ELECTROMAGNETIC INDUCTION. Faraday s Law Lenz s Law Generators Transformers Cell Phones

Transcription

1 ELECTROMAGNETIC INDUCTION Faraday s Law Lenz s Law Generators Transformers Cell Phones

2 Recall Oersted's principle: when a current passes through a straight conductor there will be a circular magnetic field around the conductor. e

3 Michael Faraday discovered an exactly opposite phenomenon: when a magnetic field moves near a conductor it makes any free charge in the conductor move. This means a changing magnetic field creates a current.

4 Faraday's law of electromagnetic induction Whenever the magnetic field in the region of a conductor is moving, or changing in magnitude, electrons are induced to flow through the conductor. The most critical word in in Faraday's Law is the word changing. If the magnetic field is not changing there is NO induced current!

5 It is important to realize that a magnetic field can change in two ways: It can move physically. This can also happen in two ways: moving a bar magnet back and forth, moves its magnetic field back and forth the magnetic field can remain stationary while the conductor is moved back and forth by some outside force. A magnetic field can also change by having its intensity or strength increased or decreased. This is most easily done with an electromagnet since all one has to do is increase or decrease the current through the coil.

6 DEMO 1. Determine what happens to the Galvanometer in each of the following: A) A wire is connected to a galvanometer and then passed through the poles of a horseshoe magnet. The needle will move slightly in one direction and then back to zero. When the wire is move out of the poles the needle will move in the opposite direction and then back to zero.

7 1.Determine what happens to the Galvanometer in each of the following: B) A bar magnet is inserted into a coil of wire which is attached to a galvanometer. The needle will move one direction and then back to zero after the bar magnet stops moving. When the bar magnet is moved out of the coil the needle will move in the opposite direction and then back to zero.

8 2.What affect does more turns have on the magnitude of the induced current? More turns means more current. 3.What affect does the relative speed between the coil and the magnet have on the induced current? More speed means more current.

9 4.A) How can the magnetic strength of the bar magnet be increased? By increasing the number of bar magnets, and aligning the poles in the same direction. B) What is the effect on the induced current of increasing the strength of the bar magnets? It increases the amount of induced current.

10 What are the factors that the induced effects are affected by? (i) the number of turns in the coil (ii) the relative speed between the magnetic field and the coil (iii) the strength of the magnetic field (iv) the orientation of the magnetic field NOTE: The orientation of the magnetic field determines the direction that the induced current will travel.

11 Direction of the Current: The following pictures show a conductor being pulled into the screen (paper) with no external magnetic field and no power source.

12 Next we make the conductor move through a magnetic field. Note that this is the same as if the conductor were still and the magnet was moving. Thus, relative to the conductor, the magnetic field is changing.

13 According to Faraday's Law a current will be induced in the conductor, provided it is a part of a circuit. You can see the circuit because there is a wire attached to the ends of the conductor.

14 The next picture shows why the current is induced as explained by Faraday.

15 The very important thing to keep in mind is this: either the conductor must move, or the magnetic field must move, or the magnetic field must change in intensity in order for current to be induced. If the conductor just sits in the field, there will be no current produced. Somehow, the conductor must "cut through" the lines of force.

16 You should realize that this is a marvelous discovery. We can produce electricity without have a battery in the circuit. All we need is relative motion between a magnetic field and a conductor in a closed loop in the field.

17 What if the conductor moved parallel to the lines of force? No current is induced!

18 Lenz's Law (You can't get something for nothing) Consider the diagram. Look at the magnetic field that the induced current produces. How does it interact with the external magnetic field?

19 On the side of the conductor away from you, the circular field and the permanent field are in the same direction (downward). (Stronger magnetic field) And on the side closest to you the fields are in the opposite direction and thus cancel out somewhat. (Weaker magnetic field) Where does the wire want to move? Towards us.

20 What does this do to the force required to pull the wire through the field? It resists the force, making it harder to do. The induced magnetic field of the induced current is fighting the motion.

21 Heinrich Lenz put it this way: The electrons of an induced current flow in such a direction that the induced magnetic field they create opposes the action of the inducing magnetic field. This is known as Lenz s Law

22 This is all very good because the Law of Conservation of energy is satisfied. That is, in order to get electrical energy out, you must put mechanical energy in. You can't get something for nothing! The mechanical energy can be supplied by falling water as in Bay d'espoir and Churchill Falls, or by expanding steam as in Holyrood. Another source of the mechanical energy that seems to be more and more desirable is wind power.

23 Determine the missing information

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25 Page 673 # 1, 2 Page # 1 4, 6, 16, 17, 18

26 Chapter Faraday s principle complements Oersted s principle. Faraday s principle or law of induction describes how a moving magnetic field or one that is changing (increasing or decreasing in strength) near a conductor causes charge to flow in that conductor. 2. The induced electromotive force in a conductor could be improved by using a magnet with a large field strength. The effect is greater if the wire is coiled because the strong magnetic field contacts a larger surface area of the conductor. Finally, the greater the rate of field change, the greater the electromotive force. 3. Inducing current to flow in a conductor requires that two conditions be met. First, a magnetic field must be present such that the field lines cut through a conductor at 90º. Second, this magnetic field must be changing either by moving the source magnet or by increasing or decreasing the strength of the electromagnetically induced field. 4. According to Lenz s law, the energy transferred to the current in the conductor comes from the kinetic energy of the source magnet or from the energy in the current of an electromagnet. Reduction in these forms of inducing energies can only be caused by an induced magnetic field. The work done to reduce the energy comes from the source of the induction. Manually moving a magnet in a coil of wire meets the resistance of the induced field. The energy lost from the source is gained by the induced current. This energy transfer from one form to another is governed by the law of conservation of energy.

