EET 251

Electricity and Electronics

Lesson Four

MAGNETISM AND ELECTROMAGNETISM

Some metals in their natural state attract small pieces of iron. This property is called magnetism. Materials that have this ability are called natural magnets. The first magnets used were called lodestones. Now, artificial magnets are made in many different strengths, sizes and shapes. Magnetism is important because it is used in electric motors, generators, transformers, relays, and many other electrical devices. The earth itself has a magnetic field like a large magnet.

Electromagnetism is magnetism which is brought about due to electrical current flow. There are many electrical machines which operate because of electromagnetism. This lesson deals with magnetism, electromagnetism and some important applications.

4.1 Permanent Magnets

Characteristics of magnets: material of magnets; poles; what happens when magnets get broken; two laws of magnetism (1) like poles repel (2) unlike poles attract

Magnetic field patterns; lines of force or magnetic flux; induced poles

Handling of permanent magnets: effect of heat on magnets; use of a keeper (sof-iron piece used to join magnetic poles) for storage; storage of bar magnets.

4.2 Magnetic Fields Around Conductors

Shape of the magnetic flux lines around a conductor

Direction of the magnetic flux lines using the left-hand rule

Use of a compass to observe presence of magnetic lines of force around a current-carrying conductor

4.3 Magnetic Field Around a Coil

Magnetic flux lines around loops of a coil

Electromagnet: a coil that has an iron or steel core inside; effect of the core on the magnetic flux density of the coil

4.4 Electromagnets

Determining poles of electromagnets, with left-hand rule or with compass

Three basic parts of an electromagnet:

(1) Iron core

(2) Wire windings

(3) Electrical power source

Magnetic strength of electromagnets:

(1) amount of current passing through the coil

(2) number of turns of wire: Calculating magnetic strength or number of ampere-turns of each coil

(3) the type of core material

Residual magnetism

Advantages of electromagnetism over magnetism with natural magnets

4.5 Ohm’s Law for Magnetic Circuits

Characteristics of magnetic circuits

(1) Magnetomotive force (MMF)

(2) Magnetic flux and reluctance

4.6 Domain Theory of Magnetism

Wilhelm Weber’s molecular theory: in magnetic materials, molecules are arranged in an orderly arrangement; in nonmagnetic materials, molecules are arranged in a random pattern

Domain theory of magnetism (modified version of Weber’s theory): domains; arrangement in magnetic/nonmagnetic materials.

4.7 Electricity Produced by MagnetismFaraday’s discovery and Faraday’s law

4.8 Magnetic Devices

Relays: use of a relay; basic construction and symbol of a relay; common contact; double-pole single-throw (DPST) relay type; determining the coil resistance of a relay; pickup current rating; dropout current rating; contact current rating.

Solenoids or actuators: construction characteristics.

Magnetic motor contractors: construction and operation.

Magnetic Circuit Breaker: construction, use and operation.

Electric Bell: construction and operation

Reed switches and reed relays: construction, common applications, operation.

Analog meter movement: basic principle and construction

Magnetic recording: common application and basic principle

Electromagnetic speakers: construction, operation.

4.9 Magnetic Terms

Maxwell: one magnetic line of force

Weber: unit to measure magnetic flux

Flux density (definition and calculation)

Tesla: unit to measure flux density

Magnetomotive force (definition and calculation)

Magnetizing force

Reluctance

Residual magnetism

Permeability and relative permeability

Retentivity

Magnetic saturation

Hysteresis

4.10 Hall Effect

Definition of the Hall effect

Application in switching and measurement circuits: for example, a gauss meter or a switch for computer keyboards.

4.11 Magnetic Levitation

Application of “maglev”: trains that do not ride on rails but “levitate” above the rails on a magnetic cushion.

4.12 Rare Earth Magnets

Material that constitute rare earth magnets: neodymium

Application and advantages: for example, brushless direct-current (dc) motors

REVIEW

1. What were the first magnets called?

2. What is electromagnetism?

3. What three materials are used in the construction of permanent magnets?

4. What are the two laws of magnetism?

5. How can a piece of iron be temporarily magnetized?

6. Why should magnets be stored in a “keeper”?

7. What is the left-hand rule of magnetic flux around a conductor?

8. What is the left-hand rule for determining the polarity of an electromagnet?

9. What are the three basic parts of an electromagnet?

10. What are three ways to increase the strength of an electromagnet?

11. What is residual magnetism?

12. Discuss the relationship of magnetomotive force, magnetic flux, and reluctance in a magnetic circuit.

13. Discuss the domain theory of magnetism.

14. What is Faraday’s law?

15. How is Faraday’s law important in the generation of electrical power?

16. What is a relay?

17. What are NO and NC contacts of relay?

18. What are pickup current and dropout current ratings of relays?

19. What is a solenoid?

20. What is a magnetic contactor?

21. What is permeability? retentivity? saturation? magnetizing force?

8. What are the voltage, current, and resistance characteristics of (a) series circuits and (b) parallel circuits?

9. How is total resistance of a series circuit measured?

10. How is total resistance of a parallel circuit measured?

11. How is total current of a series circuit measured?

12. How is total current of a parallel circuit measured?

13. How is voltage drop measured for a series circuit?

14. How is voltage drop measured for a parallel circuit?

15. What are Kirchhoff’s laws? Explain them.

16. Explain the three ways used to find total resistance of parallel circuits.

17. What are three ways to find the electrical power of a circuit?

18. What is meant by kilowatt-hour?

19. What is a voltage-divider circuit?

20. What is meant by a negative voltage?

21. Discuss each of the following problem-solving methods: (a) Kirchhoff’s voltage law; (b) Superposition; (c) Thenivin’s theorem; (d) Norton’s theorem; and (e) Bridge-circuit simplification.