How to use this guide
This guide is for Physics 2 students who know the physics but lose marks in written answers. It covers the whole course. Read the verb table, study the examples, then try Practise it before opening the model answers.
Every question here is original Marlbridge practice, not past exam material, and all data are invented. The marks shown are a suggested Marlbridge rubric to help you judge your own answers; they are not official scoring.
The free-response section at a glance
Section II has 4 questions in 95 minutes and is worth 50% of the exam. You read questions on screen and handwrite answers in a paper booklet. A four-function, scientific or graphing calculator is allowed. Each question type appears once:
| Question | Type | Points | Suggested time | What it mostly asks for |
|---|---|---|---|---|
| 1 | Mathematical Routines (MR) | 10 | 20–25 min | Derive and calculate, use a representation, then make and justify a claim |
| 2 | Translation Between Representations (TBR) | 12 | 25–30 min | Draw a diagram, derive an equation, sketch a graph, then check that they agree or predict a change |
| 3 | Experimental Design and Analysis (LAB) | 10 | 25–30 min | Design a procedure, then plot given data and use the graph to answer a question |
| 4 | Qualitative/Quantitative Translation (QQT) | 8 | 15–20 min | Make and justify a claim, derive an equation, then link the two |
The three course practices all appear: creating representations (1), mathematical routines (2) and scientific questioning and argumentation (3).
The task verbs and what each one asks for
The course lists fourteen task verbs for free-response questions. The definitions are in our own words; the length column is a guide, not a rule.
| Verb | What it asks you to do | How much to write |
|---|---|---|
| Indicate | Give the answer only: a direction, a choice, a value | A word, an arrow or a short phrase |
| Label | Add names, units, scales or parts to a diagram or graph | Labels only |
| Rank | Put quantities in order, usually by size, and show any that are equal | An ordered list, ties marked with “=” |
| Describe | Give the features that matter for the situation | Two or three sentences |
| Compare | Say how two or more things are alike or differ, and which is larger | A sentence for each point of difference |
| Justify | Give physical reasoning in words that supports your claim; equations may help but are not enough | Three to five linked sentences |
| Verify | Show that the conditions for a law or test are met, or use data to confirm an idea | A check for each condition |
| Calculate | Work through the maths to a number: equation, substitution, answer with unit and sensible significant figures | Every step written |
| Derive | Start from a basic law or a table equation and do the algebra to reach the required expression | Each algebra step, no numbers unless given |
| Determine | Reach a value or a decision, by reasoning, by reading data or by calculation | Enough working to show how you got there |
| Estimate | Find a rough value, a sign or a greater/less/equal result from data | Working not required, but a line of it helps |
| Draw | Make a diagram that shows objects and relationships accurately (free-body, ray, circuit, field) | A neat diagram; labels if asked |
| Sketch | Draw a graph or picture with the right shape and key features, not exact values | Shape, intercepts and trends must be right |
| Plot | Mark data points on a grid with a scale and units | Points, and a best-fit line if asked |
It helps to sort the fourteen into four groups:
- Answer-only verbs: Indicate, Label, Rank.
- Explanation verbs: Describe, Compare, Justify, Verify.
- Maths verbs: Calculate, Derive, Determine, Estimate.
- Representation verbs: Draw, Sketch, Plot (and Label again on diagrams and graphs).
One short example per group
Answer-only (Unit 12). Indicate the direction of the magnetic force on a proton moving in the +x-direction through a uniform field pointing in the +y-direction. Answer: the +z-direction. Nothing more is needed. An added reason with an error in it puts a false statement next to a correct answer.
Explanation (Unit 14). Compare the wavelength of a sound in air with its wavelength after it passes into water. Answer: the source sets the frequency, so it is unchanged. Sound travels faster in water, and λ = v/f, so the wavelength in water is longer. Give the direction of the difference and the reason.
