13 Magnetism Chapter 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

Topics Covered in Chapter 13 13-6: Types of Magnets 13-7: Ferrites 13-8: Magnetic Shielding 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 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 Like magnetic poles repel one another. (Fig. 13-4 b) Unlike poles attract one another. (Fig. 13-4 a) Fig. 13-4

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 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 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 × 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.

13-3: Flux Density B Flux Density Units 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/cm2 (cgs unit) Tesla (T) = 1 Wb/meter2 (SI unit)

13-3: Flux Density B Nikola Tesla was a scientist around the turn of the century, and the designation for magnetic field is named after him. Scientists measure magnetic strength with two units, tesla and gauss; one tesla equals 10,000 gauss. Your average fridge magnet is 10 gauss. The Earth's magnetic field is about 0.5 gauss (or 0.00005 tesla). A typical MRI scanner features a 1.5 tesla magnet. The world’s most powerful magnet is 45 tesla Source: http://www.magnet.fsu.edu/mediacenter/features/meetthemagnets/index.html#menu

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 V I   Conductance (G):

13-4: Induction by the Magnetic Field   Permeability ():

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 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. Fig. 13-11: Electromagnet holding nail where switch S is closed for current in coil. Copyright © The McGraw-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. The permeability is approximately 1.

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 non-conducting 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.

13-8: Magnetic Shielding A good conductor is best for two shielding functions: Preventing induction of static electric charges Shielding against the induction of a varying magnetic field. The best shield for a steady magnetic field is a good magnetic material of high permeability.