What it shows
The strength of an electromagnet grows with the number of turns of wire and with the current through it. Counting the pins it lifts for 20, 40, 60 and 80 turns gives a clear upward trend.
The science
Each turn of wire carrying current adds its magnetic field inside the coil, so the strength of the coil grows with the number of turns N and the current I (roughly in proportion to N I; for a long coil the field is B = mu0 n I, with n turns per metre). The soft iron core becomes magnetised by this field and multiplies it many times, which is what lifts the pins. More turns or more current gives a stronger field and more pins; removing the current demagnetises the soft iron, so the pins drop.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A 10 cm iron nail or bolt
- About 5 m of enamelled copper wire (28 to 30 SWG, from an electrical shop)
- Three AA cells in holders or taped end to end (used one, two and three at a time for the current test)
- A box of steel pins or paper clips
- Sandpaper to strip the wire ends
- A multimeter with a 10 A current range to measure current (optional)
Safety
For this project: The coil draws a few amps from the cells, so the coil, the nail and the cells warm quickly; connect for no more than 3 seconds per trial, let everything cool between trials, and do not hold the coil while it is connected. Pins are sharp; count them on a tray. Cells only.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
Step by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Wind 20 neat turns of wire around the nail in one direction, leaving 15 cm free at each end, and strip the enamel from the ends with sandpaper.
Connect the two cells, touch the nail tip into the pin tray, lift it, and count the pins that hang on; repeat three times and average.
Without unwinding, add 20 more turns in the same direction (in a second layer once the first is full) to make 40, and repeat the count; continue for 60 and 80 turns.
Record a table with columns for turns, trial 1, trial 2, trial 3 and the average number of pins.
Keep 80 turns and test with one, two and three cells to see the effect of current; measure the current with the meter on its 10 A range if you have one.
Plot average pins against turns, and on a second graph pins against current.
Wind 20 turns one way and 20 the other way on a second nail to show that opposite turns cancel and lift almost nothing.
Write up why soft iron is used for the core and why the pins fall off when the switch opens.
What you should see
The number of pins lifted rises steadily with turns and with current. Opposite-wound turns cancel, lifting almost nothing, which proves the field comes from the direction of current in the turns.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01Name two ways to make an electromagnet stronger.
More turns of wire and more current, plus a soft iron core.
02Why does the electromagnet drop the pins when switched off?
Soft iron loses its magnetism as soon as the magnetising field is removed.
03Which rule gives the pole of the coil?
The right-hand rule for a solenoid: curl the fingers in the current direction and the thumb points to the north pole.
04Why does the coil get warm?
The wire has resistance, so current produces heat (I squared R).
Take it further
Compare an iron core, a steel bolt and no core at 80 turns, and explain which stays magnetised after the current stops.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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