Magnetism and Field Theory

Transcription

1 Magnetism and Field Theory Section 1: Introduction In this lesson you will describe the domain theory of magnetism with special reference to dipoles state the Law(s) of magnetic forces explain how the domain theory accounts for several magnetic behaviours (e.g., magnetic induction, demagnetization, breaking a large magnet gives two smaller ones, the limiting of maximum magnetism) define and elaborate on the terms ferromagnetic, paramagnetic and diamagnetic 1

2 Origin 600 BC Greek Province of Magnesia naturally occurring magnetic ore was found the effect was called magnetism after Magnesia English referred to it as lodestone since one end, when suspended, pointed to lode, north star 3 4 th AD Chinese invented magnetic compass by the 11 th century it was used for navigation. By 12 th century Europeans knew! In 1600 De Magnete, first major account was written by William Gilbert. 2

3 Two characteristics that distinguish magnetic fields from electric fields and gravitational fields. 1 Magnetic fields occur naturally in substances that have magnetic character, such as iron, nickel and cobalt. Magnetic character is created from within matter as a result of the internal make up of the substance rather than the mere presence of matter as gravity is created by the presence of mass. 2 Magnetic forces are more versatile than the other two types of forces because they can affect magnetic substances as well as electric charges. Field Theory Describes and predicts how forces at a distance act on objects. All fields have two objects in common: an object that creates the field and another object that responds to the field. Operational Definitions 1 Gravitational field : The direction a 1 kg mass would move if placed in the field. 2 Electric field : The direction a positive charge would move if placed in the field. 3 Magnetic field : The direction a north pole would move if placed in the field. 3

4 Section 2: Magnetism and the Domain Theory When electrons are still they have an electric field around them (remember the lessons on electrostatics), but when electrons are moving they have an additional field called a magnetic field. Consider a simple atomic model: The electrons have two types of motion: spinning and revolving. Since the electrons are moving, the atom has a magnetic field around it. That is, it is like a tiny, tiny, tiny, magnet. The word for such a tiny magnet is called a dipole, where di pole means two poles, one is called a north pole and one is called a south pole. The magnetic effect is not the same for all materials. The effect would be the same, only if the electrons of all materials spun and revolved in the same way. For example, if one electron rotates clockwise while the other rotates counterclockwise, there will be a cancelling effect on the tiny magnetic field. The different possibilities for electron movement affect the magnetic nature of materials. Some materials are strongly attracted by a magnet, some are weakly attracted, and some are even slightly repelled if the magnet is strong enough. 4

5 Section1_Domain_Theory.notebook Domain Theory The little arrows represent the direction of the magnetic fields of individualdipoles. Note that in small neighborhoods the dipoles have the same orientation. These small neighborhoods are called domains. Note from the picture that a domain is also a dipole, that is, each domain has a north and south pole. This piece of iron is unmagnetized because the magnetic fields of different domains are in random directions. When a piece of iron is stroked many times with a magnet the strong magnetic field of the magnet will cause the dipoles (not the domains) of the piece of iron to line up. Not only do the domains line up some domains become larger. The piece of iron has become a magnet. We say that it has induced magnetism because the external field of the stroking magnet caused the iron domains to line up. Any magnet, whether factory made or home made, can have only a certain strength. Once all the domains are lined up or nearly lined up, that's it! It can't become any stronger. 5

6 If the piece of iron is snapped in two, you will have two magnets. The alignment of the domains takes place throughout the material so that each of the two pieces will have N and S poles. Since the magnet was created by causing the domains to line up, the piece of iron can be demagnetized by making sure that the domains take up random directions once again. Two ways to demagnetized a piece of iron: 1) heating the iron 2) submit it to some other kind of shock (ie. drop it) IMPORTANT: A piece of iron is always a magnetic substance. That is, it always has magnetic characteristics, but it becomes a magnet only when the domains are aligned. Paramagnetic substances are attracted by a magnet. In such substances the spin of one electron is not cancelled by another. If paramagnetic substances are strongly attracted, they are given a special title, ferromagnetic. Ferromagnetic substances: iron, cobalt and nickel. Paramagnetic substances (attracted to a lesser degree) oxygen, aluminum, and platinum Diamagnetic substances are weakly repelled by a magnet. In some substances electrons are paired in such a way that the spin of one cancels the spin of another. Examples: bismuth, zinc, silver and carbon. 6

7 Magnetic Phenomena and Domain Theory 7

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