Basic Electronics Ninth Edition Basic Electronics Ninth Edition ©2002 The McGraw-Hill Companies Grob Schultz

Basic Electronics Ninth Edition Basic Electronics Ninth Edition ©2003 The McGraw-Hill Companies 13 CHAPTER Magnetism

Topics Covered in Chapter 13  The Magnetic Field  Magnetic Flux   Flux Density B  Induction by the Magnetic Field  Types of Magnets

Topics Covered in Chapter 13 (continued)  Ferrites  Magnetic Shielding  The Hall Effect  Air Gap of a Magnet

Magnetic Field Around a Bar Magnet NS The magnetic field is concentrated at the poles (N & S).

Magnetic Poles 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. Like magnetic poles repel one another. Unlike poles attract one another. Repel Attract

If a bar magnet is free to rotate, it will align itself with the earth’s field. North pole South pole North-seeking pole of the bar is simply called the north pole.

Magnetic Poles and Fields 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).  A bar magnet has opposite poles at opposite ends.  A curved, or horseshoe magnet, has opposite poles on each side of the air gap.  A ring magnet has no poles.

Magnetic Flux Definition: number of lines of force Symbol:  Units:  maxwell (Mx) in CGS units  weber (Wb) in MKS and SI units

The total lines leaving the north pole is called the flux. NS The letter symbol for the flux is . A maxwell (Mx) is one field line. One weber (Wb) = 1 x 10 8 Mx (an SI unit) 1  Wb = 100 Mx (a cgs unit)

Flux Density Definition: number of lines of force per unit area Symbol:  Equation: B =  / area Units:  gauss (G) in CGS units  tesla (T) in MKS and SI units

Flux density is the number of lines per unit area. NS The letter symbol for flux density is B. One tesla (T) = 1 Wb/meter 2 (an SI unit) 1 gauss (G) = 1 line/cm 2 (a cgs unit) 1 T = 1 x 10 4 G

Magnetic Induction NSNS When an iron bar is placed in the field of a magnet, poles are induced in the iron bar. The polarity of the induced poles causes a force of attraction. Bar magnet Iron bar

N S N S The shorter the air gap, the more intense the field. Eliminating the air gap eliminates the external field. Magnets are sometimes stored with “keepers” that eliminate the external field.

Magnetic and Nonmagnetic Materials Examples of magnetic materials are:  iron, nickel, and cobalt Examples of nonmagnetic materials are:  air, paper, wood, and plastics

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.

Permeability (  ) is the ability of a material to support magnetic flux. Relative permeability (  r ) compares a material with air. Values for ferromagnetic materials range from 100 to Magnetic shields use high permeability materials to prevent external fields from interfering with the operation of a device or instrument. Magnetic shield around a meter movement.

Types of Magnets An electromagnet is made up of coils of wire, and must have an external source of current to maintain a magnetic field. A permanent magnet retains its magnetic field indefinitely.

An electromagnet produces a field via current flow. The direction of current determines the field direction.

A toroid coil has very little external field. The flux (  is concentrated in the toroid core. Toroid cores (doughnut shaped) are used to greatly reduce unwanted magnetic induction.

VH  BVH  B Hall effect sensor Ferrite bead B Additional Applications for Magnetism