Chapter 9

2. Chapter 9 Magnetism

3. Explain the basic magnetic principles. State the three laws of magnetism. Describe the link between electric current and magnetism. Explain Rowland’s law. Discuss various types of relays and the manner in which they work. Describe the use of magnetic shields. Objectives Permission granted to reproduce for educational use only.

4. Basic Magnetic Principles • First magnets were called lodestones • Ends of a magnet contain greatest magnetic force • Magnetic lines of force Permission granted to reproduce for educational use only.

5. Earth as a Magnet • Magnetic poles include north pole and south pole • Angle of declination, or angle of variation, runs between earth’s north pole and magnetic north pole Permission granted to reproduce for educational use only.

6. Laws of Magnetism • Like poles repel each other • Unlike poles attract each other Permission granted to reproduce for educational use only.

7. Creating a Magnet • Magnetic field aligns molecules and magnetizes an iron bar Permission granted to reproduce for educational use only.

8. Flux density (B) is found by dividing the number of lines of force by the cross-sectional area Unit of flux density is the gauss Φ is number of lines of force A is cross-sectional area Magnetic Flux Permission granted to reproduce for educational use only.

9. Third Law of Magnetism • The attractive force increases as the distance between the magnets decreases • The smaller the gap is, the stronger the force will be Permission granted to reproduce for educational use only.

10. Review What is a magnet? A substance that attracts iron, steel, and other magnetic materials Permission granted to reproduce for educational use only.

11. Review What are the two concentrations of force contained in every magnet? North pole and south pole Permission granted to reproduce for educational use only.

12. Review What is the difference between the earth’s true north and magnetic north poles? Angle of declination, or angle of variation Permission granted to reproduce for educational use only.

13. Review What are the first two laws of magnetism? Like poles repel each other, and unlike poles attract each other Permission granted to reproduce for educational use only.

14. Review The attractive force will increase as the distance between the magnets _____. decreases Permission granted to reproduce for educational use only.

15. Electric Current and Magnetism • Magnetic field exists around a conductor carrying an electric current • Determine the direction of the magnetic field using the left-hand rule for conductors Permission granted to reproduce for educational use only.

16. Magnetic Circuit Characteristics • Reluctance • Permeability • Field intensity Permission granted to reproduce for educational use only.

17. Rowland’s law for electrical currents Φ is the number of lines of force F is the force measured in gilberts ℜ is reluctance Reluctance Permission granted to reproduce for educational use only.

18. ℜ is reluctance l is length of path  is permeability of material A is cross-sectional area Permeability Permission granted to reproduce for educational use only.

19.  is permeability of material B is flux density H is field intensity F is the force l is length of path Field Intensity Permission granted to reproduce for educational use only.

20. Solenoids • Strength is determined by ampere-turns (At or NI) of the coil Permission granted to reproduce for educational use only.

21. Solenoids (Cont.) • Determine the polarity of a current-carrying coil by using the left-hand rule for coils Permission granted to reproduce for educational use only.

22. Electromagnets • Similar to solenoids • Better conductors of magnetic field Permission granted to reproduce for educational use only.

23. Electromagnets (Cont.) • Iron core has residual magnetism after magnet is depowered • Amount of residual magnetism depends on the core’s retentivity Permission granted to reproduce for educational use only.

24. Review What is the link between current and magnetism? A magnetic field exists around a current-carrying conductor Permission granted to reproduce for educational use only.

25. Review What rule can be used to determine the direction of the magnetic field in a conductor? The left-hand rule for conductors Permission granted to reproduce for educational use only.

26. Review What is the law stating that the number of lines of magnetic flux is proportional to the force? Rowland’s law Permission granted to reproduce for educational use only.

27. Review What is a solenoid? A current-carrying conductor wound into a coil Permission granted to reproduce for educational use only.

28. Review What is a coil with an iron core inserted in the middle? An electromagnet Permission granted to reproduce for educational use only.

29. Relays • Magnetic switch that opens and closes a circuit by moving the armature Permission granted to reproduce for educational use only.

30. Relays (Cont.) • Points made of silver, tungsten, or alloys • Can have contacts in the normally closed (NC) or normally open (NO) position Permission granted to reproduce for educational use only.

31. Relays (Cont.) • Types of relays • Time relay • Reed relay • Uses of relays • Magnetic circuit breakers • Buzzers and doorbells Permission granted to reproduce for educational use only.

32. Time Relays • To time when relays energize the coil, a time-delay on or time-delay off can be used Permission granted to reproduce for educational use only.

33. Reed Relays • Way of magnetically opening and closing contacts Permission granted to reproduce for educational use only.

34. Magnetic Circuit Breakers • Trips in an overload situation • Based on the strength of the magnetic field Permission granted to reproduce for educational use only.

35. Buzzers • Electromagnets cause device to buzz • Basis of doorbell Permission granted to reproduce for educational use only.

36. Magnetic Shields • Magnetic force flows through path of least resistance • Magnetic shielding prevents interference of magnetic fields Permission granted to reproduce for educational use only.

37. Magnetic Levitation Transportation • Elevates and propels transportation devices using magnetism • Eliminates friction of wheels on rails Permission granted to reproduce for educational use only.

38. Electric Guitar • Uses a transducer called a pickup • Vibration of steel strings against pickup causes voltage in the pickup coils • AC wave of voltage in pickup coil has frequency of musical sound Permission granted to reproduce for educational use only.

39. Review What is a relay used for? It controls a large flow of current using a low-voltage, low-current circuit Permission granted to reproduce for educational use only.

40. Review What type of contacts are closed in the de-energized position? Normally closed (NC) Permission granted to reproduce for educational use only.

41. Review What are two types of relays? Time relays and reed relays Permission granted to reproduce for educational use only.

42. Review What is magnetic shielding used for? To prevent magnetic fields from interfering with instruments or circuits Permission granted to reproduce for educational use only.

43. For Discussion • Describe some uses of magnets in everyday living. • How do compasses work in northern and southern hemispheres? Permission granted to reproduce for educational use only.