6-Mark Question Structure
Free resource by Flowxiom — Edexcel A-level Physics
Not everything. Just what’s on the paper. High-frequency topics only — covering ~80% of exam marks.
Edexcel A-level Physics | “Explain” questions | WPH11, WPH12, WPH14 & WPH15
Why Do Students Lose Marks on 6-Mark Questions?
Students often know the starting point and the final result, but struggle to bridge the gap with a precise logical chain. For 6-mark questions, there is a specific reasoning path — connecting every link earns full marks, while a broken chain leads to heavy mark loss.
Answer principles:
- Each step is an independent mark point
- Link steps with causal connectives: therefore / so / this causes
- Use precise physical terminology
Topic 1: Increase in Gas Pressure with Temperature
Question type: Sealed container, temperature rises — explain why pressure increases.
Answer Chain (6 steps)
Temperature increases → mean kinetic energy of molecules increases
Mean speed of molecules increases
Greater change in momentum per collision with the wall
By \(F = \Delta p / \Delta t\): greater force per collision
Molecules also collide with the wall more frequently
By \(P = F/A\), area unchanged → pressure increases
Logic flow:
Temp Increase → Mean KE Increase → Mean Speed Increase → (Greater Momentum Change → Greater Force per Collision) + (Higher Collision Frequency) → Greater Average Force → Pressure Increase
⚠️ Both steps 3 and 5 are required — neither can be omitted.
Topic 2: Describing SHM (displacement, velocity, acceleration)
Question type: Describe how quantities vary in SHM, or show that a motion is SHM.
Answer Chain (6 steps)
The magnitude of acceleration is proportional to the displacement from equilibrium
Acceleration is always directed towards the equilibrium position — opposite to displacement
This satisfies \(a = -\omega^2 x\) — the definition of SHM
At equilibrium: displacement = 0, velocity is maximum, acceleration = 0
At maximum displacement: velocity = 0, acceleration is maximum — directed towards equilibrium
Total mechanical energy is conserved: kinetic and potential energy interchange, sum remains constant
Phase Relationships (must know)
- Velocity leads displacement by 90°
- Acceleration is in antiphase with displacement (180° phase difference)
Topic 3: Photoelectric Effect — Why Wave Theory Fails
Question type: Explain why wave theory cannot account for a specific observation in the photoelectric effect.
Core Logic: Falsification Method
“Wave Theory Predicts → Paradox Occurs → Experiment Rejects → Particle Solution”
1. The Prediction of Wave Theory
Wave theory treats energy as continuous and proportional to intensity (amplitude). Therefore, it predicts that electrons should eventually be ejected given enough intensity or exposure time.
2. The Conflict with Experimental Facts
Experimental results completely refute these predictions: a threshold frequency exists and emission is instantaneous — both contradict the energy accumulation hypothesis.
3. Logical Correction by Photon Theory
Einstein proposed that light is quantized (photons). The core logic is the “one-to-one” interaction: a single photon’s energy is determined by its frequency (\(E=hf\)). If individual photon energy is insufficient to overcome the work function (\(\phi\)), increasing intensity (more photons) will not help.
Logic flow:
Wave Theory Prediction → Experimental Conflict → Energy Quantization (\(E=hf\)) → One-to-One Interaction → Overcomes Work Function → No Energy Accumulation Required
Exam Pitfalls
- Don’t just state the formula: Many students jump straight to \(E = hf – \phi\). Examiners look for the contrast. You must articulate the incorrect prediction of wave theory to demonstrate you understand why it fails.
Topic 4: Electromagnetic Induction — Lenz’s Law Direction
Question type: Explain the direction of the induced current and its effect on the original motion.
Answer Chain (6 steps)
Magnet approaches coil → magnetic flux through coil increases
By Lenz’s law: the induced current produces a magnetic field that opposes the increase in flux
Use the right-hand rule to determine the direction of the induced current in the coil
The magnetic field of the induced current repels the approaching magnet
Therefore the magnet experiences a force opposing its motion
By conservation of energy: work done pushing the magnet = electrical energy + thermal energy in coil
Logic flow:
Relative Motion → Flux Change → Induced E.M.F./Current → Opposing Magnetic Field → Opposing Force → Work Converted to Electrical Energy
Topic 5: Randomness and Spontaneity of Radioactive Decay
Question type: Explain what is meant by the decay being “random” and “spontaneous”.
Answer Chain (6 steps: 3 for random + 3 for spontaneous)
Random:
It is impossible to predict which specific nucleus will decay or at what time
Each nucleus has the same probability of decaying per unit time, given by the decay constant \(\lambda\)
The moment of decay of an individual nucleus is entirely random, even for identical nuclei
Spontaneous:
Decay is unaffected by external conditions: temperature, pressure and chemical state do not alter the decay rate
Decay is a process within the nucleus, governed by nuclear forces — independent of the surrounding environment
Therefore it is impossible to speed up or slow down decay by changing external conditions
⚠️ Randomness and spontaneity are distinct concepts — each requires 3 steps; any omission loses marks.
Topic 6: Doppler Effect — Frequency Increases as Source Approaches
Question type: Explain why the frequency heard by an observer changes as the source moves towards or away from them.
Answer Chain (6 steps)
As the source moves towards the observer, it moves forward before emitting the next wavefront
Therefore the distance between successive wavefronts — the wavelength — is smaller than when the source is stationary
Wave speed is determined by the medium — it is independent of the source’s speed — so wave speed is unchanged
By \(f = v/\lambda\): wave speed unchanged, wavelength decreases → observed frequency increases
Higher frequency → higher pitch heard by the observer than the frequency emitted by the source
When source recedes: wavefronts spread apart → wavelength increases → observed frequency decreases → lower pitch
⚠️ Steps 1–2 are essential: explain WHY wavelength changes — do not jump straight to “frequency increases”
⚠️ Step 3 (wave speed unchanged) is a critical logical step — do not omit it
Answer Format Template
[Starting condition]: ... (state the initial condition)
Therefore, ... (step 1)
Because ..., ... (step 2 — explain why)
This causes ... (step 3 — state the physical change)
...
Therefore, [conclusion]: ... (directly answer what the question asks)Self-Check List
After completing a 6-mark answer, check:
- [ ] Reasoning chain is complete — no steps skipped
- [ ] Every step uses precise physical terminology
- [ ] Final sentence directly answers the question
- [ ] Directions and signs are correct where relevant
- [ ] Six independent points given
Want more? Visit flowxiom.com