Magnetism Topics Covered in Chapter : 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 Chapter 13 © 2007 The McGraw-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 McGraw-Hill© 2007 The McGraw-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. 13-2b: Field indicated by lines of force. Copyright © The McGraw-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. 13-1a: The north pole on a bar magnet points to the geographic north pole of the Earth. Copyright © The McGraw-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 10 8 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 McGraw-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 × 10 8 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  l.html l.html

How about Weber? GGerman physicist who devised a logical system of units for electricity. Weber wanted to unify electricity and magnetism into a fundamental force law. HHe invented the electrodynamometer, an instrument for measuring small currents h 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  ml 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 

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 McGraw-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  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 McGraw-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 McGraw-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 McGraw-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. 13-9b: Toroid magnet. Copyright © The McGraw-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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig : Electromagnet holding nail when switch S is closed for current in coil.

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

13-6: Types of Magnets  The basis of magnetic effects is the magnetic field associated with electric charges in motion.  There are two kinds of electron motion in the atom:  Electron revolving in its orbit. This produces a weak diamagnetic effect.  Electron spinning on its axis. The spinning electron serves as a tiny permanent magnet.

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.

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 V H 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.