Physics 11–12 · Year 11
Magnetic field lines of bar magnets with iron filings, compasses and a phone magnetometer
Module 4: Electricity and Magnetism (Magnetism)
The idea
A magnet's field can be mapped by the direction a compass points, and the field is strongest where the lines are closest, at the poles.
What you need
- Two bar magnets, iron filings in a shaker, A3 paper or an overhead transparency, plotting compass
- Phone with phyphox magnetometer (the NSW Department of Education Module 4 guide activity), ruler
- Steel nail, paperclips, second magnet for stroking
How to do it
- Place the magnet under the paper and sprinkle filings; tap gently and sketch the pattern. Repeat with two magnets end to end, like poles facing and unlike poles facing.
- Remove the filings; use the compass to plot field lines point by point from the north pole around to the south.
- Lay the phone flat and locate its magnetometer (phyphox raw sensors). Record the field component along the magnet's axis with the magnet far away (Earth's field), then with the magnet's centre 12, 15, 20, 25 and 30 cm from the sensor along that axis, subtracting the background each time. Closer in, the field falls more slowly than 1/r^3, and within a few centimetres of a pole it can exceed the magnetometer's range.
- Magnetise a steel nail by stroking it 20 times in one direction with one pole; count the paperclips it lifts, then drop it on the floor several times and count again.
What you should see
Filings show closed loops leaving the north pole and entering the south, crowded at the poles; like poles give a clear gap between them. The phone reading falls steeply with distance along the axis: measured from the magnet's centre at 12 to 30 cm, a log-log plot of field against distance has a gradient close to -3 (1/r^3), while readings 2 to 10 cm from a pole fall only about as 1/r^2 because the sensor is still within about one magnet length. The stroked nail lifts several paperclips and lifts fewer after being dropped repeatedly.
What changes
- What you change
- distance from the magnet's centre along its axis
- What you measure
- magnetic field magnitude
- What you keep the same
- same magnet
- sensor orientation
- sensor kept centred on the magnet's axis
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Field lines start at the north pole and end at the south; they are closed loops that continue inside the magnet.
- Magnets attract every metal; only ferromagnetic materials such as iron, nickel and cobalt respond strongly.
Safety card
Hazards
- iron filings in eyes
- strong magnets pinching
Controls
- keep filings in the shaker, wash hands
- keep strong magnets away from bank cards and from anyone with a pacemaker
- keep the magnet away from the phone itself: the Module 4 guide warns that exposing a device to a strong magnetic field can damage it
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 2027PH11-11PH11/12-7
- Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-11-03PY-11WS-07
- 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/media/documents/physics-s6-module-4-guide-electricity-and-magnetism.docx
- phyphox.org
- curriculum.nsw.edu.au/learning-areas/science/physics-11-12-2025/outcomes