Physics 11–12 · Year 12

Force between two parallel current-carrying wires

Module 6: Electromagnetism (The Motor Effect)

Practical, model not builtMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

Each wire sits in the field of the other, so parallel currents attract and antiparallel currents repel with a force per length proportional to the product of the currents over the separation.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

a 5 A current heats the foil and leads
short circuit of the supply

Controls

current-limited supply, rheostat and fuse in circuit
pulse the current for one to two seconds only and switch off between runs

Note

Record the activity in RiskAssess (https://www.riskassess.com.au/) and follow the Science ASSIST risk management information sheet (https://asta.edu.au/resource/ais-risk-management-and-risk-assessment/).

What you need

Two 30 cm strips of household aluminium foil about 2 cm wide hung vertically 1 cm apart from a wooden frame
Current-limited low-voltage DC supply able to give 5 A, rheostat, ammeter, fuse, switch, phone video for the foil

How to do it

Connect the foil strips in series with the link running from the bottom of the first strip to the top of the second, so the current runs down both strips, and keep the link and the supply leads clear of the gap between them; pulse 5 A for one to two seconds and film the strips moving towards each other.
Move the link to join the two bottom ends, so the current runs down one strip and up the other, and film them moving apart.
Change the current with the rheostat and compare, on the video, how far the strips move.
Simulation: plot the force on a 20 cm section of two wires 5.0 mm apart against I^2 for equal currents of 2 to 10 A, and compare the gradient with mu0 L / (2 pi d).

What you should see

Same-direction currents attract, opposite currents repel, and a larger current moves the strips further. Two wires 5.0 mm apart carrying 10 A each feel 4.0e-3 N per metre, so a 20 cm section feels 8.0e-4 N, the weight of 0.082 g, and at 5 A a quarter of that; force against I^2 is a straight line of gradient 8.0e-6 N/A^2. A force this small is hard to measure with school balances, so the quantitative analysis here uses the model. Taking household foil as 16 micrometres thick, a 30 cm by 2 cm strip has a resistance of 0.025 ohms (aluminium resistivity 2.65e-8 ohm m) and at 5 A warms by about 2.7 degrees per second (density 2700 kg/m^3, c = 900 J/(kg K)), so pulses of one to two seconds keep it cool.

What changes

What you change: size and direction of the current (equal in both strips)
What you measure: direction and size of the strips' movement; force per metre in the model
What you keep the same: separation
length of parallel section
wires parallel

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Two wires with current repel like two charges of the same sign; parallel currents attract.
The force comes from the wires being charged; they are neutral and the force is magnetic.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Physics Stage 6 Syllabus (2017), current: Year 11 until the end of 2026, Year 12 until Term 3 2027PH12-13 PH11/12-2 PH11/12-3 PH11/12-5
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities