What are the key components of planning a physics experiment for Edexcel International A Levels?

When planning a physics experiment for Edexcel International A Levels, key components include defining the aim of the experiment, identifying variables (independent, dependent, and controlled), selecting appropriate equipment, outlining a step-by-step procedure, considering safety precautions, and planning for data collection and analysis. Each component ensures the experiment is structured and results are reliable.

How do I identify and control variables in a physics experiment?

To identify variables in a physics experiment, determine the independent variable (what you change), the dependent variable (what you measure), and controlled variables (what you keep constant). Controlling variables involves maintaining consistent conditions for controlled variables to ensure that any changes in the dependent variable are solely due to the independent variable, thus ensuring the experiment's validity.

Why is it important to include safety precautions in the planning of a physics experiment?

Including safety precautions in the planning of a physics experiment is crucial to prevent accidents and ensure the well-being of all participants. It involves identifying potential hazards, such as chemical spills or electrical risks, and implementing measures to mitigate them, such as wearing protective gear or ensuring proper equipment handling. This is especially important in a lab setting where various risks are present.

What strategies can I use to ensure accurate data collection in a physics experiment?

To ensure accurate data collection in a physics experiment, use calibrated and appropriate equipment, conduct multiple trials to average out anomalies, and record data systematically. Additionally, ensure that measurements are taken precisely and consistently, and consider using digital tools for data logging to minimize human error. These strategies help in obtaining reliable and reproducible results.

How can I effectively analyze and interpret data from a physics experiment?

Effective analysis and interpretation of data from a physics experiment involve organizing data into tables or graphs for clarity, identifying patterns or trends, and using statistical methods to evaluate the data. Compare the results with theoretical predictions and consider any discrepancies. Finally, draw conclusions based on the data analysis and reflect on the experiment's reliability and any potential improvements.

Why is "Stating control variables vaguely — e.g. 'keep the conditions the same' — or naming a variable without saying HOW it is kept constant" a common mistake in Practical Skills - Planning?

Why it happens: Students know a fair test needs controls but don't realise the mark is for the METHOD of control, not just naming the variable. How to avoid it: NAME each control variable and STATE how it is kept constant, e.g. 'temperature kept constant by using a small current and switching off between readings' or 'same reel of wire so the diameter is unchanged'. Source: Edexcel IAL Physics Unit 3 (WPH13) examiner reports

Why is "Writing 'be careful' or 'the experiment is dangerous' instead of a specific hazard and a matching precaution" a common mistake in Practical Skills - Planning?

Why it happens: Students fall back on a generic safety phrase when they haven't identified the actual hazard in this experiment. How to avoid it: Name a SPECIFIC hazard and a MATCHING precaution, e.g. 'hot water → wear goggles and use a heatproof mat', or 'laser → never look into the beam and use a low-power class-2 laser'. Source: Edexcel IAL Physics Unit 3 (WPH13) examiner reports

Why is "Referring to apparatus vaguely ('a ruler', 'a timer') instead of naming the specific instrument and its resolution" a common mistake in Practical Skills - Planning?

Why it happens: Students describe the measurement without thinking about which instrument best suits the size of the quantity. How to avoid it: Name the instrument and its resolution, and match it to the quantity: metre rule (1 mm) for lengths, Vernier caliper (0.1 mm) for small lengths, micrometer (0.01 mm) for thin wires, light gates + datalogger for short times. Source: Edexcel IAL Physics Unit 3 (WPH13) examiner reports

Why is "Planning too few readings, or over too narrow a range, and not repeating them" a common mistake in Practical Skills - Planning?

Why it happens: Students focus on the method and overlook that a reliable graph needs enough well-spread points. How to avoid it: Take AT LEAST SIX values of the independent variable over as wide a range as practicable, and repeat each reading and take a mean to spot anomalies and reduce random error. Source: Edexcel IAL Physics Unit 3 (WPH13) examiner reports

Why is "Choosing suitable apparatus but not JUSTIFYING the choice (e.g. not saying why a micrometer, not a ruler)" a common mistake in Practical Skills - Planning?

Why it happens: Students assume naming the right instrument is enough and forget the justification mark. How to avoid it: Justify by resolution and percentage uncertainty: 'a micrometer (0.01 mm) gives a small percentage uncertainty on a 0.4 mm wire, whereas a metre rule (1 mm) would be far too coarse'. Source: Edexcel IAL Physics Unit 3 (WPH13) examiner reports

Why is "Not linking what to plot back to the equation — or forgetting how the gradient/intercept gives the wanted quantity" a common mistake in Practical Skills - Planning?

Why it happens: Students plot 'x against y' without rearranging the governing equation into the form y=mx+c. How to avoid it: Rearrange the equation into y=mx+c, choose axes so the GRADIENT (or INTERCEPT) equals the quantity you want, and state the link, e.g. 'gradient =/A, so =gradient× A' or 'intercept = emf'. Source: Edexcel IAL Physics Unit 3 (WPH13) examiner reports

Edexcel International A Levels Physics (XPH11-YPH11)

Practical Skills - Planning

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Detailed Study Notes

Detailed notes on Practical Skills in Physics I for Edexcel International A Levels Physics, covering key concepts, explanations, examples, and exam-focused revision points.

Practical Skills — Planning (Unit 3, WPH13) — Pearson Edexcel International A Level Physics (XPH11/YPH11) Study Notes

How to plan and design a physics experiment for the Unit 3 written practical paper: choosing and controlling variables (and saying HOW each control is kept constant), selecting and justifying apparatus by resolution, describing a precise procedure over a wide range with repeats, naming a specific hazard and matching precaution, and picking what to plot so a straight-line gradient or intercept gives the required quantity — anchored in the Unit 1–2 Core Practicals and transferable to novel contexts.

At a glance

A planning question is a design task, not recall. Nail six things every time: variables, apparatus (with resolution), procedure, range + repeats, safety, and what to plot.
Variables: the independent variable is what you change, the dependent variable is what you measure, and every control variable must be NAMED and you must say HOW it is kept constant. 'Keep conditions the same' scores 0.
Apparatus must be justified by resolution: metre rule 1 mm, Vernier caliper 0.1 mm, micrometer screw gauge 0.01 mm, light gates/datalogger to remove reaction time. 'A ruler' scores less than 'a metre rule, 1 mm resolution'.
Readings: take at least six values of the independent variable over a wide range; repeat each and take a mean to spot anomalies and reduce random error. Repeats improve reliability, not accuracy.
Safety must be specific: name the hazard and a matching precaution (e.g. 'hot water → goggles and heatproof mat'). 'Be careful' earns nothing.
What to plot: rearrange the physics equation into $y=mx+c$ , choose axes so the gradient (or intercept) is the quantity you want — e.g. $s$ vs $t^2$ gives $g=2\times$ gradient; $V$ vs $I$ gives emf from the intercept.
Anchor examples: the eight Unit 1–2 Core Practicals — CP1 free-fall $g$ , CP2 viscosity, CP3 Young modulus, CP4 speed of sound, CP5 vibrating string, CP6 diffraction grating, CP7 resistivity, CP8 emf & internal resistance — plus novel setups.
Accuracy = closeness to the true value (fix by calibration); precision = closeness of repeats (set by resolution); reliability = consistency on repeating (fix by repeats + mean). Keep the three separate.

What you’ll learn

Mapped to the Pearson Edexcel International A Levels XPH11-YPH11 syllabus (2018-onwards).

Identify and distinguish the independent, dependent and control variables in an experiment, and state how each control variable is kept constant.
Select suitable apparatus and justify the choice by its resolution and the percentage uncertainty it gives.
Describe a precise measurement procedure, including how many readings to take, over what range, and how repeats are used.
State a specific hazard and a matching precaution for a given practical (risk assessment).
Choose what to plot to obtain a straight line by rearranging the governing equation into $y=mx+c$ , and explain how the gradient or intercept gives the required quantity.
Apply these planning skills to the Unit 1–2 Core Practicals (CP1–CP8) and to unfamiliar experimental contexts.

