Resistance of a wire & RP3 — AQA GCSE Physics

SPEC 8463 • MODULE 2 • LESSON 2.03 • RP3

Resistance of a Wire

Required Practical 3 — one of the most reliably-examined practicals on Paper 1. By the end of this lesson: the method, the controls, the expected graph, and the typical errors that examiners flag.

The Hypothesis

Before any practical, you state what you expect to find. For RP3, the hypothesis is short and exact:

RP3 Hypothesis

The resistance of a wire is directly proportional to its length, at constant cross-sectional area, material, and temperature.

What "Directly Proportional" Means Here

Doubling the length doubles the resistance. Trebling it trebles the resistance. Plotted as a graph: a straight line through the origin. If the line has any other shape, your hypothesis isn't supported — and either the physics or your method has a problem.

Why This Practical Matters

RP3 brings together everything from lessons 2.01 and 2.02 — circuit symbols, ammeter/voltmeter rules, and V = I R — and applies them to a single experiment. It's a frequent target for 6-mark "describe a method" questions on Paper 1.

The Apparatus

One circuit. Five components you need to draw correctly.

RP3 Apparatus — current flows through the test wire only between the crocodile clips

The Five Things to Get Right When Drawing It

1. Cell with + clearly marked.

2. Switch — so you can disconnect between readings.

3. Ammeter in series — measures the same current that flows through the wire.

4. Voltmeter in parallel across the test wire — measures pd across the wire only.

5. Crocodile clips on the test wire, both between the voltmeter's connections, so you measure the pd across the chosen length.

IV / DV / Controls

Every required practical question asks you to identify the variables. Get this triplet right and you've already scored 2–3 marks.

Variable TypeWhat It IsHow It's Set / Measured

Independent VariableLength of the wire ( L )Set by moving the crocodile clips along the metre ruler — typically 6 lengths between 10 cm and 100 cm

Dependent VariableResistance ( R )Calculated , not measured directly: take voltmeter and ammeter readings, then R = V / I for each length

Control VariablesCross-sectional area, material, temperatureSee next slide — each has a specific reason that earns the mark

Most Common Mix-Up

Resistance is the DV, not "the thing being measured directly". Stating "I will measure resistance" is wrong — you measure V and I, and calculate R from V/I. Mark schemes specifically credit students who say "calculate resistance using R = V/I" rather than "measure resistance".

The Three Controls — With Reasons

"Same wire" alone is one mark. "Same wire so that…" is two marks. The reason converts a description into an explanation.

Control 1: Cross-Sectional Area

Use the same wire throughout the experiment.

Why: a thicker wire has lower resistance, so any change in thickness would change R independently of length — confounding the result.

Control 2: Material

Use the same wire (e.g. constantan, nichrome) throughout.

Why: different materials have different resistance per unit length. Switching material would change R in a way that has nothing to do with length.

Control 3: Temperature

Keep the current low and disconnect the circuit between readings.

Why: current heats the wire, raising its temperature, which raises its resistance — invalidating the constant-temperature condition.

Why "Same Wire" Covers Two Controls

Using the same single wire automatically holds both cross-sectional area and material constant. That's why you only need one practical action ("use the same wire") to handle two controls — but you still get marks for naming both reasons separately.

The Method — Six Steps

This is the order AQA mark schemes follow. Memorise the sequence; the wording can flex but the steps cannot.

1

Set up the circuit: cell, switch, ammeter in series; voltmeter in parallel across the test wire on the metre ruler.

2

Measure the length between the crocodile clips with the ruler. Start at, e.g., 10 cm.

3

Close the switch briefly and record the readings on the ammeter ( I ) and voltmeter ( V ).

4

Open the switch immediately to let the wire cool.

5

Move one crocodile clip to a new length (e.g. 20 cm). Repeat steps 3–4 for at least six lengths up to 100 cm.

6

Calculate R = V / I for each length. Plot resistance (y-axis) against length (x-axis).

Improving Reliability

For a more reliable result: repeat each length at least 3 times and calculate the mean of V and I before working out R. Identify and exclude any anomalous readings before averaging.

The Expected Result

If your hypothesis (R ∝ L) is correct, the graph of R against length is one specific shape:

R against L: straight line through the origin

Three Things This Graph Tells You

Straight line → R is proportional to L.

Through the origin → at zero length, zero resistance (no wire to oppose current).

Gradient = R per unit length (in Ω/m) — a property of the specific wire used.

Reading the Gradient

Examiners can ask you to use the graph to find the resistance per unit length. The technique is the same as any other gradient calculation.

How to Calculate Gradient

Pick two points on the line of best fit (not on data points — the line averages the random scatter). Read off (L₁, R₁) and (L₂, R₂). Then:

gradient = (R₂ − R₁) / (L₂ − L₁)

Use a large triangle on the graph — the bigger the triangle, the smaller the percentage error in your reading.

What the Gradient Means Physically

A gradient of, say, 4 Ω/m means the wire adds 4 ohms of resistance for every metre of its length. That value depends on the wire's material and thickness — it's a fingerprint of the specific wire being tested.

The Heating Problem

The single biggest source of systematic error in RP3 — and the one mark schemes consistently reward students for addressing.

The Problem

Current flowing through the wire transfers energy to the thermal store of the wire (covered in lesson 2.07's P = I²R). The wire's temperature rises. For a metallic conductor, resistance rises with temperature. So if you leave the circuit on:

The R you calculate isn't the room-temperature R you intended to measure — it's a higher value, because the wire has heated up.

Mitigation 1: Low Current

Use a low source pd (e.g. 1.5 V cell rather than a 9 V battery) so the current stays low. Lower current = less heating per second.

Mitigation 2: Switch Off Between Readings

Open the switch immediately after recording V and I. Let the wire return to room temperature before the next reading. This is why the switch is in the circuit.

PEE Worked Example

"Explain why the student opened the switch immediately after taking each reading."2 MARKS

P

Point: Opening the switch prevents the wire from heating up between readings.

E

Evidence: When current flows, energy is transferred to the thermal store of the wire, raising its temperature.

E

Explain: Higher temperature means higher resistance. By keeping the wire cool, every reading is taken at the same temperature — so any change in R is due to the change in length, not in temperature.

✓

Mark scheme: M1 for naming the heating effect; M2 for linking heating to a change in resistance and the need for constant temperature.

Calculating R From Your Readings

Once you have V and I for each length, R follows from V = I R. The same calculation is repeated for every length.

Sample reading at L = 50 cm: voltmeter reads 0.40 V, ammeter reads 0.20 A.

Calculate the resistance of the wire at this length. 3 MARKS

E

Equation: V = I × R → R = V / I

S

Substitute: R = 0.40 / 0.20

U

Unit: ohms (Ω)

A

Answer: R = 2.0 Ω

✓

Repeat: the same calculation is done for every length — that's how the (L, R) data points for the graph are built.

Handling Anomalies

An anomalous reading is a result that doesn't fit the pattern of the others — usually caused by a misread, a poor crocodile-clip contact, or a brief temperature change.

Identifying Anomalies

Plot all your readings (or scan the table). Any point that's far from the line of best fit — much further than the typical scatter — is an anomaly. On a table, it's a value that breaks the pattern of "R increases steadily with L".

Treating Anomalies

Exclude them from the mean before calculating R. Don't ignore them silently — note them in your method as "one anomaly identified and excluded". Repeat the reading if possible to replace the bad value.

Don't Just Drag the Line

If you have several scattered points, the answer is not to draw a line that avoids them all — it's to draw a line of best fit through the average pattern. Anomalies are excluded from the mean; the line of best fit then uses the cleaned data.

What Earns The Top Marks

For a 6-mark "describe a method" question on RP3, AQA mark schemes look for these specific points. Hit them all and you reach Level 3 (5–6 marks).

Indicative Content (Mark Scheme Style)

Measure the length of the wire between the crocodile clips using the ruler

Length is varied by moving the crocodile clips

Current measured with the ammeter ; pd measured with the voltmeter

Calculate resistance for each length using R = V / I (or V = IR)

Record V and I for different lengths (typically 6 or more)

Repeat readings for each length and calculate mean values

Remove anomalous readings before averaging

Keep current low to minimise heating of the wire

Disconnect the circuit between readings (open switch)

Level 3 = 5–6 Marks

The mark scheme uses banded levelled marking. Level 3 (5–6 marks): method leads to a valid outcome, key steps identified, logically sequenced. Level 2 (3–4 marks): most steps present but not fully sequenced. Level 1 (1–2 marks): some relevant steps but unclear links. Hit the bullets above in a logical order = Level 3.

Weak vs Strong Answers

The difference between Level 1 and Level 3 isn't usually content — it's linking. Statement plus reason scores; statement alone often doesn't.

✗ Level 1 — 1 mark

Control variable: "Use the same wire."

States what to do but not why. The link to the experiment's validity is missing.

✓ Level 3 — 2 marks

Control variable: "Use the same wire — so cross-sectional area and material remain constant; the only thing changing the resistance is the length being varied."

States the action AND explains why it controls the experiment. The "so that…" or "this means…" connector earns the second mark.

✗ Level 1

"Take voltage and current and divide them to find resistance."

Vague — doesn't say which meter does what, where it's connected, or which equation is used.

✓ Level 3

"Measure the pd across the wire with the voltmeter, and the current through the wire with the ammeter. Calculate R = V / I for each length."

Specific: which meter, what it measures, where it's connected, the equation used.

Examiner Alert — RP3 in Recent Papers

RP3 was the focus of a 6-mark "describe a method" question on a recent Paper 1. The examiner report was unusually direct about what separated the strong answers from the weak ones.

