What are the key components of a successful experiment plan in Practical Skills II - Planning for Edexcel International A Levels Physics?

A successful experiment plan should include a clear aim, a detailed list of materials and equipment, a step-by-step procedure, safety considerations, and a method for data collection and analysis. It is crucial to ensure that the plan is logical and feasible within the given constraints.

How do I identify variables in a physics experiment for Practical Skills II - Planning?

In Practical Skills II - Planning, identifying variables involves distinguishing between independent, dependent, and controlled variables. The independent variable is what you change, the dependent variable is what you measure, and controlled variables are kept constant to ensure a fair test.

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

Including safety considerations in the planning phase is crucial to prevent accidents and ensure the well-being of all participants. It involves identifying potential hazards, understanding the risks, and implementing measures to mitigate them, which is an essential part of Practical Skills II - Planning in Edexcel International A Levels Physics.

How can I effectively plan for data collection and analysis in a physics experiment?

Effective planning for data collection and analysis involves deciding on the type of data to collect, the method of measurement, and the tools required. It also includes planning how to record the data systematically and how to analyze it to draw meaningful conclusions, which is a key aspect of Practical Skills II - Planning.

What strategies can I use to ensure my experiment plan is feasible and realistic?

To ensure your experiment plan is feasible and realistic, consider the availability of resources, time constraints, and the complexity of the procedures. Simplify where possible, ensure all equipment is accessible, and verify that the plan can be executed within the given timeframe, aligning with the objectives of Practical Skills II - Planning in the Edexcel International A Levels Physics curriculum.

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 II - 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. 'source–detector separation kept constant by clamping both to a fixed track', 'counting time kept constant at 5 minutes', or 'same fixed resistor in the discharge circuit'. Source: Edexcel IAL Physics Unit 6 (WPH16) examiner reports

Why is "Writing 'be careful' or 'radiation is dangerous' instead of a specific hazard and a matching precaution (especially for radioactive sources and lasers)" a common mistake in Practical Skills II - Planning?

Why it happens: Students fall back on a generic safety phrase when they haven't identified the actual IA2 hazard in this experiment. How to avoid it: Name a SPECIFIC hazard and a MATCHING precaution: 'ionising gamma radiation → handle with forceps, keep at a distance, minimise time, store in lead', or 'laser → never look into the beam, avoid reflections, use a low-power class-2 laser', or 'charged capacitor → discharge it through a resistor before touching'. Source: Edexcel IAL Physics Unit 6 (WPH16) examiner reports

Why is "Referring to apparatus vaguely ('a ruler', 'a timer', 'a meter') instead of naming the specific IA2 instrument and its resolution" a common mistake in Practical Skills II - 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: micrometer (0.01 mm) for thin absorbers, calipers (0.1 mm) for cards, GM tube + scaler for counts, voltage sensor + datalogger for a fast capacitor discharge, top-pan balance (0.01 g) for mass loss. Source: Edexcel IAL Physics Unit 6 (WPH16) examiner reports

Why is "Planning too few readings, or over too narrow a range, and not repeating them (or, in radioactivity, counting for too short a time)" a common mistake in Practical Skills II - Planning?

Why it happens: Students focus on the method and overlook that a reliable graph needs enough well-spread points, and that short counts have a large √(N) uncertainty. How to avoid it: Take AT LEAST SIX values over as wide a range as practicable, repeat each and take a mean, and — for counts — count over a long fixed time so the percentage counting uncertainty 1/√(N) is small. Source: Edexcel IAL Physics Unit 6 (WPH16) examiner reports

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

Why it happens: Students assume naming the right instrument is enough and forget the justification mark. How to avoid it: Justify by resolution/percentage uncertainty or by capability: 'a micrometer (0.01 mm) gives a small percentage uncertainty on a 1 mm absorber', or 'a datalogger samples the fast discharge automatically at ms resolution, removing reaction-time error'. Source: Edexcel IAL Physics Unit 6 (WPH16) examiner reports

Why is "Not linking the plot to the IA2 equation — plotting the raw variables of an exponential or power law instead of linearising to get a straight line" a common mistake in Practical Skills II - Planning?

Why it happens: Students plot 'the two things they measured' without taking logs or reciprocals of the governing equation to reach y=mx+c. How to avoid it: For an exponential (V=V_0 e^-t/RC, A=A_0 e^- t, C=C_0 e^- x) take natural logs and plot () against the variable, reading the constant off the gradient; for a power/inverse-square law plot against a reciprocal (I vs 1/r^2) or use a log–log plot (gradient -2). Always state how the gradient/intercept gives the wanted quantity. Source: Edexcel IAL Physics Unit 6 (WPH16) examiner reports

Edexcel International A Levels Physics (XPH11-YPH11)

Practical Skills II - Planning

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Detailed Study Notes

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

Practical Skills II — Planning — Pearson Edexcel International A Level Physics (XPH11/YPH11) Study Notes (Unit 6, WPH16)

Planning an IA2 experiment for the Unit 6 written practical paper (WPH16): identifying independent, dependent and control variables (and stating HOW each control is kept constant); selecting and justifying apparatus by resolution (micrometer, calipers, GM tube + scaler, voltage sensor + datalogger, top-pan balance); designing a precise procedure over a wide range with repeats; naming specific hazards and precautions for radioactive sources, lasers and charged capacitors; and — the IA2 discriminator — LINEARISING exponential and power laws (capacitor discharge, radioactive decay, gamma absorption, the inverse-square law and mass–spring SHM) so a straight-line graph gives the quantity from a gradient. Anchored in Core Practicals 9–16.

At a glance

Independent variable = what you deliberately change (x-axis); dependent variable = what you measure in response (y-axis); control variables = everything else that could affect the result, kept constant — and you must say HOW.
A control mark needs the METHOD, not just the name: 'source–detector separation kept constant by clamping both to a fixed track', not 'keep it the same'.
Name apparatus and its resolution, matched to the quantity: micrometer 0.01 mm (thin absorbers), calipers 0.1 mm, GM tube + scaler (counts), voltage sensor + datalogger (fast discharge), top-pan balance 0.01 g.
Take ≥6 readings over a wide range, repeat and average — to spot anomalies and reduce random error. In radioactivity, count over a long fixed time so $\sqrt{N}/N = 1/\sqrt{N}$ is small.
Safety must be SPECIFIC: gamma source → forceps, distance, minimum time, lead storage; laser → don't look into the beam, use class-2, avoid reflections; charged capacitor → discharge through a resistor before touching. 'Be careful' scores nothing.
Linearise before you plot. Exponential ⇒ take natural logs: $V=V_0 e^{-t/RC}$ ⇒ plot $\ln V$ vs $t$ (gradient $-1/RC$ ); $A=A_0 e^{-\lambda t}$ ⇒ plot $\ln A$ vs $t$ (gradient $-\lambda$ ); $C=C_0 e^{-\mu x}$ ⇒ plot $\ln C$ vs $x$ (gradient $-\mu$ ).
Power/inverse-square laws: plot against a reciprocal or use log–log. Inverse-square: plot $I$ vs $1/r^2$ (straight line through the origin) or $\log I$ vs $\log r$ (gradient $-2$ ). SHM: $T=2\pi\sqrt{m/k}$ ⇒ plot $T^2$ vs $m$ (gradient $4\pi^2/k$ ).
Always link the plot to the equation: state what the gradient (or intercept) equals and rearrange for the wanted quantity, e.g. 'gradient $=-\mu$ , so $\mu=-\text{gradient}$ '.

What you’ll learn

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

Identify the independent, dependent and control variables of an IA2 investigation and state, for each control, the specific method used to keep it constant.
Select apparatus with resolution appropriate to IA2 work (micrometer, calipers, GM tube + counter/scaler, voltage/force/motion sensors + datalogger, thermistor/thermometer, top-pan balance, signal generator, variable resistor) and justify each choice by percentage uncertainty or capability.
Design a valid, reliable procedure: at least six readings over a wide range, repeats and their purpose (mean, spot anomalies), and — in radioactivity — long counting times and background subtraction.
State specific hazards and matching precautions for IA2 apparatus, especially radioactive sources, lasers and high-voltage/charged capacitors.
Choose what to plot to obtain a straight line, LINEARISING exponential and power laws (RC discharge, radioactive decay, gamma absorption, inverse-square law, mass–spring SHM), and explain how the gradient or intercept yields the required quantity.
Apply these planning skills to Core Practicals 9–16 and to unfamiliar IA2 contexts (transferable planning).

Variables — and how controls are actually kept constant

IV changed, DV measured, controls kept constant — and the mark is for HOW.

