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 at the two ends
horseshoe magnet: bent U 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.
C. Both the same.

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!
C. both the same.

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 opposite direction cancels magnetic field of the other
electron revolution

10. Magnetic Domains 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.

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.
C. Both A and B.

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.
C. Both A and B.

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. Magnetic Force and Levitation
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

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

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.
B. direction.

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.
B. direction.

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.
A. similar devices.

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.
A. similar devices.

28. 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.
A. repulsion by the magnetic field you produce.

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.
A. repulsion by the magnetic field you produce.

36. Generators and Alternating Current
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. Transformers Everywhere
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.

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.
A. voltage.

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!
A. voltage.

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.
A. light.

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.
A. light.

51. Field Induction
Light is produced by the mutual induction of electric and magnetic fields
speed of light is the speed of emanation of these fields
too slow, the regenerating fields die out
too fast, fields build up in a crescendo of ever-increasing energy
at speed c, just right! And, there is light!