Physics 11–12 · Year 11

Magnetic field of a straight wire and a solenoid against current, measured with a phone magnetometer (syllabus practical)

Module 4: Electricity and Magnetism (Magnetism)

Practical, model not builtLow risk

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The idea

A current produces a magnetic field whose strength grows in proportion to the current and, for a solenoid, to the turns per metre.

What you need

Variable DC supply 0 to 5 A, rheostat (to prevent a short circuit, as the NSW Department of Education Module 4 guide notes), ammeter
1 m of insulated copper wire clamped straight, and a solenoid of 500 turns over 0.200 m
Phone running phyphox magnetometer, ruler, plotting compass

How to do it

Straight wire: place the phone sensor 2.0 cm from the wire; record the background field with no current, then the field at 1, 2, 3, 4 and 5 A. Subtract the background.
Plot the extra field against current; from the gradient and B = mu0 I / (2 pi r) compute mu0.
Solenoid: hold the phone against one end so its sensor sits on the axis (a phone cannot reach the centre); record B for 0.1 to 0.5 A in 0.1 A steps and plot against current.
Compare the gradient with mu0 N / (2 L), the field at the end of a long solenoid (half the central value mu0 N / L); check the direction with the compass and the right-hand grip rule.

What you should see

At 2.0 cm from a 5.0 A wire the field is 5.0e-5 T, comparable to Earth's field, so the phone shows a clear change; the field doubles when the distance halves. The 500-turn, 0.200 m solenoid carrying 0.50 A has 1.57e-3 T at its centre (B = mu0 N I / L with mu0 = 4 pi x 1e-7 N/A^2) and about 7.85e-4 T at the end of its axis; the phone reads a little less because its sensor sits beyond the end. Both plots are straight lines through the origin, and the learner knows it worked when the wire's gradient returns mu0 within about 10 per cent.

What changes

What you change: current
What you measure: magnetic field at the sensor
What you keep the same: sensor distance
sensor orientation
background field subtracted

Common misconceptions

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

The field of a wire points along the wire; it circles the wire.
More turns on a solenoid always mean a stronger field; the field depends on turns per metre.

Safety card

Low riskLearners carry it out

Hazards

wire and rheostat heating at 5 A
short circuit

Controls

switch on only while reading, rheostat in circuit
check wiring before switching on

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 2027PH11-11 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-11-03 PY-11WS-02
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities