Physics 11–12 · Year 12

Building a simple DC motor and running it backwards as a generator

Module 6: Electromagnetism (Applications of the Motor Effect)

Practical, model not builtLow risk

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The idea

A current loop in a magnetic field feels a torque that turns it, and the same coil turned by hand produces an alternating emf that a commutator can rectify.

What you need

Simple motor kit: 20 turns of enamelled copper wire on a 5 cm by 5 cm former, two paperclip or brass bearings, a ceramic magnet (about 0.1 T at the coil), 1.5 V or 3 V cell, sandpaper
Hand-crank generator or the same coil geared to a handle, an LED, a voltage sensor or a phone oscilloscope, a neodymium magnet
Aluminium pendulum plate and a magnet for magnetic braking

How to do it

Wind the coil, strip the enamel on one end fully and on the other end on one side only, so the current flows for half of each turn and is cut for the other half (a simple stand-in for a commutator); mount it in the bearings over the magnet and connect the cell.
Give the coil a flick; it should spin. Reverse the cell and confirm it spins the other way; add a second magnet and note the speed change.
Generator: crank the coil at a steady rate and log the output; the trace is a sine wave; add the commutator to get a pulsing DC.
Load the generator with the LED and feel the extra effort needed to turn it (back emf and Lenz's law).
Swing the aluminium plate between the magnet poles and compare its damping with and without the magnet.

What you should see

The motor spins steadily once flicked and reverses with the cell. At the start, 20 turns carrying 1.0 A in 0.10 T with an area of 25 cm^2 give a maximum torque of 5.0e-3 N m. Cranking a 200-turn coil of 1.0 cm by 1.0 cm at 10 rev/s in 0.050 T gives a peak emf of 0.063 V; larger kit coils light the LED. The loaded generator is harder to turn, and the plate swinging between the poles stops far sooner than it does with the magnet removed.

What changes

What you change: number of magnets (field) or supply voltage
What you measure: spin rate of the motor
What you keep the same: same coil
same bearings

Common misconceptions

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

The commutator makes the motor spin faster; it keeps the torque in one direction by reversing the current each half turn.
A generator makes electricity from nothing; the mechanical work done against the induced force is the energy source.

Safety card

Low riskLearners carry it out

Hazards

hot coil if it stalls on a shorted cell
sharp wire ends
strong magnets pinching

Controls

disconnect the cell when the coil stalls
trim wire ends

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 2027PH12-13 PH11/12-3
Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-12-02 PY-12WS-06
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities