Physics 11–12 · Year 12

Transformer turns ratio and losses with a demountable transformer

Module 6: Electromagnetism (Electromagnetic Induction)

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

An alternating current in one coil induces an emf in a second coil sharing the same core, in the ratio of their turns, while flux leakage and heating make a real transformer fall short of the ideal.

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

step-up secondaries approaching 50 V AC
hot coils and core
clamp pinch
primary current rising several-fold when the core is opened, overheating the coil or tripping the supply

Controls

primary at 6.0 V so the 800-turn secondary gives at most 48 V; switch off before touching terminals or changing coils or the core
short runs, let the core cool
open the core only with the supply at 2 V and the primary ammeter in circuit, keep the current within the coil's rating and the run to a few seconds

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

Demountable transformer kit: laminated iron C-core with clamp, coils of 100, 200, 400 and 800 turns
Low-voltage AC supply (2 to 12 V), two AC voltmeters, two AC ammeters, 12 V 6 W lamp as a load

How to do it

Fit the 100-turn coil as primary and the 200-turn coil as secondary; apply 6.0 V AC and read the secondary voltage with no load.
Repeat with the 400 and 800 turn secondaries, keeping every output at or below 48 V (inside the 50 V AC extra-low-voltage limit); plot secondary voltage against turns ratio.
Connect the lamp to the 200-turn secondary; read primary and secondary V and I and compare V I on each side.
Switch off, open the core clamp to leave an air gap, turn the supply down to 2 V and switch on with the primary ammeter in circuit; repeat for a few seconds only and note the drop in the secondary-to-primary voltage ratio and the rise in primary current.
Run for two minutes at full load and feel the core and coils for warmth.

What you should see

Secondary voltage is proportional to the turns ratio: 6.0 V on 100 turns gives ideal secondary voltages of 12.0, 24.0 and 48.0 V on 200, 400 and 800 turns unloaded, and the measured values fall a little short because some flux leaks. With the 12 V 6 W lamp on the 200-turn secondary drawing 0.50 A, an ideal transformer would draw 1.0 A from the 6.0 V supply; the measured primary current is larger, and output power over input power is the efficiency, the rest heating the coils and core. Opening the core makes an air gap that lowers the secondary voltage markedly because less of the flux links the secondary, and the primary current rises sharply because the gap cuts the primary coil's inductance several-fold, which is why that step starts at 2 V.

What changes

What you change: turns ratio
What you measure: secondary voltage
What you keep the same: primary voltage
core closed
same frequency

Common misconceptions

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

A step-up transformer gives more energy out than in; it raises voltage and lowers current so power is at best equal.
Transformers work on DC; a steady current gives no changing flux and no output.

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 PH11/12-4 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-05
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities