Lesson focus: resistanceHow much a component opposes the flow of current: resistance = potential difference ÷ current, measured in ohms (Ω). links potential difference to current and depends on material, dimensions and temperature.
Calculate resistance, plan a valid resistance investigation and interpret current–potential-difference evidence.
Build on earlier learning
Series and parallel circuitsSeries and parallel circuits changed current because their total resistance changed. This lesson quantifies that relationship.
Practical
Resistance of a wire
- Connect a length of wire with an ammeter in series and a voltmeter in parallel.
- Measure current and potential difference for different lengths.
- Resistance = potential difference ÷ current.
Record: A graph of length against resistance.
Safety: Switch off between readings so the wire does not get hot.
Resistance of a wire
Change the length of the test wire, read the meters and record each resistance.
Controls
You will be successful when you can…
- Use R = V ÷ I.
- Connect meters correctly.
- Interpret linear and curved graphs.
- Explain material, length and temperature effects.
Words that carry the science
- resistance
- A measure of how strongly a component opposes current, measured in ohms.
- ohm
- The resistance for which one volt produces one ampere.
- current–potential-difference graph
- A graph showing how current changes as potential difference across a component changes.
- ohmic conductor
- A conductor with constant resistance when physical conditions such as temperature remain constant.
- thermistor
- A component whose resistance changes strongly with temperature.
- variable resistor
- A component used to change resistance and control current.
Resistance is a ratio under stated conditions
01Resistance is a ratio under stated conditions
For a component, R = V ÷ I. One ohmThe unit of resistance (Ω). means one volt across the component produces one ampere through it.
For a metal wire at constant temperature, current is proportional to potential difference and the current–potential-difference graphA graph showing how the current through a component changes as the voltage across it changes. is a straight line through the origin. resistanceHow much a component opposes the flow of current: resistance = potential difference ÷ current, measured in ohms (Ω). is then constant.
02Structure and temperature change resistance
A longer wire usually has greater resistance because charge carriers interact across a longer path. A thicker wire usually has lower resistance because it provides more parallel conducting routes.
As a filament lamp heats, its metal ions vibrate more strongly and electron motion is impeded more, so resistance rises. The current–potential-difference graph curves and its gradient changes.
03A valid investigation controls the conductor
Connect the ammeter in series and voltmeter across the test wire. Change potential difference in small steps, record current and reverse polarity if negative values are needed.
Keep material, length, thickness and temperature controlled when testing one factor. Large currents heat the wire, so use low values, switch off between readings and monitor temperature.
Calculate and compare resistance
A component has 6.0 V across it and a current of 0.40 A.
- Use R = V ÷ I.
- Substitute R = 6.0 ÷ 0.40.
- Calculate R = 15.
- State the unit: 15 Ω.
How to make the reasoning visible: The ratio is meaningful for the stated operating condition.
Resistance investigation
The test uses the same wire material and diameter at controlled temperature.
| Length / cm | Potential difference / V | Current / A | Resistance / Ω |
|---|---|---|---|
| 20 | 2.0 | 0.80 | 2.5 |
| 40 | 2.0 | 0.40 | 5.0 |
| 60 | 2.0 | 0.27 | 7.4 |
| 80 | 2.0 | 0.20 | 10.0 |
| 100 | 2.0 | 0.16 | 12.5 |
Graph skills: SLAPUK and choosing a scaleCurrent and potential difference: decimal scales.
Describe the relationship between length and resistance.
Why must the wire be allowed to cool?
Check your understanding
Answer from memory, then check against the suggested answer.
Resistance equation?
Unit of resistance?
What happens to metal-filament resistance as temperature rises?
A resistor carries 0.25 A at 5.0 V. Find its resistance.
Why is “the graph becomes less steep” ambiguous for resistance?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A student plans to test how wire length affects resistance by connecting each wire directly to a 12 V supply for one minute.
Evaluate the plan. Identify the weaknesses.
(4)Describe an improved method.
(2)Total for this question: 6 marks
Command word: For evaluateA command word: weigh up the good points and bad points, then give your overall judgement., connect every improvement to validity, measurement quality or safety.