Unit III Electricity & Magnetism

Magnetism

Magnetism is a force field that surrounds a "magnetic" material. The field is in three dimensions and tapers off the further away from the magnet you are. A magnetic field is a force and as such can produce energy and do work. In this diagram the iron filings show the alignment of the magnetic field and the compasses help to shoe the direction of the field lines.

Where does a magnetic field come from? electrons, moving electrons. All molecules have electrons surrounding the nucleus and they can be viewded as moving around the nucleus of the atom. Surrounding the moving electron is a magnetic field, all electrons have magnetic fields, so why are some materials magnetic and some aren't?
The answer to this question is based on the concepts of domains. A domain is a small region of a material which exhibts a magnetic field. In a peice of material there is a very large number of domains, each with it's own magnetic filed. If in this material these domains can be made to all line up in one direction, this material would produce gross magnetic field effects. If the domains are random and point in different directions each individual domain's field effects may be cancelled out by another domain. Remember that magnetic fileds are forces and if on force is equal and opposite to another force the net effect of the forces is zero.
Therefor to have a magnetic material, the domains must be able to be aligned in the same direction, concentrating the field effect and prevent cancelling out of the magnetic force fields. Only iron , nickel and cobalt are able to have their domains aligned.
If domains can be aligned and they stay the way, such a material is said to form a permanent magnet.
If the domains of the substance can be made to align in an external magnet field, but return to random orientations lossing the materials gross magnetic field effect, this material is labeled a temporary magnet.
Even a permanent magnet can be made to lose its magnetic properties. Anything that disrupts the orientation of aligned domains will destroy the magnet. A sharp or severe jolt or heating will upset domain alignmet. If a magnet is heated above a certain temperature called the Curie point the magnetic domains will become permanently random and the substance will no longer be magnetic.

Surrounding every magnet is a magnetic field. Magnetic fields are made up of lines of force. They form concentric rings surrounding the magnet in three dimensions. The force field points in a direction from the North pole to the South pole of the magnet outside of the magnet and flow South to North inside the magnet.

Types of Magnetic Behavior

Definition: Magnetic permeability: is the ratio of the density of lines of force within the substance to the density of such lines in the same region in the absence of the specimen.

Ferromagnetism

• When a ferromagnetic material is placed in a magnetic field the lines of the field find it easier to pass through the material than if the ferromagnetic material was not present. Another way of saying this is that the magnetic flux density (B field) is greater inside the material than the surrounding magnetic field magnitude (H). In domain theory ferromagnetic materials allow their domains to align in one direction enhancing the external magnetic field (H). Elements that are ferromagnetic are nickel, iron and cobalt and alloys of these metals.
• Ferromagnetic effects are very large, producing magnetizations sometimes orders of magnitude greater than the applied field and as such are much larger than either diamagnetic or paramagnetic effects.
• Iron, cobalt, nickel (and various alloys of these materials) are termed ferromagnetic materials. Ferromagnetic materials are materials which can be permanently magnetized upon application of an external magnetic field. This external field is typically applied by another permanent magnet, or by an electromagnet.

Paramgnatism

• Materials that are magnetic but the ratio of flux density to magnetic field strength is not as great as in ferromagnetic materials. In a weak magnetic field they exibit no or very little magnetism. If the external field is very large of the temperature of the material is very low the material will exhibit magnetism.
  In a weak magnetic field the permeabilty is so small that it cannot overcome the thermal activity of the atoms in order to align the domains. At reduced temeratures and thermal activity less, the H field may order the domains and cause the material to be magnetic. Aluminum is a paramagnetic element.
• Paramagnetism, when present, is stronger than diamagnetism and produces magnetization in the direction of the applied field, and proportional to the applied field.
• Some materials exhibit a magnetization which is proportional to the applied magnetic field in which the material is placed. These materials are said to be paramagnetic and follow Curie's law: M = C (B/T) where M = magnetization, C = Curie,s constant, B is flux density, T is temperature in K. All atoms have inherent sources of magnetism because electron spin contributes a magnetic moment and electron orbits act as current loops which produce a magnetic field. In most materials the magnetic moments of the electrons cancel, but in materials which are classified as paramagnetic, the cancelation is incomplete.
  Examples of paramagnetic elements are uranium, platinium, aluminum and sodium.
• Paramagnetic materials are attracted toward magnets, but do not become permanently magnetized.
  If the temperature of a ferromagnetic material is raised past a certain point (called the Curie temperature) the material abruptly loses its permanent magnetization and becomes simply paramagnetic.

Diamagnetism

• Diamagnetic materials have a are less permeable than air. Elements that are diamagnetic are not magnetic and thus do not form aligned domains. Diamagnetic materials may have there domains align in the opposite direction to the applied H field resulting in a canceling out some of the lines of force due to the applied field. Materials such as copper, rubber and glass are diamagnetic.
• Diamagnetism is a property of all materials and opposes applied magnetic fields, but is very weak. Diamagnetic materials are repelled by magnets, but do not become permanently magnetized.
• The orbital motion of electrons creates tiny atomic current loops, which produce magnetic fields. When an external magnetic field is applied to a material, these current loops will tend to align in such a way as to oppose the applied field. This may be viewed as an atomic version of Lenz's law: induced magnetic fields tend to oppose the change which created them. Materials in which this effect is the only magnetic response are called diamagnetic. All materials are inherently diamagnetic, but if the atoms have some net magnetic moment as in paramagnetic materials, or if there is long-range ordering of atomic magnetic moments as in ferromagnetic materials, these stronger effects are always dominant. Diamagnetism is the residual magnetic behavior when materials are neither paramagnetic nor ferromagnetic.
  Any conductor will show a strong diamagnetic effect in the presence of changing magnetic fields because circulating currents will be generated in the conductor to oppose the magnetic field changes. A superconductor will be a perfect diamagnet since there is no resistance to the forming of the current loops.
  Examples of diamagnetic materials are mercury, silver, lead, copper, and sodium chloride

Magnetic Domains

The main implication of the domains is that there is already a high degree of magnetization in ferromagnetic materials within individual domains, but that in the absence of external magnetic fields those domains are randomly oriented. A modest applied magnetic field can cause a larger degree of alignment of the magnetic moments with the external field, giving a large multiplication of the applied field.

The complete note from which this information was taken can be found at hyperphysics

A few summary notes; add-on's

---

Continue on to Electromagneticm Click here