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1 Magnetism Intro mages/type_effects/Electro_Magnet.gif

2 Magnets About three thousand years ago, the Greeks discovered rocks which would attract iron or similar rocks (a mineral ore we call magnetite). They were found in Magnesia, an area in Asia Minor usgsweb/photogallery/images/Magnetite%202_jpg.jpg S/courses/eas170/Minerals/image s/Magnetite.jpg

3 Lodestones The Chinese later used these naturally occurring magnets, called lodestones, for ocean navigation. The first recorded description of a compass was in the Chinese Book Dream Pool Essays (1086) by Shen Kuo in the Song Dynasty, about 100 years earlier than its first record in Europe by Alexander Neekam in Source: ese/china%20culture/culture %20pic/Great%20Inventions 3.jpg

4 Magnets William Gilbert (a physician to Queen Elizabeth I) made many discoveries about magnets – including that the Earth itself is a giant magnetic sphere! He also discovered that he could make his own magnets, and coined the term “magnetic pole”.

5 Magnets Magnetism is caused by the movement of electrons within an atom. Magnets are surrounded by magnetic fields Electrons produce magnetic fields because they –orbit the nucleus –spin on their own axis. mg/bar_magnet.gif

6 !!! Chemistry Flashback !!! Electrons pair up two per orbital – one spins clockwise, the other counter clockwise. Direction of spin determines the “pole” of the electron. Diamagnetic materials – have paired electrons where the fields cancel out because all the electrons are in pairs and have opposite spins. –Material will be repelled by a magnetic field (weak effect). (1/1,000,000 weaker than iron). –The electric fields required to levitate these materials are extremely large. Source:http://www.hfml.sci.kun.nl/froglev.html Magnetic Frog Magnetic Frog

7 Paramagnetic materials - have unpaired electrons. Iron (Fe) electron configuration 4 s 2 3 d 6 –In d cloud electrons occupy 5 orbitals. –Auf bau principle – electrons must enter an empty orbital before they pair up. –The magnetic fields of the unpaired electrons combine to make each Fe atom a tiny magnet. !!! Chemistry Flashback !!!

8 But not all iron is magnetic…right? Why not? The unpaired electrons give areas in these elements small magnetic fields. When the individual areas or domains all line up, the piece of metal has a BIG magnetic field

9 Magnetic Materials Ferromagnetic – an element which is attracted to magnets and can be made into temporary magnets. –Iron, nickel and cobalt. –All are paramagnetic elements

10 Hard magnetic materials - require a strong external magnetic field to orient their domains. –Once oriented the domains stay aligned. –Permanent magnets –Alnico, an alloy of aluminum, nickel, cobalt, iron and copper common permanent magnet. –Heating or hitting can move the domains out of alignment Soft magnetic materials (nails & paper clips) are easily magnetized but demagnetized when the external field is removed. –Domains become random again when the magnet is removed. –Temporary magnetism http ://www.physics.carleton.ca/~watson/1000_level/Magnetism/1008_Perm_mags.html Magnetic Materials

11 Magnetic Poles Magnets have polarity (different parts of a magnet experience different forces) Poles of a magnet are called north and south Opposite poles attract, like poles repel. Magnetic poles always come in pairs (north and south) not possible to have only one pole –Magnetic dipole. Breaking a magnet in half forms two new magnets.

12 Magnetic Field Cutnell & Johnson, Wiley Publishing, Physics 5 th Ed. The space around a magnet has an invisible magnetic field that exerts magnetic forces This field is represented by drawing magnetic field lines or lines of magnetic flux Greater magnetic flux density (stronger field) is shown with more flux lines –closer lines stronger the magnetic field. –the flux density is the greatest at the POLES! arrows show the direction (out of the north pole and into the south pole) pass through the magnet to form loops. Earth’s Field

13 Drawing Magnetic Fields Cutnell & Johnson, Wiley Publishing, Physics 5 th Ed. Compass Method If we could scatter tiny compass needles over a piece of paper and bring a bar magnet underneath them, the needles would line up in the bar magnet’s magnetic field Iron Filing Method Sprinkling iron files near a magnet will cause the filings to line up along the magnetic flux lines

