1.
Magnetism
Intro

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

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 1190.
Source:

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
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.

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:
Magnetic Frog

7. Paramagnetic materials - have unpaired electrons.
!!! Chemistry Flashback !!!
Paramagnetic materials - have unpaired electrons.
Iron (Fe) electron configuration 4 s2 3 d6
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.

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
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. http://www. physics. carleton
Magnetic Materials
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

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. Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.
Magnetic Field
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.
Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.
Earth’s Field

13. Drawing Magnetic Fields
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
Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.

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!
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 5th Ed.

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 5th 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 5th 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.
Angle of declination
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23. 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
Earth’s Core

24. Earth’s Magnetic Field
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
Field

25. 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!
Oersted – Dutch teacher. Demonstrating that magnetism and electricity not connected. Student after class switched direction of the wire.

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

27. Magnetic Field Generation
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Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.

28. Right Hand Rule (Grip Rule)
Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.
Copywrited by Holt, Rinehart, & Winston
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:

29. 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

30. Checking Understanding
The magnetic field of a straight, current- carrying wire is
parallel to the wire.
perpendicular to the wire.
around the wire.
inside the wire.
zero.
Answer: C
Slide 24-4

31. 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?
Answer: D
Slide 24-19

32. 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?
Answer: D
Slide 24-20

33. 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.

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

35. Magnetic Force Experiments show that: F ~ the current, I
Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.
Magnetic Force
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-1m-1 or Teslas (T)

36. Magnetic Force Flux density is the force per unit
Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.
Magnetic Force
Flux density is the force per unit
length acting on a conductor
placed at 90° to the field.
If the conductor is placed at an angle θ to the field, then the force is given by:

37. Right Hand Rule (Slap Rule)

38. Magnetic Force on a Wire
Checking Understanding
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?
1) left
2) right
3) zero
4) into the page
5) out of the page I B

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

40. Magnetic Force on a Charge
On average, the charges must be moving with speed v = /t.
Therefore,
If the direction of the velocity is not at 90° to the flux lines, we use the component of the velocity which acts at 90° to the field.
and the direction of the force is at 90° (perpendicular) to both the velocity and the magnetic field.
Consider a conductor of length,  
having n free electrons per unit volume. A current, I, is flowing through it.
Recall, and
This is the sum of the forces acting on all the free charges as they move through the piece of conductor.
Therefore, force per charge, F, is given by
Charged Particle in a
Magnetic Field

41. Right Hand Rule (Slap Rule)
The direction of the magnetic field can be determined using the right hand rule.
Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.
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
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42. What direction?
A positive charge moving with a constant velocity enters a uniform magnetic field pointing out of the paper. What way will the charge move?
Continue straight
Curve upward
Curve downward
Go into the paper
What would have happened if it was a negative charge moving into a magnetic field pointing into the paper?
Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.

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

44. Checking Understanding
Magnetic Force
A positive charge enters a uniform magnetic field as shown. What is the direction of the magnetic force?
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

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

46. Charge Paths in a Magnetic Field
Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.
Three possible paths followed by a charged particle moving through a uniform magnetic field:
if q = zero the path is a straight line
if q = 90° the path is circular
if 0 < q < 90° the path is a helix.
Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.
Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.

47. Flux Density due to Current Flowing in a Long, Straight Wire
The flux density, B, at point p is directed out of the plane of the diagram.
The magnitude of B depends on
The current, I
The perpendicular distance, r
The medium in the space around the wire
Assumption: if r is small compared with the length of wire, then B does not depend on the length of the wire.
Therefore,
where µ is the permeability of the medium
µo represents air or a vacuum and has units of NA-2 or Henrys per meter (Hm-1).

48. Force per Unit Length between Two Long Parallel Conductors Carrying Current
Flux density, B, near conductor 2 due to I1 is
So, the force (F=I B) which acts on length of conductor 2 is given by
Therefore, the force per unit length is given by
Also, by Newton’s third law we can say that there is an equal but opposite force acting on conductor 1.
Now, if F/ .= 2 × 10-7 Nm-1 and r = 1m and I1 = I2, then the current in each conductor is 1 Amp.
Derivation source:

49. Formal Amp Definition
One Amp is that current which, when flowing in each of two straight, parallel, infinitely long wires, separated by 1m, in a vacuum, produces a force per unit of 2 ×10-7Nm-1.

50. Attraction or Repulsion
What will happen if two long parallel wires are carrying currents, I1 and I2, flowing in the same direction are placed next to each other?
They attract each other
They repel each other
No interaction
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Copywrited by Holt, Rinehart, & Winston
Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.

51. Attraction or Repulsion
What will happen if two long parallel wires are carrying currents, I1 and I2, flowing in the opposite direction are placed next to each other?
They attract each other
They repel each other
No interaction
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Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.

52. 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 5th Ed.

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

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

55. Right Hand Rule (Coil Rule)

56. Checking Understanding
Current Loop
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 P I

57. Magnetic Strength of a Coil
Theory suggests that the flux density, Bc, at the center of a long coil, is
Where N = # of turns
= length
Measurements shows that if the solenoid’s length ≥ 10 times its diameter, the flux density inside is uniform over most of its length.
The graph shows the variation of B along the axis of a long solenoid.

58. 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.

59. 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.

60. Simple Electric Motors
A coil, with current flowing through it, placed in a magnetic field, can experience a torque.
When a current-carrying loop is placed in a magnetic field, the loop tends to rotate such that its normal becomes aligned
with the magnetic field.
Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.
Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.

61. Simple Electric Motors
Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.
Simple Electric Motors
Torque (τ):
magnitude of the torque ~ I
depends on the angle, θ, between the field and the coil.
Torque ~ sin θ
τ =F x d for a loop of wire
d = w/2 sinΦ
where w = width of the loop
Φ is the angle between the normal to the plane of the loop and the direction of the magnetic field.
Net torque = τ = ILB(1/2 w sin Φ) + ILB(1/2 w sin Φ) = IAB sin Φ
Wire is wrapped to form a loop of N loops
t = NIABsinf

62. 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.
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63. Simple Electric Motors
In the diagram below, the force on side a – b of the coil will be directed downwards so the rotation is counter-clockwise (viewed from the front).
When the coil has rotated 180°, side d – c is on the left but, as the commutator has also rotated, the torque is still in the same sense.
DC Motors

64. Simple Electric Motors

65. Simple Electric Motors

66. Simple Electric Motors

67. Simple Electric Motors

68. Simple Electric Motors

69. Magnetic Flux
If B is at 90° to the area, A, then the total magnetic flux, Φ "passing through" A is defined to be
Φ greek letter phi
The units of flux are Tm²  1 Tm² = 1 Weber
If B is not at 90° to the area: the component of B which acts at 90° to A is B cos α.
So, the flux passing through the area is
f = AB

70. Flux Linkage
a flux, Φ, "passes through" a coil of N turns then the coil is said to be linked to the flux so we define the quantity "flux linkage" as
N = # of turns,
B = magnetic field strength
A = cross sectional area of coil
θ = angle to normal of coil
Associated with a multiple turned coil or solenoid
Flux linkage = # of turns x magnetic flux
Φ = NBA cosθ

71. 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.

72. 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.

73. 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!
IB defines EMF – work done per unit charge
Circuit EMF –in moving charge from one terminal of the battery to the other.
Motional EMF – induced as a result of the motion of a conductor in the magnetic field
But WHY??!?!?!?!

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

75. EMF Induced in a Conductor Moving through a Magnetic Field
A conductor moves a distance, Δs, at 90° to a magnetic field of flux density B, in time Δ t.
The free electrons in the conductor will experience a force causing them to move through the conductor.
Suppose that a total charge Δ Q moves past any point in the conductor in time Δ t.
The work done by the force F is given by
This work is equal to the energy given to the charge Δ Q.
Therefore the work done per unit charge is
and, work done per unit charge is the induced emf .
F Δ s/ Δ Q
W = FDs

76. EMF Induced in a Conductor Moving through a Magnetic Field
If the conductor moves at constant speed, the force F must be equal but opposite to the force acting on it due to the current, I, induced in it.
Therefore,
So,
Derivation source:

77. EMF Generation

78. EMF generation
In the case of TWO wire loops, when current is first turned on in one loop, magnetic field lines build up, cutting across the other loop – producing EMF.
When the current is switched off, the field lines collapse, again cutting across the loop.
Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.
Electrical Induction

79. Faraday’s Law of Electro-Magnetic Induction
The induced emf is directly proportional to the rate of change of magnetic flux linking the conductor.
and, if the conductor is a coil having N turns, we have:
In the case of a straight conductor moving at 90° to a uniform field, we found
If the conductor moves a distance Δs in time Δt, then the speed, v, is given by v = Δ s/ Δ t.
So, in this case, the two statements are equivalent.
Derivation source:

80. Generators
Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.
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.

81. 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)
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82. Generator Applied Induction

83. The Laws of Electro-Magnetic Induction
No current  No effect
Short circuited
current flows due to induced emf
coil is repelled from the magnet
Similarly, if the magnet is moved the opposite way, the coil "tries" to follow it.

84. Lenz’s Law
When an induced current flows in a wire, it creates a magnetic field. This magnetic field must resist the magnet’s motion, so work is done in moving it.
When the north pole of a magnet is thrust into the loop, the current must flow in a direction to make a north pole repelling the magnet.
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85. Lenz’s Law
Induced current flows in a direction to oppose the change that produced it.
law of conservation of energy  electro-magnetic induction.

