AQA GCSE Combined Science: Trilogy Physics Paper 1 (Higher), 2020: Question 3

13 marks · Standard Demand difficulty · Extended Answer

Investigate how the resistance of a wire varies with length, interpret results from a table and graphs, calculate current using resistance read from a graph and potential difference, and explain control of variables in a resistance-based glucose meter context.

Practise this question

Question

The question page contains five linked parts about resistance. Figure 2 shows a simple circuit with a cell or battery, an ammeter in series, a variable resistor in series, and a test wire, with a voltmeter connected in parallel across the wire; the first part asks why the variable resistor must be adjusted when the wire length changes. Table 1 gives three potential difference readings for a 0.10 m wire, with values X, 0.18 V and 0.15 V and a mean of 0.17 V, and asks for X. Figure 3 is a graph of resistance of wire in ohms against length of wire in metres from 0.00 to about 0.60 m, showing a straight-line increase through the origin to about 0.85 ohms at 0.50 m, followed by a question asking for the relationship between length and resistance. A later section explains that a glucometer uses resistance of a blood sample to calculate glucose concentration; Figure 4 is a downward-curving graph of resistance of blood sample in ohms against glucose concentration in grams per litre from about 0.6 to 1.3, decreasing from about 13 ohms to about 4.5 ohms, and students must use 0.90 V and a glucose concentration of 0.98 g/L to determine current, then explain why each test tube must be identical for valid results.
Question text

03 A student investigated how the resistance of a piece of wire varies with its length.

03.1 Figure 2 shows the circuit used.

Figure 2

Explain why the student needed to adjust the variable resistor each time she changed

the length of the wire.

[3 marks]

03.2 The student recorded three measurements of the potential difference across a 0.10 m

length of wire.

Table 1 shows the results.

Table 1

Potential difference in V

Length in m

12 3 Mean

0.10 X 0.18 0.15 0.17

Calculate X in Table 1.

[2 marks]

*09* 11

X = V

03.3 Figure 3 shows the results for five different lengths of the wire.

Figure 3

*10* Describe the relationship between the length of the wire and the resistance of

the wire.

[2 marks]

A glucometer uses the resistance of a blood sample to calculate the glucose

concentration in a person’s blood.

A blood sample is put into a small tube, which is put inside the glucometer. The blood

then acts like a resistance wire.

Figure 4 shows the relationship between the resistance of a blood sample and the

glucose concentration.

Figure 4

03.4 The glucometer applies a potential difference of 0.90 volts across a blood sample.

The glucose concentration of the blood sample is 0.98 grams/litre.

Determine the current in the blood sample.

[4 marks]

Current = A

03.5 A new tube is used each time a blood sample is tested.

Explain why valid results are only obtained if each tube is identical.

[2 marks]

Mark scheme

Show the mark scheme The mark scheme is a table listing answers, extra information, marks and specification references for questions 03.1 to 03.5, totalling 13 marks. It states that adjusting the variable resistor changes total circuit resistance, keeps the current the same and therefore keeps the wire temperature constant; that X in the mean calculation is 0.18 V; that resistance is directly proportional to length; that for 0.98 g/L the graph gives a resistance of about 7.5 ohms and using V equals I times R gives I = 0.12 A; and that identical tubes are needed because tube dimensions such as length, width, volume, shape or size affect resistance so only glucose concentration changes the result.

AO /

Question Answers Extra information Mark

Spec. Ref.

03.1 (the variable resistor) changes 1 AO1

the resistance of the circuit 6.2.1.3

6.2.1.4

to keep the current the same 1 RPA 15

so the temperature of the wire is allow to control the temperature 1

kept constant of the wire

03.2 X+0.18+0.15 allow X = 3 × 0.17 – 0.18-0.15 1 AO2

0.17 =

6.2.1.3

X = 0.18 (V) 1 RPA 15

03.3 resistance is directly allow length is directly 2 AO1

proportional to length proportional to resistance 6.2.1.3

RPA 15

allow as length increases

resistance increases for 1 mark

allow positive correlation for 1

mark

03.4 resistance = 7.5 (Ω) allow a range from 7.4 to 7.6 1 AO3

6.2.1.3

0.90 = I × 7.5 allow their value of R read from 1

the graph correctly substituted

0.90 allow a correct re-arrangement 1

I = using their value of R read from

7.5

the graph

I = 0.12 (A) allow a value consistent with 1

their value of R read from the

graph

03.5 the length/width/volume (of the allow length/width/volume (of 1 AO3

blood sample) affects the the blood sample) should be a 6.2.1.3

resistance of the blood sample control variable

allow shape/size of the tube

should be a control variable

ignore amount of blood

so only glucose concentration 1

affects resistance

Total 13

How to answer it

Resistance of a Wire and a Glucose Sensor

What this question tests

You need to explain a practical method, use an average from repeated measurements, describe a graph trend, read a graph to find a value, and explain why control variables matter in a biomedical test.

Question overview: electricity, graphs, and practical skills

💡 Key knowledge

  • Resistance is measured in ohms (Ω).
  • Current, resistance and potential difference are linked by V = I × R .
  • For a fair test, keep control variables the same.
  • Repeated readings can be averaged to reduce random error.

