Electromagnets

An electromagnet is a magnet made by running electric current through a coil of wire. Unlike a permanent magnet, an electromagnet can be switched on and off by switching the current on and off. It can also be made much stronger by increasing the current, adding more turns of wire or wrapping the coil around an iron core. Electromagnets are at the heart of nearly every motor, generator, transformer and loudspeaker, and they are used in everything from scrapyards to brain scans.

• What it isCoil of wire with currentBecomes a magnet when on
• DiscoveredHans Oersted, 1820Current creates magnetism
• Stronger withMore current, more turns, iron coreThree ways to boost it
• Can be switched offCut the powerMagnetism stops
• BiggestMRI scanners and particle acceleratorsField can lift cars
• Everyday usesMotors, speakers, doorbells, scrapyardsEverywhere in modern life

How they work

The basic idea was discovered in 1820 by Danish scientist Hans Christian Oersted. He noticed that a compass needle moved when held near a wire carrying electric current. The current was producing a magnetic field around the wire.

If you coil the wire into a loop, the magnetic fields from each turn of the coil add together, making a much stronger field. The field shape becomes very similar to that of a bar magnet, with a north pole at one end of the coil and a south pole at the other.

If you wrap the coil around a piece of iron (or other magnetic material), the iron becomes magnetic too, dramatically multiplying the strength of the field.

How to make one stronger

• More current: doubling the current roughly doubles the field strength.
• More turns of wire: doubling the turns doubles the strength.
• Iron core: wrapping the coil around a soft iron bar can multiply the field strength by hundreds or even thousands of times.

Fact The most powerful electromagnets ever built can generate fields of over 45 tesla, around 900,000 times stronger than Earths magnetic field. These massive machines (at facilities like the National High Magnetic Field Laboratory in Florida) are so powerful they could rip a steel wrench out of your pocket from across the room. They are used in cutting-edge physics research and consume megawatts of electric power while in operation.

Why they are so useful

Electromagnets have three advantages over permanent magnets:

• Switchable: turn the current on, the magnet is on; turn it off, the magnet is off. Very useful when you need to grab something and release it later.
• Adjustable: increase the current, get a stronger magnet. Reduce the current, get a weaker one.
• Reversible: swap the direction of the current, and the magnet flips: what was north becomes south and vice versa.

These three powers let engineers design clever machines that would be impossible with permanent magnets alone.

Where electromagnets are used

• Scrapyards: huge crane-mounted electromagnets pick up cars and old machinery, then drop them by switching off the current. (Tip: stay clear of the working area when one is operating.)
• Electric motors: every electric motor is built around an electromagnet. The interaction between the coil and a magnet (or another coil) makes the shaft turn.
• Generators: the opposite of a motor. Spinning a coil between magnets generates voltage, which drives current. All large-scale electricity generation works this way.
• Doorbells: a button completes a circuit, the electromagnet pulls a small hammer against a bell, then the circuit briefly breaks (because the hammer moved) and a spring pulls it back. Repeated rapidly, this makes the ding-ding sound.
• Loudspeakers: a coil of wire attached to a paper cone sits in front of a magnet. Audio signals (alternating current) push the coil back and forth, vibrating the cone to make sound.
• Magnetic locks: hold doors shut with an electromagnet; switch off the current to unlock.
• Maglev trains: powerful electromagnets lift the train above the track and propel it forward, frictionless.
• MRI scanners: use giant superconducting electromagnets to produce detailed pictures of the inside of the human body.
• Particle accelerators: like CERNs Large Hadron Collider, use thousands of electromagnets to steer beams of subatomic particles at near light speed.

Did you know? The tape recorder (and its modern cousin the hard disk drive) works using electromagnets and magnetic materials. A small electromagnet creates patterns of north and south on a magnetic surface (the tape or disc). Later, the same device reads back the patterns to recover the information. A few tonnes of music can be stored on a tiny memory card using just this trick, on a microscopic scale.

Build your own electromagnet

You can build a simple electromagnet at home with very little equipment:

• A long iron nail.
• About a metre of thin insulated copper wire.
• A 1.5 V battery (a single AA is fine; AA gives more punch but gets warm).

Wrap the wire tightly around the nail in a single layer, leaving about 10 cm of wire at each end. Strip the insulation from the wire ends and tape them to the two terminals of the battery. The current now flows through the coil, turning the nail into an electromagnet. Try picking up paperclips with the nail tip. Disconnect the battery and the magnetism disappears.

Try winding more turns of wire, or using a fresh battery, and see how the strength changes.

Faradays great discovery

In 1831, the English scientist Michael Faraday made an even more important discovery: a moving magnet near a coil can generate electricity, just as a current makes a magnet. He called this electromagnetic induction. The discovery led directly to the invention of the electric generator, transformer and motor: the foundations of the entire modern electrical world.

Try this Build the electromagnet described above and test it with different materials. Try a steel nail: works strongly. Try a brass screw: not magnetic. Try a copper wire: not magnetic. Try a 5p coin: probably attracted (it has steel core). Then count how many paperclips your magnet picks up. Add more battery cells (carefully, with adult help) and see if it gets stronger. You are recreating the very experiments that Oersted, Faraday and other 19th-century scientists used to launch the age of electricity.

Deeper dive: how an electric motor works

The electric motor is one of the most important inventions in modern history. It is what makes electric cars move, fridges cool, washing machines spin, lifts rise, drills drill and millions of other devices work. Inside every motor, an electromagnet (or several) is doing clever business with a permanent magnet (or another electromagnet) to turn electrical energy into rotation.

The simplest motor design is the brushed DC motor:

• A coil of wire is mounted on a spindle, free to rotate.
• The coil is positioned between the poles of a permanent magnet.
• When current flows through the coil, it becomes an electromagnet with its own north and south poles.
• The coils poles are pulled by the permanent magnet (opposite poles attract). The coil rotates.
• Just as the coil reaches the point where it would line up with the permanent magnet (and stop), a clever switch called a commutator reverses the current direction in the coil.
• The coils poles flip, so now its like poles are next to the permanent magnets poles. They repel, pushing the coil further around.
• The process repeats, keeping the coil rotating.

Modern motors come in many variations, but they all use the same basic trick: a magnetic field that can be turned on and off (or switched in direction) to push a magnet around in a circle. Bigger motors use stronger magnets and more turns of wire. The largest industrial motors (in big ships, mines and rolling mills) can produce tens of megawatts of power, enough to drive massive equipment.

The next time you hear a kitchen blender, watch a Tesla accelerating away from the lights or feel your phone vibrate, you are watching electromagnets at work, doing the same job in many different scales.

For more, see what is a magnet and what is electricity.