What a planning question is really asking

A design task with a fixed checklist — hit every item to score full marks.

Unit 3 (WPH13) is a written practical-skills paper: you are not doing the experiment, you are planning, describing and evaluating one on paper. A planning question ("Plan an investigation to determine…", "Describe how you would measure…") is a design task, and the mark scheme almost always rewards the same six ingredients. Treat them as a checklist and you will rarely drop marks:

Variables — state the independent variable (what you change), the dependent variable (what you measure), and the control variables (what you keep constant, and how).
Apparatus — name each instrument and justify it by its resolution and the percentage uncertainty it gives.
Procedure — describe, step by step, exactly how each reading is taken.
Range and repeats — at least six values over a wide range, each repeated with a mean taken.
Safety — a specific hazard with a matching precaution.
What to plot — rearrange the physics equation to $y=mx+c$ so the gradient (or intercept) gives the wanted quantity.

Every planning question rewards the same six ingredients. Walk through the checklist and you will cover the mark scheme even for an unfamiliar experiment.

The huge advantage of this approach is that it is transferable. Whether the question is a familiar Core Practical (measuring $g$ , or the resistivity of a wire) or a novel setup you have never seen, the same six-step skeleton earns the marks — because the marks are for the skills, not for recalling a specific method.

Planning questions are design tasks marked against a fixed checklist.
Six ingredients: variables, apparatus, procedure, range + repeats, safety, what to plot.
The checklist is transferable — it works for novel experiments too.

See the full worked example for practical skills - planning →

Variables — and how to control them

Name every control variable AND state how it is kept constant; 'keep conditions the same' scores nothing.

Getting the variables right is the foundation of the plan.

The independent variable (IV) is the quantity you deliberately change (e.g. the length of a wire, the mass on a spring). It goes on the x-axis.
The dependent variable (DV) is the quantity you measure in response (e.g. resistance, extension, time). It goes on the y-axis.
A control variable is any other quantity that could affect the DV, which you must keep constant so the test is fair.

The single most common way to lose marks here is with control variables. Naming a control variable is not enough — you must state HOW it is kept constant. A control mark is only awarded for the method of control.

Control variable	Vague (0 marks)	Creditworthy (states HOW)
Temperature	"keep the temperature the same"	"kept constant using a thermostatically controlled water bath at 30 °C"
Wire cross-section	"use the same wire" (borderline)	"use the same reel of wire and check the diameter with a micrometer"
p.d. / current heating	"don't let it get hot"	"use a small current and switch off between readings so the wire does not warm up"
pH (in a mixed context)	"keep pH constant"	"kept constant using a buffer solution"

Classify every quantity into one of three roles. The examiner discriminator is the control column: name each control variable AND state the method that keeps it constant.

Why does controlling variables matter? Because it makes the experiment valid: if only the IV changes, any change in the DV can be attributed to the IV alone. If a control variable drifts, you cannot tell whether the DV changed because of the IV or because of the uncontrolled factor.

IV = changed (x-axis); DV = measured (y-axis); controls = kept constant.
Every control variable must be NAMED and the method of control stated.
'Keep conditions the same' or a bare variable name scores 0.
Controlling variables makes the experiment a fair, valid test.

See the full worked example for practical skills - planning →

Choosing and justifying apparatus (resolution matters)

Match the instrument to the size of the quantity and justify it by its resolution and percentage uncertainty.

Choosing apparatus is not just naming an instrument — you must justify the choice. The justification is almost always about resolution (the smallest division the instrument can read) and the percentage uncertainty it produces. A smaller quantity needs a finer instrument.

Instrument	Typical resolution	Best used for
Metre rule	1 mm	lengths of tens of cm to ~1 m
Vernier caliper	0.1 mm (digital 0.02 mm)	small lengths, diameters of a few mm to cm
Micrometer screw gauge	0.01 mm	very thin wires (~0.1–1 mm), thin sheets
Digital balance	0.01 g (or finer)	small masses
Stopwatch (hand-timed)	0.01 s display, but ~0.2 s reaction-time limited	long time intervals (time many oscillations)
Light gates + datalogger	~1 ms	short times — removes reaction-time error
Thermometer	0.5–1 °C	temperature of a water bath

The rule of thumb: choose the instrument whose resolution keeps the percentage uncertainty small. For a wire of diameter 0.40 mm, a micrometer (0.01 mm) gives $\tfrac{0.01}{0.40}\times100\approx2.5\%$ , whereas a metre rule (1 mm) would give $\tfrac{1}{0.40}\times100=250\%$ — useless. So "a micrometer, because its 0.01 mm resolution gives a low percentage uncertainty on a 0.4 mm wire" scores; "a ruler" does not.

A resistivity setup names each instrument with its resolution: a metre rule (1 mm) for length, an ammeter and voltmeter for R = V/I, and a micrometer (0.01 mm) for the thin wire's diameter. Justifying each choice by resolution is where the marks are.

For time, remember that a hand-held stopwatch is limited by reaction time (~0.2 s), not by its display. To measure a short interval, either time many repeats (e.g. 20 oscillations of a pendulum, then divide by 20) or use light gates and a datalogger, which start and stop automatically and remove reaction-time error entirely.

Name the instrument AND justify it by resolution / percentage uncertainty.
Micrometer (0.01 mm) for thin wires; Vernier (0.1 mm) for small lengths; metre rule (1 mm) for ~cm–m.
Hand timing is limited by reaction time (~0.2 s) — time many oscillations or use light gates.
'A ruler' or 'a timer' without resolution loses the justification mark.

See the full worked example for practical skills - planning →

Procedure, range, repeats and safety

Six well-spread readings, each repeated and averaged; a specific hazard with a matching precaution.

Describe the procedure precisely. Say exactly how each reading is taken — where you start and stop, how you read the scale (e.g. at eye level to avoid parallax, or using a set square), and how you obtain each value of the DV. A reader should be able to follow your plan without seeing you do it.

Range and number of readings. Take at least six values of the independent variable, spread over as wide a range as practicable and reasonably evenly spaced. Enough well-spread points let a reliable best-fit line be drawn and a trustworthy gradient found; too few points, or a narrow range, makes the line uncertain and hides anomalies.

Repeats. Repeat each reading (typically three times) and take a mean. Repeats do two jobs:

they let you identify anomalies — a repeat that clearly doesn't fit the others is ignored;
they reduce the effect of random error, because averaging cancels scatter that lies on both sides of the true value.

Be careful with the language: repeats improve reliability, not accuracy. Accuracy (closeness to the true value) is improved by calibration and by removing systematic error, not by repeating.

Safety — be specific. A risk-assessment mark is only awarded for a named hazard with a matching precaution. "Be careful" or a generic "wear goggles" unconnected to any hazard scores nothing.

Context	Specific hazard	Matching precaution
Heating water (thermistor / CP)	hot water → scalds/burns	goggles, tongs, heatproof mat
Electrical (CP7, CP8)	wires/rheostat get hot with high current	small current, switch off between readings
Laser (CP6 diffraction)	laser light damages the eye	never look into the beam; use a low-power class-2 laser
Falling masses (CP1, springs)	mass falls onto foot	keep feet clear; place a tray/cushion below
Glycerol (CP2 viscosity)	slippery spills / irritant	wipe spills at once; wear goggles

Safety marks need a specific hazard AND a precaution that directly addresses it. 'Be careful' is never creditworthy; 'hot water can scald → use tongs and a heatproof mat' is.

Describe exactly how each reading is taken (start/stop points, avoid parallax).
Take ≥6 values over a wide range; repeat each and take a mean.
Repeats identify anomalies and reduce random error — they improve reliability, not accuracy.
Safety = a named hazard + a matching precaution; 'be careful' scores 0.

See the full worked example for practical skills - planning →

Choosing what to plot — gradient and intercept

Rearrange the equation to y = mx + c so the gradient (or intercept) is the quantity you want.

The finale of most plans is: what should you plot, and how does the graph give the answer? The master technique is to rearrange the governing physics equation into the straight-line form $y=mx+c$ , then choose the axes so the gradient (or the intercept) equals the quantity you want.

The recipe

Write the physics equation linking your measured quantities.
Rearrange it so two directly measured quantities are $y$ and $x$ , and the constants sit in the gradient (and/or intercept).
Read off what the gradient and intercept represent.
State how to extract the wanted quantity from the gradient/intercept.

Worked links from the Core Practicals:

Practical	Equation	Plot	Gives
CP1 free-fall $g$	$s=\tfrac12 g t^2$	$s$ vs $t^2$	gradient $=\tfrac12 g$ → $g=2\times$ gradient
CP3 Young modulus	$F=\dfrac{EA}{L}x$	$F$ vs $x$	gradient $=\dfrac{EA}{L}$ → $E=\text{gradient}\times\dfrac{L}{A}$
CP7 resistivity	$R=\dfrac{\rho}{A}L$	$R$ vs $L$	gradient $=\dfrac{\rho}{A}$ → $\rho=\text{gradient}\times A$
CP8 emf & internal resistance	$V=\varepsilon-Ir$	$V$ vs $I$	intercept $=\varepsilon$ ; gradient $=-r$

Choose axes so the physics quantity falls out of the graph. Sometimes it is the gradient (left: s vs t² gives g from twice the gradient); sometimes it is the intercept (right: V vs I gives the emf from the y-intercept and the internal resistance from the slope).

Two habits win marks: (1) always state the units of a gradient when you read one from a graph, and (2) use a large gradient triangle (at least half the line) so the reading uncertainty is small. And remember the intercept: not every experiment reads its answer off the gradient — in CP8 the emf is the y-intercept, a classic point that catches students who only ever look at the slope.

Rearrange the equation to $y=mx+c$ , then pick axes so the gradient/intercept is the wanted quantity.
$s$ vs $t^2$ → $g=2\times$ gradient; $R$ vs $L$ → $\rho=\text{gradient}\times A$ .
CP8: plot $V$ vs $I$ — emf is the INTERCEPT, internal resistance is $-$ gradient.
State the units of any gradient and use a large triangle to read it.

See the full worked example for practical skills - planning →

Quick recap

A planning question is a design task marked against a checklist: variables, apparatus, procedure, range + repeats, safety, what to plot.
State the IV (changed, x-axis), the DV (measured, y-axis), and every control variable NAMED with the method that keeps it constant.
Name apparatus AND justify it by resolution and percentage uncertainty (micrometer 0.01 mm, Vernier 0.1 mm, metre rule 1 mm, light gates for short times).
Take ≥6 readings over a wide range; repeat each and take a mean to identify anomalies and reduce random error (reliability, not accuracy).
Give a specific hazard with a matching precaution; 'be careful' scores nothing.
Rearrange the equation to $y=mx+c$ so the gradient (or intercept) is the required quantity: $s$ vs $t^2$ for $g$ ; $R$ vs $L$ for $\rho$ ; $V$ vs $I$ (intercept) for emf.
These skills are transferable — the same checklist works for CP1–CP8 and for novel, unfamiliar experiments.

Memorise this

Verbatim phrases and definitions Edexcel mark schemes credit.

IV = changed (x-axis); DV = measured (y-axis); control = kept constant — NAME each control AND state HOW it is kept constant.
Justify apparatus by resolution: metre rule 1 mm, Vernier 0.1 mm, micrometer 0.01 mm, light gates/datalogger to remove reaction time.
Take ≥6 readings over a wide range; repeat and average → identifies anomalies, reduces random error (improves reliability).
Safety = a NAMED hazard + a MATCHING precaution (e.g. hot water → goggles + heatproof mat). 'Be careful' = 0 marks.
What to plot: rearrange to $y=mx+c$ ; gradient (or intercept) = the wanted quantity.
CP1: plot $s$ vs $t^2$ , $g=2\times$ gradient. CP7: plot $R$ vs $L$ , $\rho=\text{gradient}\times A$ . CP8: plot $V$ vs $I$ , emf = intercept, $r=-$ gradient.
Accuracy = closeness to true value (calibrate); precision = closeness of repeats (resolution); reliability = consistency on repeating (repeats + mean).
State the units of any gradient and use a large gradient triangle to read it.

How it’s examined

Planning is assessed on the Unit 3 written practical paper (WPH13, Practical Skills in Physics I). Expect: short 'state the independent/dependent/control variable' items (1–3 marks, where control variables must be named WITH the method of control); 'suggest and justify suitable apparatus' items (2–3 marks, justified by resolution/percentage uncertainty); a 'what should you plot to obtain a straight line, and how does the graph give X?' item (2–4 marks, linking the plot to $y=mx+c$ and the gradient/intercept); a risk-assessment mark for a specific hazard + matching precaution; and an extended 'plan an investigation to determine…' question (4–8 marks) marked against the full checklist. Questions are set both in the familiar Core Practical contexts (CP1–CP8) and in NOVEL contexts to test transferable skills. Marks are lost mainly through vague controls ('keep conditions the same'), un-named or unjustified apparatus, too few readings, generic safety ('be careful'), and not linking the plotted graph to the equation.

Sources: Pearson Edexcel International Advanced Level Physics (YPH11/XPH11) specification (Issue 3, July 2021); Edexcel IAL Physics Unit 3 (WPH13) examiner reports; Edexcel IAL Physics Unit 3 sample assessment materials and mark schemes. Last reviewed 2026-07-05.

Take this whole topic with you

Step-by-step worked examples — Practical Skills - Planning

Step-by-step solutions to past-paper-style questions on practical skills - planning, written exactly the way a tutor would explain them at the board.

Question type:

1Classifying variables in a resistivity plan (3 marks)
Getting startedIdentify & classify• Core Practical 7 style• variables, Core Practical 7, resistivity, Unit 3
Question
A student plans to investigate how the resistance of a length of constantan wire depends on its length. State the independent variable, the dependent variable, and one control variable — naming how that control variable is kept constant. (3 marks)
Step-by-step solution
1. Step 1
  The independent variable is the quantity the student deliberately changes: the length $L$ of wire in the circuit (e.g. 0.20, 0.40, 0.60, 0.80, 1.00 m, set with a metre rule).
2. Step 2
  The dependent variable is what is measured in response: the resistance $R$ of that length (found from the voltmeter and ammeter readings, $R=V/I$ ).
3. Step 3
  A control variable must be NAMED with the method of control. Best answer: the cross-sectional area / diameter of the wire — keep constant by using the same reel of wire throughout (and measure the diameter with a micrometer to confirm). Also acceptable: the temperature of the wire — keep constant by using a low current / switching off between readings so the wire does not warm up.
Answer
IV = length of wire $L$ ; DV = resistance $R$ ( $=V/I$ ); control variable = wire diameter/area (kept constant by using the same reel of wire), and/or temperature (kept low by using a small current and switching off between readings).
Examiner tip
Marks: (1) IV = length; (2) DV = resistance (or the $V$ and $I$ used to find it); (3) a NAMED control with HOW it is controlled. A control stated without the method ('keep the wire the same') scores nothing — 'same reel of wire' or 'small current to avoid heating' is what earns the mark.
2Choosing apparatus to measure a wire's diameter (2 marks)
Getting startedIdentify & classify• Core Practical 7 style• apparatus, resolution, micrometer, Unit 3
Question
In a resistivity experiment a thin metal wire of diameter about 0.4 mm must be measured. State the most suitable instrument to measure the diameter and justify your choice by referring to its resolution. (2 marks)
Step-by-step solution
1. Step 1
  The wire is very thin (~0.4 mm), so a metre rule (1 mm) or Vernier caliper (0.1 mm / 0.02 mm) would give a large percentage uncertainty. Choose a micrometer screw gauge.
2. Step 2
  Justify by resolution: a micrometer reads to 0.01 mm, so on a 0.40 mm diameter the percentage uncertainty is only about $\tfrac{0.01}{0.40}\times100 \approx 2.5\%$ — far smaller than a metre rule ( $\tfrac{1}{0.4}\times100 = 250\%$ ).
Answer
Use a micrometer screw gauge (resolution 0.01 mm), because its fine resolution gives a small percentage uncertainty on a diameter of only ~0.4 mm — a metre rule (1 mm) would be far too coarse.
Examiner tip
Marks: (1) micrometer screw gauge named; (2) justified by its 0.01 mm resolution giving a small percentage uncertainty. 'A ruler' or an un-quantified 'it is more accurate' does not gain the justification mark — you must cite the resolution and link it to low percentage uncertainty.
3Hazard and precaution in an electrical experiment (2 marks)
Getting startedIdentify & classify• Core Practical 8 style• safety, hazard, Core Practical 8, Unit 3
Question
A student measures the emf and internal resistance of a battery by varying the current drawn through a rheostat (Core Practical 8). State one specific hazard and a matching precaution. (2 marks)
Step-by-step solution
1. Step 1
  Name a specific hazard: drawing a large current through the connecting wires / rheostat makes them become hot (they can burn skin or damage insulation).
2. Step 2
  Give a matching precaution: keep the current small and take readings quickly, switching the circuit off between readings so the components do not overheat (and do not touch the resistance wire while current flows).
Answer
Hazard: the wires/rheostat get hot when a large current flows (risk of burns). Precaution: use a small current and switch off between readings so nothing overheats.
Examiner tip
Marks: (1) a SPECIFIC named hazard (components getting hot, not 'electricity is dangerous'); (2) a MATCHING precaution. 'Be careful' or a generic 'wear goggles' unconnected to the hazard scores nothing — the hazard and precaution must correspond.
4Choosing the range and number of readings for measuring g (3 marks)
Building confidenceMulti-step problem• Core Practical 1 style• range, readings, Core Practical 1, Unit 3
Question
A student will measure the acceleration of free fall $g$ by dropping a steel ball from rest through various heights $s$ and timing the fall $t$ with light gates. State (a) how many readings should be taken and over what range of heights, and (b) whether any readings should be repeated, giving a reason. (3 marks)
Step-by-step solution
1. Step 1
  (a) Take at least six different fall heights spanning a wide range (e.g. from 0.20 m up to 1.50 m, roughly evenly spaced). A wide range with several points lets a straight best-fit line be drawn and a reliable gradient found.
2. Step 2
  (b) Repeat the timing at each height (e.g. three times) and take a mean time.
3. Step 3
  The reason: repeats let anomalies be identified and reduce the effect of random error in the timing, giving a more reliable value for $g$ .
Answer
(a) At least six heights spanning a wide range (~0.20–1.50 m). (b) Repeat the fall at each height (e.g. ×3) and average, to spot anomalies and reduce random error in the timing.
Examiner tip
Marks: (1) ≥6 readings over a wide/stated range; (2) repeat at each height and take a mean; (3) reason = identify anomalies / reduce random error. 'A few readings' or repeating 'to improve accuracy' are not creditworthy — repeats improve reliability, and a specific number/range is expected.
5Justifying a timing method for a pendulum plan (3 marks)
Building confidenceMulti-step problem• Edexcel IAL Physics Unit 3 style• apparatus, random error, timing, Unit 3
Question
A student plans to find the period $T$ of a simple pendulum for several lengths. The period is only about 1.4 s. Describe how the student should measure the period accurately, and justify each feature of the method. (3 marks)
Step-by-step solution
1. Step 1
  Time many oscillations, not one. Measure the time for, say, 20 complete oscillations with a stopwatch and divide by 20 to get $T$ . Timing one swing would give a huge percentage uncertainty because reaction time (~0.2 s) is a large fraction of 1.4 s.
  $T = \frac{t_{20}}{20}$
2. Step 2
  Use a fiducial marker at the centre of the swing (where the bob moves fastest) and start/stop timing as the bob passes it, so the start and end points are well defined and reaction-time error is reduced.
3. Step 3
  Repeat the timing of 20 oscillations and take a mean, to reduce random error further. (Light gates and a datalogger would remove reaction time altogether if available.)
Answer
Time ~20 oscillations and divide by 20 (reduces the effect of reaction time), use a fiducial marker at the centre of the swing (well-defined start/stop), and repeat and average. Light gates/datalogger would remove reaction time entirely.
Examiner tip
Marks: (1) time multiple oscillations ÷ number, justified by reducing percentage uncertainty from reaction time; (2) fiducial marker at the centre for a well-defined timing point; (3) repeat and mean / use light gates. Each feature must be JUSTIFIED — stating the method without the reason limits the marks.
6Choosing what to plot for the Young modulus (4 marks)
Building confidenceGraph or diagram• Core Practical 3 style• graph, gradient, Young modulus, Core Practical 3, Unit 3
Question
In Core Practical 3 a student loads a wire of natural length $L$ and cross-sectional area $A$ with force $F$ and measures the extension $x$ . Starting from $E = \dfrac{FL}{Ax}$ , state what the student should plot against what to obtain a straight line, and explain how the graph gives the Young modulus $E$ . (4 marks)
Step-by-step solution
1. Step 1
  Rearrange $E=\dfrac{FL}{Ax}$ to make it linear in the two directly measured quantities $F$ and $x$ . Solving for $F$ gives:
  $F = \left(\frac{EA}{L}\right)x$
2. Step 2
  Compare with $y=mx$ (straight line through the origin): plotting $F$ (y-axis) against $x$ (x-axis) gives a straight line whose gradient is $\dfrac{EA}{L}$ .
  $\text{gradient} = \frac{EA}{L}$
3. Step 3
  Measure $L$ (metre rule) and $A$ (from the diameter with a micrometer, $A=\tfrac{\pi d^2}{4}$ ) separately.
  $A = \frac{\pi d^2}{4}$
4. Step 4
  Rearrange the gradient for $E$ : multiply the gradient by $L$ and divide by $A$ .
  $E = \text{gradient} \times \frac{L}{A}$
Answer
Plot force $F$ (y) against extension $x$ (x): a straight line through the origin of gradient $\tfrac{EA}{L}$ . Measure $L$ and $A$ separately, then $E = \text{gradient}\times \dfrac{L}{A}$ .
Examiner tip
Marks: (1) rearrange to $F=(EA/L)x$ ; (2) plot $F$ against $x$ ; (3) gradient = $EA/L$ ; (4) $E=\text{gradient}\times L/A$ . The key skill is linking the plotted axes back to the equation so the gradient yields the wanted quantity. Plotting stress against strain (gradient $=E$ directly) is equally acceptable.
7Planning a viscosity experiment (Stokes' law) (6 marks)
StretchMulti-step problem• Core Practical 2 style• planning, viscosity, Core Practical 2, AO3, Unit 3
Question
Plan an experiment to determine the terminal velocity of a small steel ball bearing falling through a tall tube of glycerol. Identify the variables, name and justify the apparatus, describe how the measurement is taken, and refer to one safety point. (6 marks)
Step-by-step solution
1. Step 1
  Variables. IV = the position/section of the tube over which the ball falls (used to check the velocity is constant); DV = the time to fall between two marks, from which velocity is found; control variables = temperature of the glycerol (kept constant, because viscosity depends strongly on temperature) and the ball diameter/mass (use identical ball bearings, checked with a micrometer).
2. Step 2
  Apparatus, justified. Fix two rubber bands (markers) on the tube a measured distance apart, low down where the ball has reached terminal velocity. Measure that distance with a metre rule (1 mm resolution) and time the fall with a stopwatch (or, better, light gates + datalogger to remove reaction-time error). Measure the ball diameter with a micrometer (0.01 mm).
3. Step 3
  Procedure. Release the ball centrally from rest at the surface; start timing as it passes the upper marker and stop at the lower marker. Terminal velocity is $v=\dfrac{d}{t}$ where $d$ is the marker separation.
  $v = \frac{d}{t}$
4. Step 4
  Check constant velocity and repeat. Use markers at several positions to confirm the time per interval is constant (so the ball really is at terminal velocity), and repeat each drop at least three times and take a mean to reduce random error and spot anomalies.
5. Step 5
  Safety. Glycerol is slippery/an irritant: wear eye protection and wipe up spills immediately to prevent slips; retrieve balls with a magnet rather than pouring the liquid out.
Answer
IV/DV: measure the time to fall a known distance between two markers low in the tube to find terminal velocity $v=d/t$ ; controls = temperature (constant, viscosity is temperature-dependent) and identical ball bearings. Apparatus: markers + metre rule (1 mm) for distance, stopwatch/light gates for time, micrometer (0.01 mm) for diameter. Check velocity is constant using several markers, repeat and average. Safety: glycerol is slippery/irritant → goggles and wipe spills.
Examiner tip
Marks (max 6): (1) DV = time to fall a set distance → velocity; (2) control temperature (justified: viscosity depends on temperature); (3) markers set where terminal velocity is reached; (4) apparatus with resolutions justified (metre rule 1 mm, micrometer 0.01 mm, light gates to remove reaction time); (5) repeats + mean; (6) specific hazard + matching precaution. Marks are lost for un-named apparatus, vague controls, and 'be careful' safety.
8Planning a novel experiment: g from a mass-on-spring (6 marks)
StretchMulti-step problem• Edexcel IAL Physics Unit 3 (unfamiliar context)• planning, novel context, graph, AO3, Unit 3
Question
A student is given springs, masses and standard laboratory apparatus and told that the extension $x$ of a spring obeying Hooke's law when a mass $m$ hangs on it is $x=\dfrac{mg}{k}$ , where $k$ is the spring constant. Plan how the student could use this arrangement to obtain a value for $g$ . Include the variables, apparatus, what to plot, and how the graph gives $g$ . (6 marks)
Step-by-step solution
1. Step 1
  Variables. IV = the mass $m$ hung on the spring (varied over ≥6 values, e.g. 100 g to 600 g in 100 g steps, using a set of standard masses). DV = the extension $x$ of the spring. Control variable = the same spring throughout, so $k$ is constant.
2. Step 2
  Apparatus, justified. Hang the spring from a clamp/stand; place a metre rule (1 mm resolution) vertically beside it and read the pointer position; use a set square to read the scale at right angles (avoids parallax). Extension $x=$ (loaded position) − (unloaded position).
3. Step 3
  What to plot. The equation is linear in $m$ and $x$ . Comparing $x=\left(\dfrac{g}{k}\right)m$ with $y=mx$ , plot $x$ (y-axis) against $m$ (x-axis) for a straight line through the origin.
  $x = \left(\frac{g}{k}\right)m \;\Rightarrow\; \text{gradient} = \frac{g}{k}$
4. Step 4
  How the graph gives $g$ . The gradient is $\dfrac{g}{k}$ , so $g = k \times \text{gradient}$ . Find $k$ independently by a static load test or by measuring the oscillation period $T=2\pi\sqrt{m/k}$ — then $g$ follows.
  $g = k \times \text{gradient}$
5. Step 5
  Reliability. Take ≥6 masses over a wide range, repeat each extension reading and average, and draw a best-fit line so a single anomaly does not distort the gradient. Keep loads below the elastic limit so Hooke's law still holds.
Answer
IV = hanging mass $m$ (≥6 values), DV = extension $x$ , control = same spring (constant $k$ ). Apparatus: clamp, metre rule (1 mm) read with a set square to avoid parallax. Plot $x$ against $m$ : straight line through the origin of gradient $g/k$ ; find $k$ separately, so $g = k \times \text{gradient}$ . Repeat, average, best-fit line, stay below the elastic limit.
Examiner tip
Marks (max 6): (1) IV = mass, DV = extension, control = same spring; (2) metre rule with resolution + parallax precaution; (3) plot $x$ vs $m$ for a straight line; (4) gradient = $g/k$ so $g=k\times$ gradient (with $k$ found independently); (5) ≥6 readings over a range + repeats/mean; (6) best-fit line / stay within the elastic limit. Transferable planning: the mark scheme rewards linking the given equation to $y=mx$ even in an unfamiliar setup.
9Full plan: determining the resistivity of a wire (6 marks)
StretchMulti-step problem• Core Practical 7 style• planning, resistivity, Core Practical 7, AO3, Unit 3
Question
Plan an experiment to determine the resistivity $\rho$ of the metal of a wire. State the variables and how controls are kept constant, name and justify the apparatus with resolutions, state what to plot to get a straight line, and explain how the graph and further measurements give $\rho$ . (6 marks)
Step-by-step solution
1. Step 1
  Variables. IV = length $L$ of wire in the circuit (≥6 values from ~0.20 m to ~1.00 m, set on a metre rule). DV = resistance $R=V/I$ from voltmeter and ammeter readings. Controls: cross-sectional area (same wire throughout) and temperature (small current, switch off between readings so the wire does not warm and change $R$ ).
2. Step 2
  Apparatus, justified. A metre rule (1 mm) for length; a micrometer screw gauge (0.01 mm) for the diameter $d$ (thin wire, so fine resolution keeps the percentage uncertainty low); a voltmeter and ammeter (or ohmmeter) for $R$ ; crocodile clips to select the length.
3. Step 3
  What to plot. From $R=\dfrac{\rho L}{A}$ , plotting $R$ (y) against $L$ (x) gives a straight line through the origin of gradient $\dfrac{\rho}{A}$ .
  $R = \frac{\rho}{A}\,L \;\Rightarrow\; \text{gradient} = \frac{\rho}{A}$
4. Step 4
  Find the area. Measure the diameter $d$ at several points and take a mean, then $A=\tfrac{\pi d^2}{4}$ .
  $A = \frac{\pi d^2}{4}$
5. Step 5
  Get $\rho$ . Multiply the gradient by $A$ : $\rho = \text{gradient} \times A$ . Repeat readings, take means and use a best-fit line for reliability.
  $\rho = \text{gradient} \times A$
Answer
IV = length $L$ (≥6 values), DV = resistance $R=V/I$ ; controls = same wire (constant area) and low current (constant temperature). Apparatus: metre rule (1 mm), micrometer (0.01 mm) for diameter, voltmeter + ammeter. Plot $R$ against $L$ : gradient $=\rho/A$ ; measure $A=\tfrac{\pi d^2}{4}$ ; then $\rho = \text{gradient}\times A$ . Repeat, average, best-fit line.
Examiner tip
Marks (max 6): (1) IV = length, DV = resistance; (2) controls named with method (same wire; small current for constant temperature); (3) micrometer for diameter justified by resolution; (4) plot $R$ vs $L$ ; (5) gradient $=\rho/A$ ; (6) $\rho=\text{gradient}\times A$ with $A=\pi d^2/4$ . Not measuring the diameter with a micrometer, or not linking the plot to $R=\rho L/A$ , are the usual mark-losers.

