Magnetic field of a wire and a solenoid sim
Pass a current up a wire through a card ringed with plotting compasses and iron filings, reverse it and raise it with a rheostat, then thread a coil through the card and see its field look like a bar magnet's, with its N end found by the right-hand grip rule and made stronger by more current, more turns and an iron core.
Electromagnetism: magnetic effect of a current · covers the O-Level, IGCSE, GCSE topic
What the sim shows
Model. Straight wire: the field of a straight copper segment 176 mm long by the Biot–Savart law, close to B = μ₀ I ÷ (2π r) near it (beyond the syllabus). Solenoid: 10, 20, 40 turns of radius 22 mm spread evenly along 110 mm, each turn's exact loop field added up (beyond the syllabus). Soft-iron core: a 12 mm square bar 100 mm long, taken as uniformly magnetised with an apparent permeability of 25 (chosen). Earth: 30 µT horizontal, pointing north (chosen). Each compass needle points along the total field where it stands (the current's plus the Earth's), lying in the card's plane, at once, without disturbing the others. Filings line up along the field where the current's field is at least 60 µT (chosen), and only when the card is sprinkled or tapped. Circuit: 5 V (wire) or 6 V (coil) through a 0 to 10 Ω rheostat; the ammeter reads to 0.1 A. Readings: positions to 1 mm on the card's grid, needle bearings to 2° with a small seeded judging error (chosen). The leads to the wire and the coil are left out of the field. Nothing runs on its own: the needles only swing to a new field.
- The part you drag: the rheostat's slider (it sets the current)
- Card on wooden blocks, copper wire or coil, plotting compasses, power supply, reversing switch, rheostat, ammeter
Use it in your lessons
- Try it here: anyone can unlock 2 figures free, and a free account unlocks up to 5. Runs on a laptop, a Chromebook or a phone; nothing to install.
- Present it and share student links with the teacher plan, from US$5 a month, billed yearly.
- Put it on your own website or LMS for US$9, paid once, or every sim from US$29 a month.
Free guide: 12 practicals as sims, and how to run one in a lesson →
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More physics sims
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- Magnetic field of bar magnets simPlot the field round a bar magnet the classic way, moving a plotting compass and marking a pencil dot at each end of its needle, then sprinkle iron filings, put two magnets N–N (finding the neutral point X) and N–S, and add the Earth's field to see where it cancels the magnet's.
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- Thermal expansion simHeat a brass ball until it sticks in a ring and heat the ring until it drops through, watch a bimetallic strip bend away from a contact and switch a lamp like a thermostat, then warm water and air side by side in a water bath and see the gas expand far more than the liquid.
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- Using ammeters and voltmeters simDrag the meters' plugs onto a real bench circuit: put the ammeter in series and the voltmeter across the lamp, red to +, see what really happens when they are wired wrongly (a blown fuse, a dark lamp), pick the range, read the needle to half a division looking straight on, correct a zero error, compare a digital display, and work out the lamp's resistance from your own readings.