Magnetism

Season 02

Episode 01

Duration 23:05

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Stick a fridge magnet to the door and something invisible is doing the work. In this episode of Bill Nye the Science Guy, Bill picks apart that invisible something. He shows what a magnetic field looks like using iron filings, explains why only a few metals stick to a magnet, and reveals that the Earth itself, with its churning iron core, is one enormous magnet.

In this episode you will discover:

Why only iron, cobalt and nickel stick to a magnet, and nothing else
What a magnetic field is and how iron powder makes the invisible lines visible
Why every magnet has a north pole and a south pole and you can never get just one
How wrapping a wire around a nail and connecting a battery makes an electromagnet
Why the Earth’s spinning molten iron core makes the whole planet act as a giant magnet
How some animals, from honey bees to migrating birds, use the Earth’s magnetic field to find their way

Where magnetism comes from

Bill explains that magnetism is really about moving electrons. Inside a piece of iron the electrons are spinning in every direction and their tiny magnetic fields cancel out. Inside a bar magnet those spins line up, so all the little fields add together into one big one with a north end and a south end. Move a bunch of electrons another way, by pushing a current through a coil of wire, and you get a magnet you can switch on and off. That is exactly how the Earth’s core works: hot, molten iron and nickel churning around under the surface generate the planet’s magnetic field.

Key concept
Magnetic field
The invisible region around a magnet where its pulling and pushing force can be felt, mapped by lines that loop from the north pole to the south pole.

Key concept
Poles
Every magnet has a north end and a south end. Opposite poles pull together and like poles push apart, and you can never break off just one pole.

Key concept
Electromagnet
Wrap a wire into a coil, run an electric current through it, and the moving electrons create a magnetic field that switches off the moment you cut the current.

Bill Nye’s demonstrations

Bill sprinkles iron powder over a bar magnet and shakes the paper until the filings snap into the shape of the field lines. He wraps wire around a nail, hooks it to a battery, and lifts a pile of paper clips off the bench with the resulting electromagnet, then breaks the circuit and drops them the moment the current stops. He also uses the electron cars of science to show what happens when a magnet is cut in half: line the cars up nose to tail, then cut the row in the middle, and each half is still a complete magnet with its own north and south end. There is no such thing as a monopole. Finally he visits Dr Ed Weinburger for a look inside an MRI scanner, where a giant magnet is used to make detailed images of the inside of the human body.

Try this at home

The floating needle compass test. Rub a sewing needle in one direction along one end of a bar magnet, fifty or sixty times, so that the needle picks up a magnetic field of its own. Rest it on a small disc of paper or a piece of a plastic bottle top floating in a bowl of water. After a few seconds the needle swings round to line up with the Earth’s field and points north. You have made a working compass out of scrap.

The iron filing field test. Place a strong bar magnet flat on a table and lay a sheet of paper on top of it. Sprinkle iron filings (or the powder from an old wire-wool scouring pad, worked with scissors) evenly over the paper and tap gently. The powder springs into curved lines that connect the north end of the magnet to the south end, drawing out the invisible shape of the magnetic field for you to see.

Cool facts

Only three common metals are attracted to a magnet: iron, cobalt and nickel. Copper, silver, gold and aluminium are not.
The Earth’s magnetic field is strongest near the poles, and it is where charged particles from the Sun crash into that field that we get the aurora borealis and aurora australis.
Jupiter’s magnetic field is about twenty thousand times stronger than the Earth’s, while the Moon and Venus have almost no measurable field at all.
Michael Faraday in Britain and Joseph Henry in the United States both worked out how to generate electricity by moving a magnet through a coil of wire in 1831, independently of each other.

Science update: since this episode aired

Magnetic north is on the move. Since it was first pinned down by Sir James Clark Ross in the Canadian Arctic in 1831 it has drifted more than 2,200 kilometres towards Siberia, and in the 2000s the pace briefly hit 55 kilometres a year, fast enough that the joint US and UK team responsible for the World Magnetic Model released an emergency update ahead of schedule to keep aircraft navigation accurate. The current model was released in December 2024. NOAA National Centers for Environmental Information, 2024
How migratory birds actually sense the Earth’s field has finally been pinned down. A joint Oxford and Oldenburg team has shown that a protein in the retina of the European robin, cryptochrome 4, responds to weak magnetic fields through a genuine quantum mechanical effect involving electrons hopping between four amino acids. It is the strongest evidence so far that birds “see” the magnetic field. University of Oxford, 2021
On 5 September 2021 a team at MIT and Commonwealth Fusion Systems ramped up a large, high temperature superconducting electromagnet to a field of 20 tesla, the strongest sustained field ever made in a device of that size. It is the key technology for a compact fusion reactor called SPARC that aims to produce more energy than it consumes. MIT News, 2021

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Curriculum focus

Built on NGSS MS-PS2-3 and MS-PS2-5 (electric and magnetic forces, and how they act at a distance). Complementing that, it is pitched at KS3 physics (magnetism and electromagnets) and works as an introduction or revision aid for GCSE and IGCSE physics on permanent magnets, magnetic fields, the Earth’s field and electromagnets, across boards such as AQA, Edexcel, OCR and Cambridge IGCSE. The worksheet and critical thinking questions take these ideas further than the episode alone.

NGSS MS-PS2
KS3 physics
GCSE revision
IGCSE revision
Magnetism
Electromagnets