Magnetic Effects of Electric CurrentIntroduction
It is the last lab period of the week, and Riya and Aman are packing up the circuit from the electricity chapter: a battery, a plug key and a long, thick copper wire. Riya has left her scout compass on the bench, right under the wire. Aman pushes the key back in to check the circuit one last time, and the compass needle jumps sideways. He pulls the key out, and it swings back to north. There is no magnet anywhere near it, only a copper wire carrying a current. So what moved the needle?
In the last chapter you met one effect of an electric current: it heats the wire it flows through. A current has another effect, and you need nothing more than a compass and a wire to see it. This is Activity 12.1. A thick copper wire XY is connected in a circuit with a battery and a plug key, and a small compass is placed close to the wire.
Make a prediction first. Then plug in the key, take it out again, and watch the needle.
The needle turns as soon as a current flows through the copper wire, and swings back to north when the current stops. Only a magnet can turn a compass needle, so the wire carrying a current must be acting like a magnet. In other words, electricity and magnetism are linked: an electric current produces a magnetic effect. In this chapter you will see what that magnetic effect looks like and how to find its direction, what happens to a wire carrying a current when it sits in the field of a magnet, and how all of this keeps the wiring in your home safe.
An Accident in a Lecture Hall
The same thing once happened by accident, in front of an audience. In 1820 the Danish scientist Hans Christian Oersted (1777–1851) had a battery and a wire on his bench during a lecture, and a compass happened to be lying nearby. When the current flowed, the needle moved.
Oersted repeated the experiment carefully until he was sure. What did his jumping needle prove?
Oersted's discovery started the study of electromagnetism, and the technologies it led to include the radio, television and fibre optics. The unit of magnetic field strength, the oersted, is named in his honour. In this chapter we study magnetic fields, the magnetic effect of an electric current, and electromagnets: magnets made by an electric current.