Every IA2 plan begins by sorting the quantities into three roles. Getting this exactly right sets up the whole experiment and earns easy marks.

The independent variable (IV) is the quantity you deliberately change — e.g. the thickness of a lead absorber, the mass on a spring, or the time since a capacitor began discharging. It normally goes on the x-axis.
The dependent variable (DV) is the quantity you measure in response — e.g. the count rate, the period, or the p.d. across the capacitor. It normally goes on the y-axis.
A control variable is anything else that could affect the DV and so must be kept constant for a fair, valid test.

The examiner's discriminator at IA2 is not naming the control but stating the METHOD of control. "Keep the temperature the same" earns nothing; "keep the temperature constant by using a small sensing current and stirring the water bath so the thermistor does not self-heat" earns the mark. Some worked control statements:

Radioactivity (CP15): keep the source–detector geometry constant by clamping both the source and the GM tube to a fixed track; keep the counting time constant by counting for the same fixed interval (e.g. 5 min) at every thickness.
Capacitor discharge (CP11): keep the resistance constant with the same fixed resistor, the capacitance with the same capacitor, and the starting voltage by charging from the same supply each run.
Boyle's law (CP14): keep the temperature constant by compressing slowly and letting the gas settle to room temperature before each reading; keep the mass of gas constant by using a sealed column so none leaks.

A variables table for the gamma-absorption Core Practical. Notice every control names both the quantity AND the method of keeping it constant — that is what earns the mark.

IV = deliberately changed (x-axis); DV = measured in response (y-axis).
A control mark needs the METHOD, not just the name.
Fixed geometry (clamp to a track), same counting time, same source — typical IA2 controls.
Subtracting the background count is a systematic correction, not a control.

See the full worked example for practical skills ii - planning →

Selecting and justifying apparatus by resolution

Name the instrument, quote its resolution, and link it to a small percentage uncertainty.

An IA2 plan must name the specific instrument, its resolution, and WHY it suits the quantity — never "a ruler" or "a timer". The justification is almost always about keeping the percentage uncertainty small, $\%\,\text{uncertainty}=\dfrac{\Delta x}{x}\times100$ .

Quantity	Instrument	Resolution	Why
Thin absorber / wire diameter	Micrometer screw gauge	0.01 mm	Tiny $\Delta x$ on a ~1 mm value ⇒ <1%
Card / small length	Vernier calipers	0.1 mm	Fine on cm-scale lengths
Source separation $r$	Metre rule on a track	1 mm	Spans the range, defined measuring point
Radioactive counts	GM tube + counter/scaler	1 count	Direct, and long counts cut $\sqrt{N}/N$
Fast p.d. (discharge)	Voltage sensor + datalogger	ms sampling	Captures the whole curve, no reaction time
Mass boiled/melted	Top-pan balance	0.01 g	Small mass changes resolved
Temperature	Thermometer / temp sensor	~0.5 °C	Set and read the bath temperature

A model justification: "A micrometer (0.01 mm) is used for the 1.2 mm lead sheet because its absolute uncertainty is under 1% of the reading, whereas a metre rule (1 mm) would give ~80% — and since absorption depends exponentially on thickness, that error would badly distort the graph."

Two IA2-specific justifications worth memorising:

Why a datalogger, not a stopwatch, for a capacitor discharge? The p.d. falls continuously and fast, so hand-timing is dominated by reaction time (~0.2 s) and misses the rapid early fall; a voltage sensor + datalogger samples automatically at millisecond resolution and captures the whole curve.
Why count for a long time in radioactivity? The count uncertainty is $\sqrt{N}$ , so the fractional uncertainty is $\dfrac{\sqrt{N}}{N}=\dfrac{1}{\sqrt{N}}$ — it shrinks as the total count grows, so long fixed counting times give a smaller percentage uncertainty.

Always: name the instrument + its resolution + why (percentage uncertainty or capability).
Micrometer 0.01 mm, calipers 0.1 mm, metre rule 1 mm, balance 0.01 g.
Datalogger beats a stopwatch for a fast discharge (captures the curve, no reaction time).
Long counting times cut the fractional counting uncertainty $1/\sqrt{N}$ .

See the full worked example for practical skills ii - planning →

Procedure, reliability and SPECIFIC safety

≥6 readings over a wide range, repeat and average — and name a real hazard with a matching precaution.

Range, readings and repeats. A reliable graph needs at least six values of the IV over as wide a range as practicable, reasonably evenly spaced. Repeat each reading (e.g. ×3) and take a mean — this lets you identify anomalies and reduce random error, improving reliability (repeats do NOT improve accuracy; accuracy comes from removing systematic error, such as subtracting the background count or zeroing an instrument).

Reaching steady conditions. Many IA2 measurements need the system to settle first: let the water bath reach a stable temperature (stir it) before reading a thermistor's resistance; let the water boil steadily before timing mass loss; let the trapped gas return to room temperature after compression.

Safety at IA2 must be SPECIFIC — a real hazard AND a matching precaution. "Be careful" or "radiation is dangerous" scores nothing. Learn these:

Radioactive source: hazard — ionising radiation damages living cells; precaution — handle only with long-handled forceps/tongs, keep it at a distance and pointed away from the body, expose it for the minimum time, and store it in a lead-lined container.
Laser light: hazard — a laser beam can permanently damage the eye; precaution — never look into the beam or a reflection, use a low-power class-2 laser, and put up a warning sign.
Charged capacitor / high voltage: hazard — a charged capacitor stores charge and can give a shock or discharge suddenly; precaution — use a safe low-voltage supply and discharge the capacitor through a resistor (not a short circuit) before touching the terminals.
Hot apparatus (latent heat, Boyle's law heating): hazard — hot water/steam scalds; precaution — use a lid/shield, tongs and a heatproof mat, and keep clear of escaping steam.

A gamma-absorption plan (Core Practical 15). The source and GM tube are clamped to a fixed track (a control on geometry), lead sheets of measured thickness x go in the gap, and the counter records over a fixed time. The safety plan is specific: forceps, distance, minimum exposure and lead storage.

≥6 readings over a wide range; repeat and average to spot anomalies and cut random error.
Repeats improve RELIABILITY; accuracy needs systematic errors removed (background, zero error).
Let systems reach steady conditions (stable bath temperature, steady boiling) before reading.
Safety = SPECIFIC hazard + MATCHING precaution (forceps/distance/lead for sources; class-2 laser; discharge a capacitor first).

See the full worked example for practical skills ii - planning →

Choosing what to plot — linearising IA2 laws

Take logs of an exponential, a reciprocal of a power law: reach y = mx + c so the gradient carries the answer.

The single biggest IA2 planning skill is turning the governing equation into the straight-line form $y=mx+c$ so a gradient (or intercept) gives the quantity you want. At IA1 you mostly rearranged; at IA2 many laws are exponential or power laws, so you must linearise.

Exponential laws — take natural logs.

Capacitor discharge (CP11): $V=V_0 e^{-t/RC}$ . Taking logs, $\ln V = \ln V_0 - \dfrac{1}{RC}\,t$ . Plot $\ln V$ against $t$ : gradient $=-\dfrac{1}{RC}$ , so $RC=-\dfrac{1}{\text{gradient}}$ (and $R=RC/C$ ); the intercept is $\ln V_0$ .
Radioactive decay: $A=A_0 e^{-\lambda t}$ . Plot $\ln A$ against $t$ : gradient $=-\lambda$ , so $\lambda=-\text{gradient}$ , and $t_{1/2}=\dfrac{\ln 2}{\lambda}$ .
Gamma absorption (CP15): $C=C_0 e^{-\mu x}$ . Plot $\ln C$ against $x$ : gradient $=-\mu$ , so $\mu=-\text{gradient}$ .
Thermistor (novel): $R=A e^{B/T}$ . Plot $\ln R$ against $\dfrac{1}{T}$ : gradient $=B$ , intercept $=\ln A$ .

Power / inverse-square laws — reciprocal or log–log.

Inverse-square (CP15 geometry, or light): $I=\dfrac{k}{r^2}$ . Plot $I$ against $\dfrac{1}{r^2}$ : a straight line through the origin of gradient $k$ confirms $I\propto 1/r^2$ . Or plot $\log I$ against $\log r$ : gradient $=-2$ .
Mass–spring SHM (CP16): $T=2\pi\sqrt{\dfrac{m}{k}}$ . Square it: $T^2=\dfrac{4\pi^2}{k}\,m$ . Plot $T^2$ against $m$ : gradient $=\dfrac{4\pi^2}{k}$ , so $k=\dfrac{4\pi^2}{\text{gradient}}$ .

Sometimes the intercept carries a wanted quantity (e.g. the constant heat-loss power $h$ in a latent-heat plan of $P$ against $\Delta m/t$ , where gradient $=L$ and intercept $=h$ ). Always state explicitly what the gradient and intercept equal — a plot with no link back to the equation earns no method marks.

You cannot read a constant off a curve. Taking natural logs of the decay law turns the exponential into a straight line of ln A against t whose gradient is −λ, so the decay constant is minus the gradient. The same trick linearises RC discharge (gradient −1/RC) and gamma absorption (gradient −μ).

Exponential ⇒ take logs: $\ln V$ vs $t$ (RC), $\ln A$ vs $t$ (decay), $\ln C$ vs $x$ (absorption).
Inverse-square ⇒ plot $I$ vs $1/r^2$ (line through origin) or log–log (gradient $-2$ ).
SHM ⇒ square it: $T^2$ vs $m$ , gradient $4\pi^2/k$ .
State what the gradient AND intercept equal, then rearrange for the wanted quantity.

See the full worked example for practical skills ii - planning →

Putting a full IA2 plan together

Variables → apparatus (justified) → procedure with repeats → what to plot → safety.

A full-mark IA2 plan (typically 6 marks) follows the same skeleton every time — build the habit so you never miss a mark.

Variables and controls. State the IV, the DV, and each control with its method (e.g. same source, fixed geometry, same counting time).
Apparatus, justified. Name each instrument, its resolution, and why (percentage uncertainty or capability — a datalogger to capture a fast change, a scaler to count).
Procedure. Describe the measurement precisely; take ≥6 readings over a wide range; repeat and average; reach steady conditions first; and apply any systematic correction (subtract the background count, allow for a zero error).
What to plot. Linearise the governing equation and state the axes; say what the gradient (and/or intercept) equals and rearrange for the wanted quantity.
Safety. One specific hazard + matching precaution appropriate to the apparatus.

Worked skeleton — RC discharge (find $RC$ , hence $R$ ). IV = time $t$ ; DV = p.d. $V$ ; controls = same $R$ , $C$ , $V_0$ . Apparatus: voltage sensor + datalogger (captures the fast fall automatically). Procedure: charge fully, switch to discharge, log $V$ over several time constants, repeat and average. Plot: $\ln V$ against $t$ (from $\ln V = \ln V_0 - t/RC$ ); gradient $=-1/RC$ , so $RC=-1/\text{gradient}$ and $R=RC/C$ . Safety: discharge the capacitor through a resistor before touching the terminals.

$V = V_0\,e^{-t/RC} \;\Rightarrow\; \ln V = \ln V_0 - \frac{1}{RC}\,t \;\Rightarrow\; \text{plot } \ln V \text{ vs } t,\ \ RC = -\frac{1}{\text{gradient}}.$

Whether the context is a Core Practical (CP9–16) or an unfamiliar setup, the same five-step skeleton — and the same linearisation trick — earns the marks. The examiner rewards naming, justifying, and linking the plot to the equation far more than description.

Five-step skeleton: variables/controls → justified apparatus → procedure+repeats → what to plot → safety.
Apply systematic corrections (background subtraction, zero-error) in the procedure.
Linearise and state what the gradient/intercept equals, then rearrange for the answer.
The same skeleton works for novel contexts — transferable planning.

See the full worked example for practical skills ii - planning →

Quick recap

IV = deliberately changed; DV = measured in response; controls = kept constant — and you must state HOW each control is held constant.
Name apparatus + resolution + justification: micrometer 0.01 mm, calipers 0.1 mm, GM tube + scaler, voltage sensor + datalogger, top-pan balance 0.01 g.
Take ≥6 readings over a wide range, repeat and average (reliability); count for a long fixed time in radioactivity so $1/\sqrt{N}$ is small; subtract the background (accuracy).
Safety must be specific: gamma source → forceps/distance/minimum time/lead; laser → don't look into the beam, class-2; charged capacitor → discharge through a resistor first.
Linearise exponentials by taking logs: $\ln V$ vs $t$ (RC, gradient $-1/RC$ ), $\ln A$ vs $t$ (decay, gradient $-\lambda$ ), $\ln C$ vs $x$ (absorption, gradient $-\mu$ ).
Power/inverse-square: plot $I$ vs $1/r^2$ (line through origin) or log–log (gradient $-2$ ); SHM: plot $T^2$ vs $m$ (gradient $4\pi^2/k$ ).
Always state what the gradient and intercept equal and rearrange for the wanted quantity — a plot not linked to the equation earns no method marks.

Memorise this

Verbatim phrases and definitions Edexcel mark schemes credit.

Control variable mark = NAME + METHOD (e.g. 'fixed geometry: clamp source and GM tube to a track').
Capacitor discharge: $V=V_0 e^{-t/RC}$ ⇒ plot $\ln V$ vs $t$ ; gradient $=-1/RC$ .
Radioactive decay: $A=A_0 e^{-\lambda t}$ ⇒ plot $\ln A$ vs $t$ ; gradient $=-\lambda$ ; $t_{1/2}=\ln 2/\lambda$ .
Gamma absorption: $C=C_0 e^{-\mu x}$ ⇒ plot $\ln C$ vs $x$ ; gradient $=-\mu$ ; subtract background first.
Inverse-square: plot $I$ vs $1/r^2$ (straight line through origin) or $\log I$ vs $\log r$ (gradient $-2$ ).
Mass–spring SHM: $T=2\pi\sqrt{m/k}$ ⇒ plot $T^2$ vs $m$ ; gradient $=4\pi^2/k$ , so $k=4\pi^2/\text{gradient}$ .
Counting uncertainty $=\sqrt{N}$ , fractional $=1/\sqrt{N}$ — count for longer to reduce it.
Radioactive-source safety = forceps + distance + minimum time + lead storage (NOT 'be careful').

How it’s examined

Planning is assessed on the Unit 6 written practical paper (WPH16, Practical Skills in Physics II), an IA2 paper that draws its contexts from Units 4 and 5. Typical formats: identify/classify variables for a given IA2 experiment (2–3 marks, with a control mark reserved for stating the METHOD of control); choose and justify apparatus by resolution/percentage uncertainty (2 marks); state a specific hazard and matching precaution — very often for a radioactive source or laser (2 marks); describe range, repeats and their purpose (2–4 marks); and, the IA2 discriminator, state what to plot to obtain a straight line by LINEARISING an exponential or power law and explain how the gradient/intercept gives the quantity (2–4 marks). A full planning question is usually 6 marks and expects the five-step skeleton. Marks are routinely lost for vague controls, 'be careful' safety, un-named apparatus, too few readings, un-justified apparatus, and plotting the raw variables of an exponential instead of linearising. Where numeric, quote 3 significant figures with units.

Sources: Pearson Edexcel International Advanced Level Physics (YPH11/XPH11) specification (Issue 3, July 2021); Edexcel IAL Physics Unit 6 (WPH16) examiner reports; Edexcel IAL Physics Unit 6 sample assessment materials and mark schemes. Last reviewed 2026-07-06.

Take this whole topic with you

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

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

Question type:

1Classifying variables in a capacitor-discharge plan (3 marks)
Getting startedIdentify & classify• Core Practical 11 style• variables, Core Practical 11, capacitor discharge, Unit 6
Question
A student plans to investigate how the potential difference $V$ across a capacitor falls as it discharges through a fixed resistor. 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/measures against: the time $t$ since discharge began (measured with a stopwatch, or automatically with a datalogger).
2. Step 2
  The dependent variable is what is measured in response: the potential difference $V$ across the capacitor (read on a voltmeter or logged by a voltage sensor).
3. Step 3
  A control variable must be NAMED with the method of control. Best answer: the resistance $R$ in the discharge circuit — keep it constant by using the same fixed resistor throughout (do not adjust it). Also acceptable: the capacitance $C$ — keep constant by using the same capacitor, or the starting/charging voltage — keep constant by fully charging from the same supply before each run.
Answer
IV = time $t$ since discharge began; DV = potential difference $V$ across the capacitor; control variable = discharge resistance $R$ (kept constant by using the same fixed resistor), and/or the same capacitor $C$ and the same starting voltage from the same supply.
Examiner tip
Marks: (1) IV = time; (2) DV = p.d. across the capacitor (or the voltage-sensor reading); (3) a NAMED control with HOW it is controlled. 'Keep the circuit the same' scores nothing — 'same fixed resistor' or 'same capacitor charged from the same supply' is what earns the mark.
2Choosing apparatus to measure source–detector separation (2 marks)
Getting startedIdentify & classify• Core Practical 15 style (inverse-square)• apparatus, resolution, gamma, Unit 6
Question
In an inverse-square-law experiment the distance $r$ from a gamma source to a GM tube (a few centimetres up to about 40 cm) must be measured. State the most suitable instrument and justify your choice by referring to its resolution. (2 marks)
Step-by-step solution
1. Step 1
  The separation ranges from a few cm to ~40 cm, so a calibrated track / clamped metre rule (millimetre scale) is appropriate — it comfortably spans the range while resolving to 1 mm.
2. Step 2
  Justify by resolution: a metre rule reads to 1 mm, so even at the smallest separation (~5 cm) the percentage uncertainty is only $\tfrac{1}{50}\times100 = 2\%$ ; at larger $r$ it is smaller still. Because count rate depends on $1/r^2$ , small errors in $r$ matter, so a defined reference point (front face of the source to front of the tube) must be used.
Answer
Use a millimetre metre rule fixed to a track (resolution 1 mm): it spans the whole range yet resolves to 1 mm, giving a percentage uncertainty of only a couple of percent even at the closest separation, and $r$ is measured consistently from the source face to the tube face.
Examiner tip
Marks: (1) metre rule / calibrated track (mm scale) named; (2) justified by 1 mm resolution giving a small percentage uncertainty, with a defined measuring point. 'A ruler' with no resolution, or 'it is accurate' unquantified, does not gain the justification mark.
3Hazard and precaution with a radioactive source (2 marks)
Getting startedIdentify & classify• Core Practical 15 style• safety, hazard, radioactivity, Unit 6
Question
A student plans to measure how the count rate from a gamma source varies with the thickness of lead absorber (Core Practical 15). State one specific hazard and a matching precaution. (2 marks)
Step-by-step solution
1. Step 1
  Name a specific hazard: the ionising radiation from the gamma source can damage living cells / increase cancer risk on prolonged exposure.
2. Step 2
  Give a matching, SPECIFIC precaution: handle the source only with long-handled forceps/tongs, keep it at arm's length pointing away from the body, expose it for the minimum time, and store it in a lead-lined container when not in use. ('Be careful' scores nothing.)
Answer
Hazard: ionising gamma radiation harms body cells. Precaution: handle the source with forceps/tongs, keep it at a distance and pointed away, minimise exposure time, and return it to its lead castle when not measuring.
Examiner tip
Marks: (1) a SPECIFIC named hazard (ionising radiation harming cells, not 'radiation is dangerous'); (2) a MATCHING, specific precaution (forceps + distance + minimum time + lead storage). 'Be careful' or a generic 'wear gloves' unlinked to the radiation hazard scores nothing.
4Range and repeats when measuring specific latent heat (3 marks)
Building confidenceMulti-step problem• Core Practical 13 style• range, readings, Core Practical 13, Unit 6
Question
A student measures the specific latent heat of fusion of ice by supplying electrical energy to melt ice and finding the mass melted, using an electrical heater at several different powers $P$ . State (a) how many power settings should be used and over what range, and (b) whether readings should be repeated, giving a reason. (3 marks)
Step-by-step solution
1. Step 1
  (a) Take at least six different power settings spanning a wide range (e.g. from a low power up to the heater's safe maximum, roughly evenly spaced), so a straight best-fit line of melted-mass rate against power can be drawn and its gradient found reliably.
2. Step 2
  (b) Repeat the measurement at each power (e.g. three times), timing a fixed interval and re-weighing the collected melt-water, then take a mean.
3. Step 3
  The reason: repeats let anomalies be identified and reduce random error (in timing and weighing), giving a more reliable value. A control run with the heater off subtracts melting due to the warm surroundings — a systematic error the graph method removes as an intercept.
Answer
(a) At least six power settings over a wide range up to the safe maximum. (b) Repeat each measurement (e.g. ×3) and average, to spot anomalies and reduce random error; include a heater-off control to correct for background melting.
Examiner tip
Marks: (1) ≥6 settings over a wide range; (2) repeat at each setting and take a mean; (3) reason = identify anomalies / reduce random error (credit the heater-off control for background melting). 'A few settings' or repeating 'to improve accuracy' are not creditworthy — repeats improve reliability.
5Justifying a data-logging method for a collision plan (3 marks)
Building confidenceMulti-step problem• Core Practical 10 style (conservation of momentum)• apparatus, random error, datalogger, Core Practical 10, Unit 6
Question
A student plans to test conservation of momentum in a collision between two trolleys on a track. Each trolley moves quickly, so velocities must be found accurately. Describe how the student should measure the velocities, and justify each feature of the method. (3 marks)
Step-by-step solution
1. Step 1
  Use light gates and a datalogger, not a stopwatch. Attach an interrupt card of measured length $\ell$ to each trolley; the light gate measures the time the card blocks the beam, and the datalogger computes velocity as $v=\ell/t$ automatically. Hand timing a fast trolley would be dominated by reaction time (~0.2 s).
  $v = \frac{\ell}{t}$
2. Step 2
  Measure the card length with calipers (0.1 mm) and keep the track level/friction-compensated, so the only forces are the collision forces (a control on the surface). Use light gates before AND after the collision to get initial and final velocities of both trolleys.
3. Step 3
  Repeat each collision several times and take a mean, to reduce random error, and check the total momentum before equals that after within uncertainty. (A motion sensor logging velocity directly is an acceptable alternative.)
Answer
Use light gates + datalogger with a card of measured length so $v=\ell/t$ is found automatically (removes reaction-time error), measure the card with calipers, compensate the track for friction, gate before and after the collision, and repeat and average. Each feature is justified: automatic timing removes human error; friction-compensation isolates the collision forces; repeats reduce random error.
Examiner tip
Marks: (1) light gates + datalogger with card so $v=\ell/t$ , justified by removing reaction-time error; (2) measure the card precisely / compensate the track (a named control); (3) repeat and mean, gating before and after. Each feature must be JUSTIFIED — describing the method without the reason limits the marks.
6Choosing what to plot for the spring constant from SHM (4 marks)
Building confidenceGraph or diagram• Core Practical 16 style (mass–spring)• graph, gradient, SHM, linearisation, Core Practical 16, Unit 6
Question
A mass $m$ on a spring of constant $k$ oscillates with period $T = 2\pi\sqrt{\dfrac{m}{k}}$ . A student varies $m$ and measures $T$ . State what the student should plot against what to obtain a straight line, and explain how the graph gives the spring constant $k$ . (4 marks)
Step-by-step solution
1. Step 1
  The period equation contains a square root, so square both sides to make it linear in $m$ :
  $T^2 = \frac{4\pi^2}{k}\,m$
2. Step 2
  Compare with $y = mx$ (straight line through the origin): plotting $T^2$ (y-axis) against $m$ (x-axis) gives a straight line whose gradient is $\dfrac{4\pi^2}{k}$ .
  $\text{gradient} = \frac{4\pi^2}{k}$
3. Step 3
  Measure $T$ by timing 20 oscillations and dividing by 20 (to reduce the effect of reaction time), for ≥6 masses over a wide range.
  $T = \frac{t_{20}}{20}$
4. Step 4
  Rearrange the gradient for $k$ : $k = \dfrac{4\pi^2}{\text{gradient}}$ .
  $k = \frac{4\pi^2}{\text{gradient}}$
Answer
Plot $T^2$ (y) against $m$ (x): a straight line through the origin of gradient $\dfrac{4\pi^2}{k}$ , so $k = \dfrac{4\pi^2}{\text{gradient}}$ . Time 20 oscillations to find each $T$ , over ≥6 masses.
Examiner tip
Marks: (1) square to $T^2 = (4\pi^2/k)m$ ; (2) plot $T^2$ against $m$ ; (3) gradient $=4\pi^2/k$ ; (4) $k = 4\pi^2/\text{gradient}$ . The key skill is linearising the square-root law so the gradient yields $k$ . Plotting $T$ against $m$ (a curve) is the classic error.
7Planning to find the time constant of an RC discharge (6 marks)
StretchMulti-step problem• Core Practical 11 style• planning, capacitor discharge, linearisation, Core Practical 11, AO3, Unit 6
Question
Plan an experiment to determine the time constant $RC$ of a capacitor discharging through a resistor, and hence check the value of $R$ . The discharge obeys $V = V_0\,e^{-t/RC}$ . Identify the variables, name and justify the apparatus, describe the procedure, state what to plot to get a straight line, and refer to one safety point. (6 marks)
Step-by-step solution
1. Step 1
  Variables. IV = time $t$ since discharge began; DV = p.d. $V$ across the capacitor. Controls: the same resistor $R$ and same capacitor $C$ throughout (do not change them), and the same starting voltage $V_0$ (charge fully from the same supply each run).
2. Step 2
  Apparatus, justified. A voltage sensor + datalogger (or a high-resistance digital voltmeter) records $V$ against $t$ automatically — essential because $V$ falls continuously, so manual readings would miss the fast early part. A stopwatch is a fallback for slow discharges. Use a large $C$ and $R$ so the time constant is a few seconds and easy to sample.
3. Step 3
  Procedure. Fully charge the capacitor, then switch to discharge and start logging. Record $V$ at regular time intervals for at least ≥6 well-spread points spanning several time constants. Repeat the whole discharge and average corresponding readings.
4. Step 4
  What to plot (linearise). Take natural logs of $V = V_0 e^{-t/RC}$ : $\ln V = \ln V_0 - \dfrac{1}{RC}\,t$ . Plotting $\ln V$ (y) against $t$ (x) gives a straight line of gradient $-\dfrac{1}{RC}$ , so $RC = -\dfrac{1}{\text{gradient}}$ ; dividing by the known $C$ gives $R$ .
  $\ln V = \ln V_0 - \frac{1}{RC}\,t \;\Rightarrow\; \text{gradient} = -\frac{1}{RC}$
5. Step 5
  Safety. A charged capacitor stores charge and can give a shock / discharge suddenly: use a safe low voltage supply, and discharge the capacitor through a resistor (not a short circuit) before touching the terminals.
Answer
IV = time $t$ , DV = p.d. $V$ ; controls = same $R$ , same $C$ , same $V_0$ . Apparatus: voltage sensor + datalogger (records the continuous fall automatically). Charge fully, switch to discharge, log $V$ vs $t$ over several time constants, repeat and average. Plot $\ln V$ against $t$ : straight line, gradient $=-1/RC$ , so $RC = -1/\text{gradient}$ and $R = RC/C$ . Safety: discharge the capacitor through a resistor before touching terminals.
Examiner tip
Marks (max 6): (1) IV = time, DV = p.d., controls named (same $R$ , $C$ , $V_0$ ); (2) datalogger/voltage sensor justified (continuous automatic recording); (3) ≥6 points over several time constants + repeats/mean; (4) take logs to get $\ln V = \ln V_0 - t/RC$ ; (5) plot $\ln V$ vs $t$ , gradient $=-1/RC$ ; (6) specific capacitor-discharge safety point. Not linearising (plotting $V$ vs $t$ and trying to read a straight line) is the usual mark-loser.
8Full plan: absorption of gamma rays by lead (6 marks)
StretchMulti-step problem• Core Practical 15 style• planning, gamma absorption, linearisation, radioactivity, Core Practical 15, AO3, Unit 6
Question
Plan an experiment to determine the linear attenuation (absorption) coefficient $\mu$ of lead for gamma rays, where the corrected count rate obeys $C = C_0\,e^{-\mu x}$ for absorber thickness $x$ . State the variables and controls, name and justify the apparatus with resolutions, describe how background is handled, state what to plot for a straight line, and give one safety point. (6 marks)
Step-by-step solution
1. Step 1
  Variables. IV = thickness $x$ of lead (≥6 values, using sheets stacked to a wide range of thicknesses, each measured with a micrometer/calipers). DV = count rate from a GM tube + counter/scaler. Controls: the same source and same fixed source–detector geometry (do not move the source or tube), and the same counting time for every reading.
2. Step 2
  Apparatus, justified. A GM tube + digital scaler/counter to count gammas; micrometer (0.01 mm) or calipers (0.1 mm) for the sheet thickness (thin sheets, so fine resolution keeps the percentage uncertainty low); a clamped metre rule to fix the geometry. Count for a long fixed time (e.g. several minutes) at each thickness so the statistical (counting) uncertainty $\sqrt{N}$ is a small fraction of $N$ .
3. Step 3
  Background correction. With the source removed, measure the background count rate over the same time and subtract it from every reading to get the corrected count rate $C$ — this removes a systematic error.
4. Step 4
  What to plot (linearise). Take natural logs of $C = C_0 e^{-\mu x}$ : $\ln C = \ln C_0 - \mu x$ . Plotting $\ln C$ (y) against $x$ (x) gives a straight line of gradient $-\mu$ , so $\mu = -\text{gradient}$ .
  $\ln C = \ln C_0 - \mu x \;\Rightarrow\; \text{gradient} = -\mu$
5. Step 5
  Safety. Ionising radiation harms cells: handle the source with forceps, keep it at a distance and pointed away, minimise exposure time, and store it in a lead container. Repeat counts and take means for reliability.
Answer
IV = lead thickness $x$ (≥6 values, micrometer/calipers), DV = count rate (GM tube + scaler); controls = same source, fixed geometry, same counting time. Subtract the background count rate to get corrected count $C$ . Count for a long fixed time so $\sqrt{N}$ is small. Plot $\ln C$ against $x$ : straight line, gradient $=-\mu$ , so $\mu = -\text{gradient}$ . Safety: forceps, distance, minimum time, lead storage.
Examiner tip
Marks (max 6): (1) IV = thickness, DV = count rate, controls (same source, fixed geometry, same counting time); (2) named apparatus with resolutions (GM tube + scaler; micrometer 0.01 mm for thickness); (3) subtract background to get corrected count + long counting time for small $\sqrt{N}$ ; (4) take logs to $\ln C = \ln C_0 - \mu x$ ; (5) plot $\ln C$ vs $x$ , gradient $=-\mu$ ; (6) specific radioactive-source safety. Forgetting to subtract background, or not linearising, are the classic mark-losers.
9Planning a novel experiment: thermistor B-value from resistance (6 marks)
StretchMulti-step problem• Edexcel IAL Physics Unit 6 (unfamiliar context, CP12 style)• planning, novel context, thermistor, linearisation, AO3, Unit 6
Question
A student is told that a thermistor's resistance $R$ depends on absolute temperature $T$ through $R = A\,e^{B/T}$ , where $A$ and $B$ are constants. Plan how the student could use standard laboratory apparatus to obtain a value for the constant $B$ . Include the variables, apparatus with resolutions, what to plot, and how the graph gives $B$ . (6 marks)
Step-by-step solution
1. Step 1
  Variables. IV = temperature $T$ of the thermistor (varied over ≥6 values across a wide range, e.g. by placing it in a water bath heated in stages). DV = resistance $R$ of the thermistor. Control = the same thermistor throughout (so $A$ and $B$ are unchanged), and ensure the thermistor and water are at the same, stable temperature before reading (stir the bath).
2. Step 2
  Apparatus, justified. A water bath + thermometer/temperature sensor (resolution ~0.5 °C) to set and read the temperature; a digital ohmmeter (or voltmeter + ammeter in a potential divider) to measure $R$ ; a stirrer so the temperature is uniform. Measure $R$ with a small sensing current to avoid self-heating (a control on the thermistor's own temperature). Convert every temperature to kelvin: $T/\text{K} = \theta/{}^\circ\text{C} + 273$ .
3. Step 3
  What to plot (linearise). Take natural logs of $R = A\,e^{B/T}$ : $\ln R = \ln A + B\left(\dfrac{1}{T}\right)$ . Comparing with $y = c + mx$ , plot $\ln R$ (y-axis) against $\dfrac{1}{T}$ (x-axis) for a straight line.
  $\ln R = \ln A + B\left(\frac{1}{T}\right)$
4. Step 4
  How the graph gives $B$ . The gradient equals $B$ (in kelvin), and the y-intercept equals $\ln A$ . Draw a best-fit line and find the gradient from a large triangle.
  $\text{gradient} = B, \quad \text{intercept} = \ln A$
5. Step 5
  Reliability. Use ≥6 temperatures over a wide range, let the thermistor reach thermal equilibrium at each, repeat the resistance reading and average, and use a best-fit line so a single anomaly does not distort the gradient.
Answer
IV = temperature $T$ (≥6 values in kelvin), DV = resistance $R$ , control = same thermistor at a stable stirred temperature with a small sensing current. Apparatus: water bath + thermometer (~0.5 °C), digital ohmmeter, stirrer. Linearise $R = A e^{B/T}$ to $\ln R = \ln A + B(1/T)$ ; plot $\ln R$ against $1/T$ : gradient $=B$ , intercept $=\ln A$ . Repeat, average, best-fit line.
Examiner tip
Marks (max 6): (1) IV = temperature, DV = resistance, control = same thermistor at equilibrium; (2) named apparatus with resolutions + small sensing current to avoid self-heating; (3) convert to kelvin and take logs; (4) plot $\ln R$ vs $1/T$ ; (5) gradient $=B$ ; (6) ≥6 readings over a range + repeats/best-fit line. Transferable planning: the mark scheme rewards taking logs of a given exponential law to get $y=mx+c$ even in an unfamiliar setup — plotting $R$ vs $T$ scores nothing for the graph.

Model Answers — Practical Skills II - Planning

High-scoring sample answers for practical skills ii - 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 6 style3 marks
Q (3 marks). Explain what is meant by the independent variable, the dependent variable and a control variable in an IA2 physics experiment.
Model answer
The independent variable is the quantity the experimenter deliberately changes during the investigation (for example, the thickness of a lead absorber, the mass on a spring, or the time since a capacitor began to discharge). 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 count rate, the period, or the potential difference across the capacitor). 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 — such as the source–detector geometry, the counting time, or the discharge resistor — so that only the independent variable is responsible for the change observed. This is what makes it a fair, valid test.
Why this scores
Marks: (1) IV = quantity deliberately changed; (2) DV = quantity measured in response; (3) control = kept constant for a fair test, with an IA2 example. Defining a term using itself ('the variable that is controlled') is not credited.
Question 2
Core Practical 15 style2 marks
Q (2 marks). A student needs to measure the thickness of a thin lead absorber, about 1.2 mm. State a suitable instrument and justify the choice by referring to its resolution.
Model answer
A micrometer screw gauge (resolution 0.01 mm) — or a digital caliper reading to 0.02 mm — is suitable.

Its fine resolution means the absolute uncertainty (~0.01 mm) is a very small fraction of the 1.2 mm thickness, giving a percentage uncertainty of under 1%. A metre rule (resolution 1 mm) would be far too coarse for a 1.2 mm measurement (~80% uncertainty), which matters because the attenuation depends exponentially on thickness.
Why this scores
Marks: (1) a named fine-resolution instrument (micrometer or caliper); (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 0.01 mm resolution.
Question 3
Core Practical 15 style2 marks
Q (2 marks). A student uses a sealed gamma source to investigate absorption by aluminium. State one specific hazard and a matching precaution.
Model answer
Hazard: the ionising gamma radiation from the source can damage or kill living cells and, with prolonged exposure, increase the risk of cancer.

Precaution: handle the source only with long-handled forceps or tongs, keep it at arm's length pointing away from the body, expose it for the minimum time needed, and store it in a lead-lined container when not in use.
Why this scores
Marks: (1) a SPECIFIC hazard (ionising radiation harming cells), not 'radiation is dangerous'; (2) a MATCHING precaution (forceps + distance + minimum time + lead storage). 'Be careful' scores nothing, and the precaution must correspond to the radiation hazard.
Question 4
Core Practical 15 style4 marks
Q (4 marks). A student investigates how the count rate from a gamma source varies with the thickness of a lead absorber. State two control variables and, for each, describe how it should be kept constant. Explain why controlling them matters.
Model answer
Control variable 1 — the source–detector geometry (separation). Keep it constant by clamping both the source and the GM tube to a fixed track and inserting the absorbers in the gap between them without moving either. If the separation changed, the count rate would change by the inverse-square law for a reason other than thickness, spoiling the result.

Control variable 2 — the counting time. Keep it constant by counting for the same fixed interval (e.g. 5 minutes) at every thickness, using a timer/scaler. Comparing counts over different times would not be a fair comparison of count rate, and the counting (statistical) uncertainty $\sqrt{N}$ depends on the total counted.

Controlling these variables ensures that any change in count rate is due only to the change in absorber thickness — that is, it makes the experiment a fair, valid test. (The background count rate must also be measured and subtracted as a systematic correction.)
Why this scores
Marks: (1) geometry/separation named with method (clamp source and tube to a fixed track); (2) counting time named with method (same fixed interval); (3) reason each affects the count rate (inverse-square / fair comparison of rate); (4) controlling them makes it a fair, valid test. A control named without HOW it is kept constant scores nothing.
Question 5
Edexcel IAL Physics Unit 6 style (radioactivity)4 marks
Q (4 marks). A student measures how the activity $A$ of a radioactive sample changes with time $t$ , where $A = A_0\,e^{-\lambda t}$ . (a) State what the student should plot against what to obtain a straight line. (b) Explain how the graph gives the decay constant $\lambda$ , and why the background count rate must be subtracted first.
Model answer
(a) Take natural logs of $A = A_0\,e^{-\lambda t}$ :

$\ln A = \ln A_0 - \lambda t$

Comparing with $y = c + mx$ , the student should plot $\ln A$ (y-axis) against $t$ (x-axis), which gives a straight line.

(b) The gradient of that line equals $-\lambda$ , so $\lambda = -\text{gradient}$ (and the half-life follows from $t_{1/2}=\ln 2/\lambda$ ). Using the whole best-fit line averages out random (counting) error and makes any anomaly obvious.

The background count rate must be subtracted first because the detector always registers some counts from cosmic rays and nearby materials. If it is not removed, the corrected activity is systematically too high, the log plot bends away from a straight line at long times, and the gradient — and hence $\lambda$ — is wrong.
Why this scores
Marks: (a) 2 marks — take logs and plot $\ln A$ against $t$ , straight line; (b) 2 marks — gradient $=-\lambda$ so $\lambda=-\text{gradient}$ , AND subtract background to avoid a systematic error/curved plot. Plotting $A$ against $t$ (a curve) or forgetting the minus sign are the classic errors.
Question 6
Edexcel IAL Physics Unit 6 style4 marks
Q (4 marks). In planning a radioactivity 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 counts should be taken over a long fixed time and repeated, and how the repeats are used.
Model answer
(a) The student should take at least six values of the independent variable (e.g. absorber thickness), spread over as wide a range as practicable and reasonably evenly spaced. A wide range with several points lets a reliable straight-line trend (after linearising) 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 counted over a long fixed time so that the total count $N$ is large; the fractional counting uncertainty is $\sqrt{N}/N = 1/\sqrt{N}$ , which shrinks as $N$ grows, so long counts give a smaller percentage uncertainty. Each measurement should also be repeated and a mean taken, to (i) identify anomalies and (ii) reduce the effect of the random, statistical nature of radioactive decay, improving reliability.
Why this scores
Marks: (a) 2 marks — ≥6 values over a wide range, so a reliable trend/gradient can be found; (b) 2 marks — long counting time makes $\sqrt{N}/N$ small, and repeats + mean identify anomalies / reduce random error. Saying repeats 'improve accuracy' is wrong — repeats improve reliability; accuracy comes from removing systematic error (background).
Question 7
Core Practical 11 style4 marks
Q (4 marks). A student plans to record how the p.d. across a capacitor falls during a discharge that lasts only about a second. (a) Explain why reading a voltmeter by hand and timing with a stopwatch would be unsatisfactory. (b) State a better method and explain how it improves the measurement.
Model answer
(a) During a fast discharge the p.d. changes continuously and quickly, so reading a voltmeter by eye and timing with a stopwatch relies on the student's reaction time (~0.2 s) to note both the value and the instant together. Over a ~1 s discharge this gives a large percentage uncertainty and the student cannot capture the rapid early fall, where $V$ changes fastest — the readings are sparse and mistimed.

(b) A far better method is to use a voltage sensor connected to a datalogger (or a storage oscilloscope). It samples the p.d. automatically at short, regular intervals (millisecond resolution), so it captures the whole discharge curve — including the fast early part — with the time of each reading known precisely and no human reaction-time error. The full data set can then be linearised (plot $\ln V$ against $t$ ) and the discharge repeated quickly to take a mean.
Why this scores
Marks: (a) 2 marks — continuous fast change + reaction time ~0.2 s → large uncertainty and sparse/mistimed readings; (b) 2 marks — voltage sensor + datalogger samples automatically at high resolution, capturing the whole curve and removing reaction-time error. Naming the apparatus (voltage sensor/datalogger) and the mechanism (automatic high-rate sampling) is essential.
Question 8
Core Practical 14 style6 marks
Q (6 marks). Plan an experiment to verify Boyle's law — that for a fixed mass of gas at constant temperature the pressure $p$ is inversely proportional to the volume $V$ . Identify the variables and controls, name and justify the apparatus, describe the procedure, and state what to plot and how the graph confirms the law.
Model answer
Variables. The independent variable is the volume $V$ of the trapped gas (changed by compressing it, ≥6 values over a wide range). The dependent variable is the pressure $p$ of the gas. The control variables are the temperature (kept constant — compress slowly and allow the gas to return to room temperature before each reading, because heating changes $p$ ) and the fixed mass of gas (a sealed column, so none leaks).

