Magnetism Electromagnetism Electromagnetic Induction

Transcription

1 3D Magnetism Electromagnetism Electromagnetic Induction 1

2 Recall: What determines the direction of magnetic fields? A compass. Draw the magnetic field between the magnets. N S N 2

3 What causes a magnetic field?? Movement of electric charges, such as electrons. 3

4 ELECTROMAGNETISM The study of the relationship between electricity and magnetism is called Electromagnetism Before, 1819 it was believed that there was no connection between electricity and magnetism. Who discovered the connection? Hans Christian Oersted 4

5 While demonstrating to students that the current passing through a wire produces heat, Danish professor Hans Christian Oersted ( ) noticed that the needle of a nearby compass deflected each time the circuit was switched on. This experiment led Oersted to the important conclusion that there is a relationship between electricity and magnetism, at a time when electricity and magnetism were considered separate phenomena. He proved that electric current was a cause of magnetism. 5

6 Although permanent magnets receive a lot of exposure, we use and depend on electromagnets much more in our everyday lives. Electromagnetism is essentially the foundation for all of electrical engineering. We use electromagnets to: generate electricity store memory on our computers generate pictures on a television screen diagnose illnesses and in just about every other aspect of our lives that depends on electricity. 6

7 Electromagnetism works on the principle that an electric current through a wire generates a magnetic field. This magnetic field is the same force that makes some metal objects stick to permanent magnets. In a bar magnet, the magnetic field runs from the north to the south pole. In a wire, carrying a current, the magnetic field forms around the wire. magneticfieldwire_en.htm 7

8 Magnetic fields are generated by moving electric charges. wire out in 8

9 The direction of the magnetic field around a current carrying conductor can be found using the first Left Hand Rule. First Left Hand Rule. (LHR#1) The thumb points in the direction of the electron flow. The fingers wrap around the electron flow in the direction of the magnetic field. 9

10 NOTE: There are right hand rules: These work for conventional flow. Recall that conventional flow is the opposite of electron flow. The magnetic field is still in the same direction. 10

11 Example: Draw the magnetic field around the following 1.A) e - B) C) out in 11

12 2. At each point labeled below on the circuit. A C B 12

13 3. Which way would a compass point in the circuit below? 13

14 4. Indicate the direction of the electron flow in each of the following: A) B) C) 14

15 SO, suppose there is a wire carrying electrical current outside of your house. Would you be exposed to the magnetic field while in your house? What would affect the strength of the magnetic field? Amount of current Distance from house Is it possible to determine the strength of this magnetic field?? Yes! Biot s Law! 15

16 Formulae for Electromagnetism Biot's Law B I 2 r 16

17 Biot's Law. Biot's Law states that the magnetic field strength (B) is directly proportional to the current in a straight conductor, and inversely proportional to the perpendicular distance (r) away from the conductor. B I r 17

18 The constant of proportionality being µ/(2 ), where the µ is the magnetic permeability of the substance in which the field is located. NOTE: If the field is in free space, the µ is written as µ o where µ o = 4 x 10-7 T m/a ( T = tesla, m = metre and A = ampere) Air can be considered to be free space. 18

19 Mathematically: B I 2 r This law is attributed to Jean- Baptiste Biot ( ). 19

20 Practice 1. Find the magnetic field strength (B) in air 15.0 m away from a straight conductor in which there is a current of 157 A. (Typical of an electric wire outside of your home.) I 7Tm A o B A 2 r 2 (15 m) 20

21 2.The magnetic field surrounding the currentcarrying wire shown below has a magnitude T, and is directed into the page at point P. Calculate the magnitude and direction of the current in the wire. 21

22 3.When a potential difference of 12.0 V is applied to a straight conductor, the magnetic field strength (B) 2.0 cm from the conductor is 3.0 x 10-5 T. What is the resistance of the conductor in ohms? o 2 rb B I I 2 r o 5 2 (0.02 m)( T ) I 7Tm 4 10 A V R 12V 4W I 3A 3A Answer: The resistance of the conductor is 4.0 W. 22

23 4.Two parallel wires each carry 5.0 A of current in opposite directions. A) Which way are the wires forced to move? Answer: The wires will move apart because there is a stronger magnetic field in between the wires. 23

24 B B 3.Two parallel wires each carry 5.0 A of current in opposite directions. B) What is the magnetic field strength midway between the wires if the wires are 10 cm apart? 1 7Tm 4 10 o I A 5.0A 2 r 2 2 ( m) T 5 5 B 2 10 T 2 10 T T Answer: The magnetic field between the wires is T 24

25 4. Repeat practice exercise #3 with the current in the wires running in the same direction. Solution: A) The wires will move together. B) At mid-distance between the wires, B l and B 2 cancel each other so that the net magnetic field strength is 0.0 T. 25

26 Cell Phone Towers The location of cellphone towers and electric substations in relation to residences is a concern to many homeowners due to the exposure of electromagnetic radiation. 26

27 Cell Phone Towers in Canada 27

28 Cell Phone Towers Pros Cons 28

29 Cell towers in Quebec s Eastern townships Cell tower petition Response 1 Response 2 29

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31 Research activity For the remainder of the class we look for answers to the following questions. You can use these sites, or you can search for other sources 31

32 1.What are some of the dangers due to the exposure of electromagnetic radiation(emf)? Name three. Sunburn Cataracts Cancers 32

33 2. A) What is the difference between ionizing and non-ionizing EMF? Ionizing can cause atoms to become ions. B) Give an example of each type of EMF: Ionizing X-rays UV-light Non-ionizing Microwaves Visible Light C) What is the danger of each type to living organisms? Ionizing Cancers Non-ionizing warming of living tissue Radiation Sickness

34 3 A) What evidence is given to support the claims that non-ionizing electromagnetic radiation from devices such as cell phone towers is causing physical harm? B) What evidence is given to support the claims that non-ionizing electromagnetic radiation from devices such as cell phone towers is NOT causing physical harm? 34

35 4. How is the location of cell phone towers determined? For example what are the considerations in the placement of towers based on A) Topography B) Capacity (# of users) C) Government Regulations D) Esthetics E) Health RISKS 35

36 5. What is the minimum distance your cell phone should be from your body? 36

37 Magnetic Fields in a Curved wire or in a Loop. Draw the current and the magnetic field around the cross section of the loop of wire as shown below TOP VIEW back wire front wire 37

38 What do you notice? The magnetic field inside the loop point in the same direction. Looking from the bottom: 38

39 We can apply this to a series of loops wrapped around a toilet paper roll. Draw the current on the front of the loops. 39

40 Now imagine that we cut the loops as below. 40

41 In your mind's eye, discard the half of the coil nearest you and draw the magnetic field around the cross section of the loop of wire as shown below. Top wire Bottom wire What do you notice? The magnetic field inside ALL of the loops point in the same direction. 41

42 In fact a series of loops provide a magnetic field similar to one found in a bar magnet. 42

43 Second Left Hand Rule The direction of this magnetic field in a coil is determined by: wrapping the fingers of your left hand around the coil in the direction of the electron flow. the thumb points in the direction of the magnetic field inside the coil. NORTH (The thumb is the pole of the magnetic field) 43

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46 Note: A large number of loops can be classified as a coil, helix or solenoid. Current flowing through a coil produces a magnet. This is how electromagnets are produced. Magnetic field strength is measured in a unit called tesla (T) Most classroom magnets are 0.1 T The earth's magnetic field is 10-5 T Junkyard magnets used to pick up old cars produce fields of about 1 T. This is a VERY strong magnet!! DEMO 46

47 Motor Principle Electric motors may look complicated when you look at them (don't dismantle Mom's mixer!), but the underlying principle is as simple as this: if two magnetic fields are in the same vicinity, they will act on each other. Depending on how they are brought near, their magnetic fields either attract each other, or repel each other. That is, one magnet can make the other move. This is the basic operation of a motor! 47

48 Example: Determine the direction of the wire below. Two magnets are align as shown and the circle represents a current carrying conductor. N S Draw the magnet fields. What do you notice? The magnetic fields on top are acting in the opposite direction, the magnetic fields on the bottom are acting in the same direction Which way will the wire move? UP 48

49 Rather than have to draw the magnetic fields each time, we can determine the direction of movement of the wire by the, yet again, another Left Hand Rule. 49

50 Third Left Hand Rule: With an opened left hand The thumb points in the direction of the electron flow The fingers point in the direction of the external magnetic field The palm faces the direction of the force applied to the wire. 50

51 Third Left Hand Rule (Notice the 3 right-angles) 51

52 Example: 1. Determine the missing force, electron flow, and poles. A) B) C) S N 52

53 2. Determine the direction of the coil Note: This is a simple DC electric motor. 53

54 Single loop motor "g and "h" are brushes. "e" and f make up a split-ring commutator with a halfring attached to each end of the loop inside the motor. The brushes allow the current to pass into and out of the loop via the split rings (e & f). 54

55 Apply the third left hand rule to both sides of the wire. Since the left-hand side is moving upwards, and the right-hand side is moving downwards, the loop (when properly mounted on bearings) will spin. We have motion, an electric motor, made from interactions between magnetic and electric fields! 55

56 Not much has been said about the splitring commutator. It is a simple but necessary device for a DC motor. (DC means direct current - that is, current which travels in one direction only). What can you say about the current in the loop when the loop has rotated 1/4 of a complete rotation from its present position? When the loop rotates 90 from its present position, the gaps between the rings will be touching the brushes. No current will flow for that split second. 56

57 Will the loop stop after making 1/4 turn? No!! Inertia will keep it moving. Is the inertia required to make the loop rotate very far? Hardly anywhere. How come? Because almost immediately split ring "e" comes in contact brush "h", and split ring "f comes in contact with brush "g". When that happens, the current flows in the loop once more. What fascinating thing has happened to the current in the loop? It has reversed! 57

58 When the loop started, "e" was in contact with "g" and "f" was in contact with "h". (Look and see.) That meant that the current entered through split ring "e" and left via split ring "f". 58

59 After ¼ turn the current is entering via split ring "f" and leaving via split ring "e". Since the half rings are riveted to the ends of the loop, the current in the loop has reversed. Why is it important for the current to be reversing? It is only in that way that the left hand side of the loop will always be forced upward, and the right hand side of the loop always forced downward 59

60 AC Motor AC motors differ from DC motors only by the way they are connected to the external power supply AC motors have: slip rings they rotate with the coil contact brushes they are in constant contact with the slip rings 60

61 Let s work out the motion of the conductor. Consider the red (left) conductor, which is horizontal on the diagram. Suppose the current is heading in through this side. What is the direction of the force on the wire? UP How far would the wire rotate? It would reach the top(vertical). 61

62 What is the direction of the force on the other side Blue (right) wire? Down Note: The magnetic field provided by permanent magnets is not going to change its direction. Thus the wire would remain stuck in the vertical position 62

63 However, the current does change direction. Why? AC electricity At that instant, in AC, the current cycle reverses, and so does the force acting on the wire. You have your RED conductor moving down and the Blue moves up This repeats and you have a rotation. Motor works! 63

64 Mathematical Formulas for Motor Force on Moving Charges Lorentz s Formula F Principle Force on Current carrying Wires Bqv sin F BIL sin 64

65 Magnetic Force on Moving Charges F Bqv sin where B is the magnetic field strength in tesla (T), q is the magnitude of the charge in coulombs (C) that is moving at a velocity v in m/s, and is the angle between v and B 65

66 Direction of the Force To determine the direction of the force on a negative charge that is passing through a magnetic field, just apply left-hand rule #3: Point your fingers in the direction of the magnetic field, Your thumb in the direction that the charge is moving, Your palm points in the direction of the force on the charge. 66

67 Direction of the Force While the left hand rule will work to show the deflection of a stream of electrons, the right hand rule will be useful for a stream of positive particles. Again, your thumb in the direction that the charge is moving. 67

68 Example: 1.A) An electron is moving vertically upward when it enters a magnet field directed North. In what direction is the electron forced in that instant? THINK 3D B) An alpha particle (charge 2+) is moving East upward when it enters a magnet field directed North. In what direction is the alpha particle forced in that instant? 68

69 2. A cathode-ray tube aims electrons parallel to a nearby wire that carries current in the same direction. What will happen to the cathode rays in terms of deflection? 69

70 3. A magnetic field of 44.0 T is directed into the smart board (your paper). A particle with a negative charge of 1.92 x C is shot into the field from the right, making an angle of 90.0 o with the field lines. If the particle is moving at 5.40 x 10 7 m/s, what magnetic force does it experience? F Bqv sin 70

71 4. An electron moving at m/s through a 1.50 T magnetic field experiences a force of N. What is the angle between the electron s path and the magnetic field lines? 71

72 5. A proton moves at m/s at a distance of 5.00 cm from a straight conductor carry a current of 1860 A. Find the magnitude and direction of the force on the proton when it is moving parallel to the wire A) In the opposite direction of current. B) In the same direction of current. 72

73 Practice: Text page A proton with a charge of 1.60 x10-19 C is travelling with a speed of 3.50 x 10 4 m/s perpendicularly through an external magnetic field of magnitude 4.20 x 10-4 T. Determine the magnitude of the magnetic deflecting force on the proton. ANS: 2.35 x N 73

