Science 7–10 · Year 9
Resistance of a wire: how it changes with length
Physical sciences — Energy, content group Electrical energy (NSW Stage 5 focus area; ACARA places electric circuits in Year 6, AC9S6U03, and has no Year 9 or Year 10 content description on circuits, so only Year 9 inquiry codes are cited)
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The idea
The resistance of a uniform wire is proportional to its length, because each extra centimetre adds the same opposition to the flow of charge.
What you need
- nichrome wire 26 SWG (0.457 mm diameter), 1.10 m taped along a metre rule, 1
- nichrome wire 22 SWG (0.711 mm diameter), 1.10 m taped along a second metre rule, 1
- DC power supply set to 2.0 V, or a single 1.5 V cell, 1
- digital multimeters as ammeter and voltmeter, 2
- crocodile clip on a flying lead, 1
- switch, 1
- 4 mm leads, 6
How to do it
- Connect the supply, switch and ammeter in series with the full wire. Connect the voltmeter between the wire's start and a flying crocodile clip.
- Clip the flying lead at 10.0 cm, close the switch, read current and voltage, open the switch at once so the wire stays cool.
- Repeat at 20.0, 30.0, 40.0, 50.0, 60.0, 80.0 and 100.0 cm. Take each reading twice.
- Calculate R = V / I for each length and plot resistance (ohm) against length (m).
- Find the gradient in ohm per metre and predict the resistance of 1.50 m of the same wire.
- Repeat with a thicker wire of the same metal (22 SWG, 0.711 mm) and compare gradients.
What you should see
Resistance is proportional to length: a straight line through or very near the origin. For 26 SWG nichrome (resistivity about 1.10 x 10^-6 ohm m, a standard tabulated value; cross-section 1.64 x 10^-7 m^2) the gradient is about 6.7 ohm per metre, so 0.500 m reads about 3.35 ohm, 1.000 m about 6.71 ohm and the predicted 1.50 m is about 10.1 ohm. The whole 1.10 m wire carries the current wherever the clip is, about 0.27 A at 2.0 V (2.0 V across 7.38 ohm), or 0.20 A from a single 1.5 V cell, so the wire barely warms. Any small intercept comes from contact resistance at the clips. The 22 SWG wire gives a gradient about 2.4 times smaller (2.77 ohm per metre) because its cross-section is 2.4 times larger.
What changes
- What you change
- length of wire between the contacts (m)
- What you measure
- resistance V / I (ohm)
- What you keep the same
- same wire and diameter
- switch closed only briefly so temperature stays constant
- same supply voltage
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- A longer wire lets more current through because there is more metal.
- Resistance is a property of the material only, not the shape.
- A thicker wire has more resistance.
Safety card
Hazards
- wire ends puncturing skin
- a short circuit if the flying lead touches the supply terminals
Controls
- switch closed for under 5 seconds per reading
- supply no higher than 2 V
- tape the wire ends
Note
Heat or electrical energy is involved. Complete the school's risk assessment for the activity before the lesson, using CSIS 1.7 (Risk assessment – a pre-requisite for risk control) from the department's Chemical Safety in Schools package (2021 Technical Update), which the NSW Department of Education Science safety and compliance page names for risk assessment advice.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7–10 Syllabus (2023)SC5-EGY-01SC5-WS-04SC5-WS-05SC5-WS-06
- Australian Curriculum v9AC9S9I02AC9S9I04AC9S9I06
Sources
The pages the author read to write this activity.