Maths (Unit 15). Calculate the energy of a photon of wavelength 600 nm. E = hc/λ = (6.63 × 10⁻³⁴ J·s)(3.00 × 10⁸ m/s) ÷ (600 × 10⁻⁹ m) = 3.32 × 10⁻¹⁹ J (about 2.07 eV). If the verb were Estimate, “about 3 × 10⁻¹⁹ J” would do.
Representation (Unit 11). Sketch the current in the resistor of a charging RC circuit against time, from the moment the switch closes. A good sketch starts at ℰ/R on the labelled I axis, falls steeply, then levels off towards zero without crossing it. A straight line, or a curve starting at zero, loses the shape marks.
Reading a multi-part question
Spend the first minute reading all parts, not just part (a).
- Mark the verb in every part: “answer only”, “words”, “algebra” or “graph”.
- Find the allowed symbols. If a derivation must be “in terms of m, q, ΔV and B”, any other symbol in the final answer loses credit.
- Look for links. A late part often asks whether your graph agrees with your equation, or what changes if a property changes. Keep earlier work clear so you can use it.
- Note conditions. “Ideal”, “constant volume” and “long after the switch closes” each remove a term or fix a quantity.
- Judge claims part by part. A claim may be partly true. Say which part holds and which does not, with a reason for each.
Planning your 95 minutes
40 points in 95 minutes is about 2.4 minutes per point. The suggested times add up to 85–105 minutes, so you cannot take the maximum on every question. One workable plan:
| Step | Minutes | Notes |
|---|---|---|
| Read all four questions | 3 | Note verbs and which question looks easiest |
| MR (10 points) | 22 | Leave a gap if a derivation stalls; come back |
| TBR (12 points) | 27 | Most points; do not rush the graph |
| LAB (10 points) | 25 | Write the procedure as numbered steps |
| QQT (8 points) | 16 | Short, but the justification must be complete |
| Final check | 2 | Units, symbols, unanswered parts |
| Total | 95 |
You may answer in any order; starting with your easiest question builds a cushion. If a part takes twice its share of time, write what you have and move on.
Showing your work on calculations
- Start from an equation from the table or a basic principle, before any numbers.
- Solve in symbols first when you can; a later part may need the expression.
- Substitute with units, so the reader sees what each number is.
- Keep extra digits until the end, then give 2 or 3 significant figures to match the data.
- Finish with a unit and, for vectors, a direction.
- Carry forward. A correct method using a wrong value from part (a) can still earn credit in part (b). Say which value you are using.
What earns and what loses credit
| Earns credit | Loses credit |
|---|---|
| A derivation that begins with a named law or table equation | Starting from the answer, or from a memorised result with no source |
| Reasoning that names a principle and applies it to this situation | “Because of the equation” with no physics in words |
| Graph shapes and diagrams that agree with your equations | A graph that contradicts your own derivation |
| A clear single answer | Two answers, one right and one wrong (“it increases or stays the same”) |
| Ranking with ties shown | Forcing an order on quantities that are equal |
Worked example 1: Calculate, then justify (Unit 9)
A closed steel tank of fixed volume holds an ideal gas at 250 K and 1.5 × 10⁵ Pa. It is warmed to 350 K. (a) Calculate the new pressure. (b) Justify, using a molecular model, why the pressure rises.
Weak answer. (a) 2.1 × 10⁵. (b) The pressure goes up because PV = nRT and T goes up.
What it misses. Part (a) has no equation, substitution or unit. Part (b) only repeats the equation; it gives no molecular reasoning.
Strong answer. (a) V and n are constant, so P/T is constant: P₂ = P₁T₂/T₁ = (1.5 × 10⁵ Pa)(350 K) ÷ (250 K) = 2.1 × 10⁵ Pa. (b) Temperature measures the average kinetic energy of the molecules, so at 350 K they move faster on average. Faster molecules hit the walls more often, and each collision changes their momentum by more. So the walls feel a larger force on each square metre: a higher pressure.
| Suggested Marlbridge rubric | Points |
|---|---|
| Uses P/T constant (or PV = nRT with n, V fixed) and kelvin temperatures | 1 |
| Correct answer 2.1 × 10⁵ Pa with unit | 1 |
| Links higher T to greater average molecular speed or kinetic energy | 1 |
| Links faster molecules to more frequent and harder collisions, so greater force per area | 1 |
Worked example 2: Rank, then justify (Unit 11)
An ideal 12 V battery is connected to resistor R₁ = 4.0 Ω in series with a parallel pair, R₂ = 6.0 Ω and R₃ = 12 Ω. Rank the power dissipated in the three resistors, from greatest to least. If any are equal, say so. Justify your ranking.
Weak answer. P₃ > P₂ > P₁, because P = I²R and the largest resistor gets the most power.
What it misses. The ranking is reversed. P = I²R only compares resistors carrying the same current, and these three do not.
Strong answer. P₁ > P₂ > P₃. R₂ and R₃ are in parallel, so they have the same potential difference. With ΔV fixed, P = ΔV²/R, so the smaller resistance gets more power: P₂ > P₃. The parallel pair has an equivalent resistance of 4.0 Ω, the same as R₁, and both carry the full battery current, so R₁ and the pair each take half of the 12 V. R₁ and R₂ therefore have the same ΔV (6.0 V), and R₁ has the smaller resistance, so P₁ > P₂.
Check: the current is 1.5 A; the powers are 9.0 W, 6.0 W and 3.0 W, adding to the battery’s 18 W.
| Suggested Marlbridge rubric | Points |
|---|---|
| Correct ranking with no false ties | 1 |
| Uses equal ΔV across the parallel pair to compare P₂ and P₃ | 1 |
| Compares R₁ with the pair through equal ΔV (or equal current with resistances) | 1 |
Worked example 3: Derive, then predict (Unit 12, using Unit 10)
An ion of mass m and charge +q starts at rest and is accelerated through a potential difference ΔV. It then enters a uniform magnetic field of magnitude B at right angles to its velocity. (a) Derive an expression for the radius r of its path, in terms of m, q, ΔV, B and constants as needed. Begin with a fundamental principle or a table equation. (b) Indicate the factor by which r changes if ΔV is made four times larger.
Weak answer. r = mv/(qB) = m(2qΔV/m)/(qB), so r = 2ΔV/B. (b) r is 4 times larger.
What it misses. The speed is not derived from a starting law, a square root is lost and the units are wrong. Part (b) carries the error forward.
Strong answer. (a) Energy conservation for the acceleration: qΔV = ½mv², so v = √(2qΔV/m). In the field the magnetic force is the centripetal force: qvB = mv²/r, so r = mv/(qB). Substitute v: r = (m/(qB))·√(2qΔV/m), which simplifies to r = (1/B)·√(2mΔV/q). (b) r ∝ √ΔV, so r changes by a factor of √4 = 2.
| Suggested Marlbridge rubric | Points |
|---|---|
| Starts with energy conservation, qΔV = ½mv² | 1 |
| Sets magnetic force equal to mv²/r | 1 |
| Correct final expression in the given symbols only | 1 |
| Factor of 2, consistent with the expression | 1 |
Worked example 4: Describe a procedure, then determine (Unit 14)
You have a tall tube open at the top, with water inside whose level can be lowered, a set of tuning forks of known frequencies and a metre rule. (a) Describe a procedure to collect data that could be graphed to find the speed of sound in air. (b) Describe how you would use the graph. (c) A student’s best-fit line of L against 1/f has a slope of 85 m/s. Determine the speed of sound.
Weak answer. Hit a tuning fork, find where it is loud, measure the length, use v = fλ. Repeat to be accurate.
What it misses. It never says what is changed or measured, and names no graph to analyse.
Strong answer. (a) 1. Strike a fork and hold it above the tube. 2. Lower the water from the top until the sound is loudest (first resonance). 3. Measure L, the air column from water surface to tube top. 4. Repeat for at least five forks. 5. Take three readings of each L and use the mean. (b) At first resonance the air column is a quarter wavelength, so L = v/(4f). Plot L (vertical) against 1/f (horizontal). The line’s slope is v/4; a small intercept does not change it. (c) v = 4 × slope = 4 × 85 m/s = 340 m/s.
| Suggested Marlbridge rubric | Points |
|---|---|
| Changes frequency (fork), measures resonance length with the rule | 1 |
| Repeats readings or uses several forks to reduce uncertainty | 1 |
| Names a linear graph (L against 1/f, or equivalent) | 1 |
| Links slope to v and gets 340 m/s | 1 |
Common mistakes
- Answering Justify with an equation and nothing in words.
- Adding an explanation to Indicate that contradicts the right answer.
- Ranking without checking for ties, or ranking in the wrong direction.
- Starting a derivation from a result instead of a law or table equation.
- Leaving symbols in a derived answer that the question did not allow.
- Sketching a graph that disagrees with an equation you derived earlier in the same question.
- Describing a procedure with no clear changed quantity, measured quantity and repeat.
Practise it
Task 1 (Unit 10). Two point charges, each +5.0 nC, are fixed 0.60 m apart. Determine the electric field and the electric potential at the point halfway between them, and justify your answer.
Model answer
Each charge is 0.30 m away. The two fields are equal in size and point in opposite directions (each away from its own charge), so the net field is zero. Potential is a scalar, so the two terms add: V = 2kq/r = 2(8.99 × 10⁹)(5.0 × 10⁻⁹) ÷ 0.30 ≈ 300 V. Zero field does not mean zero potential.
Task 2 (Unit 13). An object is 8.0 cm from a converging lens of focal length 12 cm. Calculate the image distance and the size of the magnification, and indicate whether the image is real or virtual.
Model answer
1/sᵢ = 1/f − 1/sₒ = 1/12 − 1/8.0 = −1/24, so sᵢ = −24 cm. |M| = |sᵢ/sₒ| = 24/8.0 = 3.0. The image is virtual (and upright), because sᵢ is negative.
Task 3 (Unit 14). A string 0.65 m long, fixed at both ends, vibrates in its fundamental mode at 196 Hz. Estimate the wave speed on the string.
Model answer
In the fundamental, λ = 2L ≈ 1.3 m, so v = fλ ≈ 200 × 1.3 ≈ 260 m/s. (Exactly 255 m/s; 250–260 m/s is a fine estimate.)
Task 4 (Unit 15). Verify that the beta-minus decay ¹⁴₆C → ¹⁴₇N + ⁰₋₁e + ν̄ obeys the conservation laws for nuclear decay.
Model answer
Nucleon number: 14 = 14 + 0 + 0. Charge: 6 = 7 + (−1) + 0. Lepton number: 0 = (+1) + (−1). All three are conserved; each needs its own check.
Task 5 (Unit 9). A fixed amount of ideal gas is held at constant pressure by a freely moving piston. Sketch its volume against its absolute temperature, and describe two features of your sketch.
Model answer
A straight line through the origin. Feature 1: V = (nR/P)T, so V ∝ T in kelvin; doubling T doubles V. Feature 2: the slope is nR/P, so at a higher fixed pressure the line would be less steep.
Where to practise next
Quick questions
If a question says Indicate, will I lose credit for explaining?
Usually not, but extra writing costs time, and a wrong extra statement can contradict a correct answer. Give the answer clearly and move on unless the question also says Justify.
What should I do if I cannot finish an early part?
Keep going. Later parts are often marked on their own, and many can be done with a value or expression you name yourself. Write, for example, 'using my value of v from part (a)' so that your method can still be followed.
Is a correct number with no working enough for Calculate?
No. Calculate asks for the steps: the equation, the substitution and the answer with a unit. A bare number gives a reader nothing to credit if it is slightly wrong.