14 Magnetic Fields Bar Magnet Two Like Magnets Two Unlike Magnets

15 Magnetic Field Between two like poles.

16 Magnetic Field Between two opposite poles.

17 Magnetic Field Around a rotating pole.

18 Magnetic Field Field Variations.

19 Earth’s Magnetism The earth has a magnetic field! Scientists theorize it is caused by electric currents circulating in the liquid outer core. The earth has both a north and a south magnetic pole, like a magnet. Notice the earth’s magnetic field lines! Same shape as a bar magnet Is the Earth’s North Pole a North or a South magnetic pole? Cutnell & Johnson, Wiley Publishing, Physics 5 th Ed. Earth’s Core

20 Compass If suspended, a magnet’s north pole will point toward the earth’s geographic north. A compass is a magnetic needle on a pivot. Cutnell & Johnson, Wiley Publishing, Physics 5 th Ed.

21 North Pole Fallacy Is the Earth’s North Pole a North or South magnetic pole? You already know that like poles _______and that unlike poles _______. But think about this… The north end of a magnet is attracted to the south end of a second magnet. The north end of a compass needle points to the geographic NORTH pole of the earth…. So…the earth’s geographic NORTH pole must be a magnetic SOUTH pole. Cutnell & Johnson, Wiley Publishing, Physics 5 th Ed.

22 Earth’s Magnetic Field Since the earth is tilted, the magnetic poles of the earth don’t line up exactly with the geographic poles. This discrepancy is called magnetic declination. Copywrited by Holt, Rinehart, & Winston /ch11.htm Angle of declination

23 Cosmic rays (atomic nuclei stripped bare of their electrons) would continuously bombard Earth’s surface if most of them were not deflected by Earth’s magnetic field. Given that Earth is, to an excellent approximation, a magnetic dipole, the intensity of cosmic rays bombarding it surface is greatest at the A.poles B.mid-latitudes C.equator

24 Importance of Earth’s Magnetic Field Navigation –Animal migration –Navigational systems Protects the earth from solar winds –Charged particles from the sun –Aurora Borealis – Northern Light –Aurora Australis – Southern Lights g

25 Earth’s Magnetic Field Over the past 150 years, the main component of the Earth's magnetic field has decayed by nearly 10%, a rate ten times faster than expected This is centered around an area in the south Atlantic Ocean that has a field 35% weaker than expected. source: Earth’s Magnetic Earth’s Magnetic Field Field

26 Are Magnetism and Electricity Connected? Until 1820 everyone thought electricity and magnetism were completely separate. Hans Oersted discovered that a compass needle is deflected by an electric current. Electricity and Magnetism are just different aspects of the same thing! gy/descent/photos/H.C.Oersted.1822.jpg magnetostatics/images/35-wirecompass320.jpg

27 Magnetic Field Generation Moving charges create magnetic fields. The magnetic field of a current through a straight wire makes circles perpendicular to the current.

28 Magnetic Field Generation Cutnell & Johnson, Wiley Publishing, Physics 5 th Ed. Copywrited by Holt, Rinehart, & Winston

29 Right Hand Rule (Grip Rule) To calculate the direction of the magnetic field produced by a current carrying wire: Point your right thumb in the direction of the conventional current flow. Your fingers curl in the direction of the magnetic field. Current Convention: Copywrited by Holt, Rinehart, & Winston Cutnell & Johnson, Wiley Publishing, Physics 5 th Ed.

30 RHR Practice What direction would the magnetic field be around a current coming out of the screen? A. Clockwise B. Counterclockwise C. Into the screen D. Out of the screen

31 The magnetic field of a straight, current- carrying wire is A.parallel to the wire. B.perpendicular to the wire. C.around the wire. D.inside the wire. E.zero. Slide 24-4 Checking Understanding

32 Point P is 5 cm above the wire as you look straight down at it. In which direction is the magnetic field at P? Slide Checking Understanding

33 Slide Checking Understanding Point P is 5 cm above the wire as you look straight down at it. In which direction is the magnetic field at P?

34 Attraction or Repulsion What will happen if two long parallel wires are carrying currents, I 1 and I 2, flowing in the same direction are placed next to each other? a.They attract each other b.They repel each other c.No interaction Cutnell & Johnson, Wiley Publishing, Physics 5 th Ed. Copywrited by Holt, Rinehart, & Winston

35 Attraction or Repulsion What will happen if two long parallel wires are carrying currents, I 1 and I 2, flowing in the opposite direction are placed next to each other? a.They attract each other b.They repel each other c.No interaction Cutnell & Johnson, Wiley Publishing, Physics 5 th Ed. Copywrited by Holt, Rinehart, & Winston

36 Magnetic Force A charge moving in a magnetic field feels a force. The force on the charge is a “sideways” force - perpendicular to the field line and to the charge’s velocity.

