Chapter 13 Magnetism Topics Covered in Chapter 13

Chapter 13 Magnetism Topics Covered in Chapter 13 13 -1: The Magnetic Field 13 -2: Magnetic Flux Φ 13 -3: Flux Density B 13 -4: Induction by the Magnetic Field 13 -5: Air Gap of a Magnet © 2007 The Mc. Graw-Hill Companies, Inc. All rights reserved.

Topics Covered in Chapter 13 § 13 -6: Types of Magnets § 13 -7: Ferrites § 13 -8: Magnetic Shielding § 13 -9: The Hall Effect Mc. Graw-Hill © 2007 The Mc. Graw-Hill Companies, Inc. All rights reserved.

13 -1: The Magnetic Field § Magnetic Field Lines § Every magnet has two poles (north and south). § The magnetic field, or strength of the magnet, is concentrated at the poles. § The field exists in all directions but decreases in strength as distance from the poles increases. Fig. 13 -2 b: Field indicated by lines of force. Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

13 -1: The Magnetic Field § Field Lines § Magnets have an invisible field (made up of lines of force). § These lines of force are from the north to the south pole of the magnet (external field). § Field lines are unaffected by nonmagnetic materials, but become more concentrated when a magnetic substance (like iron) is placed in the field.

13 -1: The Magnetic Field § Like magnetic poles repel one another. § Unlike poles attract one another. Fig. 13 -4

13 -1: The Magnetic Field § North and South Magnetic Poles § Earth is a huge natural magnet. § The north pole of a magnet is the one that seeks the earth’s magnetic north pole. § The south pole is the one that is opposite the north pole.

13 -1: The Magnetic Field § North and South Magnetic Poles § If a bar magnet is free to rotate, it will align itself with the earth’s field. § North-seeking pole of the bar is simply called the north pole. Fig. 13 -1 a: The north pole on a bar magnet points to the geographic north pole of the Earth. Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

13 -2: Magnetic Flux Φ § Magnetic flux is defined as the number of lines of force flowing outward from a magnet’s north pole. § Symbol: Φ § Units: § maxwell (Mx) equals one field line § weber (Wb) One weber (Wb) = 1 x 108 lines or Mx

13 -2: Magnetic Flux Φ Fig. 13 -5: Total flux Φ is 6 lines or 6 Mx. Flux density B at point P is 2 lines per square centimeter or 2 G. Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

13 -2: Magnetic Flux Φ § Systems of Magnetic Units § CGS system: Centimeter-Gram-Second. This system defines small units. § Mx and μWb (100 Mx) are cgs units. § MKS system: meter-kilogram-second. This system defines larger units of a more practical size. § Wb (1 × 108 Mx) is an MKS unit. § SI: Systeme Internationale. Basically another name for the metric system. SI units provide a worldwide standard in mks dimensions; values are based on one ampere of current.

Who is Maxwell? § Scottish mathematician and physicist who published physical and mathematical theories of the electromagnetic field. § Maxwell proved that electromagnetic phenomena travel in waves at the speed of light § http: //scienceworld. wolfram. com/biography/Maxwel l. html

How about Weber? § German physicist who devised a logical system of units for electricity. Weber wanted to unify electricity and magnetism into a fundamental force law. § He invented the electrodynamometer, an instrument for measuring small currents § http: //scienceworld. wolfram. com/biography/Weber. Wil helm. html

13 -3: Flux Density B § Flux density is the number of lines per unit area of a section perpendicular to the direction of flux. § Symbol: B § Equation: B = Φ / area § Flux Density Units § Gauss (G) = 1 Mx/cm 2 (cgs unit) § Tesla (T) = 1 Wb/meter 2 (SI unit)

Who is Gauss? § German mathematician who is sometimes called the “prince of mathematics”. § Set up the first telegraph in Germany § http: //scienceworld. wolfram. com/biography/Gauss. ht ml

How about Tesla? § Eccentric Serbian-American engineer who made many contributions to the invention of electromagnetic devices. § Tesla’s ac power became the worldwide power standard § http: //scienceworld. wolfram. com/biography/Tesla. html

13 -4: Induction by the Magnetic Field § Induction is the electric or magnetic effect of one body on another without any contact between them. § When an iron bar is placed in the field of a magnet, poles are induced in the iron bar. § The induced poles in the iron have polarity opposite from the poles of the magnet.

13 -4: Induction by the Magnetic Field Fig. 13 -7: Magnetizing an iron bar by induction. Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

13 -4: Induction by the Magnetic Field § Magnetic Permeability § Magnetic permeability is the ability to concentrate lines of magnetic force. § Ferromagnetic materials have high permeability. § Magnetic shields are made of materials having high permeability. § Symbol: r (no units; r is a comparison of two densities)

13 -4: Induction by the Magnetic Field § Permeability ( ) is the ability of a material to support magnetic flux. § Relative permeability ( r) compares a material with air. Ferromagnetic values range from 100 to 9000. § Magnetic shields use highly permeable materials to prevent external fields from interfering with the operation of a device or instrument. Magnetic shield around a meter movement. Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

13 -5: Air Gap of a Magnet § The air space between the poles of a magnet is its air gap. § The shorter the air gap, the stronger the field in the gap for a given pole strength. Fig. 13 -8: The horseshoe magnet in (a) has a smaller air gap than the bar magnet in (b). Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

13 -5: Air Gap of a Magnet § The shorter the air gap, the more intense the field. Eliminating the air gap eliminates the external field. This concentrates the lines within the field. § Magnets are sometimes stored with “keepers” that eliminate the external field. Fig. 13 -9: Example of a closed magnetic ring without any air gap. (a) Two PM horseshoe magnets with opposite poles touching. Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

13 -5: Air Gap of a Magnet § A toroid coil has very little external field. § Toroid cores (doughnut shaped) are used to greatly reduce unwanted magnetic induction. Fig. 13 -9 b: Toroid magnet. Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

13 -6: Types of Magnets § There are two main classes of magnets: § An electromagnet is made up of coils of wire, and must have an external source of current to maintain a magnetic field. § Applications: buzzers, chimes, relays (switches whose contacts open or close by electromagnetism), tape recording. § A permanent magnet retains its magnetic field indefinitely.

13 -6: Types of Magnets § An electromagnet produces a field via current flow. § The direction of current determines the field direction. § Left-hand rule: Thumb points toward N if hand is curled around coil in direction of current Fig. 13 -11: Electromagnet holding nail when switch S is closed for current in coil. Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

13 -6: Types of Magnets § Classification of Magnetic and Nonmagnetic Materials § Magnetic materials: § Ferromagnetic materials include iron, steel, nickel, cobalt, and certain alloys. They become strongly magnetized in the same direction as the magnetizing field, with high values of permeability. § Paramagnetic materials include aluminum, platinum, manganese, and chromium. They become weakly magnetized in the same direction as the magnetizing field. The permeability is slightly more than 1.

13 -6: Types of Magnets § Classification of Magnetic and Nonmagnetic Materials § Diamagnetic materials include copper, zinc, mercury, gold, silver, and others. They become weakly magnetized in the opposite direction from the magnetizing field. The permeability is less than 1. § Nonmagnetic materials: § air, paper, wood, and plastics

13 -7: Ferrites § Ferrites are nonmetallic materials that have the ferromagnetic properties of iron. § They have high permeability. § However, a ferrite is a nonconducting ceramic material. § Common applications include ferrite cores in the coils for RF transformers, and ferrite beads, which concentrate the magnetic field of the wire on which they are strung. http: //www. mag-inc. com/ferrites. asp

13 -8: Magnetic Shielding § Shielding is the act of preventing one component from affecting another through their common electric or magnetic fields. § Examples: § The braided copper wire shield around the inner conductor of a coaxial cable § A shield of magnetic material enclosing a cathode-ray tube.

How does Magnetic Shielding Work? § When magnetic lines of flux encounter high permeability material, the magnetic forces are both absorbed by the material and redirected away from its target. § The most effective shields are constructed as enclosures such as boxes or better yet, cylinders with end caps.

What is EMI? § EMI is the abbreviation for Electro Magnetic Interference. § EMI is an electrical or magnetic disturbance that causes unwanted interference.

13 -9: The Hall Effect § A small voltage is generated across a conductor carrying current in an external magnetic field. This is known as the Hall effect. § The amount of Hall voltage VH is directly proportional to the value of flux density B. § To develop Hall effect voltage, the current in the conductor and the external flux must be at right angles to each other. § Some gaussmeters use indium arsenide sensors that operate by generating a Hall voltage.

13 -9: The Hall Effect § Additional Applications for Magnetism The ferrite bead concentrates the magnetic field of the current in the wire. This construction serves as a simple RF choke that will reduce the current just for an undesired radio frequency. The semiconductor material indium arsenide is generally used as a Hall effect sensor.