Model Answers — Practical Skills - Planning

High-scoring sample answers for practical skills - planning on the Cambridge IGCSE paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question 1
Edexcel IAL Physics Unit 3 style3 marks
Q (3 marks). Explain what is meant by the independent variable, the dependent variable and a control variable in a physics experiment.
Model answer
The independent variable is the quantity the experimenter deliberately changes during the investigation (for example, the length of a wire or the mass hung on a spring). It is normally plotted on the x-axis.

The dependent variable is the quantity that is measured in response to changing the independent variable (for example, the resistance, the extension, or the time of fall). It is normally plotted on the y-axis.

A control variable is any other quantity that could affect the dependent variable and must therefore be kept constant, so that only the independent variable is responsible for the change observed — this is what makes it a fair test.
Why this scores
Marks: (1) IV = quantity deliberately changed; (2) DV = quantity measured in response; (3) control = kept constant for a fair test. Defining a term using itself ('the variable that is controlled') is not credited.
Question 2
Edexcel IAL Physics Unit 3 style2 marks
Q (2 marks). A student needs to measure the thickness of a metal sheet, about 1.5 mm. State a suitable instrument and justify the choice by referring to its resolution.
Model answer
A Vernier caliper (resolution 0.1 mm, or a digital caliper reading to 0.02 mm) — or a micrometer screw gauge (resolution 0.01 mm) — is suitable.

Its fine resolution means the absolute uncertainty (~0.05 mm or less) is a small fraction of the 1.5 mm thickness, giving a percentage uncertainty of only a few percent. A metre rule (resolution 1 mm) would be far too coarse for a 1.5 mm measurement.
Why this scores
Marks: (1) a named fine-resolution instrument (Vernier caliper or micrometer); (2) justified by its resolution giving a low percentage uncertainty. 'A ruler' or an unquantified 'more accurate' does not gain the second mark — cite the resolution.
Question 3
Edexcel IAL Physics Unit 3 style2 marks
Q (2 marks). A student heats water to investigate how the resistance of a thermistor changes with temperature. State one specific hazard and a matching precaution.
Model answer
Hazard: the hot water (and hot beaker) can cause scalds or burns to the skin.

Precaution: wear eye protection, handle the hot beaker with tongs or a heatproof glove, stand it on a heatproof mat, and keep it away from the edge of the bench so it cannot be knocked over.
Why this scores
Marks: (1) a SPECIFIC hazard (hot water / burns), not 'the experiment is dangerous'; (2) a MATCHING precaution (goggles, tongs, heatproof mat). 'Be careful' scores nothing, and the precaution must correspond to the stated hazard.
Question 4
Core Practical 7 style4 marks
Q (4 marks). A student investigates how the resistance of a length of wire depends on its length. State two control variables and, for each, describe how it should be kept constant. Explain why controlling them matters.
Model answer
Control variable 1 — cross-sectional area (diameter) of the wire. Keep it constant by using the same reel of wire for every length tested (checking the diameter with a micrometer at several points). If the area changed, the resistance would change for a reason other than length, since $R=\dfrac{\rho L}{A}$ .

Control variable 2 — temperature of the wire. Keep it constant by using a small current and switching the circuit off between readings, so the wire does not warm up (a metal's resistance rises with temperature). Warming would make the resistance readings drift upward and spoil the straight-line relationship.

Controlling these variables ensures that any change in resistance is due only to the change in length — that is, it makes the experiment a fair, valid test.
Why this scores
Marks: (1) area/diameter named with method (same reel of wire); (2) temperature named with method (small current, switch off between readings); (3) reason each affects R via $R=\rho L/A$ or the temperature dependence of resistance; (4) controlling them makes it a fair test / valid. A control named without HOW it is kept constant scores nothing.
Question 5
Core Practical 1 style4 marks
Q (4 marks). A student times a steel ball falling from rest through various heights $s$ to measure the acceleration of free fall $g$ . (a) State what the student should plot against what to obtain a straight line. (b) Explain how the graph gives a value for $g$ , and why this graphical method is better than calculating $g$ from a single drop.
Model answer
(a) Because the ball starts from rest, $s=\tfrac{1}{2}g t^2$ . Comparing this with $y=mx$ , the student should plot $s$ (y-axis) against $t^2$ (x-axis), which gives a straight line through the origin.

(b) The gradient of that line equals $\tfrac{1}{2}g$ , so:

$g = 2 \times \text{gradient}$

The graphical method is better than a single drop because using the whole best-fit line averages out random errors in the timing, and any anomalous point stands out clearly and can be ignored. A single drop would carry the full random error of one timing.
Why this scores
Marks: (a) 2 marks — plot $s$ against $t^2$ , straight line through the origin; (b) 2 marks — gradient $=\tfrac12 g$ so $g=2\times$ gradient, and best-fit line averages random error / reveals anomalies. Plotting $s$ against $t$ (a curve) or forgetting the factor of 2 are the classic errors.
Question 6
Edexcel IAL Physics Unit 3 style4 marks
Q (4 marks). In planning an experiment a student is asked to describe how the readings should be taken. Explain (a) how many values of the independent variable should be used and over what range, and (b) why readings should be repeated and how the repeats are used.
Model answer
(a) The student should take at least six values of the independent variable, spread over as wide a range as practicable and reasonably evenly spaced. A wide range with several points lets a reliable straight-line (or curve) trend be drawn and a gradient found; too few points, or a narrow range, makes the line uncertain and any anomaly hard to spot.

(b) Each reading should be repeated (for example three times). The repeats are used to (i) identify anomalies — a repeat that does not fit the others is ignored — and (ii) calculate a mean, which reduces the effect of random error and so improves the reliability of the result.
Why this scores
Marks: (a) 2 marks — ≥6 values over a wide range, so a reliable trend/gradient can be found; (b) 2 marks — repeat and take a mean to identify anomalies and reduce random error. Saying repeats 'improve accuracy' is wrong — repeats improve reliability; accuracy comes from calibration.
Question 7
Core Practical 1 style4 marks
Q (4 marks). A student plans to measure the very short time (about 0.1 s) for a card to pass through a single point. (a) Explain why timing this by hand with a stopwatch would be unsatisfactory. (b) State a better method and explain how it improves the measurement.
Model answer
(a) Timing by hand relies on the student's reaction time, which is about 0.2 s. Because the event lasts only ~0.1 s, the reaction time is larger than the quantity being measured, giving an enormous percentage uncertainty (well over 100%). Reaction time also varies from trial to trial, adding a large random error.

(b) A far better method is to use a light gate connected to a datalogger (or electronic timer). The light gate starts and stops timing automatically as the card interrupts and clears the beam, so it removes the human reaction-time error entirely and measures the short interval to a resolution of about a millisecond. This makes the timing both more accurate and more precise, and the reading can be repeated quickly to take a mean.
Why this scores
Marks: (a) 2 marks — reaction time ~0.2 s is comparable to/larger than the 0.1 s event → huge percentage uncertainty; (b) 2 marks — light gate + datalogger removes reaction-time error and times automatically to high resolution. Naming the apparatus (light gate/datalogger) and the mechanism (removes reaction time) is essential.
Question 8
Core Practical 4 style6 marks
Q (6 marks). Plan an experiment to determine the speed of sound in air using a resonance tube (a tube closed at one end whose length can be varied) and a tuning fork of known frequency. Identify the variables, name and justify the apparatus, describe the procedure, and state what to plot and how the graph gives the speed of sound.
Model answer
Variables. The independent variable is the frequency $f$ of the tuning fork (use a set of forks of different known frequencies, ≥6 values). The dependent variable is the length $L$ of the air column at the first (fundamental) resonance. The control variable is the air temperature, kept constant because the speed of sound varies with temperature (do the experiment in one session in a draught-free room).

Apparatus, justified. A tube of adjustable length (raised/lowered in a water reservoir, or a piston); a metre rule (1 mm resolution) clamped beside the tube to read $L$ ; a set of tuning forks of known frequency. The metre rule's 1 mm resolution keeps the percentage uncertainty in $L$ small.

Procedure. Strike a fork, hold it over the open end and slowly change the column length until the sound is loudest (first resonance). Record $L$ for that fork. Repeat for each frequency; repeat each reading and take a mean $L$ to reduce random error.

What to plot and how it gives the speed. For a closed tube the resonance condition is $L = \dfrac{v}{4f}$ (ignoring the small end correction). Rearranging, plot $L$ (y-axis) against $\dfrac{1}{f}$ (x-axis):

$L = \frac{v}{4}\left(\frac{1}{f}\right) \;\Rightarrow\; \text{gradient} = \frac{v}{4}$

so the speed of sound is $v = 4 \times \text{gradient}$ . Using a best-fit line averages out random error.
Why this scores
Marks (max 6): (1) IV = frequency, DV = resonance length; (2) control temperature (justified); (3) metre rule with resolution / adjustable tube; (4) find resonance = loudest sound + repeat/mean; (5) plot $L$ vs $1/f$ (straight line); (6) gradient $=v/4$ so $v=4\times$ gradient. Linking the plotted axes to the resonance equation is the discriminator; un-named apparatus and vague controls lose marks.
Question 9
Core Practical 6 style6 marks
Q (6 marks). A student is given a laser, a diffraction grating of known line spacing $d$ , and standard laboratory apparatus. Plan how to determine the wavelength $\lambda$ of the laser light. Include the variables measured, the apparatus with a safety point, the procedure, and how the results give $\lambda$ .
Model answer
Quantities measured. For a grating, $n\lambda = d\sin\theta$ , where $\theta$ is the angle of the $n$ -th order maximum. The student measures the angle $\theta$ to several orders $n$ (the independent aspect is the order number $n$ ; the measured/dependent quantity is the position of each bright spot). The grating spacing $d$ is known.

