Magnets are fascinating objects for anyone ranging from the child to the engineer. They can be a source of fun and we can also probe and quantify their curious properties
Everyone has observed magnets attracting iron objects and knows that magnetic fields are invisible force fields. When iron filings are used, magnets disturb the filings to reveal lines of force (Fig 1). What is less familiar is the source of the magnetism.
Common Properties of Magnets
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A feature of magnets is the ends acting differently from each other. While one end attracts, the other end repels. Thus magnet ends are called “poles”, and in particular, a magnet itself is referred to as a “dipole”.
An interesting aspect of magnets is when suspended by a string, they always align themselves in a North-South direction. For this reason, magnet poles are correspondingly labeled North and South. Lines of force emanate from the North pole and return to the South pole.
Source of Magnetism
If one was inquisitive, it would be tempting to see what happens if a magnet is chopped in pieces. Surprisingly, each piece retains its magnetic properties, no matter how small the piece. This gives the idea that at the microscopic level, there must be some fundamental magnetic domain (Fig 2).
- When an iron object is unmagnetized, the domains consisting of N-S dipoles, lie in random directions
- When the object is placed in a magnetic field, the internal domains line up with each other, giving rise to a net magnetic field
This magnetic field is said to be induced by the external field of the magnet. As soon as the external magnet is removed, the object is observed to lose its magnetism: domains return to their random states and the induced field disappears. The object only makes a temporary magnet.
So how are permanent magnets created? Permanent magnets, or lodestones, exist naturally or permanent magnets are produced from the tempering process of ferrous metals, such as iron, iron alloys, nickel, and cobalt.
External Magnetic Fields
For a suspended magnet located in the Southern Hemisphere of the earth, the North pole of the magnet is observed to point North and up slightly; conversely, if the magnet is located in the Northern Hemisphere, the North pole of the magnet will still point North but down. This phenomenon is due to the lines of force in the external magnetic field; that of the earth.
To determine the direction of an external magnetic field, whether from the earth or some other magnetic field, a “test” magnet can be used (c.f. a compass). When the test magnet is placed at some angle to the field, torque on the magnet is observed which aligns it in the direction of the external field. By convention, the direction of the external field is that which the North pole of the test magnet is pointing to.
Magnets in Science and Daily Life
The study of magnets and magnetic fields is of great interest not only to scientists and engineers but also to everybody ranging from the ordinary person to the electronics enthusiast.
Electromagnetic Field
An electric current creates a curling magnetic field. When placed in an external field, a force acts on the wire. This magnetic force is what drives the electric motor.
Once basic properties of magnets were known, early experimenters sought new ways, other than using natural magnets, of producing a magnetic field. In 1820, the Danish physicist Hans Christian Oersted was the first to discover the existence of a magnetic field around a current-carrying wire. The interaction of the wire’s field with an external magnetic field produced a force on the wire, and this eventually led to the invention of the electric motor.