Ages 8–12 · about 7 minutes

The Metal That Lets Electricity Run Free

What is a superconductor, and what is it good for?

Do this first · Try this at home

Follow the energy on paper

Start with your hands. The reading makes far more sense afterwards.

You need: Paper, a pencil and an adult to discuss your drawing with

Tick each step as you do it

Draw three boxes: a power supply, a superconducting magnet and a cooling machine. This is a diagram, not equipment to build.
Draw an arrow from the power supply to the magnet and label it 'energy to start the current'. Draw another arrow to the cooling machine and label it 'energy for cooling'.
Beside the magnet, write 'no resistance heating in the superconducting wire for steady current'. Circle the two energy-input arrows that still remain.

0 of 3 steps done

What should I notice?

Notice: Removing one way of turning energy into heat does not remove every energy input. Your drawing tracks this distinction; it does not measure a real magnet.

Now read on to find out why it happens.

The big idea

A superconductor can carry a steady electric current with zero electrical resistance. Starting the current and keeping the material cold still require energy.

current: The flow of electric charge.
resistance: A material's opposition to electric current, which can turn electrical energy into heat.
superconductor: A material with zero electrical resistance under the right temperature, current and magnetic-field conditions.
magnetic field: A field through which magnets and moving electric charges exert forces.

Electric current is moving electric charge. In an ordinary wire, electrical resistance turns some electrical energy into heat. That heat is useful in a heater, but it is an unwanted loss in many other devices.

Certain materials enter a superconducting state below their own critical temperature. They then have zero electrical resistance for a steady direct current. This is a change in the material's state, not a wire with an empty tunnel inside.

Zero resistance has conditions

Temperature matters, but so do the current and magnetic field. If those conditions exceed a material's limits, it can stop being superconducting.

A current can keep circulating in a closed superconducting loop without resistance draining its energy into heat. The current had to be started first. Equipment also uses energy to keep practical superconducting magnets cold. Zero resistance does not mean a whole machine needs no power.

A child in a woolly hat and mittens watches a small metal disc hang in the air above a frosted disc on a table, with cold mist pouring off it. — An illustration of magnetic levitation, not a photograph of an experiment. Real cold-material demonstrations need trained people and laboratory equipment; our activity uses only paper.

A floating magnet still feels gravity

A superconductor can develop currents whose magnetic field opposes an applied field. Keeping a weak field out is called the Meissner effect. Some superconductors also allow magnetic field to enter in narrow regions while remaining superconducting.

In a suitable arrangement, magnetic force can balance the weight of a magnet and hold it above the material. Earth still pulls the magnet downward. The upward magnetic force supports it; gravity has not disappeared.

Superconducting coils make strong magnets for MRI scanners and particle accelerators. Avoiding resistance heating in the wire is useful, even when the cooling system needs energy.

A material is not a vacuum

A superconductor is matter. A laboratory vacuum contains very few particles. In an accelerator, charged particles supplied by a source can travel through a vacuum, and electric fields give them energy. Emptying the tube does not supply that energy.

Electromagnetic waves, including light, can carry energy through a vacuum without a material medium. A current in a superconducting wire, a particle beam and a light wave are different physical situations. Drawing them separately helps keep their energy sources clear.

DOE explains superconductivity.