86. Lenz’s Law
This circuit is completed by the falling conducting rod. As it falls the magnetic flux decreases inducing a current.
The force due to the induced current is upward, slowing the fall of the rod.
Copyright ©2007 Pearson Prentice Hall, Inc.
Copyright ©2007 Pearson Prentice Hall, Inc.

87. Eddy Currents Induced currents in bulk conductors.
Desired Effect: used as powerful brakes.
Undesired Effect: source of energy loss (heat)
Copyright ©2007 Pearson Prentice Hall, Inc.

88. Eddy Currents in Motors
As DC motor begins to rotate, an emf (back emf) will be induced in the loop due to the changing magnetic flux in the loop.
Lenz’s law states that this emf will oppose the change in the flux that created it.
The induced current will flow in the loop in the opposite direction to the direction of the current fed by the battery.
The current in the loop when it is rotating will be lower than initially before the loop has started to rotate.
Lights dimming when the refrigerator motor turns on

89. AC Generation
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Copywrited by Holt, Rinehart, & Winston
AC Generator – coil of wire is made to rotate in a magnetic field causing the magnetic flux in the wire to change with its rotation producing an alternating EMF.

90. AC Generation Flux Linkage =
Copywrited by Holt, Rinehart, & Winston
If the coil turns at a constant angular speed,
Therefore,
Faraday’s Law
Substituting,
Flux Linkage =
where N is number of coils, B is magnetic field, and θ is the angle between the magnetic field and the normal of the coil.

91. AC Generation where represents
the slope of a graph of cos (ωt) against t.
Slope is (calculus)
EMF becomes
(max EMF)
Note: EMF is zero when the flux is a max or min and flux is zero when the EMF is a max or min
----- Meeting Notes (9/15/15 14:16) -----

92. AC Generation
How does the speed of the coil rotation in a generator effect the output power?
P=ε I
If the speed is doubled what will be the change in output power?

93. AC Generation
If an identical light was burning with the same intensity in the DC and AC circuits shown below, would the average current be the same?

94. AC Generation From Ohm’s Law: So, or But, average IAC is zero
Average or mean AC
Power must be the
same as the DC Power
for the light to have the
same intensity.
Physics for the IB Diploma 5th Edition (Tsokos) 2008

95. Root Mean Square Since P = I2R, then Where, or
This is called the root mean square (rms)
The r.m.s. value of an a.c. supply is analogous to the steady d.c.value which would convert heat at the same rate in a given resistance.

96. AC Notation
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97. Transformer
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A device used to change voltage in an alternating current from one value to another.
Constructed with two sets of independent coils (primary and secondary) of wire around a common iron core.
Copyright ©2007 Pearson Prentice Hall, Inc.
Copywrited by Holt, Rinehart, & Winston

98. The Ideal Transformer Assumptions of an ideal transformer:
the coils have zero resistance
all the magnetic flux, Φ, produced by the primary current, Ip, is linked with the secondary coil
When Ip changes, Φ changes.
From Faraday’s law, we have:
Combining these two statements gives
Copywrited by Holt, Rinehart, & Winston
Transformers

99. Transformer
Power in both the primary and secondary circuit of the transformer is the same.
Transformer Equations
P = primary quantities
S = secondary quantities
Challenges:
Eddy currents are produced in the iron core because the electrons in the iron move due to the magnetic field.
These cause the material to heat up.
By laminating the core into thin sandwiches of iron these are eliminated.

100. Transformer Example
A TV contains a transformer with primary windings of 200 turns and a secondary winding of 50 turns. If 120 V is supplied to the primary winding what is the secondary output voltage?
What is the secondary output current?
Transformer
Examples
Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.

101. AC vs DC Power Distribution

102. AC vs DC Power – Current Wars
DC – Thomas Edison
AC – Nikola Tesla
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
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 George Westinghouse
Won contract to provide electricity to Chicago World Fair in 1893

103. 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
looks like:
Rload
Rwire X

104. DC Power Distribution – Feasibility?
AC Electricity
02/07/2008
DC Power Distribution – Feasibility?
120 Watt
Light bulb
12 Volt
Connection Box
Power Plant
on Colorado River
150 miles
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, Plost = I2R
0.001 W/m  2105 m = 20 Ohms
P = VI so I = P/V
I = 120 W/12 V = 10 A
Plost = I2R = (10A)2 x 20Ω = 2000 W
Lecture 7

105. DC Power Distribution – Feasibility?
AC Electricity
02/07/2008
DC Power Distribution – Feasibility?
150 miles
120 Watt
Light bulb
Power Plant
on Colorado River
12 Volt
Connection Box
Rwire
Calculate Total Power Required
Total Power = Power Lost + Power Required
Calculate the efficiency (e)? e = Pout/Pin
What could we change in order to do better?
Rload
Total Power = 2 (2000) = 4120 W X
Rwire
e = Po/Pi= 120 W/4120 W = 0.3%
Lecture 7