Apparatus + safety. Mount the grating perpendicular to the laser beam on an optical bench; shine the beam onto a screen a measured distance $D$ away (measured with a metre rule, 1 mm). Measure the distance $x_n$ from the central (zero-order) spot to the $n$ -th order spot with a metre rule, then $\tan\theta_n = x_n/D$ . Safety: a laser can damage the eye — never look into the beam or its reflection, use a low-power (class 2) laser, and put up a warning sign.

Procedure. For each visible order $n$ , measure $x_n$ (repeat and average for each), and calculate $\theta_n$ from $\tan\theta_n = x_n/D$ . Using a large $D$ makes the spots well separated and reduces the percentage uncertainty in $x_n$ .

How the results give $\lambda$ . From $n\lambda = d\sin\theta$ , plotting $\sin\theta$ (y-axis) against $n$ (x-axis) gives a straight line through the origin of gradient $\dfrac{\lambda}{d}$ , so:

$\lambda = d \times \text{gradient}.$

Alternatively, calculate $\lambda = \dfrac{d\sin\theta}{n}$ for each order and take a mean.
Why this scores
Marks (max 6): (1) use $n\lambda=d\sin\theta$ ; (2) measure order distances $x_n$ and screen distance $D$ with a metre rule; (3) laser eye hazard + matching precaution (don't look into beam / low power); (4) $\tan\theta=x/D$ procedure with repeats; (5) plot $\sin\theta$ vs $n$ (straight line); (6) gradient $=\lambda/d$ so $\lambda = d\times$ gradient. Omitting the laser safety point or not linking the plot to the grating equation loses marks.
Question 10
Core Practical 8 style6 marks
Q (6 marks). Plan an experiment to determine the emf $\varepsilon$ and internal resistance $r$ of a cell. State the circuit, the variables, the apparatus, and explain fully how a graph of the results gives both $\varepsilon$ and $r$ . Include one precaution to protect the cell.
Model answer
Circuit and variables. Connect the cell to a variable resistor (rheostat) in series with an ammeter, with a voltmeter across the cell's terminals. The independent variable is the current $I$ (changed with the rheostat); the dependent variable is the terminal p.d. $V$ across the cell.

Apparatus. Ammeter and voltmeter (digital, for clear readings), a rheostat to vary the current, and the cell under test. Take at least six pairs of readings of $V$ and $I$ over as wide a range of currents as the rheostat allows.

Precaution for the cell. Keep the current small and take readings quickly, switching off between readings, so the cell does not run down (which would change $\varepsilon$ ) and the connecting wires do not overheat.

How the graph gives $\varepsilon$ and $r$ . The terminal p.d. obeys:

$V = \varepsilon - Ir$

Comparing with $y = c + mx$ (with $y=V$ , $x=I$ ), a plot of $V$ (y-axis) against $I$ (x-axis) is a straight line. The y-intercept equals the emf $\varepsilon$ (the terminal p.d. when $I=0$ ), and the gradient equals $-r$ , so the internal resistance $r = -\text{gradient}$ (i.e. the magnitude of the negative slope).

Repeat readings, take means, and use a best-fit line so both the intercept and the gradient are reliable.
Why this scores
Marks (max 6): (1) circuit with rheostat, ammeter, voltmeter across terminals; (2) IV = current, DV = terminal p.d.; (3) ≥6 readings over a range + precaution (small current / switch off, to protect the cell); (4) use $V=\varepsilon - Ir$ ; (5) plot $V$ vs $I$ → intercept $=\varepsilon$ ; (6) gradient $=-r$ so $r=-$ gradient. This is the key INTERCEPT case — reward recognising that the emf comes from the intercept, not the gradient.

Key Formulae — Practical Skills - Planning

The formulae you need to memorise for practical skills - planning on the Cambridge IGCSE paper, with every variable defined in plain English and a note on when to use it.

Percentage uncertainty of a reading
$\%\,\text{uncertainty} = \frac{\Delta x}{x} \times 100$
$\Delta x$
absolute uncertainty (½ smallest division for analogue; ± smallest division for digital)
$x$
the measured value
When to use
Justifying apparatus choice — a finer-resolution instrument gives a smaller $\Delta x$ and so a smaller percentage uncertainty. Also for judging which measurement dominates the overall uncertainty.
Example
Diameter 0.40 mm with a micrometer (0.01 mm): $\tfrac{0.01}{0.40}\times100 = 2.5\%$ .
What to plot to find g (free fall, CP1)
$s = \tfrac{1}{2}g\,t^2 \;\Rightarrow\; \text{plot } s \text{ vs } t^2,\ \ g = 2\times\text{gradient}$
$s$
fall distance / m (y-axis)
$t^2$
time squared / s² (x-axis)
$g$
acceleration of free fall / m s⁻²
When to use
Planning Core Practical 1: linearise $s=\tfrac12 gt^2$ so a straight-line graph through the origin gives $g$ from twice the gradient.
Example
Gradient $4.90\ \text{m s}^{-2}$ ⇒ $g = 2\times4.90 = 9.80\ \text{m s}^{-2}$ .
What to plot for the Young modulus (CP3)
$F = \frac{EA}{L}\,x \;\Rightarrow\; \text{plot } F \text{ vs } x,\ \ E = \text{gradient}\times\frac{L}{A}$
$F$
applied force / N (y-axis)
$x$
extension / m (x-axis)
$E$
Young modulus / Pa
$A$
cross-sectional area / m²
$L$
natural length / m
When to use
Planning Core Practical 3: a force–extension graph has gradient $EA/L$ ; equivalently a stress–strain graph has gradient $E$ directly.
Example
Gradient $=EA/L$ , so $E = \text{gradient}\times L/A$ after measuring $L$ and $A$ .
What to plot for resistivity (CP7)
$R = \frac{\rho}{A}\,L \;\Rightarrow\; \text{plot } R \text{ vs } L,\ \ \rho = \text{gradient}\times A$
$R$
resistance / Ω (y-axis)
$L$
length of wire / m (x-axis)
$\rho$
resistivity / Ω m
$A$
cross-sectional area / m² ( $=\pi d^2/4$ )
When to use
Planning Core Practical 7: an $R$ – $L$ graph has gradient $\rho/A$ ; find $A$ from the diameter and multiply to get $\rho$ .
Example
Gradient $\rho/A$ , area $A=\pi d^2/4$ , so $\rho = \text{gradient}\times A$ .
What to plot for emf and internal resistance (CP8)
$V = \varepsilon - Ir \;\Rightarrow\; \text{plot } V \text{ vs } I,\ \ \varepsilon = \text{intercept},\ r = -\text{gradient}$
$V$
terminal p.d. / V (y-axis)
$I$
current / A (x-axis)
$\varepsilon$
emf / V (y-intercept)
$r$
internal resistance / Ω ( $=-$ gradient)
When to use
Planning Core Practical 8: a $V$ – $I$ graph gives emf from the intercept and internal resistance from (minus) the gradient — a case where the INTERCEPT, not the gradient, carries the wanted quantity.
Example
Intercept $1.5\ \text{V}$ ⇒ $\varepsilon=1.5\ \text{V}$ ; gradient $-0.80\ \Omega$ ⇒ $r=0.80\ \Omega$ .
Equation of a straight line
$y = mx + c$
$m$
gradient (carries the physics quantity when the axes are chosen well)
$c$
y-intercept (can also carry a wanted quantity)
When to use
The master tool for planning: rearrange the governing equation into the form $y=mx+c$ , then choose the axes so the gradient (or intercept) equals the quantity you want.
Example
$s=\tfrac12 gt^2$ matches $y=mx$ with $y=s$ , $x=t^2$ , gradient $=\tfrac12 g$ .

