Chapter 9 Magnetism

Objectives After completing this chapter, the student should be able to: Identify three types of magnets. Describe the basic shapes of magnets. Describe the difference between permanent magnets and temporary magnets. Describe how the earth functions as a magnet. State the laws of magnetism.

Explain magnetism based on the theory of atoms and electron spin. Explain magnetism based on the domain theory. Identify flux lines and their significance. Define permeability. Describe the magnetic effects of current flowing through a conductor. Describe the principle of an electromagnet. Describe how to determine the polarity of an electromagnet using the left-hand rule.

Define magnetic induction. Define retentivity and residual magnetism. Define a magnetic shield. Describe how magnetism is used to generate electricity. State the basic law of electromagnetism. Describe how the left-hand rule for generators can be used to determine the polarity of induced voltage.

Describe how AC and DC generators convert mechanical energy into electrical energy. Describe how a relay operates as an electromechanical switch. Discuss the similarities between a doorbell and a relay. Discuss the similarities between a solenoid and a relay. Describe how a magnetic phonograph cartridge works.

Describe how a loudspeaker operates. Describe how information can be stored and retrieved using magnetic recording. Describe how a DC motor operates.

Magnets Natural magnet Artificial magnet Electromagnet Derived from magnetite. Artificial magnet Created by rubbing a piece of soft iron with a piece of magnetite. Electromagnet Created by current flowing through a coil of wire.

Permanent magnets Temporary magnets Retain their magnetic properties. Retain only a small portion of their magnetic properties.

The earth is a huge magnet.

The color code for magnets The laws of magnetism Unlike magnetic poles attract each other. Like magnetic poles repel each other. The color code for magnets Red for the North Pole. Blue for the South Pole.

Magnetism Can be traced to the atom. As electrons orbit the nucleus, they also spin on their axis. This electrostatic charge produces a magnetic field. The direction of the magnetic field is the same as the electron’s direction of spin.

Ferromagnetic materials Materials that respond to magnetic fields. Atoms combine into domains or groups. When unmagnetized, the domains are random.

When magnetized, the domains align in a common direction and the material becomes a magnet.

Magnetic field Flux lines The invisible lines of force that surround a magnet. These are called flux lines. Flux lines Have polarity from North to South. Always form a complete loop. Do not cross each other. Tend to form the smallest possible loop.

Permeability The ability of a material to accept magnetic lines of force.

Electricity and Magnetism A magnetic field is generated when current flows through a wire.

The principle of the electromagnet. Electromagnets Composed of many turns of wire close together. The principle of the electromagnet. When wire is twisted into a loop: The flux lines are brought together. The flux lines are concentrated at the center of the loop. A North and South Pole are established.

The strength of the magnetic field can be increased three ways. The more turns of wire, the more flux lines are added together. The greater the current, the greater the number of flux lines generated. A ferromagnetic core is inserted into the center of the coil, usually iron.

Magnetic induction Residual magnetism Retentivity The effect a magnet has on an object without physical contact. Residual magnetism The magnetic field that remains when an object is separated from a magnet. Retentivity The ability of a material to retain its magnetic field after the magnetizing force is removed.

Magnetic shields Low-reluctance materials Used to protect electronic equipment from magnetic flux lines.

Electromagnetic induction The principle behind the generation of electricity. A current is produced when a conductor passes or is passed by a magnetic field. As the conductor passes through the magnetic field, a deficiency of electrons is created. This results in a difference of potential between the ends of the conductor.

When the conductor is removed from the magnetic field, the free electrons return to their parent atoms.

Faraday’s law The induced voltage in a conductor is directly proportional to the rate at which the conductor cuts the magnetic lines of force.

The left-hand rule

Magnetic and Electromagnetic Applications AC generator Converts mechanical energy to electrical energy by utilizing the principle of electromagnetic induction.

DC generator Functions like an AC generator with the exception that it converts the AC voltage to DC voltage.

Relay An electromagnetic switch that opens and closes with an electromagnetic coil. Used where it is desirable to have one circuit control another circuit. It electrically isolates the two circuits. Also used to control several circuits some distance away. Doorbell

Solenoid A coil, when energized, pulls a plunger that does some mechanical work. Door chimes Automotive starters

Phonograph pickups Use the electromagnetic principle. A magnetic field is produced by a permanent magnet attached to the stylus. The stylus tracks through the groove of a record in response to the audio signal recorded. The movement induces a small voltage that varies at the audio signal response. The induced voltage is amplified and used to drive a loudspeaker, producing the audio signal.

Loudspeakers Constructed of a moving coil around a permanent magnet. The magnet produces a stationary magnetic field. The current passes through the coil, producing a magnetic field that varies at the rate of the audio signal. The magnetic field of the coil is attracted and repelled by the field of the magnet.

The coil is attached to a cone that moves in response to the audio signal. The cone reproduces the audio signal.

Magnetic recording Uses the electromagnetic principle to store information. A signal is stored on tape or disk with a record head, to be read back later with a playback head. Some are combined in one package. They may be on the same head. The record and playback heads are a coil of wire with a ferromagnetic core.

A tiny gap between the ends of the core is a magnetic field. A piece of material covered with iron oxide, is pulled across the record head, magnetizing it. Information is written in a magnetized pattern. To play back or read the information, the material is moved past the gap in the playback head. The magnetic field induces a small voltage into the coil winding. When amplified, the information is reproduced.

Examples- Cassette recorders Video recorders Reel-to-reel recorders Floppy disk drives Hard disk drives

DC motor Operation depends on the principle that a current-carrying conductor, placed in and at right angles to a magnetic field, tends to move at right angles to the direction of the field.

( A ) The magnetic field extending between a north and south pole.

( B ) The magnetic field that exists around a current-carrying conductor.

( C ) The conductor placed in the magnetic field.

( D ) If the current through the conductor is reversed, the direction of the magnetic flux around the conductor is reversed.

TV, radar, computer terminals Uses the current-carrying principle. The conductor carrying current is deflected by a magnetic field. The electrons travel through a vacuum to strike a phosphor screen where they emit light. By varying the electron beam over the surface of the picture screen, a picture can be created. Two magnetic fields deflect the beam.

One field moves the beam from side to side. One beam moves the beam up and down.

In Summary Magnet Two theories of magnetism Three kinds Many shapes Unlike poles attract, like poles repel Two theories of magnetism Electron spin Domains

Determining direction of flux lines. Electromagnets Permeability Magnetic field Determining direction of flux lines. Electromagnets Strength Polarity Retentivity

Electromagnetic induction Faraday’s law Induced voltage is directly proportional to the rate at which the conductor cuts the magnetic lines of force. The left-hand rule for generators AC and DC generators Relays

Electromagnetic principles are applied in Doorbells Solenoids Phonograph pickups Loudspeakers Magnetic recordings DC motors and meters Use the same principles

Electron beams Television Radar Oscilloscopes Computer terminals