Chapter 9: MAGNETISM AND ELECTROMAGNETIC INDUCTION

1. Chapter 9: MAGNETISM AND ELECTROMAGNETIC INDUCTION

2. This lecture will help you understand: • Magnetic Poles • Magnetic Fields • Magnetic Domains • Electric Currents and Magnetic Fields • Magnetic Forces on Moving Charges • Electromagnetic Induction • Generators and Alternating Current • Power Production • The Transformer—Boosting or Lowering Voltage • Field Induction

3. Magnetic Poles Magnetic poles are in all magnets: • you can’t have one pole without the other • no single pole known to exist Example: • simple bar magnet—poles atthe two ends • horseshoe magnet: bentU shape—poles at ends

4. Magnetic Poles Magnetic force • force of attraction or repulsion between a pair of magnets depends on which end of the magnet is held near the other • behavior similar to electrical forces • strength of interaction depends on the distance between the two magnets

5. Magnetic Poles Magnetic poles • give rise to magnetic force • two types interacting with each other • north pole (north-seeking pole) • south pole (south-seeking pole) Rule for magnetic forces between magnetic poles: • Like poles repel; opposite poles attract

6. Magnets CHECK YOUR NEIGHBOR A weak and strong magnet repel each other. The greater repelling force is by the • A. stronger magnet. • weaker magnet. • Both the same. • None of the above.

7. Magnets CHECK YOUR ANSWER A weak and strong magnet repel each other. The greater repelling force is by the • A. stronger magnet. • weaker magnet. • Both the same. • None of the above. • Explanation: • Remember Newton’s third law!

8. Magnetic Fields Magnetic fields: • occupy the space around a magnet • produced by moving electric charges Field shape revealed by magnetic field lines that spread from one pole, curve around magnet, and return to other pole Lines closer together  field strength is greater

9. Magnetic Fields Magnetic fields • produced by two kinds of electron motion • electron spin • main contributor to magnetism • pair of electrons spinning in same direction creates a stronger magnet • pair of electrons spinning in oppositedirection cancels magnetic field of theother • electron revolution

10. Magnetic domains • clustered regions of aligned atoms oriented in random fashion — magnetic fields produced by each can cancel the fields of other. When oriented in one direction, then the substance containing them is a magnet • Magnet strength depends on number of magnetic domains that are aligned. Magnetic Domains

11. Magnetic Domains

12. Electric Currents and Magnetic Fields Connection between electricity and magnetism Magnetic field forms a pattern of concentric circles around a current-carrying wire • when current reverses direction, the direction of the field lines reverse

13. Electric Currents and Magnetic Fields

14. Electric Currents and Magnetic Fields Magnetic field intensity • increases as the number of loops increase in a current-carrying coil

15. Electric Currents and Magnetic Fields CHECK YOUR NEIGHBOR An electromagnet can be made stronger by • A. increasing the number of turns of wire. • increasing the current in the coil. • Both A and B. • None of the above.

16. Electric Currents and Magnetic Fields CHECK YOUR ANSWER An electromagnet can be made stronger by • A. increasing the number of turns of wire. • increasing the current in the coil. • Both A and B. • None of the above.

17. Magnetic Forces on Moving Charges Charged particles moving in a magnetic field experience a deflecting force—greatest when moving at right angles to magnetic field lines.

18. Magnetic Forces Current-Carry Wires

19. When an upward magnetic force is greater than gravity, then an object can levitate. A magnetically levitated vehicle is shown in the figure to the right – a magplane. No friction, no vibrations Magnetic Force and Levitation

20. Earth’s magnetic field deflects many charged particles that make up cosmic radiation. Magnetic Force in Space

21. Magnetic Force CHECK YOUR NEIGHBOR The magnetic force on a moving charged particle can change the particle’s • A. speed. • direction. • Both A and B. • Neither A nor B.

22. Magnetic Force CHECK YOUR ANSWER The magnetic force on a moving charged particle can change the particle’s • A. speed. • direction. • both A and B. • neither A nor B. • Explanation: • Only an electric force can change the speed of a charged particle. Since the magnetic force acts at right angles to velocity, it can only change the direction of a moving charged particle.

23. Magnetic Force on Moving Charges Electric meters • detect electric current Examples: • magnetic compass • compass in a coil of wires

24. Magnetic Force on Moving Charges Galvanometer • current-indicating device named after Luigi Galvani • called ammeter when calibrated to measure current (amperes) • called voltmeter when calibrated to measure electric potential (volts)

25. Magnetic Force on Moving Charges Electric motor • different from galvanometer in that each time the coil makes a half rotation, the direction of the current changes in cyclic fashion to produce continuous rotation

26. Motor and Generator CHECK YOUR ANSWER A motor and a generator are • A. similar devices. • very different devices with different applications. • forms of transformers. • energy sources.

27. Motor and Generator CHECK YOUR ANSWER A motor and a generator are • A. similar devices. • very different devices with different applications. • forms of transformers. • energy sources.

28. Electromagnetic Induction Electromagnetic induction • discovered by Faraday and Henry • voltage is induced with change of magnetic field strength in a coil of wire

29. Electromagnetic Induction Electromagnetic induction (continued) • induced voltage can be increased by • increasing the number of loops of wire in a coil • increasing the speed of the magnet entering and leaving the coil • slow motion produces hardly any voltage • rapid motion produces greater voltage

30. Electromagnetic Induction Induction occurs whether the magnetic field moves past the wire or the wire moves through the magnetic field.

31. Electromagnetic Induction More loops; more induction

32. Electromagnetic Induction Faraday’s law • the induced voltage in a coil is proportional to the number of loops, multiplied by the rate at which the magnetic field changes within those loops • amount of current produced by electromagnetic induction is dependent on • resistance of the coil • circuit that it connects • induced voltage

33. Electromagnetic Induction More difficult to push the magnet into a coil with many loops because the magnetic field of each current loop resists the motion of the magnet.

34. Electromagnetic Induction CHECK YOUR ANSWER The resistance you feel when pushing a piece of iron into a coil involves • A. repulsion by the magnetic field you produce. • energy transfer between the iron and coil. • Newton’s third law. • resistance to domain alignment in the iron.

35. Electromagnetic Induction CHECK YOUR ANSWER The resistance you feel when pushing a piece of iron into a coil involves • A. repulsion by the magnetic field you produce. • energy transfer between the iron and coil. • Newton’s third law. • resistance to domain alignment in the iron.

36. Generators and Alternating Current Generator • opposite of a motor • converts mechanical energy into electrical energy via coil motion • produces alternating voltage and current

37. Generators and Alternating Current The frequency of alternating voltage induced in a loop is equal to the frequency of the changing magnetic field within the loop.

38. Power Production Using Faraday and Henry’s discovery of electromagnetic induction, Nikola Tesla and George Westinghouse showed that electricity could be generated in sufficient quantities to light cities.

39. The Transformer—Boosting or Lowering Voltage • input coil of wire —primary powered by AC voltage source • output coil of wire —secondary connected to external circuit

40. The Transformer Transformer (continued) • both wound on a common iron core • then magnetic field of primary passes through secondary • uses ac in one coil to induce ac in second coil

41. The Transformer • Transformer relationship:

42. This common transformer lowers 120V to 6V or 9V. It also converts AC to DC by means of a diode inside. A common neighborhood transformer that typically steps 2400V down to 240V. Transformers Everywhere

43. Transformer Power CHECK YOUR ANSWER A step-up transformer in an electrical circuit can step up • A. voltage. • energy. • Both A and B. • Neither A nor B.

44. Transformer Power CHECK YOUR NEIGHBOR A step-up transformer in an electrical circuit can step up • A. voltage. • energy. • Both A and B. • Neither A nor B. • Explanation: • Stepping up energy is a conservation of energy no-no!

45. Electric Power • Electric power is equal to the product of the voltage and current.

46. Electric Grid uses Transformers • Voltage generated in power stations is stepped up with transformers prior to being transferred across the country by overhead cables. • Then other transformers reduce the voltage before supplying it to homes, offices, and factories.

47. Transformer Power • Neglecting heat losses, power into a transformer = power out of transformer.

48. Electric Field Induction Basic to electromagnetic induction is that electric and magnetic fields can induce each other. An electric field is induced in any region of space in which a magnetic field is changing with time. or A magnetic field is induced in any region of space in which an electric field is changing with time.

49. Electric and Magnetic Field Induction CHECK YOUR NEIGHBOR The mutual induction of electric and magnetic fields can produce • A.light. • energy. • sound. • None of the above.

50. Electric and Magnetic Field Induction CHECK YOUR ANSWER The mutual induction of electric and magnetic fields can produce • A. light. • energy. • sound. • None of the above.