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Science 7–10 · Year 7

Building an electromagnet: how turns of wire and current change its strength

Physical sciences — Forces, content group Magnets in everyday life (NSW Stage 4 focus area; ACARA places magnetic forces in Year 4, AC9S4U03, and has no Year 7 to 10 content description on magnetism, so only Year 7 inquiry codes are cited)

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

An electric current in a coil produces a magnetic field, and the field grows with more turns of wire and with more current.

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

  • a near short circuit: several amps flow through the low-resistance coil, so the cell, leads and wire heat within seconds
  • wire ends scratching skin

Controls

  • adult supervision throughout
  • close the switch only while counting, for under 10 seconds, and let the wire and cell cool between trials
  • use a switch rather than holding wires on the cell
  • no rechargeable or lithium cells

Note

Heat or electrical energy is involved. Complete the school's risk assessment for the activity before the lesson, using CSIS 1.7 (Risk assessment – a pre-requisite for risk control) from the department's Chemical Safety in Schools package (2021 Technical Update), which the NSW Department of Education Science safety and compliance page names for risk assessment advice.

What you need

  • large steel nail 100 mm or soft-iron rod, 1
  • PVC-insulated copper wire 0.5 mm diameter, 2 m with stripped ends, 1
  • 1.5 V D cells in holders, 2
  • switch, 1
  • steel paperclips, 50
  • multimeter on its 10 A range, or an ammeter reading to at least 5 A, 1
  • crocodile leads, 4

How to do it

  1. Wind 10 neat turns of wire around the nail, leaving free ends. Connect nail coil, switch, one cell and the ammeter in series.
  2. Close the switch, touch the nail tip to the paperclip pile, lift and count the clips held. Open the switch at once. Record the current and the count; repeat three times.
  3. Wind to 20, 30 and 40 turns (same direction) and repeat, keeping one cell so the current stays about the same.
  4. Keep 40 turns and add the second cell in series to raise the current; repeat the count and the current reading.
  5. Open the switch and test whether the nail still holds any clips (it keeps some magnetism because it is steel).
  6. Plot clips held against number of turns; describe the effect of current separately.

What you should see

The count rises with the number of turns. The whole 2 m of wire stays in the circuit whatever the number of turns, so the current stays about the same through the turns series: 2 m of 0.5 mm copper has a resistance of only about 0.17 ohm (resistivity 1.68 x 10^-8 ohm m), so the current is set mainly by the cell and the connections, and the wire warms within seconds, which is why the switch is closed only while counting. Adding the second cell raises the current and the count rises again. A steel nail keeps a few clips after the switch opens; a soft-iron rod drops almost all of them. The learner knows it worked when the count rises steadily with turns and falls to almost zero when the switch is opened.

What changes

What you change
number of turns (or the current)
What you measure
number of paperclips held
What you keep the same
  • same nail
  • same wire and winding direction
  • switch closed for the same short time

Common misconceptions

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

  • The nail itself is the magnet and the wire is only a holder.
  • Winding more turns works regardless of direction.
  • An electromagnet switches off instantly and completely for every core material.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Sources

The pages the author read to write this activity.

  1. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/outcomes
  2. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/content/stage-4/fa71c2a852
  3. spark.iop.org/simple-electromagnet
  4. spark.iop.org/making-stronger-electromagnets
  5. instructional-resources.physics.uiowa.edu/5g2070-electromagnet

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