🧠 Exam technique

  • Use short, clear scientific statements.
  • When using a graph, quote the value from the graph before calculating.
  • Show working in steps so you can earn method marks.

❌ Common errors

  • Not explaining why the variable resistor is needed.
  • Giving only “the current stays the same” without linking to temperature.
  • Reading values inaccurately from a graph.
  • Forgetting units, especially V, Ω, and A.

Part (a) 03.1 — Why adjust the variable resistor?

3 marks

✅ Correct answer

The variable resistor changes the resistance of the circuit so that the current stays the same. This keeps the temperature of the wire constant.

Marks are given for: changing resistance, keeping current constant, and controlling temperature.

💡 Key knowledge

  • When current flows through a wire, it heats up.
  • Heating changes the wire’s resistance.
  • If temperature changes, the test is not fair because resistance is affected by both length and temperature.

🧠 How to score full marks

  • Link the variable resistor to current, not just “changing the circuit”.
  • Always finish with the purpose: keeping the wire at the same temperature.

❌ Common errors

  • Saying only “to change resistance” without explaining the effect.
  • Confusing resistance in the circuit with resistance of the test wire.
  • Not mentioning temperature, which is the key control variable here.

Part (b) 03.2 — Calculate X from the mean

2 marks

📐 Calculation steps

Given: mean = 0.17 V, and the three readings are X , 0.18, 0.15

  1. Use the mean formula: mean = total ÷ number of values
  2. Substitute: 0.17 = (X + 0.18 + 0.15) ÷ 3
  3. Multiply by 3: 0.51 = X + 0.33
  4. Subtract 0.33: X = 0.18 V

✅ Correct answer

X = 0.18 V

You can also write: X = 3 × 0.17 − 0.18 − 0.15

💡 Key knowledge

  • The mean is the total of all readings divided by the number of readings.
  • Because the answer is a voltage, the unit must be V.

❌ Common traps

  • Adding 0.18 and 0.15 first but forgetting to subtract from 3 × 0.17.
  • Writing the answer without units.
  • Rounding too early.

Part (c) 03.3 — Describe the relationship between length and resistance

2 marks

✅ Correct answer

As the length of the wire increases, the resistance increases. The resistance is directly proportional to the length of the wire.

💡 Key knowledge

  • A longer wire has more material for electrons to pass through.
  • This causes more collisions, so resistance increases.
  • The graph shows a straight line through the origin, which supports direct proportion.

🧠 What the examiner wanted

  • For 1 mark, say resistance increases with length.
  • For full marks, add “directly proportional”.
  • Using the graph evidence makes your answer stronger.

❌ Common errors

  • Saying “they are related” without describing the trend.
  • Writing “inverse proportional” — this is wrong here.
  • Not mentioning the straight-line pattern.

Part (d) 03.4 — Determine the current in the blood sample

4 marks

📐 Calculation steps

  1. Read the resistance from the graph at 0.98 grams/litre.
  2. The resistance is about 7.5 Ω (accept 7.4 to 7.6 Ω).
  3. Use the equation: V = I × R
  4. Substitute: 0.90 = I × 7.5
  5. Rearrange: I = 0.90 ÷ 7.5
  6. Calculate: I = 0.12 A

✅ Correct answer

Current = 0.12 A

Marks are awarded for: reading the graph correctly, using the equation, rearranging correctly, and giving the final answer with units.

💡 Key knowledge

  • Potential difference is measured in volts (V).
  • Current is measured in amperes (A).
  • Resistance is measured in ohms (Ω).
  • Always check that the numbers you use match the graph and the question.

❌ Common errors

  • Using the wrong graph value for resistance.
  • Rearranging incorrectly: I = R ÷ V is wrong.
  • Forgetting that V = I × R , not V = I + R .
  • Leaving the answer without units.

Part (e) 03.5 — Why must each tube be identical?

2 marks

✅ Correct answer

The length, width, or volume of the blood sample must be the same each time because these affect the resistance of the blood sample. This means only the glucose concentration changes the resistance.

💡 Key knowledge

  • Identical tubes make the test fair.
  • Tube size/shape is a control variable.
  • Without this, you would not know whether resistance changed because of glucose concentration or because of the tube itself.

🧠 Examiner insight

  • Top answers identified the control variable and explained the reason.
  • Strong responses linked the physical property of the blood sample/tube to resistance.

❌ Common errors

  • Saying “to make it fair” with no explanation.
  • Talking about the amount of blood only, when the mark scheme focused on length/width/volume/shape.
  • Not stating that glucose concentration should be the only variable affecting resistance.

Big exam takeaways

💡 Remember

  • Fair tests need control variables.
  • Mean = total ÷ number of readings.
  • Use graph readings carefully before calculating.
  • Resistance questions often test V = I × R .

🧠 Best strategy

  • Answer in the order the marks are likely to be awarded.
  • Use “because” to explain science, not just describe.
  • Check units at the end of every calculation.

❌ Final traps to avoid

  • Misreading graph values.
  • Using the wrong equation.
  • Not linking resistance changes to temperature in the practical.
  • Leaving out the word “directly proportional” when the graph supports it.

Topics

Physics · Required Practicals · P2: Electricity · Physics Required Practicals

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Physics Paper 1 (Higher), 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.