74 2. An ion with a charge of 3.20 x C and a speed of 2.30 x 10 5 m/s enters an external magnetic field of 2.20 x 10-1 T, at an angle of 30 o, as shown in the figure below. Calculate the magnitude of the magnetic deflecting force on the ion. ANS: 8.10 x N 74

75 3. A negatively charged sphere travels from west to east along Earth s surface at the equator. What is the direction of the magnetic deflecting force on the sphere? ANS: Downward toward Earth s surface 75

76 More fun stuff! If a charge is shot in horizontally (or perpendicular to the lines of force), it will follow a circular path. This is because the moving charge has a circular field concentric around its direction of motion. This circular field and the permanent field reinforce each other on one side of the charge, therefore forcing it into circle. 76

77 N - Pole N - Pole e e S - Pole S - Pole Electron shot in perpendicular to magnetic field Electron shot in at angle to magnetic field 77

78 Why are auroras seen only at higher latitudes? 78

79 What would happen if the charged particle was a proton? N - Pole N - Pole S - Pole S - Pole 79

80 Describe the behaviour of beams of charged particles passing through a magnetic field. N - Pole e S - Pole This is how a TV picture tube works. 80

81 Centripetal Magnetic Force For a particle moving at a constant speed and experiencing a constant magnetic force at 90 o to its motion traces a circular path. What is providing the centripetal force? The magnetic force is. 81

82 Example 1: An electron is shot perpendicularly into a magnetic field of strength T with a velocity of m/s. What is the radius of the electron s path inside the magnetic field? F net Fcentripetal 2 mv r mv Bqv 2 sin F magnetic F Bqv magnetic sin r r Bq S ince r mv sin mv Bq 90 o 82

83 Example 1: An electron is shot perpendicularly into a magnetic field of strength T with a velocity of m/s. What is the radius of the electron s path inside the magnetic field? r mv Bq 31 6 ( kg)( m / s ) 5 ) T C ) 83

84 2. A) What is the velocity of an alpha particle moving in a circular path of radius 10.0 cm in a plane perpendicular to a 1.7 T magnetic field? Solution: An alpha particle is a particle consisting of two neutrons and two protons that is identical to the helium nucleus and is emitted during certain radioactive transformations. What is the charge on an alpha particle? q C 2 ( ) C 19 84

85 m kg r 10.0cm 0.100m B 1.7T mv r mv rbq Bq v v rbq m 19 m T C v kg m s 85

86 B) If this alpha particle is accelerated by the application of an electric field over a set of parallel plates, what voltage is required to accelerate the alpha particle from rest? Recall: V E q and Kinetic Energy is given by 2 mv V 2q V kg C 27 6 m s 2 E 1 mv V 86

87 TEXT p. 601 #3, 4, 6, 7(Fun One), 10 87

88 Formula for Magnetic Force Note: If a wire, or conductor is parallel to the magnetic field, the wire will not experience any force. Max Force Max Force NO Force 88

89 On page 641 there is a formula for calculating the force produced on a current carrying conductor by the motor principle F- Force (N) B Magnetic field strength (T) I Current (A) F BILsin L length of conductor (m) - angle made between conductor and external magnetic field 89

90 If = 90 o then sin 1, which will produce the maximum force. Conductor is perpendicular to magnetic field If = 0 o then produce NO force. sin 0, which will Conductor is parallel to magnetic field 90

91 The unit for B is named to honour the Croatian-born, American engineer, Nikola Tesla ( ) 1 tesla (T) is the magnitude of the magnetic field strength (B) that causes a conductor of length 1.0 m to experience a force of 1.0 N when the conductor is carrying a current of 1.0 A and is perpendicular to the magnetic field. 91

92 Practice: 1.Calculate the magnitude of the force on a 2.1 m wire that is carrying a current of 5.0 A perpendicular to a magnetic field of strength 1.4 x 10-4 T. 92

93 2. The wire (and current) in practice exercise 1 is running from east to west. The magnetic field direction is from north to south. In which direction will the force be exerted on the conductor (ignore gravity). The net force on the conductor in this case will be Down 93

94 3. Determine by what factor the force diminishes if a conductor changes its orientation from being perpendicular to the magnetic field to a new position that makes an angle of 45 o with the field. sin 45 o A certain current-carrying conductor which is perpendicular to a magnetic field experiences a force of magnitude F 1. If both the current and length of the conductor are tripled, but the conductor moves from a perpendicular orientation to an angle of 30 o with the field, the new force is F 2. Compare the magnitudes of the forces. F 2 is 4.5 times larger than F 1 94

