28.1:
Magnetism
Magnetism was first observed over 2500 years ago near the ancient city of Magnesia.
Magnets can be found in everyday objects like speakers, credit cards, compasses, and memory devices.
A bar magnet has a North and South pole. When an unmagnetized iron nail is brought in contact with it, it becomes magnetized.
Like poles of a magnet repel each other, while unlike poles attract each other.
A magnet's poles cannot be separated by breaking them into two pieces. Instead, smaller magnets, each with a north and south end, are obtained. Hence, magnetic monopoles are never observed.
In a navigation compass, the needle is a thin bar magnet suspended at its center.
The north pole of a compass is attracted close to the Earth's geographic North Pole.
Earth also acts like a huge bar magnet, with its magnetic south pole near the geographic North Pole and its magnetic north pole near the geographic South pole.
The axis of the Earth’s magnet and the geographical axis do not coincide. This deviation is called magnetic declination.
28.1:
Magnetism
Magnets are commonly found in everyday objects, such as toys, hangers, elevators, doorbells, and computer devices. Experimentation on these magnets shows that all magnets have two poles: one is labeled north (N) and the other south (S). Magnetic poles repel if they are alike and attract if unlike. Moreover, both poles of a magnet attract unmagnetized pieces of iron.
An individual magnetic pole cannot be isolated. No matter how small, every piece of a magnet contains a north pole and a south pole. This property of magnets differs from the property of electric charges, in which both positive and negative charges can exist in isolation.
An example of a magnet is a compass needle. It is a thin bar magnet suspended at its center, so it is free to rotate in a horizontal plane. The Earth also acts like a giant magnet, with its south-seeking pole near its geographic North Pole. The north pole of a compass is attracted toward Earth’s geographic North Pole because the Earth’s geographic North Pole is near to its magnetic south pole.
Back in 1819, the Danish physicist Hans Oersted was performing a lecture demonstration for some students and noticed that a compass needle moved whenever current flowed in a nearby wire. Further investigation of this phenomenon convinced Oersted that an electric current could somehow cause a magnetic force.
Today, magnetism plays many important roles in our lives. Physicists' understanding of magnetism has enabled the development of technologies that affect both individuals and society. For example, the electronic tablet in the purse or backpack wouldn’t have been possible without the applications of magnetism and electricity on a small scale.
All electric motors contain magnets. Generators, whether producing hydroelectric power or running bicycle lights, use magnetic fields. Recycling facilities employ magnets to separate iron from other refuse. Research into using magnetic containment of fusion as a future energy source has continued for several years. Magnetic resonance imaging (MRI) has become an important diagnostic tool in the field of medicine, and the use of magnetism to explore brain activity is a subject of contemporary research and development. The list of applications also includes computer hard drives, tape recording, detection of inhaled asbestos, and levitation of high-speed trains.
Suggested Reading
- OpenStax. (2019). University Physics Vol. 2. [Web version], section 11.1, pages 494–496. Retrieved from https://openstax.org/books/university-physics-volume-2/pages/11-1-magnetism-and-its-historical-discoveries