Physics 11–12 · Year 12
The motor effect measured on a digital balance: force on a current-carrying wire
Module 6: Electromagnetism (The Motor Effect)
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The idea
A conductor carrying current across a magnetic field feels a force proportional to the current, the field and the length in the field, at right angles to both.
What you need
- Digital balance reading to 0.01 g, a strong horseshoe magnet or a pair of ceramic magnets on a steel yoke placed on the pan
- Stiff copper wire taped to an insulating frame of packaging foam or timber so a straight horizontal section sits in the gap (the NSW Department of Education Module 6 guide, Appendix 2)
- DC supply 0 to 5 A, rheostat, ammeter, switch, leads
How to do it
- Set the magnet on the pan and zero the balance with the wire in the gap and no current.
- Switch on 1.0 A and record the balance change in grams; reverse the current and confirm the sign flips.
- Record the reading at 0.5 A steps to 4.0 A; three readings each.
- Convert grams to newtons (1 g = 0.0098 N) and plot force against current; the gradient is B L, so B follows from the magnet's width as the length in the field.
- Rotate the frame so the wire runs along the field and confirm the force falls to zero.
What you should see
Force is proportional to current and reverses with it. With B = 0.10 T over a 5.0 cm gap, 2.0 A gives 0.010 N, which the balance shows as a 1.02 g change; the force against current plot is a straight line through the origin, and the gradient returns the field strength. A wire parallel to the field shows no change.
What changes
- What you change
- current
- What you measure
- force on the wire (balance reading)
- What you keep the same
- length of wire in the field
- field strength
- wire perpendicular to the field
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- The force acts along the wire or along the field; it is perpendicular to both.
- A stronger magnet always gives a bigger reading whatever the wire's direction; along the field the force is zero.
Safety card
Hazards
- wire heating at 4 A
- strong magnets pinching fingers and affecting the balance
Controls
- current on only while reading
- zero the balance with the magnet in place
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/).
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 2027PH12-13PH11/12-2PH11/12-3PH11/12-5
- Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-12-02PY-12WS-03
- Australian Curriculum v9No Australian Curriculum v9 code is listed.
Sources
The pages the author read to write this activity.
- www.nsw.gov.au/sites/default/files/noindex/2025-03/physics-stage-6-syllabus-2017.docx
- education.nsw.gov.au/content/dam/main-education/teaching-and-learning/curriculum/key-learning-areas/science/s-6/physics/12Physics_-module-6-guide.docx
- curriculum.nsw.edu.au/learning-areas/science/physics-11-12-2025/outcomes