Chapter 6

Magnets and Magnetism

Objectives

After studying this chapter, you will be able to:

•Describe various types of magnets

•Describe the nature of magnetic fields and forces

•Explain the theories of magnetism

Objectives (cont’d.)

• List the various uses for magnetism and methods of controlling magnetic forces

• Explain the relationship between magnetism and electricity

Magnets

• Materials that attract other metals

• Three classes: natural, artificial and electromagnets

• Laws of magnetic attraction and repulsion– Like magnetic poles repel each other– Unlike magnetic poles attract each other– The closer together the magnets, the greater

the attraction or repulsion

Magnetic Fields and Forces

• Magnetic lines of force– Causes iron filings to form a pattern when

placed on paper over a magnet

• Magnetic field is the space in which a magnetic force exists

Magnetic Theories

• Molecular theory of magnetism– A magnetic substance is composed of

molecular magnets

• Electron theory of magnetism– Magnetic field is produced by more electrons

spinning in one direction than the other direction

Magnetic Materials• Permanent magnets retain their magnetic

properties for many years• Temporary magnets lose magnetism once

removed from magnetizing influence• Magnetic Shields

– Magnetism passes through some materials more readily than others

– Surrounding sensitive instruments with iron provides a magnetic path around them

Electromagnetism

• An electric current produces a magnetic field

• Left-hand rule for a single conductor– Can determine direction of lines of force by

grasping conductor with left hand with thumb in the direction of the current flow

• Fingers will point in the direction of lines of force

Electromagnetism (cont’d.)

• Magnetic forces– If two magnetic forces are within reach of

each other, their fields will react according to laws of attraction and repulsion

Electromagnetism (cont’d.)

• Left-hand rule for a coil– Determine magnetic polarity of a coil by

grasping coil in left hand with fingers pointing in direction of current flow

• Extended thumb will point in direction of lines of force

Electromagnetism (cont’d.)

• Electromagnets– Consist of a coil– Magnetic force can be turned on and off with

the current– Field strength can vary– Inserting iron core into coil will increase field

strength– Solenoid is electromagnet with movable core

Electromagnetism (cont’d.)

• Application of electromagnets– Used for lifting large amounts of iron and steel– Magnetic clutches to connect and disconnect

the load from the driving source– Solenoids used to open and close valves– Other examples: relays, circuit breakers and

door chimes

Magnetic Circuits and Measurements

• Electromagnet strength depends on its ability to conduct magnetism

• Amount of magnetic flux is measured in maxwells (Mx)

• Magnetomotive force (mmf)– Influences amount of flux produced in an

electromagnet; measured in gilberts (Gb)

Magnetic Circuits and Measurements (cont’d.)

• Reluctance– Opposition to the magnetic flux

• Permeability– Ability of a material to conduct lines of force

• Electromagnet strength can be varied by changing the mmf– Increasing or decreasing coil current

Magnetic Circuits and Measurements (cont’d.)

• Other factors to consider when designing magnets– Retentivity– Residual magnetism– Magnetic saturation– These depend on type, size and length of the

core

Magnetic Circuits and Measurements (cont’d.)

• Flux depends on mmf and reluctance and can be calculated using the following equation

Solenoids

• Type of electromagnet with moveable core called armature– Made by winding magnet wire on hollow fiber

or plastic form– When coil is energized, the current’s magnetic

field pulls core into the coil• Commonly used in doorbells (door chimes)

Summary

• Magnets can be classified into natural, artificial or electromagnets

• Laws of magnetic attraction and repulsion govern behavior

• Magnetic field is the space in which magnetic force exists

• There are two main theories of magnetism: molecular and electron

Summary (cont’d.)

• An electric current produces a magnetic field; this fact is used in electromagnets

• Applications of electromagnets include lifting large metal objects, clutches, and opening and closing valves

• A solenoid is a type of electromagnet that has a movable core in its coil

• Iron in the core increases the field strength