Moving Charges and MagnetismIntroduction
In the summer of 1820, during a lecture demonstration in Copenhagen, the Danish physicist Hans Christian Oersted noticed something odd. Whenever a current flowed in a wire on his bench, a compass needle close by swung away from north. Electricity and magnetism had been known for more than two thousand years, but always as two separate things. That small twitch of a needle was the first sign that they are one.
Aman and Riya repeat the experiment in their school lab. A long straight wire runs straight down through the bench, and a ring of compasses sits round it. Switch the current on, reverse it, and sprinkle some iron filings.
With no current, every needle points north. With the current on, the needles line up along a circle round the wire, tangent to it, and reversing the current turns every one of them round. The iron filings show the same pattern everywhere: circles with the wire at the centre. The needles swing further for a bigger current and closer to the wire. Oersted's conclusion: moving charges, or currents, produce a magnetic field in the space around them.
Dots and Crosses
Experiments poured in after Oersted. In 1864 James Clerk Maxwell joined the laws of electricity and magnetism into one theory and realised that light is an electromagnetic wave. Hertz discovered radio waves, and Jagadish Chandra Bose and Guglielmo Marconi were producing them by the end of the 19th century. Nearly all of modern electronics grew out of that understanding.
In this chapter we see how a magnetic field pushes on moving charges, such as electrons, protons and the charges in a current-carrying wire, and how currents make magnetic fields. We end with the galvanometer, the instrument inside every analogue ammeter and voltmeter. Many of our pictures look at wires and fields head-on, so we need one convention:
A dot is a current or a field coming out of the page, towards you, like the tip of an arrow. A cross is one going into the page, like the feathered tail of an arrow flying away. A current drawn as a cross flows
Right: the cross is the tail of the arrow. Field lines get the same marks, so a page covered in small purple crosses shows a magnetic field pointing into the page.