ELECTROMAGNETISM The study of the relationship between electricity and magnetism is called

Transcription

1 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 1

2 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. 2

3 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 metal objects stick to permanent magnets. In a bar magnet, the magnetic field runs from the north to the south pole. In a wire, the magnetic field forms around the wire. 3

4 Magnetic fields are generated by moving electric charges. wire out in 4

5 The direction of the magnetic field around a current carrying conductor can be found using the first Left Hand Rule. First Left Hand Rule. The thumb points in the direction of the electron flow. The fingers wrap around the electron flow in the direction of the magnetic field. 5

6 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. 6

7 Example: Draw the magnetic field around the following 1.A) e - B) C) out in 7

8 2. At each point labeled below on the circuit. A C B 8

9 3. Which way would a compass point in the circuit below? 9

10 4. Indicate the direction of the electron flow in each of the following: A) B) C) 10

11 Page 638 #1, Page 663 #

12 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 12

13 What do you notice? The magnetic field inside the loop point in the same direction. Looking from the bottom: 13

14 We can apply this to a series of loops wrapped around a toilet paper roll. Draw the current on the front of the loops. 14

15 Now imagine that we cut the loops as below. 15

16 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. 16

17 In fact a series of loops provide a magnetic field similar to one found in a bar magnet. 17

18 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) 18

19 Page #2,3 19

20 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 20

21 Factors Affecting the Magnetic Field of a Coil. (page 636) The strength of the electromagnet can be increased by: increasing the current in the coil increasing the number of coils. The more tightly wound the coil is, the stronger will be the field. decreasing the size of the coil. A smaller diameter results in a stronger magnetic field. The more ferromagnetic the material within the coil, the greater the magnet s strength. 21

22 The reason is that the domains within the ferromagnetic core line up due to the magnetic field passing through the centre of the coil. The fields associated with the domains then reinforce the applied field. Would you use soft or hard ferromagnetic material? The best ferromagnetic core is soft iron because not only do the domains line up when the current is flowing, they also go back to their random state when the current is shut off. 22

23 Why is this an ideal control condition for a magnetic crane that moves car wrecks from one location to another? The load can be deposited simply by shutting down the power to the electromagnet. 23

24 What is a special term that accounts for the change in the strength of an electromagnetic according to the type of material that comprises the core? Magnetic permeability. It is represented by the Greek letter m (mu) 24

25 Note: You can think of permeability in a general way. For example, a rock is not very permeable as far as water is concerned, but a sponge is very, very permeable! So, too, some materials are very permeable as far as magnetic lines are concerned. Iron and nickel are examples. And, as you know, paper and wood have very little permeability for magnetic lines. 25

26 So, it is easy to compare substances according to how well magnetic lines travel through them. You could even say the magnetic lines are "trapped" very well in substances with high permeability. The relative magnetic permeability of a substance is a ratio of the strength of the magnet when the core is made of that substance to the strength when there is no core at all (AIR CORE). 26

27 Ferromagnetic substances have a high permeability. You can find the magnetic permeability of some familiar substances in Table 15.3 on p. 637 of your textbook. However, here is a cautionary note: the permeability coefficients in the table are not absolute values, but rather appear to be relative permeabilities. 27

28 Notice that µ for a vacuum and oxygen (and you can assume also for air) is taken to be 1. For iron, µ is given as 6100, which we should interpret to mean that if an iron core is place inside a coil, the magnetic field strength of the coil will increase by a factor of

29 By what factor would the following increase or decrease the magntic strength of an electromagnet with an air core? A) An iron core 6100 times increase B) A core of aluminium There would be almost no effect on the electromagnet s strength C) A core of copper There would be a slight decrease in the magnetic strength WHY?? Copper is diamagnetic! 29

30 Magnetism (Revisited) The Nitty Gritty of Magnetism What do electrons have around them when they are still? They have an electric field around them What additional field do electrons have around them when they are moving? They have an additional magnetic field. 30

31 Are the electrons moving in an atom or are they sitting still? They are moving. In fact they have two types of motion: Spinning and Revolving 31

32 Since the electrons are moving, what can we say about the atom? The atom has a magnetic field around it. That is, it is like a tiny, tiny, tiny, tiny magnet! What is the word for such a tiny magnet? Dipole, where di-pole means two poles. One is called a North pole and one is called a South pole. Is this magnetic effect the same for all materials? No! The effect would be the same only if the electrons of all materials spinned and revolved in the same way. 32

33 As you can see from the picture, a couple of electrons in the same region can have a canceling effect if they rotate in opposite ways. That is, if one rotates clockwise while the other rotates counterclockwise, there will be a canceling effect on the tiny magnetic field. 33

34 How does the different possibilities for electron movement affect the magnetic nature of materials? Some materials are strongly attracted by a magnet, some are weakly attracted, and some are even repelled if the magnet is strong enough. Paramagnetic substances are attracted by a magnet. In such substances the spin of one electron is not cancelled by another. If paramagnetic substances are strongly attracted, they are given a special title, ferromagnetic 34

35 What are some ferromagnetic substances? Iron Nickel Cobalt Alloys made including these 3 elements. Other paramagnetic substances that are attracted to a lesser degree are: Oxygen Aluminum Platinum 35

36 What is the name given to substances that are repelled by a magnet? Diamagnetic WHY are they not magnetic? In some substances electrons are paired in such a way that the spin of one cancels the spin of another. Examples are: Water Copper Bismuth Zinc Silver carbon 36

37 A cautionary note: Some textbooks state that the term paramagnetism is applied only to substances that experience a weak attraction to a magnet. In this view ferromagnetism is not under the umbrella of paramagnetism, but instead is a separate category. 37

38 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! 38

39 If the inside part is then connected to beaters, you can make a If the inside part is connected to a saw, you can cut a. Who first devised the first motor that produced continuous motion using a form of energy? Michael Faraday 39

40 What was the energy? Electric and Magnetic When was this? 1821 Basic Idea: A current carrying conductor placed in a magnetic field experiences a force. WHY? 40

41 Consider the following: In the picture below you can see only one magnetic field. There is no sign of a circular field yet. This is because the switch is. OPEN There is no current flowing, and no circular magnetic field around the straight conductor which is hanging like a motionless swing. 41

42 Below shows what happens when the switch closed. In this picture the switch is closed (i.e., the motor is turned on), the current flows and a circular magnetic field is created around the conductor according to left-hand rule #1. Look at the two magnetic fields to the left of the conductor. They are pointing in the same direction. This means the two fields are repelling each other to the left of the conductor. WHY? Like Poles repel! 42

43 Below shows what happens when the switch closed. The magnetic fields to the right of the conductor are in opposite directions. Here the fields are attracting each other. WHY? Different poles attract The end result is that the conductor is kicked to the right. We have made motion out of electricity and magnetism! 43

44 Example: Determine the direction of the wire below. Two magnets are align as shown and the circle represents a current carrying conductor. N S Hint: 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 44

45 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. 45

46 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. 46

47 (Notice the 3 right-angles) 47

48 Example: Determine the missing force, electron flow, and poles. A) B) C) S N 48

49 That magnetic swing was not much of a motor. However, can you remember being in a swing with someone pushing you really hard? Were you ever scared that the swing would make a complete circle and come down on the other side? If we had the right set-up, the magnetic fields could make the straight conductor do that. This would result in rotary motion, which is the motion that all motors make. 49

50 If you have ever taken a motor apart, you know that the inside piece that rotates consists of many, many turns of wire. This is too complicated to draw here, but we can show why the rotor" rotates by looking at just one of the turns of wire. 50

51 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. 51

52 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. 52 Now, this is more like a motor!

53 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, split, split second. 53

54 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! 54

55 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". 55

56 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 56

57 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 57

58 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) q q- angle made between conductor and external magnetic field 58

59 If q = 90 o then sin q 1, which will produce the maximum force. Conductor is perpendicular to magnetic field If q = 0 o then produce NO force. sin q 0, which will Conductor is parallel to magnetic field 59

60 What are the units for T? Solve the equation for B. F BILsin q B IL F sin q Sub in the units. N N Ns T Am C m s Cm 60

61 The unit for B is named to honour the Croatian-born, American engineer, Nicola 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. 61

62 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. 62

63 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 63

64 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 64

65 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 q I q m N T 3.0m sin90 o Solution: 56 A. However such a large current would probably cause something to catch on fire! 65

66 In your textbook: on p do #1, #2. on p do #10, #11. on p do #25, #26 on p do #37 66