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AQA GCSE Electricity

Resistance and I-V Characteristics

Resistance is the ratio V/I for a component at a particular point. A fixed resistor is approximately ohmic at constant temperature, while a filament lamp and diode have non-linear current-potential difference relationships.

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The revision snapshot

Resistance is the ratio V/I for a component at a particular point. A fixed resistor is approximately ohmic at constant temperature, while a filament lamp and diode have non-linear current-potential difference relationships.

By the end, you should be able to:

  • Explain how resistance changes.
  • Apply the electricity model, evidence or method to an unfamiliar exam context.
  • Recognise and correct this wrong turn: Using one constant resistance for a filament lamp.

Before you revise

Diagnostic question

Choose an answer from memory. Your result tells you what to watch for in the guide.

Resistance is calculated using…

Core revision guide

Learn the model, then use it

Resistance and I-V Characteristics questions become manageable when the central model, evidence and exam method are kept together. Read the explanation, test the misconception and then apply the idea without looking back.

How resistance changes

Resistance is the ratio V/I for a component at a particular point. A fixed resistor is approximately ohmic at constant temperature, while a filament lamp and diode have non-linear current-potential difference relationships.

Build the physics picture

For an ohmic conductor at constant temperature, doubling potential difference doubles current, so the I-V graph is a straight line through the origin. A filament lamp heats as current rises, increasing resistance; a diode mainly allows current in one direction.

The relationship for this guide

Resistance: V = IR (V, A, Ω). Choose each relationship from the physical change described, convert quantities into compatible units and keep the unit beside the final answer.

Labelled current-potential difference graphs show a fixed resistor, filament lamp and diode.
A fixed resistor is linear at constant temperature, a lamp curve changes gradient, and a diode conducts mainly in one direction.Open the full-size exam diagram
ResistanceV = IRV, A, Ω

Misconception clinic

Replace the tempting answer

Read each belief, say what is wrong with it, then compare your correction.

Tempting idea: Using one constant resistance for a filament lamp.

Use instead: Resistance is the ratio V/I for a component at a particular point. A fixed resistor is approximately ohmic at constant temperature, while a filament lamp and diode have non-linear current-potential difference relationships.

Tempting idea: Confusing graph gradient with resistance without checking which variable is on each axis.

Use instead: For an ohmic conductor at constant temperature, doubling potential difference doubles current, so the I-V graph is a straight line through the origin. A filament lamp heats as current rises, increasing resistance; a diode mainly allows current in one direction.

Tempting idea: Forgetting a diode characteristic includes negative p.d.

Use instead: Read the graph shape, identify the component, describe the physical cause and use V = IR only when the relationship is appropriate.

Retrieval practice

Quick checks

1. A fixed resistor at constant temperature has an I–V graph that is…

2. Why does a filament lamp graph curve?

3. A diode mainly allows current…

4. What does a steeper I–V graph mean?

5. When is V = IR useful?

Explore the relationship

Resistance and I-V curve

Change potential difference or resistance, then use I = V ÷ R to predict the current through a fixed resistor.

What to notice: use the readout to describe how one variable changes when the other is controlled.

Worked examples

See the method being built

These examples expose the difference between a secure how resistance changes method and the common error “Using one constant resistance for a filament lamp.”.

Example 1 · Fixed resistor

A fixed resistor has 6.0 V across it and 0.50 A through it. Calculate resistance.

  1. Use R = V ÷ I.
  2. Substitute 6.0 ÷ 0.50.
  3. Give ohms.
Show the answer

R = 12 Ω.

Example 2 · Read a lamp graph

A lamp’s current rises less for each extra volt. Explain the curve.

  1. Describe the graph shape.
  2. Link current to filament temperature.
  3. Link temperature to resistance.
Show the answer

As current rises the filament heats up; its resistance increases, so current no longer rises proportionally with potential difference.

Example 3 · Diode choice

Choose between a diode and fixed resistor for a circuit that must conduct in one direction only.

  1. Identify the required behaviour.
  2. Match it to the component characteristic.
  3. State the direction condition.
Show the answer

Use a diode because it has very high resistance in the reverse direction and conducts mainly when forward biased.

Exam precision

Exam technique: How resistance changes

Do this: Read the graph shape, identify the component, describe the physical cause and use V = IR only when the relationship is appropriate.

Independent practice

Exam-style questions

Write an answer before opening the marking guidance. Then edit the exact phrase or step that would gain the next mark.

1. Calculate the resistance of a component with 9.0 V and 3.0 A.

[3 marks]
Show marking guidance and model answer

Marking guidance: Award one mark for the correct relationship, one for a valid substitution and one for the final answer with its unit.

Model answer: R = 9.0 ÷ 3.0 = 3.0 Ω.

2. Describe the I–V graph of a fixed resistor at constant temperature.

[2 marks]
Show marking guidance and model answer

Marking guidance: 2 marks are available for relevant, linked physics points that match the model answer.

Model answer: A straight line through the origin because current is proportional to potential difference.

3. Explain why a filament lamp’s resistance increases as it heats.

[3 marks]
Show marking guidance and model answer

Marking guidance: Award up to 3 marks for distinct physics points joined into a clear cause-and-effect chain.

Model answer: Metal ions vibrate more, causing more collisions with electrons.

4. State one difference between the I–V graphs of a diode and a resistor.

[1 mark]
Show marking guidance and model answer

Marking guidance: Award one mark for the precise statement shown in the model answer.

Model answer: A diode has almost no current in reverse bias; a fixed resistor has a straight line through the origin in both directions.

5. A component has 4.0 V and 0.20 A. Calculate resistance.

[3 marks]
Show marking guidance and model answer

Marking guidance: Award one mark for the correct relationship, one for a valid substitution and one for the final answer with its unit.

Model answer: R = 4.0 ÷ 0.20 = 20 Ω.

6. Explain why quoting one resistance for a filament lamp can be misleading.

[3 marks]
Show marking guidance and model answer

Marking guidance: Award up to 3 marks for distinct physics points joined into a clear cause-and-effect chain.

Model answer: Its resistance changes with temperature/current, so V ÷ I is different at different points.

Questions pupils ask

Resistance and I-V Characteristics FAQs

What is the main idea in Resistance and I-V Characteristics?

Resistance is the ratio V/I for a component at a particular point. A fixed resistor is approximately ohmic at constant temperature, while a filament lamp and diode have non-linear current-potential difference relationships.

What mistake should I avoid in Resistance and I-V Characteristics?

Using one constant resistance for a filament lamp. For an ohmic conductor at constant temperature, doubling potential difference doubles current, so the I-V graph is a straight line through the origin. A filament lamp heats as current rises, increasing resistance; a diode mainly allows current in one direction.

Useful next steps

Connect this guide to your next task

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