Section 35.2 Permanent and Temporary Magnets
Magnetism in magnets can be temporary or permanent. You are already familiar with permanent magnets, such as the refrigerator magnets. Permanent magnets are magnetized materials that have magnetism already built into them and whose magnetism is not easily destroyed.
Magnetic materials near a magnet also act as it they were magnets. For instance, a paper clip hanging from a magnet attracts other paper clips as illustrated on the right. The first paper clip is an example of a temporary magnet.
Similarly, electromagnets have magnetic property when current is flowing in the coil surrounding them. These are examples of Temporary magnets since their magnetism is not permanent.
Magnetizable materials, such as iron, cobalt, and nickel, can be transformed into permanent magnets by stroking the material with a permanent magnet as illustrated in Figure 35.5.
First, place an unmagnetized iron bar on a table. Now, take a permanent magnet and rub the iron bar with the same pole of the magnet along the length of the iron bar in the same direction over and over again as indicated. The iron bar will become permanently magnetized!
Why do magnetizable materials become magnetized while other materials do not? You can trace the origin of magnetism in materials to the atomic level. Many atoms are tiny magnets themselves. In many materials, called ferromagnets or ferrimagnets, atomic magnets interact with each other such that they tend to align each other’s poles.
In ferromagnets, the interatomic forces tend to align their magnetic poles, but thermal vibration tends to randomize them. If temperature is low enough, we find domains where atomic magnets are aligned overwhelmingly in one direction as shown in Figure 35.6. When the domains are small and randomly oriented, the piece of material does not act as a permanent magnet, but when the domains are large and overall oriented in similar directions, the piece would be a permanent magnet.
When a material with atomic magnets is placed near a magnet, say near the South Pole of a permanent magnet, then the force of the external magnet tends to preferentially align the atomic magnets so that North Pole of the atomic magnet faces the South Pole of the external magnet. This leads to growth of magnetic domains that are aligned with the external magnet and large magnetic domains form that are oriented more or less in the direction of the external Poles.
Subsection 35.2.1 Electromagnets
In April of 1820 Hans Christian Øersted of University of Copenhagen (Denmark) published a remarkable discovery that united the subjects of electricity and magnetism. It is said that, while performing physics demonstrations for students, Øersted noticed that, whenever he turned on the current in the wire, a magnetic needle placed nearby also reacted to the current as illustrated in Figure 35.7.
An electromagnet makes use of the magnetism of moving charges in current carrying wires. We will study this fundamental aspect of magnetism in the succeeding chapter.
To enhance the impact of current, the wire carrying a current is wound in loops such that current in every loop flow in the same direction. Then, magnetic field of each loop of current add together and thus enhance their magnetic effect.
The magnetism of electromagnets can be demonstrated rather starkly by bringing a permanent magnet near an electromagnet as shown in Figure 35.8. The electromagnet acts just like a bar magnet with two poles since it attracts the North pole of the suspended permanent magnet when the current is one direction and repels the magnet when the orientation of the current is reversed. Electromagnet is an example of a temporary magnet that acts as a magnet when current through the material is turned on and as a non-magnet when the current is off.
Often, current carrying wires are wound around magnetizable material. when you do that, then the magnetic field of current in the loops tend to magnetize the material in such a way that the two sources of magnetic field, that of the current in the wires and of the newly formed magnet, vectorially add. This produces a net magnetic field that is hundreds of times more powerful than that of current in the loops alone. As a matter of fact, this is another way to transform a magnetizable material such as an iron bar into a permanent magnet, which is preferred way of making new permanent magnets than by stroking the material by another magnet.