27 6. In Fig , the conductor moving in a magnetic field would have no induced current moving through it. The field lines are parallel, meaning that the motion from the north pole to the south pole would not cause the strength of the field to change sufficiently to cause current flow. Induced current would flow if the conductor were moved either up or down.

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31 Consider the arrangement below: If a current in one coil is inducing an emf (electromotive force) and current in another coil then:

32 (i) the induced effects in the second coil only occur at the instant of opening and closing of the switch in the circuit of the first coil. It is only then that there is relative motion due to the collapsing and expanding magnetic field around the first coil.

33 (ii) the direction of the induced current in the second coil will depend on whether or not the switch in the first coil is opening or closing. This is because as the switch opens, the magnetic field around the first coil is "collapsing", and as the switch closes the magnetic field around the first coil is "expanding", i.e., these two fields move in opposite directions. In any case, the direction of the induced current will cause a magnetic field that OPPOSES the magnetic field in the first coil.

34 Show how the two compasses and the galvanometer will point when : A) The switch closes

35 Show how the two compasses and the galvanometer will point when : B) The switch is opens

36 Generators Describe the construction and operation of an AC/DC electric generator Sketch the characteristic graph of the current. (16.3)

37 Generators A generator is any device which converts mechanical energy of motion into electrical energy. They were originally called dynamos. Generators inside Hoover Dam

38 Every generator has the following components: 1. magnetic field - either permanent or electromagnet 2. conductor in motion - a spinning coil 3. external force to move the conductor - examples are: wind - wind generators falling water - hydroelectricity expanding steam - nuclear power plants gas engine - alternator in cars, portable generators for cabins and Rvs

39 Operation of a simplified AC generator (page 674) From Lenz's Law the induced current must produce a force (motor principle) that opposes the motion of the external force. slip rings they rotate with the coil contact brushes they are in constant contact with the slip rings There MUST be an external force that rotates the coil.

40 Describe how the generator works. The opposite occurs on the left side, the external force is acting downwards, so the induced current must exert a force upwards. Thus the induced current is heading out on the LEFT. On the right side, the external force is acting upwards, so the induced current must exert a force downwards. Thus the induced current is heading in on the right.

41 Where is the maximum induced voltage (current) produced? When the coil is moving perpendicular to the magnetic field. Where is induced voltage (current) zero? At the top and bottom. Here the external force acts parallel to the magnetic field. What happens when the coil reverses position from right to left? The current also changes direction.

42 The graph below shows induced current vs. rotation of the coil. max I or V 0 rotation min position of coil

43 This current alternates from positive (direction) to negative (direction). What is it know as? Alternating Current (AC) For household AC current we have 60 Hz (or 60 cycle per second) This means for every second there are 60 complete waves of electricity.

44 What is the frequency of the electricity in Europe? 50 Hz For 60 Hz, what is the period of one cycle?

45 The following is a sketch a graph of voltage vs. time for the AC electricity available for common household lighting and equipment. What is the amplitude of this graph? 110 V

46 This is the graph of current vs. time for a light bulb of connected to a household AC supply. What is the power rating of the bulb? 100 W

47 Sketch a graph of current vs. time for a 60 W light bulb of connected to a household AC (120 V) supply.

48 Slightly Less Simplified AC generator

49 DC Generator An AC generator can be changed into a DC generator by using a commutator, instead of slip rings. By using split rings (commutator) the current from each brush always leaves the generator in the same direction during the complete cycle.

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51 What type of current always flows in the same direction? Direct Current The graph of current vs. rotation of the coil look like this: Vab time

52 NOTE: This DC electricity has a serious disadvantage over a battery because batteries deliver a constant current The DC generator above drops its current to zero every half cycle. This would not be good for any device, such as a computer, which always expects a constant current.

53 The ripple effect can be reduced by: using several separate coils, with each coil having its own pair of split rings. using a capacitor. A capacitor behaves like an electrical sponge. If a sponge is drier than its surroundings it soaks up water. If it is wetter than its surroundings the water leaks out of the sponge.

54 Similarly with a capacitor: If the capacitor contains less voltage than the circuit it "soaks up" or stores electric charge. The capacitor is a voltage drop. When the circuit's voltage drops lower than what is stored in the capacitor, the capacitor "leaks off" some of its charge. The capacitor becomes a voltage source.

55 Schematic of circuit with DC generator and a capacitor and the graph of the resulting voltage a DC C F R L Vab b time

56 Next Topic But not these type of transformers!!

57 Churchill Falls

58 Churchill falls before development Churchill falls after development

59 Cross Section Powerhouse (Churchill Fall (Labrador) Corporation Limited)

60 Three Mile Island BACK

61 Car Alternator Portable Gas Generator BACK