks before WA 2.
Students: print the First Law equation and stick it on your water bottle — then apply it to every past-year Qn you meet.

Last updated 14 Jul 2025. Next review: upon release of the 2027 draft syllabus.