Key Definitions and Keywords — Practical Skills - Planning

Definitions to memorise and the exact keywords mark schemes credit for practical skills - planning answers — sharpened from recent examiner reports for the 2026 Cambridge IGCSE sitting.

Independent variable
Examiner keyword
The quantity the experimenter deliberately changes in an investigation to test its effect (normally plotted on the x-axis).
Example
Length of wire, mass on a spring, frequency of a tuning fork.
Dependent variable
Examiner keyword
The quantity measured in response to changing the independent variable (normally plotted on the y-axis).
Example
Resistance, extension, time of fall, resonance length.
Control variable
Examiner keyword
Any quantity, other than the independent variable, that could affect the dependent variable and so must be kept constant for a fair test. It must be named AND the method of keeping it constant stated.
Related: validity, fair test
Fair test
Examiner keyword
An investigation in which only the independent variable is changed while all control variables are kept constant, so any change in the dependent variable can be attributed to the independent variable alone.
Resolution
Examiner keyword
The smallest change in a quantity that an instrument can detect or display (its smallest scale division) — e.g. metre rule 1 mm, Vernier caliper 0.1 mm, micrometer 0.01 mm. Finer resolution lowers the percentage uncertainty.
Range and interval
Examiner keyword
The range is the spread of the independent variable from its lowest to highest value; the interval is the gap between successive values. A wide range with at least six values (evenly spaced) lets a reliable trend and gradient be found.
Accuracy
Examiner keyword
How close a measurement (or its mean) is to the true value; improved by calibrating apparatus and removing systematic error, NOT by repeating.
Precision
Examiner keyword
How close repeated readings are to one another (a small spread); largely set by the resolution of the apparatus, and independent of accuracy.
Reliability
Examiner keyword
How consistent the results are when the measurement is repeated; improved by taking more repeats and calculating a mean, which also reveals anomalies.
Validity
Examiner keyword
Whether the experiment actually measures what it is intended to — a fair test in which all relevant control variables are kept constant and a suitable procedure is used.
Random error
Examiner keyword
An error that scatters readings unpredictably on both sides of the true value (e.g. reaction time, small fluctuations); reduced by taking more repeats and a mean.
Systematic error
Examiner keyword
An error that shifts every reading in the same direction by a similar amount (e.g. a zero error on an instrument); not reduced by repeating, but removed by recalibrating/zeroing.
Zero error
Examiner keyword
A systematic error where an instrument does not read zero when it should (e.g. a micrometer reading 0.02 mm with its jaws closed); corrected by subtracting the zero reading from every measurement.
Repeat readings / anomaly
Examiner keyword
Taking a measurement several times and averaging; an anomaly is a repeat that clearly does not fit the pattern of the others and is excluded from the mean (but not deleted from the raw data).
Percentage uncertainty
Examiner keyword
(absolute uncertainty ÷ reading) × 100. Choosing finer-resolution apparatus and measuring larger quantities both reduce it; when quantities are combined by multiplication/division the percentage uncertainties add.
Fiducial marker
Examiner keyword
A fixed reference mark used to define exactly where a timing or reading is taken (e.g. a marker at the centre of a pendulum's swing, where the bob moves fastest), reducing timing error.

Common Mistakes and Misconceptions — Practical Skills - Planning

The traps other students keep falling into on practical skills - planning questions — taken from recent Cambridge IGCSE examiner reports and mark schemes — and how to avoid them.

✕Stating control variables vaguely — e.g. 'keep the conditions the same' — or naming a variable without saying HOW it is kept constant
Edexcel IAL Physics Unit 3 (WPH13) examiner reports
Why it happens
Students know a fair test needs controls but don't realise the mark is for the METHOD of control, not just naming the variable.
How to avoid it
NAME each control variable and STATE how it is kept constant, e.g. 'temperature kept constant by using a small current and switching off between readings' or 'same reel of wire so the diameter is unchanged'.
✕Writing 'be careful' or 'the experiment is dangerous' instead of a specific hazard and a matching precaution
Edexcel IAL Physics Unit 3 (WPH13) examiner reports
Why it happens
Students fall back on a generic safety phrase when they haven't identified the actual hazard in this experiment.
How to avoid it
Name a SPECIFIC hazard and a MATCHING precaution, e.g. 'hot water → wear goggles and use a heatproof mat', or 'laser → never look into the beam and use a low-power class-2 laser'.
✕Referring to apparatus vaguely ('a ruler', 'a timer') instead of naming the specific instrument and its resolution
Edexcel IAL Physics Unit 3 (WPH13) examiner reports
Why it happens
Students describe the measurement without thinking about which instrument best suits the size of the quantity.
How to avoid it
Name the instrument and its resolution, and match it to the quantity: metre rule (1 mm) for lengths, Vernier caliper (0.1 mm) for small lengths, micrometer (0.01 mm) for thin wires, light gates + datalogger for short times.
✕Planning too few readings, or over too narrow a range, and not repeating them
Edexcel IAL Physics Unit 3 (WPH13) examiner reports
Why it happens
Students focus on the method and overlook that a reliable graph needs enough well-spread points.
How to avoid it
Take AT LEAST SIX values of the independent variable over as wide a range as practicable, and repeat each reading and take a mean to spot anomalies and reduce random error.
✕Choosing suitable apparatus but not JUSTIFYING the choice (e.g. not saying why a micrometer, not a ruler)
Edexcel IAL Physics Unit 3 (WPH13) examiner reports
Why it happens
Students assume naming the right instrument is enough and forget the justification mark.
How to avoid it
Justify by resolution and percentage uncertainty: 'a micrometer (0.01 mm) gives a small percentage uncertainty on a 0.4 mm wire, whereas a metre rule (1 mm) would be far too coarse'.
✕Not linking what to plot back to the equation — or forgetting how the gradient/intercept gives the wanted quantity
Edexcel IAL Physics Unit 3 (WPH13) examiner reports
Why it happens
Students plot 'x against y' without rearranging the governing equation into the form $y=mx+c$ .
How to avoid it
Rearrange the equation into $y=mx+c$ , choose axes so the GRADIENT (or INTERCEPT) equals the quantity you want, and state the link, e.g. 'gradient $=\rho/A$ , so $\rho=\text{gradient}\times A$ ' or 'intercept $=$ emf'.

Practical Skills - Planning — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Practical Skills - Planning

Detailed Study Notes

At a glance

What you’ll learn

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1Classifying variables in a resistivity plan (3 marks)

2Choosing apparatus to measure a wire's diameter (2 marks)

3Hazard and precaution in an electrical experiment (2 marks)

4Choosing the range and number of readings for measuring g (3 marks)

5Justifying a timing method for a pendulum plan (3 marks)

6Choosing what to plot for the Young modulus (4 marks)

7Planning a viscosity experiment (Stokes' law) (6 marks)

8Planning a novel experiment: g from a mass-on-spring (6 marks)

9Full plan: determining the resistivity of a wire (6 marks)

Percentage uncertainty of a reading

What to plot to find g (free fall, CP1)

What to plot for the Young modulus (CP3)

What to plot for resistivity (CP7)

What to plot for emf and internal resistance (CP8)

Equation of a straight line

Independent variable

Dependent variable

Control variable

Fair test

Resolution

Range and interval

Accuracy

Precision

Reliability

Validity

Random error

Systematic error

Zero error

Repeat readings / anomaly

Percentage uncertainty

Fiducial marker

✕Stating control variables vaguely — e.g. 'keep the conditions the same' — or naming a variable without saying HOW it is kept constant

✕Writing 'be careful' or 'the experiment is dangerous' instead of a specific hazard and a matching precaution

✕Referring to apparatus vaguely ('a ruler', 'a timer') instead of naming the specific instrument and its resolution

✕Planning too few readings, or over too narrow a range, and not repeating them

✕Choosing suitable apparatus but not JUSTIFYING the choice (e.g. not saying why a micrometer, not a ruler)

✕Not linking what to plot back to the equation — or forgetting how the gradient/intercept gives the wanted quantity

Practical Skills - Planning — frequently asked questions