Magnetic Field Patterns

1. Magnetic Field Patterns

2. A Quick Review of Magnetic Fields • http://www.youtube.com/watch?v=uj0DFDfQajw

3. When an electric current passes along a wire, a magnetic field is set up around the wire. • For a straight wire, the magnetic field is a circle around the wire.

4. We can use our right hand to determine the direction of the magnetic field.

5. If a compass is placed next to a current carrying conductor, what would happen if the current were reversed? • What would happen if you moved the compass away from the current carrying conductor?

6. When a long wire is wrapped in a coil it is called a solenoid.

7. Prediction: What do you think the magnetic field around a solenoid would look like? • To break it down we could imagine what the field lines would look like for one loop.

8. If we increase the number of loops next to each other we can see that the magnetic field would look like this:

9. Describe the properties of the magnetic field in a solenoid:

10. Conclusion • The magnetic field lines around a wire are circles centered on the wire. • The magnetic field of a solenoid is uniform inside the solenoid and like a bar magnet on the outside. • Increasing the current increases the strength of the magnetic fields; reversing the current reverses the magnetic field lines. • Homework pg. 203 #1,2

11. The Motor Effect

12. When a current passes through a wire in a magnetic field a force is exerted on the wire. This is called the motor effect.

13. We can use our left hand to determine the direct of the Kinetic movement, Field, and Current (KFC). Kinetic Movement Field Current

14. Use the left hand rule to determine the direction of motion for the following:

15. How could we increase the force? • Increase the current • Use a stronger magnet • How could we reverse the direction of the force? • Reverse the direction of the field • Reverse the direction of the current

16. The Force depends on the angle between the wire and the magnetic field lines. The greatest force occurs when the wire is perpendicular to the field. • The direction of the force is always at right angles to the wire and field lines.

17. The Motor Effect • A current carrying conductor in a magnetic field will experience a force (the motor effect). • In the motor effect, the force: • Is increased if the current or the strength of the magnetic field is increased. • Is at right angles to the direction of the magnetic field and to the wire. • Is reversed if the direction of the current or the magnetic field is reversed.

18. The Electric Motor

19. The motor effect can be used to create an electric motor.

20. The following is a diagram of a simple motor: Brushes (metal or graphite) Split-ring commutator

21. What direction is the current flowing in the wire? Brushes (metal or graphite) Split-ring commutator

22. What is the direction of the magnetic field? Brushes (metal or graphite) Split-ring commutator

23. Use the left hand rule to determine the way the wire will move. Notice that the current in the wire is flowing in two different directions between the magnets. Brushes (metal or graphite) Split-ring commutator

24. These opposing forces will cause the wire to turn. Brushes (metal or graphite) Split-ring commutator

25. This video demonstrates a working motor: http://www.youtube.com/watch?v=yJyVTd_O-vw&feature=related

26. Why does the motor need brushes and a split-ring commutator? Brushes (metal or graphite) Split-ring commutator

27. The split-ring commutator reverses the current round the coil every half-turn. This means the coil is pushed in the same direction every half-turn. Brushes (metal or graphite) Split-ring commutator

28. What would happen if we increase the current? • What would happen if we reverse the current? Brushes (metal or graphite) Split-ring commutator

29. The Electric Motor • A simple electric motor has a rectangular coil of wire that spins in a magnetic field when a current passes through a coil. • The speed of an electric motor is: • Increased if the current is increased • Reversed if the current is reversed • Homework: pg. 207 #1,2

30. Electromagnetic Induction

31. For the electric motor, when a current passes through a magnetic field a force is created. • For a generator, when a wire moves across a magnetic field line a current is induced in the wire. Motor Field Current Movement Generator Field Movement Current

32. If a wire is moved through a magnetic field we will produce a current. • Or, if a field is moved through a wire we will produce a current. Motor Field Current Movement Generator Field Movement Current

33. Electromagnetic Induction • When a wire passes through the lines of a magnetic field, an emf is induced in a wire. • If the wire is part of a complete circuit, the induced emf causes a current in the circuit. • The current is increased if the wire moves faster or a stronger magnet is used. • The direction of an induced current opposes the change that causes it. • Homework pg.213 #1,2

34. An AC Generator

35. AC Generator Graph

36. The AC Generator • The simple ac generator consists of a coil that spins in a uniform magnetic field. • The slip rings and brush contacts enable the coil to stay connected to the circuit. • The peak value of the induced emf is when the sides of the coil cut directly across the magnetic field lines. • When the sides of the coil move parallel to the field lines, the induced emf is zero.

37. Transformers

38. Not these ones…

39. These ones.

40. Remember: • When a current passes through a solenoid, a magnetic field is created. • The more coils the solenoid has the stronger the magnetic field. • A changing magnetic field creates an alternating current. This creates an alternating voltage in the conductor. • The more turns in a coil, the larger the value of the alternating voltage.

41. A Basic Transformer • A transformer has two coils of insulated wire, both wound round the same iron core as shown below:

42. A Basic Transformer • When an alternating voltage is applied to the primary coil and magnetic field is created in the iron core.

43. A Basic Transformer • This magnetic field will induce an alternating voltage in the secondary coil.

44. Questions Q: Will the alternating voltage in the secondary coil be more or less than the primary coil? A: It will be less because there are less turns in the coil.

45. Questions Q: What would happen if both coils had the same number of turns? A: The primary and secondary voltage would be the same.

46. Questions Q: Why does the primary voltage have to be alternating? A: If it were only in one direction, the magnetic field would not change. Only a changing magnetic field induces a current.

47. The Transformer Equation Where: Vp – primary voltage Vs – secondary voltage Np – number of primary turns Ns – number of secondary turns

48. Types of Transformers • A step-up transformer has a secondary voltage that is greater than the primary. • A step-down transformer has a secondary voltage that is less than the primary.

49. Example Problem: • A transformer with 500 turns in the primary coil and 100 turns in the secondary coil has a secondary voltage of 12V. What is the primary voltage? What type of transformer is this?

50. Summary • A transformer consists of a primary coil and a secondary coil wrapped on the same iron core. • Transformers only work using alternating current. • The alternating current in the primary coil creates an alternating magnetic field in the iron core which induces an alternating voltage in the secondary coil. • The transformer equation is: