Plate 39 · Matter
The Invisible Lines Around a Magnet
A magnet pulls a nail towards it without touching it, so something invisible must surround it. Lay a sheet of paper over a magnet, sprinkle fine iron filings on it and tap gently, and the filings arrange themselves into curved lines. Those lines draw the shape of the magnetic field.
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It Pulls Through Paper
A magnet can pull iron through paper, through a tabletop, even through your hand. The space around it has been changed, and scientists call that change a magnetic field.
You can’t see or touch a magnetic field, but you can make it show itself.
A bar magnet on a desk · the red end is the north pole
Filings Fall Into Line
Sprinkle on iron filings and tap the paper. Each filing becomes magnetised, a tiny magnet itself, and turns to lie along the field, end to end with its neighbours, forming lines.
The lines leave one end of the magnet, curve round, and come back to the other end.
Crowded at the Ends
The lines crowd together most at the two ends, where the field outside the magnet is strongest. These are its poles: a north pole at one end, a south pole at the other.
Set down a few small compasses and their needles all point along the lines: outside the magnet, from the north pole round to the south.
Two Magnets
North facing north: the lines swerve away from each other, leaving a spot in the middle with few filings, where the two magnets’ fields cancel each other out. The two magnets push apart.
Turn one round so north faces south, as you can in the lab below, and lines run straight from one magnet to the other; the two magnets pull together.
The Earth Is a Magnet Too
The Earth has a magnetic field of its own, shaped much like a giant bar magnet’s. A compass’s north end points north because the Earth’s northern magnetic pole is, in fact, a south-type pole.
The Earth’s field also protects us: it turns aside most of the charged particles blowing from the Sun; a few are steered down into the air near the poles, where they make the aurora.
Your Turn
Try a different setup: one magnet, opposite poles facing, or like poles facing. Press “Tap” and watch the filings rearrange; switch on the compasses to see where they point.
Try thisChoose “N to N” and press “Tap”: what happens to the filings midway between the magnets?
What the model leaves out
The field is worked out from a simple model with magnetic “charge” on each end face of the magnet. For an evenly magnetised bar this gives the field outside it exactly; here each face is a row of a few points. The filings are laid along the calculated field lines with some randomness; real filings also pull on each other and clump into thicker chains. The field is really three-dimensional; the paper shows one slice of it.
A common misreading
Field lines aren’t real strings. Filings line up because each one becomes a little magnet: the field turns it to lie along the field, and neighbours attract end to end; where lines are drawn closer together, the field is simply stronger. And magnets don’t attract every metal: iron, cobalt and nickel are pulled, but aluminium, copper and gold hardly at all.
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Follow the question
Plate 23Where a Compass PointsA compass doesn’t point at the North Pole. It lines up with Earth’s magnetic field — and the magnetic pole it heads for keeps moving.
Plate 36Where Is the Electron?Textbooks often draw electrons as little balls circling the nucleus. In fact an electron has no orbit. It is more like a cloud: all we can say is where it is likely to be.