95 5. A 3.0 m wire has a linear density of kg/m. The wire is sitting in a magnetic field of strength 3.5 x 10-3 T and the wire is perpendicular to the lines of force. How large a current would be required in order for the wire to be suspended against gravity in the magnetic field? kg N kg F mg m N F BILsin F BL sin I m N T 3.0m sin90 o Solution: 56 A. 95

96 6. How far from a conductor carrying 5.0 A of current is a second wire with a current of 10.5 A if the force between the two wires is 2 x 10-5 N/m 96

97 In your textbook: on p do #

98 ELECTROMAGNETIC INDUCTION Faraday s Law Lenz s Law Generators Transformers

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

100 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.

101 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!

102 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.

103 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.

104 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.

105 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.

106 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.

107 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.

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

109 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.

110 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.

111 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.

112 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.

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

114 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?

115 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.

116 What does this do to the force required to pull the wire through the field? It resists, or opposes the force, making it harder to do. The induced magnetic field of the induced current is opposing the motion. (NOT repelling it)

117 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

118 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.

119 Determine the missing information

120

121 5. Determine the movement of the bar magnet to produce the induced current

122 6. A bar magnet is moved away from a coil as shown. What is the direction of the induced current through the resistor and the polarity of the left end of the coil? 122

123 7.What is the direction of the induced current in the wire? a) into the page b) left c) out of the page d) right 123

124 Page 620 # 6, 7 Page 623 # 20

125 Typical Exam Question Two identical magnets are dropped simultaneously through the hollow tubes as shown below. A student observes that both magnets fall at different rates. Which magnet should fall faster? Using your knowledge of physics principles, how could this observation be explained? (2 marks)

126 Solution: The faster speed of fall would occur in the glass rod. (1) An induced current would be produced around the metal tube as each pole of the magnet moves through it. The direction of these currents will be such to produce a magnetic force to oppose the motion that produced it in accordance with Lenz s Law, thereby making it more difficult to fall down through the metal tube. No such current is induced in the glass as charges do not easily move through insulators.

127 127

128 Generators

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

130 A generator converts mechanical energy of rotation into electrical energy. The only difference between a DC (or AC) motor and a DC (or AC) generator is that some mechanical force causes the rotation and an electric current is produced. 130

131 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

132 Operation of a simplified AC generator 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.

133 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 right. 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.

134 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.

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

136 This current alternates from positive (direction) to (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.

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

138 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

139 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

140 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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142 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

143 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.

144 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.

145 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.

146 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

147 Electric Cars 147

148 Electric Cars So, what do you know about of electric cars? Can you identify specific issues with electric cars that are relevant to this province? Some arguments for electric cars include: Cleaner source of energy Renewable source of energy Quieter Cheaper to operate 148

149 Some of the arguments against electric cars include Number of charging stations Range Design (normally smaller) Initial Cost 149

150 Note: The technology of regenerative braking being used in newer electric and hybrid vehicles. When the accelerator is released, the car s engine will actually run in reverse, acting like a generator to charge the vehicle's battery. This reverse motion of the engine also slows down the vehicle to assist in braking (allowing less use of the car s braking system) 150

151 MOST IMPORTANT QUESTION OF WHOLE COURSE How do you charge (boost) a dead car battery? 151

152 Next Topic But not these type of transformers!!

153 Transformers

154 Transformers Transformers are a necessary component of the electricity transmission grid. The principle behind the operation of a transformer comes from a simple device made by Faraday called Faraday s Ring Apparatus.

155 Faraday's Ring Apparatus: (page 615) primary circuit soft iron ring secondary circuit primary coil secondary coil Note: There is NO DIRECT connection between the two circuits. When the switch is closed the entire ring becomes magnetized and a current is temporarily induced in the secondary circuit.

156 This is the same as inserting a magnet into a coil of wire. When the switch is opened, the entire ring becomes demagnetized and once again there is a temporary induced current in the secondary circuit but in the opposite direction. This is the same as removing a magnet from a coil of wire.

157 The following graphs represent the action of Faraday's Ring Apparatus. (These graphs ignore the phase lags associated with induced currents and magnetic fields)

158 Magent ic Fiel d in Ring Secondar y Coil Pr imar y Coil Sw it ch Cl oses Sw it ch Opens Note: The secondary current only occurs as the magnetic field changes. As the change in the magnetic field gets smaller the induced current gets less. When the change in magnetic field reverses so does the direction of the induced current. The induced current alternates in direction. (AC)

159 Important: The problem with DC is that the switch must be opened and closed to get a secondary current. However if AC is used, the electricity is always being turned on and off so that there will always be current produced in the secondary circuit. The following graphs illustrate what happens with Faraday's Ring connected to AC.

160 Primary Coil Magentic Field in Ring Secondary Coil NOTE: The induced current is NOT in the same phase as the inducing current. Thus to get a transformer (Faraday s Ring) to constantly produce current in the secondary circuit the primary circuit MUST be connected to AC.

161 The Victory of AC over DC The Transformer

162 1. Why the following generating stations must be built in areas that are usually a long distance from the areas that use the electricity? A) hydroelectricity They must be built in places where there is fast moving, or falling water. This does not usually occur close to large cities.

163 B) Fossil fuel and nuclear plants They need to be near a large body of cooling water. They also need to be away from a densely populated area, to decrease pollution, and the risk of radiation (nuclear plants) 2. Why does the electrical energy have to be at a high voltage when it leaves the generating station? To reduce the power loss in the transmission lines.

164 3. Why does the energy have to be reduced in voltage before it can be used by the consumer? To reduce the risk of electrocution. 4. What device is used to increase the voltage at the generating station and reduce it so that we can safely use it? A transformer

165 What is a transformer? A transformer is a device that is capable of changing electric potentials and currents. Schematics for : (i) an iron core transformer (ii) an air core transformer

166 Transformers apply both Faraday s and Oersted s principles to transform current and voltage in a circuit. The primary side of a transformer uses an alternating current to produce an electromagnet. (Oersted s principle) The changing magnetic field on the secondary side uses Faraday s principle to produce a new alternating current. If the number of turns in the primary and secondary sides of the transformers are different, then the voltage on the primary and secondary sides are also different. An increase in the number of turns in the secondary compared to the primary increases the secondary voltage proportionally. 166

167 There are three types of transformers: RED is primary BLUE is secondary (i) isolation transformers The number of coils on primary and secondary coils are the same. Voltages are the same on both sides Used in razor outlets it prevents the user from being exposed to a large current

168 (ii) step-up transformers More windings on secondary coil than on primary coil. Used to increase voltage in secondary coil. Used at generating stations to increase voltage (and decrease current) before power transmission.

169 (iii) step-down transformers More windings on primary coil than on secondary coil. Used to decrease voltage in secondary coil. Used at substations, light poles, AC adapter blocks, etc. to decrease voltage (and increase current) for safe power usage.

170 Modern transformers are made to be energy efficient by: making the copper coils have a low resistance to reduce power loss. One simple way this can be done is by keeping the transformers cool A core of high permeable, soft ferromagnetic material is used. The core quickly becomes magnetized to decrease the energy required to change the magnetic field in the core. The core is given a size and shape that will ensure a large amount of mutual induction between the coils.

171 Describe and explain the role of transformers in the distribution of electrical energy. At the generating station you need stepup transformers to increase the voltage before the energy is sent to the users. WHY? To decrease the power loss in the transmission lines.

172 At the district transformer stations the voltage is reduced, using step-down transformers, and the reductions continue, at the substations, to the pole/neighbourhood transformers and finally into your home.

173 Before Muskrat Falls 173

174 174

175 Equation that is used with transformers The ratio of the number of turns on the primary and secondary coils is equal to the ratio of the voltage supplied to the primary and the voltage obtained at the secondary. V N P P V N V P - voltage on primary V S - voltage at secondary S N P - number of turns on primary coil N S - number of turns on secondary coil S

176 Questions: 1. A) Determine the secondary voltage and give the name of this transformer. V V P S N N P S 176

177 1. B) Determine the number of turns on the primary and give the name of this transformer. V V P S N N P S 177

178 2. Which is the most efficient way to transfer electricity from remote generating sites?

179 3. (Probable Exam question) Calculate the Power loss in transmission given the current and or voltage and the resistance of the transmission lines. Ex: 4.5 MW of power is generated at 20.0 kv. Compare the power loss in transmitting through a 50.0 Ohm line without stepping up the voltage with transmitting after passing through a transformer having secondary windings and 400 primary winds. 179

180 Summary of Unit 3: Fields The table outlines all three fields (Gravitational, Electric and Magnetic) Write the equations for: Field Strength on Row 1 Force of the Field in Row 2 180

181 Churchill Falls

182 Churchill falls before development Churchill falls after development

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

184 Three Mile Island BACK

185 Car Alternator Portable Gas Generator BACK