106. AC Electricity
02/07/2008
The Tradeoff
Major Problem: high current through the (fixed resistance) transmission lines
Need less current
Power Loss is I2R I2 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.
Lecture 7

107. DC Power Distribution – Feasibility?
AC Electricity
02/07/2008
DC Power Distribution – Feasibility?
150 miles
120 Watt
Light bulb
Power Plant
on Colorado River
12,000 Volt
Connection Box
Calculate
Current
Power Lost in Transmission Line
Total Power Required
Efficiency (e)? e = Pout/Pin
More Power Delivered  More Profit!
I = P/V = I = 120 W/12,000 V = 0.01 A
Plost = I2R = (0.01A)2 x 20Ω = W
Total Power = 2 (0.002W) + 120W = W
e = Po/Pi= 120 W/ W = %
Lecture 7

108. AC Electricity
02/07/2008
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
Transformer
not possible with DC, Why?
Works only with AC (Tesla wins)
Lecture 7

109. A way to provide high efficiency, safe low voltage:
AC Electricity
02/07/2008
Power Transmission
A way to provide high efficiency, safe low voltage:
step-up to 500,000 V
step-down,
back to 5,000 V
step-down to 120 V
~5,000 Volts
High Voltage Transmission Lines
Low Voltage to Consumers
Lecture 7

110. Transmission structures
AC Electricity
02/07/2008
Transmission structures
three-phase “live” wires
to house
500, , , ,000 7–13,000
long-distance neighborhood
Lecture 7

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

112. 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?
Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.

113. 120 VAC is a root-mean-square number: peak-to-peak is 340 Volts!
AC Electricity
04/25/2006
= 170 Volts
= -170 Volts
Spring 2006
UCSD: Physics 8; 2006
120 VAC is a root-mean-square number: peak-to-peak is 340 Volts!
Lecture 8

114. 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.

115. AC Receptacle Receptacles have three holes each
AC Electricity
04/25/2006
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
Cutnell & Johnson, Wiley Publishing, Physics 5th Ed.
Lecture 8

116. AC  DC - Rectification Half-wave Rectification A single diode convert
AC into a pulsating DC
Diode
Electrical gate
Allows only positive current through (forward biased)
Blocks negative current (reverse biased)
Output limitation
Electrical energy in AC negative cycle not used.

117. AC  DC - Rectification Full-wave Rectification
A diode bridge converts
AC into a pulsating DC signal using all the energy of the AC signal

118. AC  DC - Rectification Full-wave Rectification
Positive half cycle, current flows A-C
Negative half cycle, current flows B-D

119. AC  DC - Rectification Full-wave Rectification Drawing the circuit
Diodes on parallel sides point in the same direction
AC signal is fed to the point where opposite ends of two diodes join.
Positive output comes from the junction of the negative side of two diodes
Negative output comes from the junction of the negative side of two diodes

120. AC  DC – Smoothing Circuit
Turns the pulsating output of a rectifying circuit into a more steady DC output.
Uses a capacitor in parallel with the output of rectifying circuit.
Smoothed half-wave Rectification

121. AC  DC – Smoothing Circuit
Turns the pulsating output of a rectifying circuit into a more steady DC output.
Smoothed full-wave Rectification

122. AC  DC – Smoothing Circuit
Output ripple – slight fluctuation
Capacitor
Short term energy storage
Constantly charging and discharging
Slow discharge requires a large capacitance C so the time constant
τ is significantly large
τ=RC where R = resistance

123. Wheatstone Bridge Arrangement used to estimate an unknown resistor
Used primarily with DC circuits
Four resistors
Two fixed resistors
One variable resistor
One unknown resistor (RX)
Variable resistor always paired with unknown resistor.
Galvanometer (Ammeter) bridges pair of resistors
Adjust variable resistor so galvanometer
reads 0 A

124. Wheatstone Bridge
When galvanometer reads 0 A, no potential difference across BD.
Therefore, the potential difference across R1 and R3 are identical and the same is true for R2 and RX.
V1=I1R1=I3R3 and V2=I1R2=I3RX
So,

125. Wien Bridge
Modification of Wheatstone bridge allow identification of resistance and capacitance values for an unknown component
Used with AC power supply
Current in center arm adjusted to zero.
Adjusting R2, C2 and the frequency of the supply to minimize current.
Unknown values of RX and CX can
be calculated.

126. Sources:
Homer, D. and Bowen-Jones, M.,(2014) Physics: Course Companion, Oxford University Press
Tsokos, K.A.,(2014) Physics for the IB Diploma,6th edition, Cambridge University Press
University of California at San Diego (2008). AC Electricity: Why AC Distribution? Retrieved from