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⚡ Physics Practical Master Guide
Exam success blueprint for Question 1 (Electricity) & Question 2 (Mechanics)
⚡ Question 1: Electricity (Potential Dividers)
🔧 Core knowledge & precision skills
- PRECISION Voltmeter range: Always set to 2V / 2000mV range. Record readings to 3 decimal places (e.g., 1.450V).
- COMPONENT LOGIC Keep the 22Ω resistors in holders X. Only swap the variable resistors in holder R.
- THERMAL SAFETY Keep the switch OPEN except when taking a reading → prevents resistor heating & drift.
⚠️ Pitfalls & error identification
| Limitation (The Problem) | Solution (The Improvement) |
|---|---|
| Battery Sag: Voltage drops as cell drains during use. | Use a regulated DC power supply for constant EMF. |
| Contact Resistance: Loose wires/dirty terminals cause fluctuating readings. | Use gold-plated connectors or clean terminals with emery cloth. |
| Thermal Drift: Resistance changes as resistors warm up. | Use a momentary switch → current flows only for 1–2 seconds. |
💡 Pro tip: Connect voltmeter across the variable resistor (R2) in potential divider. Record Vout for different R values, plot Vout vs. R.
⏱️ Question 2: Mechanics (Oscillations / SHM)
🎯 Essential measurement techniques
- The 20-Oscillation Rule: Never time a single swing. Measure total time for 20 complete oscillations → divide by 20 to find period T (reduces reaction uncertainty).
- Fiducial markers: Use a pin/tape at the equilibrium position to start/stop the timer accurately.
- SHM consistency: Ensure mass moves strictly vertically (spring-mass system). Stop any side-to-side pendulum motion before timing.
📐 Uncertainty analysis
% Uncertainty = (Absolute Uncertainty / Measured Value) × 100
📏 Absolute uncertainty for standard ruler: typically 0.1 cm (or ±0.05 cm if using vernier). For stopwatch: ±0.1 s human reaction can be reduced by 20 oscillations.
📏 Absolute uncertainty for standard ruler: typically 0.1 cm (or ±0.05 cm if using vernier). For stopwatch: ±0.1 s human reaction can be reduced by 20 oscillations.
🚨 Critical limitations & advanced solutions
| Limitation (The Problem) | Solution (The Improvement) |
|---|---|
| Human Reaction Time: Delay in starting/stopping stopwatch. | Use light gates or a motion sensor for automatic timing → precision ±0.001 s. |
| Parallax Error: Misjudging the mass position against ruler scale. | Attach a set square / pointer to align mass with the ruler markings. |
| Air Resistance / Damping: Friction slows oscillations over time. | Increase mass-to-surface-area ratio (use heavier mass) or perform experiment in low air disturbance. |
🧪 Period formula for spring-mass system: T = 2π √(m/k) → plot T² vs. m to find spring constant k from gradient.
⚖️ The "Verdict" Question (Final 4 Marks)
When asked "Do your results support the suggestion / theoretical relationship?", apply the 10% Rule:
📐 % Difference = [ |k₁ - k₂| / ( (k₁ + k₂)/2 ) ] × 100
➕ Compare result to 10%.
➕ Compare result to 10%.
- ✅ If % Difference ≤ 10%: "The difference is within the limits of experimental accuracy; the results support the suggestion."
- ❌ If % Difference > 10%: "The difference exceeds the 10% limit; the results do NOT support the suggestion."
🔍 Exam-ready note: Always mention systematic errors (e.g., friction, resistance of wires) and random errors (reaction time, parallax) when justifying your verdict.
📌 Quick Commandments for Full Marks
🔋
Electricity
Switch OFF between readings → keep resistors cool. Use short connecting wires.
📏
Oscillations
Measure length from suspension point to center of mass. Use fiducial marker at equilibrium.
📊
Graphs
Label axes with units, choose scales that use >75% of grid, plot points clearly.
