Physics 11–12 · Year 12

Electromagnetic induction: magnet and coil, and Lenz's law with a magnet falling through a copper tube

Module 6: Electromagnetism (Electromagnetic Induction)

Practical, model not builtLow risk

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The idea

A changing magnetic flux through a coil induces an emf proportional to the rate of change, in a direction that opposes the change producing it.

What you need

Coils of 200, 400 and 800 turns, bar magnet and a strong neodymium disc magnet, centre-zero galvanometer or a voltage sensor with data logger sampling at 1 kHz
1 m copper or aluminium tube (the NSW Department of Education Module 6 guide demonstration) and a matching non-magnetic slug, stopwatch
Second coil and a switched DC supply for coil-to-coil induction

How to do it

Push the magnet into the 400-turn coil slowly, then quickly; record the peak reading each time and note the sign; pull it out and note the reversal.
Repeat with the 200 and 800 turn coils at the same speed; plot peak emf against turns.
Log the emf against time as the magnet passes right through the coil. Integrate the first lobe alone, up to the zero crossing when the magnet is centred in the coil, to get the flux change N delta phi; then integrate the whole pass and see the two lobes cancel, because the flux is zero before and after.
Drop the neodymium magnet and the plain slug through the tube; time both.
Coil-to-coil: switch a current on and off in the primary and watch the secondary galvanometer kick at each change only.

What you should see

Peak emf grows with speed and with turns: a flux change of 1.0e-3 Wb through 100 turns in 0.10 s gives 1.0 V. The sign reverses on withdrawal and with the other pole. The magnet takes far longer to fall through the 1 m tube than the slug, which falls in 0.45 s; the integral of the first lobe is N times the magnet's flux through the coil in size and is the same for a fast or slow pass, while the integral over the whole pass is zero because the two lobes cancel. The secondary coil responds only while the primary current changes.

What changes

What you change: speed of the magnet (rate of flux change), then number of turns
What you measure: induced emf
What you keep the same: same magnet
same coil (series 1) or same speed (series 2)

Common misconceptions

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

A magnet sitting inside a coil induces a current; only a changing flux does.
The copper tube attracts the magnet because it is magnetic; copper is not magnetic and the braking comes from induced currents.

Safety card

Low riskLearners carry it out

Hazards

strong magnets pinching fingers and affecting pacemakers or cards
brittle neodymium magnet shattering

Controls

keep magnets apart and away from steel, wear eye protection with neodymium magnets
catch the magnet on foam

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 PH11/12-1 PH11/12-3 PH11/12-5
Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-12-02 PY-12WS-01
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities