Section 38.1 Magnets
Magnets are ubiquitous now a days. We find them all around us, in toys, hangers, lifts, door bells, computers, just to name a few things where you can find magnets.
Magnets have been around for a long time. Ancient Greeks knew about a rare mineral called lodestones found near the city of Magnesia in Asia Minor, in present day Turkey, that had a strange power of attracting iron (Figure 38.1.1).
Around 1100 AD Chinese discovered that when lodestone or iron magnets were placed in floating containers on water, the magnetics always pointed in the North-South direction (Figure 38.1.2). This led to the development of magnetic compass. The discovery of magnetic compass in China quickly reached Arabs and from there to the Europeans.
Despite its use in navigation, the strange property of magnetism behind the compass remained a mystery until William Gilbert (1544-1603) in England demonstrated with his experiments on a terrella, a spherically shaped magnet, that Earth was actually a giant magnet and magnetic compass responded to Earth's magnetism. Gilbert published his findings in the seminal publication on magnetism called De Magnete in 1600.
Uniformly magnetized magnets are polarized in the sense that if one end of a magnet is attracted to one end of another magnet, the other end of the first magnet is then repelled by this same end of the second magnet. This can be demonstrated by marking the ends of two magnets and hanging them from a support as shown in Figure 38.1.3. We identify the ends that are attracted as opposite poles.
When magnets are hung from a support so that they can rotate freely about the vertical axis or placed in a fluid so that they can float freely, one of the like-ends of all magnets point towards North pole of Earth.
Actually, they point towards a point near the Geographic North Pole, which is called Magnetic North Pole. We mark the end of a magnet that points towards the Magnetic North Pole, the North Pole of the magnet. Because opposite poles of magnets attract, we deduce that the Earth as a magnet has its South pole near the Geographic North Pole at the Magnetic North Pole.
The magnetic pole of a magnet that is near Earth's geographic North-pole is, of course, magnetic South-pole of Earth because opposite poles of a magnet are attracted to each other. To confuse the matter more people using compass for directions call the south pole of earth as “magnetic north”, i.e. towards which north pole of a compass points. The magnetic south pole of earth or “magnetic north of a compass” is not at the geographic north, which is also called “true north”.
The angle between the “magnetic North of compass” at a place and the true geographic North pole of Earth is called declination. Magnetic declination depends on the location on earth. A declination map of Earth is a very useful guide in navigation.
Furthermore, there is geological evidence that the magnetic poles reverse directions. Magnetic reversals of Earth's magnetic field occurs due to the fluid dynamics of the interior of the Earth. The reversals in Earth's magnetic poles come at irregular intervals, averaging about 450,000 years. The last time the reversal happened was 780,000 years ago. NASA scientists in USA keep track of the movement of magnetic North pole, which has been moving between 10 km and 40 km every year in the direction of Northwest.