Magnetic field of bar magnets sim
Plot 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.
Magnetism: magnetic fields · covers the O-Level, IGCSE, GCSE topic
What the sim shows
Model (chosen, not a real magnet's measured field). Each bar magnet, 80 × 15 × 8 mm, is two point poles of ±1.5 A m, 6 mm in from its ends; their fields (B = μ₀ q ÷ 4π r², beyond the syllabus) add as vectors, worked out in the level plane through the poles, where the compass needle is taken to lie. Inside a magnet this model gives the wrong direction, so nothing is drawn there. The Earth: 30 µT horizontal, pointing north, the same everywhere (chosen); it can be left out in set-ups A to C, as textbook drawings do. The needle points along the total field at once; where the field is under 1 µT (chosen) it is not turned and stays as it was. Filings line up where the field is at least 60 µT (chosen), only when sprinkled or tapped. The model's field lines start round each magnet in equal shares of the field leaving it; the Earth's lines are drawn every 2.8 cm and are not to the same scale. Nothing runs on its own: the needle only swings to a new field after you move it.
- The part you move: the plotting compass (its needle's red end is its N end)
- Bar magnets (N pole red, S pole blue), squared paper on a drawing board, iron filings, pencil, keepers, a compass to find north
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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