Physics 11–12 · Year 11
Electromagnet strength against current and turns: magnetising an iron core
Module 4: Electricity and Magnetism (Magnetism)
The idea
A current in a coil aligns the domains of an iron core so the core becomes a magnet whose strength grows with current and turns until the domains are all aligned.
What you need
- Large iron nail or soft-iron C-core, 2 m of insulated copper wire, 6 V DC supply with rheostat and ammeter
- Box of steel paperclips, electronic balance or spring balance for a pull-off test, switch
How to do it
- Wind 20 turns on the nail; at 1.0 A count how many paperclips hang in a chain from the tip; repeat three times.
- Repeat at 0.5, 1.5 and 2.0 A.
- Rewind with 40 and then 60 turns and repeat at 1.0 A.
- Switch off and test whether the nail keeps any magnetism; compare a steel nail with a soft-iron core.
What you should see
Paperclip count rises with current and with turns because the coil's field is proportional to N I; the rise can flatten at the highest settings as the core approaches saturation. The soft-iron core loses nearly all its magnetism when the current stops; the steel nail keeps some and can be demagnetised by dropping it.
What changes
- What you change
- current, then number of turns
- What you measure
- number of paperclips held
- What you keep the same
- same core
- same paperclips
- turns fixed while current varies
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- The core stores the current; it is magnetised by the coil's field and returns to unmagnetised when the field goes.
- Doubling the current always doubles the strength; the core saturates.
Safety card
Hazards
- coil heating at 2 A
Controls
- switch on only while counting
- keep runs short
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 2027PH11-11PH11/12-5
- Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-11-03PY-11WS-02
- 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
- curriculum.nsw.edu.au/learning-areas/science/physics-11-12-2025/outcomes