What The Examiner Report Said

Students who had physically completed RP3 in class found the question straightforward. The mean mark on a 6-mark question was around 4 — meaning roughly half the candidates scored under 4. 44% of students reached Level 3 (5 or 6 marks); the rest were stuck in Level 1 or Level 2 because their methods were vague.

~4 / 6

Mean Mark (recent P1)

44%

Reached Level 3 (5–6)

~37%

Stuck at Level 2 (3–4)

The Lesson

If your school does the practical, do it. The hands-on memory of moving crocodile clips, watching the meters, and switching off between readings is exactly what makes a 6-mark answer specific. If you're revising without lab access, walk through the method on paper several times — drawing the circuit, listing the steps, naming the controls — until the sequence is automatic.

Common Errors

Students often writeCorrect versionWhy it matters

"Take voltage and current and divide""Measure V across wire, I through wire, calculate R = V / I"Method must specify what is measured where

Length labelled as a control variableLength is the IV (the thing being varied)IV / control confusion loses 2 marks

Anomalies kept in the meanAnomalies excluded before averagingSkews the mean; affects the line of best fit

"Resistance is measured directly"R is calculated from V and IMark schemes credit "calculate"; not "measure"

No mention of switching off between readingsOpen switch immediately to prevent heatingMisses 1–2 marks tied to temperature control

Voltmeter shown in seriesVoltmeter in parallel across the test wireWrong measurement; whole circuit fails

Practice Question

Describe a method the student could use.

The equipment they will use

How they will vary the length

What measurements they will take and how

How they will calculate the resistance

One step that controls the temperature of the wire

6 MARKS

1

Equipment: connect a cell, switch, and ammeter in series with the test wire (laid alongside a metre ruler with a crocodile clip at each end). Connect a voltmeter in parallel across the section of test wire being measured.

2

Vary length: set the length under test by moving one of the crocodile clips along the metre ruler. Use at least 6 different lengths between 10 cm and 100 cm.

3

Measurements: for each length, close the switch briefly, record the voltmeter reading (V) and the ammeter reading (I).

4

Calculate R: for each length, calculate the resistance using R = V / I. Repeat each length 3 times and use the mean values; exclude any anomalies.

5

Control temperature: open the switch immediately after each reading so the wire does not heat up. Use a low-voltage source so the current stays low.

✓

Level 3 anchor points: ammeter & voltmeter named with what each measures · R = V/I · ≥6 lengths · repeats and mean · anomalies excluded · open switch / low current for temperature control. Hit these and you score 5–6.

Three Things to Remember

1. Resistance Is Calculated, Not Measured

You measure V with the voltmeter and I with the ammeter. R is then calculated using R = V / I. Mark schemes are explicit on this — saying "I will measure resistance" loses the methodology mark.

2. Every Control Needs a Reason

"Same wire" alone is half a mark. "Same wire so that cross-sectional area and material stay constant" is the full credit. The "so that…" connector is the difference between Level 1 and Level 3.

3. Switch Off Between Readings

The wire heats when current flows. Hot metal has higher resistance. The way to keep R measurements at one temperature is to open the switch immediately after each reading and use a low source pd. Both should appear in any 6-mark method answer.

Summary

The Investigation

Hypothesis: R is directly proportional to L (constant area, material, temperature)

IV: length of wire · DV: resistance (calculated as V / I) · Controls: cross-sectional area, material, temperature

Apparatus: cell, switch, ammeter (series), voltmeter (parallel across test wire), test wire on metre ruler with crocodile clips

The Method (Order Matters)

Set up circuit · vary L by moving crocodile clips · ≥6 lengths from 10 cm to 100 cm

For each L: close switch, record V and I, open switch immediately

Repeat each length, calculate mean V and I, exclude anomalies

Calculate R = V / I for each L · plot R against L

The Result

Graph: R against L — straight line through the origin

Confirms R ∝ L · gradient = R per unit length (Ω/m)

The Marks

Always say " calculate R " not "measure R"

Every control gets a " so that… " reason

Open switch + low current → temperature control (worth 1–2 marks every time)

≥6 lengths · repeats and mean · anomalies excluded

Checkpoint

You should now be able to:

✅ Draw the RP3 circuit from memory: cell, switch, ammeter (series), voltmeter (parallel across test wire), test wire on metre ruler with two crocodile clips.

✅ State the IV, DV, and three controls with reasons for each — and recognise that resistance is calculated, not measured.

✅ Describe the method in 6 ordered steps, including ≥6 lengths, switching off between readings, repeating, averaging, and excluding anomalies.

✅ Sketch the expected graph (R vs L: straight line through origin) and explain that the gradient gives resistance per unit length.

✅ Identify and reduce the heating error — explain why hot metal has higher resistance, and how a low source pd plus a switch addresses it.

Next Lesson

2.04 — IV characteristics & RP4. Three different components, three different graph shapes: the linear ohmic resistor, the S-curve filament lamp, and the diode's one-way current. We'll cover the explanations and the practical that produces each one.