A-Level · 26 September 2026 · 6 min read
Edexcel IAL Physics Unit 4 vs Unit 5: what to study and practise
A2 Physics can feel like several new subjects arriving at once. A better map starts with the principles that connect them: force changes momentum, fields describe interactions, energy moves between stores, and mathematical models predict how quantities vary. Unit labels organise the assessment; they should not isolate your thinking.
This guide concerns the 2018 Edexcel International qualification used by current cohorts. Confirm the specification version with your centre if your course starts later. All short diagnostic examples here are original teaching questions, not extracted examination material.
Map the assessment without confusing raw marks and UMS
Under the 2018 specification, both WPH14 and WPH15 are 90-mark written papers lasting 1 hour 45 minutes. Each contributes 20% of the full IAL. Unit 6 is another required A2 component, so a plan containing only Units 4 and 5 leaves a practical-skills gap. Keep paper codes on your folders and completed-paper records.
Raw scores from a practice paper help identify errors. They do not directly equal the UMS contribution to the qualification. Use the official conversion for the particular series when interpreting a completed official paper. For planning revision, a list of recurring mistakes usually tells you more than repeatedly converting a mock percentage into a hoped-for final grade.
Unit 4: connect force, motion and fields
Build your Unit 4 practice around decisions. When a particle moves in a circle, identify what provides the inward resultant force. When charges move through a field, distinguish a force calculation from an energy calculation. When interpreting a track, distinguish charge information from momentum information. The same drawing can support several different questions.
Try this diagnostic: a 0.20 kg object moves at 3.0 m s⁻¹ in a horizontal circle of radius 0.50 m. The inward resultant is mv²/r = 3.6 N. If the speed doubles at unchanged radius, it becomes 14.4 N. The useful learning is the square dependence and the direction, not memorising the first number. A force diagram must still show which real interaction supplies that resultant.
Unit 5: connect microscopic models with measurable quantities
Unit 5 asks you to move between scales: molecular behaviour and gas pressure, nuclear changes and released energy, oscillating systems and graphs, or astronomical observations and a model. Practise saying what can be measured directly and what is inferred. That distinction helps with both calculation and evaluation questions.
For a fixed quantity of ideal gas at constant volume, heating from 300 K to 450 K multiplies the pressure by 1.5. Heating from 30°C to 45°C does not. The error is not an arithmetic problem; it is using the wrong temperature scale in a proportionality. Explain the particle model as well: faster average molecular motion changes momentum transfers at the walls.
Use earlier knowledge as a tool rather than a separate revision block
Before a difficult A2 question, identify its AS prerequisite. A particle-track calculation may require rearrangement and circular motion. A nuclear-energy question requires powers of ten and unit conversion. A gas graph requires gradients and controlled variables. Repairing a small prerequisite can remove a large apparent A2 difficulty.
Do not respond to every mistake by restarting the entire AS course. If the problem was converting electronvolts to joules, practise that conversion in three different contexts and return to the original question. If the force model itself was wrong, draw and explain several diagrams before repeating the algebra. Match the repair to the first failed decision.
Practise a changed condition after every worked example
After completing a capacitance question, change the resistance and predict the effect on the time constant before calculating. After a decay question, double the initial number of nuclei and ask which quantities change and which remain fixed. After a circular-motion question, hold period constant instead of speed and reconsider the dependence on radius.
This habit exposes whether your method depends on the actual physics or on matching the appearance of a remembered solution. Record a sentence such as “I assumed speed was fixed, but the question held angular speed fixed”. That is a more reusable correction than “careless mistake on question 5”.
Build a balanced A2 practice cycle
Choose one Unit 4 question family, one Unit 5 family and one practical reasoning task for your next cycle. Attempt them without notes, mark the reasoning, then return later to a changed version. Include complete timed papers once you can sustain the component skills. Timing should reveal where your method breaks down, rather than become a substitute for understanding.
Keep the next task small and observable: interpret a magnetic track, explain a pressure change, or justify a graph used to find a constant. Use the linked topic guides when that task is unclear. If you need course support, bring your unit code and a recent attempted question so discussion can focus on the model or explanation you need to improve.
Questions, explained
Choose a question for a direct answer, then explore the explanation and supporting resources. Each answer has its own link to save or share.
What is the difference between Edexcel IAL Physics Unit 4 and Unit 5?
In the 2018 specification, Unit 4 develops further mechanics, fields and particles. Unit 5 develops thermodynamics, radiation, oscillations and cosmology. Both connect to earlier knowledge. Use their distinct topic lists to organise practice, but keep shared skills such as graphs, units and mathematical modelling active across both.
Do I need AS Physics knowledge for Unit 4?
Yes. Earlier ideas and mathematical skills remain useful and can be assessed in synoptic contexts. Diagnose the particular prerequisite behind an error rather than assuming every A2 difficulty requires restarting AS. Forces, energy, graph interpretation and algebra commonly support later reasoning.
Is Unit 5 easier than Unit 4?
That depends on your skills and the particular questions. A student confident with fields may find Unit 4 more accessible, while another prefers thermal or nuclear reasoning. Compare your errors across actual question families rather than using a general difficulty label to decide which unit to neglect.
How long are Unit 4 and Unit 5 Physics exams?
For the Edexcel IAL Physics 2018 specification, each paper lasts 1 hour 45 minutes and has 90 raw marks. Check your entry and specification version with your centre. The paper’s raw maximum is different from its UMS maximum and should not be used as a direct substitute.
Can I prepare only Units 4 and 5 for A2 Physics?
No: the Edexcel IAL route also includes Unit 6 practical skills. In addition, practical reasoning supports theoretical questions. Include experimental design, uncertainty, graphs and evaluation in your A2 plan, and confirm all required entries with the exams officer.