Apparatus, justified. A Boyle's-law apparatus: a sealed tube of trapped air (oil-sealed) whose length gives the volume, with a scale (metre rule, 1 mm resolution) to read the air-column length $L$ (volume $\propto L$ for uniform bore), and a pressure gauge / Bourdon gauge to read $p$ . The pump compresses the gas in stages.

Procedure. Increase the pressure in steps; at each step wait for the temperature to settle (so it stays constant), then record $p$ and the column length $L$ . Take ≥6 well-spread readings, and repeat each, taking a mean, to reduce random error.

What to plot and how it confirms the law. Boyle's law is $pV = \text{constant}$ , i.e. $p = \text{constant}\times\dfrac{1}{V}$ . Plotting $p$ (y-axis) against $\dfrac{1}{V}$ (x-axis):

$p = (\text{constant})\left(\frac{1}{V}\right) \;\Rightarrow\; \text{a straight line through the origin}$

confirms $p \propto 1/V$ at constant temperature. A best-fit line through the origin (and a constant $pV$ product) is the confirmation.
Why this scores
Marks (max 6): (1) IV = volume, DV = pressure; (2) control temperature (justified: compress slowly / let it settle) and fixed sealed mass of gas; (3) named apparatus with resolution (metre rule for column length, pressure gauge); (4) ≥6 readings over a range + repeats/mean; (5) plot $p$ vs $1/V$ ; (6) a straight line through the origin confirms $p\propto 1/V$ . Plotting $p$ vs $V$ (a curve) and calling it a confirmation is the classic error; vague temperature control loses a mark.
Question 9
Core Practical 15 style (inverse-square)6 marks
Q (6 marks). A student is given a gamma source, a GM tube with a counter, and a metre-rule track. Plan how to test whether the count rate obeys an inverse-square law with distance, $C \propto 1/r^2$ . Include the variables measured, the apparatus with a safety point, the procedure (including background), and how the results confirm the law.
Model answer
Quantities measured. The independent variable is the source–detector separation $r$ (≥6 values from a few cm up to ~40 cm). The dependent variable is the count rate from the GM tube + counter. Control the same source and same counting time at each separation, and measure $r$ consistently from the source face to the tube window.

Apparatus + safety. Clamp the source and GM tube to a metre-rule track so $r$ is read to 1 mm and the alignment stays fixed; use a scaler/counter to count over a fixed time. Safety: the gamma source emits ionising radiation that harms cells — handle it with forceps, keep it at a distance and pointed away, minimise exposure time, and store it in a lead container.

Procedure (with background). First, with the source removed, count the background over the same fixed time and find the background count rate. Then, at each separation $r$ , count for the same fixed time, and subtract the background to get the corrected count rate $C$ . Repeat each measurement and take a mean.

How the results confirm the law. If $C = \dfrac{k}{r^2}$ , then plotting $C$ (y-axis) against $\dfrac{1}{r^2}$ (x-axis) should give a straight line through the origin:

$C = k\left(\frac{1}{r^2}\right).$

A straight line through the origin confirms the inverse-square law and its gradient is $k$ . (Equivalently, a log–log plot of $\log C$ against $\log r$ should have gradient $-2$ .)
Why this scores
Marks (max 6): (1) IV = separation $r$ , DV = count rate, controls (same source, same counting time); (2) clamp source + tube to a track, metre rule 1 mm, defined measuring point; (3) radiation hazard + matching precaution (forceps / distance / minimum time / lead); (4) measure and subtract background + repeat/mean; (5) plot $C$ vs $1/r^2$ ; (6) straight line through the origin confirms $C\propto 1/r^2$ (or log–log gradient $-2$ ). Omitting the safety point, the background subtraction, or the correct linearised plot each loses marks.
Question 10
Core Practical 13 style6 marks
Q (6 marks). Plan an experiment to determine the specific latent heat of vaporisation $L$ of water using an electrical heater. State the variables, the apparatus, and explain fully how a graph of the results gives $L$ while cancelling the heat lost to the surroundings. Include one safety point.
Model answer
Circuit and variables. An electrical heater of measured power $P = VI$ (from a voltmeter and ammeter) boils water in an insulated container standing on a top-pan balance; steam escapes so the water mass falls. The independent variable is the heater power $P$ (set with a variable resistor/supply, ≥6 values); the dependent variable is the rate of mass loss $\dfrac{\Delta m}{\Delta t}$ (from balance readings timed with a stopwatch) once boiling is steady.

Apparatus. Voltmeter and ammeter (digital) to find $P=VI$ ; a top-pan balance (resolution 0.01 g) to measure the mass boiled off; a stopwatch for the timing interval; the water kept boiling steadily at each power before readings.

Precaution. The water and steam are very hot and can scald: use a lid/shield, tongs and a heatproof mat, and keep clear of the escaping steam.

How the graph gives $L$ (cancelling heat loss). In a time $t$ the energy supplied equals the energy to vaporise the water plus the constant heat lost to the surroundings $h$ :

$P\,t = L\,\Delta m + h\,t \;\Rightarrow\; P = L\left(\frac{\Delta m}{t}\right) + h.$

Plot $P$ (y-axis) against the rate of mass loss $\dfrac{\Delta m}{t}$ (x-axis): a straight line whose gradient is $L$ and whose intercept is the (constant) heat-loss power $h$ . Because $L$ comes from the gradient, the steady heat loss to the surroundings is automatically cancelled out. Repeat and use a best-fit line.
Why this scores
Marks (max 6): (1) IV = power $P=VI$ , DV = rate of mass loss; (2) named apparatus with resolutions (balance 0.01 g, voltmeter/ammeter); (3) ≥6 powers over a range + steady boiling + repeats; (4) specific scald hazard + matching precaution; (5) use $P = L(\Delta m/t) + h$ ; (6) plot $P$ vs $\Delta m/t$ → gradient $=L$ , intercept $=h$ , so heat loss cancels. This is a key GRADIENT-with-intercept case — reward recognising that the graph method removes the constant heat loss.

Key Formulae — Practical Skills II - Planning

The formulae you need to memorise for practical skills ii - 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 (micrometer 0.01 mm, calipers 0.1 mm) gives a smaller $\Delta x$ and so a smaller percentage uncertainty. At IA2 also used for counting uncertainty $\sqrt{N}$ in radioactivity.
Example
Lead sheet 2.00 mm with a micrometer (0.01 mm): $\tfrac{0.01}{2.00}\times100 = 0.5\%$ .
What to plot for the RC time constant (CP11)
$V = V_0\,e^{-t/RC} \;\Rightarrow\; \ln V = \ln V_0 - \tfrac{1}{RC}\,t;\ \ \text{plot } \ln V \text{ vs } t,\ \ RC = -\tfrac{1}{\text{gradient}}$
$\ln V$
natural log of the capacitor p.d. (y-axis)
$t$
time since discharge began / s (x-axis)
$RC$
time constant / s ( $=-1/\text{gradient}$ )
When to use
Planning Core Practical 11: take logs of the exponential discharge so a straight-line graph of $\ln V$ against $t$ has gradient $-1/RC$ ; divide the known $C$ into $RC$ to find $R$ .
Example
Gradient $-0.50\ \text{s}^{-1}$ ⇒ $RC = 2.0\ \text{s}$ .
What to plot for the decay constant (radioactivity)
$A = A_0\,e^{-\lambda t} \;\Rightarrow\; \ln A = \ln A_0 - \lambda t;\ \ \text{plot } \ln A \text{ vs } t,\ \ \lambda = -\text{gradient}$
$\ln A$
natural log of the (background-corrected) activity/count rate (y-axis)
$t$
time / s (x-axis)
$\lambda$
decay constant / s⁻¹ ( $=-\text{gradient}$ )
When to use
Planning a radioactive-decay experiment: linearise the decay law so a straight-line graph of $\ln A$ against $t$ has gradient $-\lambda$ ; the half-life follows from $t_{1/2}=\ln 2/\lambda$ .
Example
Gradient $-0.030\ \text{s}^{-1}$ ⇒ $\lambda = 0.030\ \text{s}^{-1}$ , $t_{1/2}=\tfrac{\ln 2}{0.030}\approx 23\ \text{s}$ .
What to plot for the inverse-square law (CP15)
$I = \frac{k}{r^2} \;\Rightarrow\; \text{plot } I \text{ vs } \frac{1}{r^2}\ (\text{straight line, gradient } k),\ \text{ or } \log I \text{ vs } \log r\ (\text{gradient } -2)$
$I$
corrected count rate / intensity (y-axis)
$1/r^2$
reciprocal of separation squared / m⁻² (x-axis)
$r$
source–detector separation / m
When to use
Planning an inverse-square experiment (gamma count rate or light intensity): plotting $I$ against $1/r^2$ gives a straight line through the origin, confirming the inverse-square law; a log–log plot has gradient $-2$ .
Example
A straight line of $I$ vs $1/r^2$ through the origin confirms $I\propto 1/r^2$ .
What to plot for a mass–spring (SHM, CP16)
$T = 2\pi\sqrt{\tfrac{m}{k}} \;\Rightarrow\; T^2 = \tfrac{4\pi^2}{k}\,m;\ \ \text{plot } T^2 \text{ vs } m,\ \ k = \tfrac{4\pi^2}{\text{gradient}}$
$T^2$
period squared / s² (y-axis)
$m$
oscillating mass / kg (x-axis)
$k$
spring constant / N m⁻¹
When to use
Planning Core Practical 16: square the period equation so $T^2$ against $m$ is a straight line through the origin of gradient $4\pi^2/k$ .
Example
Gradient $8.0\ \text{s}^2\text{kg}^{-1}$ ⇒ $k = \tfrac{4\pi^2}{8.0}\approx 4.9\ \text{N m}^{-1}$ .
Equation of a straight line (the master tool)
$y = mx + c$
$m$
gradient (carries the physics quantity when the axes are chosen well)
$c$
y-intercept (can also carry a wanted quantity, e.g. $\ln A$ or $\ln V_0$ )
When to use
The master planning tool: rearrange OR take logs of the governing IA2 equation into the form $y=mx+c$ , then choose the axes so the gradient (or intercept) equals the quantity you want. Exponential ⇒ plot $\ln(\ldots)$ ; power law ⇒ plot log–log or the reciprocal.
Example
$V=V_0 e^{-t/RC}$ becomes $\ln V = \ln V_0 - \tfrac{1}{RC}t$ , matching $y=c+mx$ with $y=\ln V$ , $x=t$ .

Key Definitions and Keywords — Practical Skills II - Planning

Definitions to memorise and the exact keywords mark schemes credit for practical skills ii - 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
Absorber thickness, mass on a spring, time since discharge began.
Dependent variable
Examiner keyword
The quantity measured in response to changing the independent variable (normally plotted on the y-axis).
Example
Count rate, period, potential difference across a capacitor.
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, calipers 0.1 mm, micrometer 0.01 mm, balance 0.01 g. Finer resolution lowers the percentage uncertainty.
Linearisation
Examiner keyword
Rearranging a non-linear law into the straight-line form $y=mx+c$ so a graph gives the wanted quantity from its gradient or intercept. For an exponential ( $V=V_0 e^{-t/RC}$ ) take natural logs to plot $\ln V$ against $t$ ; for a power law take a log–log plot or plot against a reciprocal (e.g. $I$ vs $1/r^2$ ).
Related: straight line
Straight-line (y = mx + c) method
Examiner keyword
The master planning tool: choose the axes (after any linearisation) so the gradient $m$ or intercept $c$ equals the physics quantity required, then find it from a large best-fit-line triangle with units.
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 (e.g. subtracting the background count rate), 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 (and, in radioactivity, by counting over a long time), 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, or the statistical nature of radioactive decay); reduced by taking more repeats and a mean, and by counting over longer times.
Systematic error
Examiner keyword
An error that shifts every reading in the same direction by a similar amount (e.g. a zero error on a micrometer, or an unsubtracted background count); not reduced by repeating, but removed by recalibrating/zeroing or by correcting (subtracting the background).
Counting (statistical) uncertainty
Examiner keyword
For a radioactive count of $N$ , the uncertainty is $\sqrt{N}$ , so the fractional uncertainty is $\sqrt{N}/N = 1/\sqrt{N}$ . Counting over a longer time gives a larger $N$ and a smaller percentage uncertainty.
Percentage uncertainty
Examiner keyword
(absolute uncertainty ÷ reading) × 100. Choosing finer-resolution apparatus, measuring larger quantities, and (for counts) counting for longer all reduce it; when quantities are combined by multiplication/division the percentage uncertainties add.

Common Mistakes and Misconceptions — Practical Skills II - Planning

The traps other students keep falling into on practical skills ii - 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 6 (WPH16) 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. 'source–detector separation kept constant by clamping both to a fixed track', 'counting time kept constant at 5 minutes', or 'same fixed resistor in the discharge circuit'.
✕Writing 'be careful' or 'radiation is dangerous' instead of a specific hazard and a matching precaution (especially for radioactive sources and lasers)
Edexcel IAL Physics Unit 6 (WPH16) examiner reports
Why it happens
Students fall back on a generic safety phrase when they haven't identified the actual IA2 hazard in this experiment.
How to avoid it
Name a SPECIFIC hazard and a MATCHING precaution: 'ionising gamma radiation → handle with forceps, keep at a distance, minimise time, store in lead', or 'laser → never look into the beam, avoid reflections, use a low-power class-2 laser', or 'charged capacitor → discharge it through a resistor before touching'.
✕Referring to apparatus vaguely ('a ruler', 'a timer', 'a meter') instead of naming the specific IA2 instrument and its resolution
Edexcel IAL Physics Unit 6 (WPH16) 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: micrometer (0.01 mm) for thin absorbers, calipers (0.1 mm) for cards, GM tube + scaler for counts, voltage sensor + datalogger for a fast capacitor discharge, top-pan balance (0.01 g) for mass loss.
✕Planning too few readings, or over too narrow a range, and not repeating them (or, in radioactivity, counting for too short a time)
Edexcel IAL Physics Unit 6 (WPH16) examiner reports
Why it happens
Students focus on the method and overlook that a reliable graph needs enough well-spread points, and that short counts have a large $\sqrt{N}$ uncertainty.
How to avoid it
Take AT LEAST SIX values over as wide a range as practicable, repeat each and take a mean, and — for counts — count over a long fixed time so the percentage counting uncertainty $1/\sqrt{N}$ is small.
✕Choosing suitable apparatus but not JUSTIFYING the choice (e.g. not saying why a micrometer not a ruler, or why a datalogger not a stopwatch)
Edexcel IAL Physics Unit 6 (WPH16) 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/percentage uncertainty or by capability: 'a micrometer (0.01 mm) gives a small percentage uncertainty on a 1 mm absorber', or 'a datalogger samples the fast discharge automatically at ms resolution, removing reaction-time error'.
✕Not linking the plot to the IA2 equation — plotting the raw variables of an exponential or power law instead of linearising to get a straight line
Edexcel IAL Physics Unit 6 (WPH16) examiner reports
Why it happens
Students plot 'the two things they measured' without taking logs or reciprocals of the governing equation to reach $y=mx+c$ .
How to avoid it
For an exponential ( $V=V_0 e^{-t/RC}$ , $A=A_0 e^{-\lambda t}$ , $C=C_0 e^{-\mu x}$ ) take natural logs and plot $\ln(\ldots)$ against the variable, reading the constant off the gradient; for a power/inverse-square law plot against a reciprocal ( $I$ vs $1/r^2$ ) or use a log–log plot (gradient $-2$ ). Always state how the gradient/intercept gives the wanted quantity.

Practical Skills II - Planning — frequently asked questions

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

Practical Skills II - 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 capacitor-discharge plan (3 marks)

2Choosing apparatus to measure source–detector separation (2 marks)

3Hazard and precaution with a radioactive source (2 marks)

4Range and repeats when measuring specific latent heat (3 marks)

5Justifying a data-logging method for a collision plan (3 marks)

6Choosing what to plot for the spring constant from SHM (4 marks)

7Planning to find the time constant of an RC discharge (6 marks)

8Full plan: absorption of gamma rays by lead (6 marks)

9Planning a novel experiment: thermistor B-value from resistance (6 marks)

Percentage uncertainty of a reading

What to plot for the RC time constant (CP11)

What to plot for the decay constant (radioactivity)

What to plot for the inverse-square law (CP15)

What to plot for a mass–spring (SHM, CP16)

Equation of a straight line (the master tool)

Independent variable

Dependent variable

Control variable

Fair test

Resolution

Linearisation

Straight-line (y = mx + c) method

Range and interval

Accuracy

Precision

Reliability

Validity

Random error

Systematic error

Counting (statistical) uncertainty

Percentage uncertainty

✕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 'radiation is dangerous' instead of a specific hazard and a matching precaution (especially for radioactive sources and lasers)

✕Referring to apparatus vaguely ('a ruler', 'a timer', 'a meter') instead of naming the specific IA2 instrument and its resolution

✕Planning too few readings, or over too narrow a range, and not repeating them (or, in radioactivity, counting for too short a time)

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

✕Not linking the plot to the IA2 equation — plotting the raw variables of an exponential or power law instead of linearising to get a straight line

Practical Skills II - Planning — frequently asked questions