AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER
Current, resistance and potential difference
Electricity · Lesson 2 of 10
Warm-up
Answer each one, then check.
1. Write the unit of resistance.
Ohm (Ω)
2. What is the unit of potential difference?
Volt (V)
3. Where is a voltmeter connected?
In parallel across the component
4. What does a longer wire do to resistance?
Increases it
5. What is a current?
A flow of charge
Learning Objectives
1. Explain that current depends on resistance and potential difference.
2. Recall and apply V = IR.
3. Describe how to investigate the resistance of a wire and of combinations of resistors (Required Practical 3).
4. Interpret graphs of resistance against length.
V = Ir
potential difference = current × resistance V = IR
The greater the resistance of a component, the smaller the current for a given potential difference across it. Questions use the term potential difference; voltage also gains credit.
Required Practical 3: Resistance of a Wire
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Change only the length of wire between the clips. |
A circuit with a cell, switch and ammeter in series with a wire on a metre rule, and a voltmeter across the length of wire being tested.
Method: Resistance and Length of a Wire
Required Practical 3, part 1.
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1 Set up Connect the circuit with the ammeter in series and the voltmeter across the wire. |
2 Set the length Clip the crocodile clips at 0.20 m, and record the pd and current. |
3 Switch off between readings This keeps the wire at a constant temperature. |
4 Repeat Take readings at 0.40 m, 0.60 m, 0.80 m and 1.00 m. |
5 Calculate For each length R = V ÷ I. |
6 Plot Draw a graph of resistance against length: a straight line through the origin. |
Method: Resistors in Series and Parallel
Required Practical 3, part 2.
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1 Series Connect known resistors in series, measure V and I, calculate R = V ÷ I. |
2 Parallel Repeat with the same resistors in parallel. |
3 Compare The series combination has a larger resistance than either resistor; the parallel combination has a smaller one. |
Calculating Current
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A 12 V battery is connected across a 4.0 Ω resistor. Calculate the current. |
1. Rearrange
I = V ÷ R
2. Substitute
I = 12 ÷ 4.0
3. Answer
I = 3.0 A
Answer: 3.0 A
Calculating Potential Difference
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A current of 0.50 A flows through a 20 Ω resistor. Calculate the potential difference across it. |
1. Write the equation
V = IR
2. Substitute
V = 0.50 × 20
3. Answer
V = 10 V
Answer: 10 V
Calculating Resistance
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A voltmeter reads 6.0 V and an ammeter reads 20 mA. Calculate the resistance. |
1. Convert the current
20 mA = 0.020 A
2. Rearrange
R = V ÷ I
3. Substitute
R = 6.0 ÷ 0.020
4. Answer
R = 300 Ω
Answer: 300 Ω
Getting Good Results
Why we switch off between readings.
▸ Heating. A current makes the wire hotter, and a hotter metal wire has a higher resistance.
▸ Constant temperature. Use a low current and switch off between readings.
▸ Zero errors. Check the meters read zero before starting.
▸ Repeat. Repeat and average to reduce random errors.
Key Terms
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Resistance How much a component opposes the flow of current. |
Ohm The unit of resistance (Ω). |
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Potential difference The energy transferred per coulomb; measured in volts. |
Voltmeter Measures pd; connected in parallel. |
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Constant temperature Kept the same so the resistance does not change for other reasons. |
Directly proportional Doubling one quantity doubles the other. |
Your Task: Predict the Graph
10 minutes
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A student measures the resistance of 20 cm, 40 cm, 60 cm, 80 cm and 100 cm of a wire. What does the graph of resistance against length look like? A 30 cm length has a resistance of 1.8 Ω; what is the resistance of 90 cm? 1. Sketch the axes. 2. Use the ratio of lengths. |
A good answer shows: A straight line through the origin: resistance is directly proportional to length. 90 cm is three times as long, so R = 5.4 Ω.
Can I...?
☐ Recall V = IR.
☐ Rearrange for I or R.
☐ Convert mA to A.
☐ Describe the resistance of wire method.
☐ Explain why the wire is switched off between readings.
☐ Draw a graph of R against length.
☐ Say what happens to resistance in series and parallel.
☐ Use the correct units.
Summary
✓ V = IR.
✓ Ammeter in series, voltmeter in parallel.
✓ Longer wire means greater resistance.
✓ Switch off between readings to keep the temperature constant.
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EXAM FOCUS A potential difference of 9.0 V is applied across a resistor and the current is 0.30 A. Calculate the resistance. (2 marks) State the equation, substitute and give the unit Ω. |