Invisible Force Fields: Magnetism and Induction

Hold a compass near a wire carrying electricity and watch the needle swing. That should be impossible — the wire is not made of iron, yet it deflects the compass just like a magnet would. In 1820, the scientist Hans Christian Ørsted noticed exactly this by accident during a lecture. It changed physics forever. Before we explain why, think about what you already know: what do you believe a magnet actually does to the space around it?

**Magnetic fields** Every magnet is surrounded by a magnetic field — an invisible region of space where magnetic forces act. We can map this field using field lines, which curve from the magnet's north pole to its south pole. Where the lines are close together (near the poles), the field is strongest. Magnetic fields are not just produced by permanent magnets made of iron or neodymium. Ørsted's discovery showed that any electric current — moving electric charges — creates a magnetic field around it. The field circles the wire like rings around a bullseye. This is why electricity and magnetism are deeply linked; together they form electromagnetism. The more current that flows, or the more times a wire is coiled, the stronger the magnetic field produced. Wrapping a wire into many loops (a solenoid) makes a very strong, bar-magnet-shaped field. Add an iron core inside and you have an electromagnet — a magnet you can switch on and off. **Electromagnetic induction** Faraday and Henry independently discovered in 1831 that the opposite is also true: a changing magnetic field can produce — or induce — an electric current in a nearby conductor. This is called electromagnetic induction. The key word is *changing*. A steady magnetic field sitting still does nothing. But move a magnet towards a coil of wire, or move the coil towards the magnet, and a current flows. Pull the magnet away and the current flows in the opposite direction. The faster the change, the greater the induced current. Faraday's Law states: the size of the induced voltage is proportional to the rate at which the magnetic field through the coil changes. More loops in the coil, a stronger magnet, or faster movement all increase the induced voltage. **Why does this matter?** Every electrical generator in the world — from a wind turbine to the dynamo on a bicycle — works by spinning a coil inside a magnetic field, continuously changing the field passing through the coil and inducing a current. Without electromagnetic induction, you would have no mains electricity.