🧪 Common systematic errors & how to reduce them
| Error type | Example in practical | Improvement strategy |
|---|---|---|
| Zero error | Ammeter/voltmeter not reading zero when circuit open. | Check & adjust zero before experiment; record offset. |
| Loading effect | Voltmeter internal resistance alters potential divider voltage. | Use high-impedance digital voltmeter (≥10 MΩ). |
| Systematic damping | Air currents affect oscillation amplitude decay. | Enclose apparatus in transparent shield; use heavier bob. |
💡 Uncertainty propagation: For T = t_total / 20 → absolute uncertainty in T = (uncertainty in stopwatch) / 20. If stopwatch ±0.2 s over 20 cycles → ΔT = ±0.01 s.
🎯 Verdict in action – solved example
Scenario: Two values of spring constant obtained: k₁ = 24.3 N/m , k₂ = 26.7 N/m. Does experiment support Hooke's law prediction that k is constant?
Average = (24.3 + 26.7)/2 = 25.5 N/m
% difference = (|24.3 - 26.7| / 25.5) × 100 = (2.4 / 25.5) × 100 ≈ 9.41%
✅ 9.41% < 10% → "The results support the suggestion within experimental precision."
% difference = (|24.3 - 26.7| / 25.5) × 100 = (2.4 / 25.5) × 100 ≈ 9.41%
✅ 9.41% < 10% → "The results support the suggestion within experimental precision."
✏️ If % diff = 14.2% → verdict: "Difference exceeds typical 10% uncertainty; results do NOT support suggestion."
📘 Complete Success Guide: Physics Paper 3 (Questions 1 & 2)
Cambridge AS & A Level Physics (9702) – Practical Skills
This guide covers Electrical Circuits (Q1) and Springs & Oscillations / Equilibrium (Q2) – including theory, practical techniques, graph linearisation, error handling, and typical exam questions.
1. Core Skills Common to Both Questions
| Skill | What you must be able to do |
|---|---|
| Following instructions | Set up circuits or mechanics exactly as shown in diagram. |
| Recording data | Table with headings, units, consistent decimal places. |
| Graph plotting | Scale > half grid, accurate points (crosses), line of best fit. |
| Graph analysis | Gradient (large triangle) and y‑intercept from graph. |
| Linearisation | Transform equations into Y = mX + c form. |
| Uncertainty | Estimate % uncertainty, justify significant figures. |
| Units | Always include units in table headers, axes, final answers. |
2. Question 1: Electrical Circuits
🔧 Typical Apparatus
- 1.5 V d.c. power supply (dry cell)
- Digital voltmeter (0–2 V, reads to 0.001 V)
- 2 × 22 Ω resistors (holders labelled X)
- 1 × empty component holder labelled R
- Resistors: 12, 15, 18, 22, 27, 33, 39 Ω (±5% tolerance)
- Connecting leads, switch
🧠 Required Theory
- Ohm’s Law: V = I × R
- Series resistance: Rtotal = R1 + R2 + R3
- Voltage division: Vn = Vtotal × (Rn / Rtotal)
- Internal resistance: Terminal voltage V = E − I·r (less than EMF under load)
📊 Graph Linearisation (Past Paper Examples)
Example 1 (2019):
Given: \( \frac{R}{d} = A R + B \)
Y-axis = \( \frac{R}{d} \), X-axis = \( R \)
Gradient = \( A \), Y-intercept = \( B \)
Given: \( \frac{R}{d} = A R + B \)
Y-axis = \( \frac{R}{d} \), X-axis = \( R \)
Gradient = \( A \), Y-intercept = \( B \)
Example 2 (2020):
Given: \( \frac{1}{V} = A \cdot \frac{1}{R} + B \)
Y-axis = \( \frac{1}{V} \), X-axis = \( \frac{1}{R} \)
Gradient = \( A \), Y-intercept = \( B \)
Given: \( \frac{1}{V} = A \cdot \frac{1}{R} + B \)
Y-axis = \( \frac{1}{V} \), X-axis = \( \frac{1}{R} \)
Gradient = \( A \), Y-intercept = \( B \)
Example 3 (2025):
Given: \( \frac{E-V}{L} = P V - Q \)
Y-axis = \( \frac{E-V}{L} \), X-axis = \( V \)
Gradient = \( P \), Y-intercept = \( -Q \)
Given: \( \frac{E-V}{L} = P V - Q \)
Y-axis = \( \frac{E-V}{L} \), X-axis = \( V \)
Gradient = \( P \), Y-intercept = \( -Q \)
📝 Common Questions & Model Answers
- Q: Total resistance when R = 18 Ω in series with two 22 Ω resistors?
A: 22 + 22 + 18 = 62 Ω - Q: Current if voltage across 18 Ω is 0.435 V?
A: I = 0.435 / 18 = 0.02417 A - Q: Why is measured battery voltage less than 1.5 V when circuit closed?
A: Internal resistance causes a voltage drop I·r. - Q: Voltmeter reads 0.000 V across R – two possible faults?
A: Switch open; resistor not making contact; voltmeter leads shorted.
3. Question 2: Springs & Oscillations / Equilibrium
🔧 Apparatus
- 2 stands, 2 clamps, 2 bosses
- 3 expendable springs (≈25 N·m⁻¹ each)
- Protractor (1° divisions), metre rule (mm scale)
- Stopwatch (0.1 s), 100 g mass hanger, 2 × 100 g slotted masses
🧠 Required Theory
- Hooke’s Law: F = k·x (where F = mg, x = extension)
- Springs in series: 1/ktotal = 1/k1 + 1/k2
- Springs in parallel: ktotal = k1 + k2
- Period of oscillation: T = 2π √(m/k) → T² = (4π²/k) · m
- Angle relation (sample from 2020 paper): sin θ = C · (n²/2 − n)
📊 Graph Linearisation (Springs)
For T² vs m (oscillation experiment):
T² = (4π²/k) · m
Y-axis = T², X-axis = m
Gradient = 4π²/k → k = 4π² / gradient
T² = (4π²/k) · m
Y-axis = T², X-axis = m
Gradient = 4π²/k → k = 4π² / gradient
For sin θ vs (n²/2 − n) (angle experiment):
sin θ = C · (n²/2 − n)
Y-axis = sin θ, X-axis = (n²/2 − n)
Gradient = C
sin θ = C · (n²/2 − n)
Y-axis = sin θ, X-axis = (n²/2 − n)
Gradient = C
📝 Common Questions & Model Answers
- Q: Calculate spring constant k from graph of T² vs m.
A: k = 4π² / gradient (units: N/m). - Q: Why measure time for 10 oscillations instead of 1?
A: To reduce percentage uncertainty – human reaction time is constant, so larger total time gives smaller % error. - Q: Percentage uncertainty in T if total time for 10 oscillations = 12.5 s (±0.1 s)?
A: (0.1 / 12.5) × 100 = 0.8% - Q: Two improvements to oscillation experiment?
A: Use a motion sensor / light gate; repeat and average; use fiducial marker; ensure small amplitude vertical oscillations.
4. Pitfalls to Avoid (Both Questions)
- Voltmeter in series → reads nearly battery voltage. Always connect in parallel.
- Ignoring internal resistance → calculated Rtotal appears too high.
- Loose connections / switch open → zero or unstable readings.
- Timing too few oscillations → high % uncertainty. Use ≥10 oscillations.
- Large amplitude oscillations → not SHM, period varies. Keep amplitude small.
- Parallax error with protractor/ruler → read from directly above.
- Not repeating readings → cannot identify random errors.
5. Error Identification & Correction
Types of Errors
- Systematic: constant bias (zero error). Correction: calibrate / subtract zero.
- Random: unpredictable variations (reaction time). Correction: repeat and average.
- Parallax: eye not perpendicular. Correction: use mirror scale or position eye correctly.
- Environmental: temperature affects resistance/spring constant. Correction: control conditions.
| Observation | Likely Error |
|---|---|
| All voltage readings too high | Voltmeter zero error or fresh battery |
| Graph line does not pass through origin (should) | Systematic error (contact resistance, spring initial tension) |
| One point far from line of best fit | Random error (misreading or recording mistake) |
| Calculated k varies widely | Timing inconsistency or amplitude too large |
6. Uncertainty & Significant Figures Guide
% uncertainty = (absolute uncertainty / measured value) × 100
- Metre rule (mm) → ±0.1 cm
- Protractor (1°) → ±0.5° to ±1°
- Stopwatch (human reaction) → ±0.1 s to ±0.2 s
- Digital voltmeter (0–2 V) → ±0.001 V
Significant figures: Measured values → instrument precision. Calculated constants → 2 or 3 SF. Always justify (e.g., "mass to 0.1 g, time to 0.1 s, so k given to 3 SF").
7. Improvements & Limitations (4‑mark Questions)
Common Limitations
- Electricity: contact resistance, non‑uniform wire, voltmeter loading, resistor tolerance, battery internal resistance.
- Springs: non‑Hookean behaviour, non‑vertical oscillations, reaction timing, parallax in length measurement.
Improvements for Any Experiment
- Repeat measurements and average → reduces random error.
- Use digital sensors (light gate, motion sensor) → eliminates reaction time.
- Measure larger quantities (e.g., 20 oscillations) → lower % uncertainty.
- Calibrate instruments before use → eliminates systematic errors.
- Use fiducial marker for timing → consistent start/stop.
- Ensure vertical alignment with plumb line.
8. Quick Revision Formula Sheet
Electricity (Q1)
V = IR, Rseries = R₁ + R₂ + ... , Vn = VT × (Rn/RT), P = I²R, Vterminal = E − I·r
Springs & Oscillations (Q2)
F = k·x, T = 2π √(m/k) → T² = (4π²/k)·m
Series: 1/kT = 1/k₁ + 1/k₂, Parallel: kT = k₁ + k₂
Uncertainty
% unc = (abs unc / value) × 100, mean ± range/2
V = IR, Rseries = R₁ + R₂ + ... , Vn = VT × (Rn/RT), P = I²R, Vterminal = E − I·r
Springs & Oscillations (Q2)
F = k·x, T = 2π √(m/k) → T² = (4π²/k)·m
Series: 1/kT = 1/k₁ + 1/k₂, Parallel: kT = k₁ + k₂
Uncertainty
% unc = (abs unc / value) × 100, mean ± range/2
9. Pre-Exam Checklist
- ☐ I can set up a series circuit with voltmeter in parallel.
- ☐ I can measure period by timing ≥10 oscillations.
- ☐ I can use protractor and ruler without parallax.
- ☐ I record readings in a table with units and consistent decimals.
- ☐ I plot graph: scales > half grid, accurate points, line of best fit.
- ☐ I calculate gradient using two points ON the line (not data points).
- ☐ I can rearrange given equation into Y = mX + c.
- ☐ I can determine constants (A, B, k, P, Q, E) from gradient/intercept.
- ☐ I can suggest 4 improvements / limitations and justify significant figures.
🎯 Final Tip – Paper 3 Pattern: Every question follows:
① Measure something → ② Repeat with different values → ③ Plot graph → ④ Find gradient & intercept → ⑤ Use given equation to find constants → ⑥ Answer uncertainty/improvements.
Master this pattern = master Paper 3.
① Measure something → ② Repeat with different values → ③ Plot graph → ④ Find gradient & intercept → ⑤ Use given equation to find constants → ⑥ Answer uncertainty/improvements.
Master this pattern = master Paper 3.
© Learner Revision Resource – based on Cambridge 9702 syllabus and past papers (2019–2025).