37 Magnetic Force Charge not moving? Charge moving with field line? The charge must move ACROSS the field lines to feel a force NO FORCE Cutnell & Johnson, Wiley Publishing, Physics 5 th Ed.

38 Magnetic Force Magnets exert forces on moving charges Force is proportional to: 1. intensity of magnetic field 2. speed of the moving charged particle

39 Cutnell & Johnson, Wiley Publishing, Physics 5 th Ed. Copywrited by Holt, Rinehart, & Winston Right Hand Rule (Slap Rule) Force direction calculation Thumb direction of (+) charge Point fingers in the direction of the magnetic field The palm indicates the direction of force Alternative method Is there a left hand rule? Yes, when a (–) charge is involved The direction of the magnetic field can be determined using the right hand rule.

40 Magnetic Force 1) out of the page 2) into the page 3) downward 4) to the right 5) to the left A positive charge enters a uniform magnetic field as shown. What is the direction of the magnetic force? x x x x x x v q Checking Understanding

41 1) out of the page 2) into the page 3) downward 4) upward 5) to the left x x x x x x v q Magnetic Force A positive charge enters a uniform magnetic field as shown. What is the direction of the magnetic force? Checking Understanding

42 1) out of the page 2) into the page 3) zero 4) to the right 5) to the left  v q Magnetic Force A positive charge enters a uniform magnetic field as shown. What is the direction of the magnetic force? Checking Understanding

43 Experiments show that: F ~ the current, I F ~ the length of conductor in the field, F = (a constant) I x the constant is called the magnetic flux density (B) when current flows at 90° to the field. B is a measure of the strength of the magnetic field B units are NA -1 m -1 or Teslas (T) Magnetic Force Cutnell & Johnson, Wiley Publishing, Physics 5 th Ed.

44 Right Hand Rule (Slap Rule)

45 Magnetic Force on a Wire A horizontal wire carries a current and is in a vertical magnetic field. What is the direction of the force on the wire?A horizontal wire carries a current and is in a vertical magnetic field. What is the direction of the force on the wire? 1) left 2) right 3) zero 4) into the page 5) out of the page B I Checking Understanding

46 B I 1) left 2) right 3) zero 4) into the page 5) out of the page Magnetic Force on a Wire A horizontal wire carries a current and is in a vertical magnetic field. What is the direction of the force on the wire?A horizontal wire carries a current and is in a vertical magnetic field. What is the direction of the force on the wire? Checking Understanding

47 Flux Density inside a Long Coil (Solenoid) Current flowing through a conductor produces a magnetic field. For a long straight wire, then the field is distributed over a large region of space. If the wire is used to make a coil, the magnetic field is concentrated into a smaller space and is therefore stronger. Cutnell & Johnson, Wiley Publishing, Physics 5 th Ed.

48 Wire Loop Bending a current carrying wire into a circle gives a magnetic field like this: Cutnell & Johnson, Wiley Publishing, Physics 5 th Ed.

49 Solenoid Winding many turns makes a solenoid coil with a magnetic field like a bar magnet. Cutnell & Johnson, Wiley Publishing, Physics 5 th Ed.

50 Right Hand Rule (Coil Rule)

51 Current Loop P I What is the direction of the magnetic field at the center (point P) of the square loop of current? 1) left 2) right 3) zero 4) into the page 5) out of the page Checking Understanding

52 Electromagnet A coil of wire with a current passing through it creates a magnetic field just like a bar magnet. Can be turned on/off or reversed by controlling the current flow.

53 Electromagnet How can you strengthen an electromagnet? Putting an iron core in the center of the coil strengthens the magnetic field. Adding more coils makes the magnetic field stronger. Increasing the current through the wire strengthens the magnetic field.

54 Simple Electric Motors A simple d.c. electric motor consists of a coil of wire placed in a magnetic field. When current flows through the coil, a torque is produced. The brushes and commutator conduct the current from the supply to the coil. Each of the carbon brushes makes contact with one half of the commutator. The commutator rotates with the coil. This arrangement ensures that the torque produces a constant sense of rotation. Copywrited by Holt, Rinehart, & Winston

55 Simple Electric Motors

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60 Can a Magnetic Field produce a Current? For 12 years after Oersted’s discovery that electric current creates a magnetic field, scientists looked for a way for magnetic fields to create a current. In 1832, Michael Faraday made a suggestion: “Move the Magnet!” Doing so “induced” an electric current!!!! The result of Faraday’s work became known as electromagnetic induction.

61 Electromagnetic Induction Thrusting a magnet into a loop of wire induces current. Holding the magnet still does not! It doesn’t matter whether the magnetic field moves or the wire moves. It works either way! Faraday described this by saying that electromotive forces are generated in the wire whenever field lines cut across the wire. When the magnet is thrust into the loop, its field lines cut across the wire generating EMF that produces current. Ditto when the loop is moved over the magnet.

62 EMF Clarification Warning: The term EMF can be misleading!!!!! The electromotive forces that are generated are not really “forces”. They are actually increases in electrical potential (voltage) and are therefore measured in VOLTS! But WHY??!?!?!?!

63 EMF Generation No relative motion between the conductor and the magnetic field no emf. Copywrited by Holt, Rinehart, & Winston

64 EMF Generation

65 Cutnell & Johnson, Wiley Publishing, Physics 5 th Ed. Generators Generator action requires a conductor, a magnetic field, and relative motion. Generators and motors are almost identical in construction. Both consist of wire loops wrapped around an iron form - the armature - placed in a strong magnetic field. Convert energy in opposite directions. –Electric generators convert mechanical energy to electric energy. –Electric motors convert electrical energy into mechanical energy.

66 Copywrited by Holt, Rinehart, & Winston Generator vs Motor In a generator, as the armature turns, current is induced in the wire. This current cuts across the magnetic field lines and produces an EMF (voltage). In a motor, a voltage is placed across an armature coil that is in a magnetic field. The voltage causes current to flow in the coil creating a magnetic force which causes the armature to turns (torque)

67 Generator Applied Induction

68 AC Power AC is like a battery where the terminals exchange sign periodically! AC sloshes back and forth in the wires Recall when we hooked up a bulb to a battery, the direction of current flow didn’t affect its brightness Although net electron flow over one cycle is zero, can still do useful work! –Imagine sawing (back & forth), or rubbing hands together to generate heat

69 Spring 2006UCSD: Physics 8; = 170 Volts = -170 Volts 120 VAC is a root-mean-square number: peak-to-peak is 340 Volts! AC Power

70 AC vs DC Power Distribution

71 AC vs DC Power – Current Wars DC – Thomas Edison Invented the light bulb Owned the patents for DC Power Distribution Provided DC Power to 59 customers in NYC, September 4, 1882 Company became General Electric AC – Nikola Tesla Invented the AC Motor, radio (Marconi given credit), radio control. Owned the patents for AC Power Distribution Worked for Edison then went to work for Westinghouse Won contract to provide electricity to Chicago World Fair in 1893

72 What if Edison had won the current wars? If so Homes powered with DC Power. –Power plants needed to be within a few miles of home. Currently, power plants are far from homes. Why? Long power lines Feasibility? power plant home appliance long transmission line R load R wire X looks like:

73 Estimate resistance of power lines: ~ Ohms per meter length 200 km Calculate current required by a single bulb 120 W light bulb 12 Volt connection box using Calculate power lost in transmission line Recall, P lost = I 2 R 120 Watt Light bulb 12 Volt Connection Box Power Plant on Colorado River 150 miles DC Power Distribution – Feasibility?  /m  2  10 5 m = 200 Ohms P = VI so I = P/V I = 120 W/12 V = 10 A P lost = I 2 R = (10A) 2 x 200Ω = 20,000 W

74 Calculate Total Power Required Total Power = Power Lost + Power Required Calculate the efficiency (  P out /P in What could we change in order to do better? 120 Watt Light bulb 12 Volt Connection Box Power Plant on Colorado River 150 miles DC Power Distribution – Feasibility? R load R wire X  P o /P i = 120 W/40,120 W = 0.3% Total Power = 2 (20,000) = 40,120 W

75 The Tradeoff Major Problem: high current through the (fixed resistance) transmission lines Need less current –Power Loss is I 2 R  I 2  increases exponentially with current Appliances require minimum amount of power –P = VI so less current demands higher voltage Solution: High Voltage transmission Repeat the power calculation for a 120 W light bulb with 12,000 Volts or 12kV delivered to the house.

76 Calculate Current Power Lost in Transmission Line Total Power Required Efficiency (  P out /P in More Power Delivered  More Profit! 120 Watt Light bulb 12,000 Volt Connection Box Power Plant on Colorado River 150 miles DC Power Distribution – Feasibility?  P o /P i = 120 W/ W = % Total Power = 2 (0.002W) + 120W = W I = P/V = I = 120 W/12,000 V = 0.01 A P lost = I 2 R = (0.01A) 2 x 20Ω = W

77 DANGER High Voltage! High voltage in each household is a recipe for disaster –sparks every time you plug something in –risk of fire –not cat or kid friendly Need a way to step-up/step-down voltage

78 How electricity gets to your home…. Step-up Voltage Power station National Grid Step-down Voltage Homes, businesses and factories etc Power Transmission HOW?

79 Transformer Transformer is just wire coiled around metal. –Magnetic field is generated by current in primary coil –Iron core channels magnetic field through secondary coil. –Changing magnetic field induces a current in the secondary coil. –The size of the output voltage depends upon the ratio of the turns on the primary and secondary coils. Transformer Equations Only works with AC, not DC, Why? P = primary quantities S = secondary quantities

80 Spring 2006UCSD: Physics 8; Typical Transformers

81 More/less turns on the primary coil than the secondary coil. Primary Secondary Step-down Transformer

82 More/less turns on the primary coil than the secondary coil. Primary Secondary Step-up Transfomer

83 Transformer Problem A transformer has 200 turns on its primary coil and 50 turns on its secondary coil. The input voltage is 920V. a)Is a step-up or step-down transformer? b)What is the output voltage? Step-down 230V

84 Transformer Problem A transformer has 100 turns on its primary coil. It has an input voltage of 35V and an output voltage of 175V. a)Is a step-up or step-down transformer? b)How many turns are on the secondary coil? Step-up 500 turns

85 Understanding Check A transformer has 30 turns on its primary coil and 240 turns on its secondary coil. If the output voltage is 128V, what is the input voltage? A.8V B.16V C.1024V D.512V

86 A way to provide high efficiency, safe low voltage: High Voltage Transmission Lines Low Voltage to Consumers step-up to 500,000 V step-down, back to 5,000 V step-down to 120 V ~5,000 Volts Power Transmission

87 Transmission structures three-phase “live” wires 500, , ,000 69,000 7–13,000 long-distance neighborhood to house

88 How electricity gets to your home…. Step-up transformer Power station National Grid Step-down transformer Homes, businesses and factories etc Power Transmission

89 Cutnell & Johnson, Wiley Publishing, Physics 5 th Ed. Power Transmission Power Generated at Power Plant Power stepped up in voltage and sent to high voltage transmission lines Power stepped down in voltage through transformers prior to sending it to your home. Example: 480kW Plant Transmission Lines transmit electricity at 240kV What is the current in the line? If the resistance of the cable is 100 Ω what is the power lost in the transmission? What would be the power lost if the lines transmitted power at 120kV?

90 Spring 2006UCSD: Physics 8; = 170 Volts = -170 Volts 120 VAC is a root-mean-square number: peak-to-peak is 340 Volts!

91 Power Transmission Most U.S. voltages are 120 V. It used to be 110-V because early light bulbs couldn’t handle higher voltages. Most other countries have switched to 220 V, which transmits power more efficiently.

92 Cutnell & Johnson, Wiley Publishing, Physics 5 th Ed. AC Receptacle Receptacles have three holes each Lower (rounded) hole is earth ground –connected to pipes. –green wire Larger slot is “neutral” –for current “return” –never far from ground –white wire –if wired correctly Smaller slot is “hot” –swings to +170 and  170 –black wire –dangerous one

93 Sources: Tsokos, K.A.,Physics for the IB Diploma,5 th edition, Cambridge University Press, University of California at San Diego (2008). AC Electricity: Why AC Distribution? Retrieved from res.html res.html


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