1. Magnetism The term "magnet" comes from Magnesia
Legend has it that a Greek shepherd in 900 BC had the nails in his shoes pulled out by the rock he was standing on; probably not true
Later (by 2000 years) these rocks were placed on leaves and floated in water and found that they rotated to point to North; came to be called "lodestones" , or "leading stones"

2. Substances that exhibit strong magnetic materials are
called ferromagnetic materials
3 elements: iron, nickel, and cobalt
Also: neodymium (a rare earth element)
Nd2Fe14B

3. Some alloys can also be permanently magnetized: alnico
Paramagnetic materials are weakly attracted
to a strong magnet
ex. liquid oxygen
Diamagnetic materials are weakly repelled by a
strong magnet
ex. gold, mercury

4. Where does magnetism come from?
magnetism comes from the orbital motions of
electrons around the nucleus
; tend to cancel out in atoms, as orbits are random
- most comes from the spin of electrons
; can spin clockwise or counterclockwise
; so each electron is
itself a magnet
; in most atoms, spins cancel out

5. Iron has four electrons that spin in one direction that are
not paired off; these unpaired spins add up to give
iron its ferromagnetic properties
Iron:

6. General Points:
- Like poles repel, unlike poles attract
Magnetic poles cannot be separated; if you break a bar
magnet in half, get two bar magnets, each with
a N and S pole
- Magnetic force is a non-contact force; has a field

7. Field around a bar magnet:
- to map, use an independent N pole
- field lines begin on N pole and end on S pole
- field strongest at poles
- no field in the middle

8. Field around unlike poles:

9. Field around like poles:

10. Only a magnet can affect another magnet
Suspended bar magnet was affected by Earth
So Earth is a magnet

11. Where does Earth's magnetic field come from?
A giant bar magnet
inside Earth?
Earth's magnetic field is generated by currents in the liquid outer core

12. Earth's geographic north pole is a
magnetic south pole
It's located about 1000 miles away, in Northern Canada
The magnetic pole is not located
at the geographic pole

13. The difference in where the compass points compared to true north is called magnetic declination

14. There is also magnetic inclination; the field lines of Earth are only parallel to Earth's surface at the equator

15. And: the location of the magnetic pole is not stable;
it changes location over time

17. And: it appears as if the poles “reverse polarity"
every years or so
The evidence is from lava flows on the seafloor

18. Domain Theory of Magnetism
Each iron atom is a magnet; atoms will cluster in regions
called domains
Each domain is about 1 mm on a side, containing
millions of atoms
Within each domain, the magnetic fields of the atoms
will line up
In a ferromagnetic material that is not magnetized (an
iron nail), the domains are randomly oriented

19. Nail's domains are
randomly oriented

20. Nail's domains are
randomly oriented
By placing magnet on nail,
domains are forced to line up
S N
Induced magnetism
Temporary; when magnet is removed, domains in
nail will randomize again

21. In permanent magnets, the domains are aligned by breaking and
re-forming bonds; requires energy
Domains will randomize over time; energy required
to break bonds comes from heat, or striking it with
a hammer, dropping it, etc.

22. Electromagnetism
In 1821, Hans Christian Oersted (Danish) showed that a
current-carrying wire affected a compass needle
re-creation of his
experiment:
Field lines form
concentric circles
around wire

23. Direction of field can be found
from a Right-Hand-Rule:
Thumb of right hand points in the direction of
the current I
; then fingers wrap around wire in
the direction of the field lines B B
out of page
into page

24. I B Consider a horizontal wire: Pass current from right to left
What is the direction of the magnetic field
above and below the wire? (Use the RHR) I
into page B
wire
out of page

25. Consider a loop of wire:B
Consider a loop of wire:
Send current through loop as shown; find direction of B field through loop B B I
What is the direction of the induced field
(1) on the inside of the loop
(2) on the outside of the loop
at different places on the loop?

26. Electromagnet Now have a series of loops, and make a coil:
When current is flowing, there is a
magnetic field generated in the coil I
Electromagnet N S
Direction of field can be found from a Right-Hand-Rule for coils:
then thumb points to North pole of coil
Fingers wrap around coil in direction of I;

28. 3 ways to make a stronger electromagnet:
- more current (each amp adds to field)
- more coils (each coil contributes to field)
insert a ferromagnetic core (domains line up,
adding to field)

29. Forces on Current-Carrying Wires
Consider a wire that is hanging in a magnetic field created by a
horseshoe magnet: B B S N
Pass current up through the wire: I
Current will generate a magnetic field around the wire
Magnetic field from current will interact with field from horseshoe magnet

30. Forces on Current-Carrying Wires
Direction of force can be found
by a Right-Hand-Rule: B B S N
Fingers: North to South
Thumb: in direction of I
Palm: shows direction of force I I B F

31. The speaker:

32. Motor Principle Put a loop of wire in a magnetic
field; pass current through the wire
A force will be exerted that will
cause the loop to rotate
Check this out:

33. F = B I L Force on wire depends on: - strength of magnetic field
- current
- length of wire in field
F = force
B = strength of magnetic field
I = current
L = length of wire in field
F = B I L

34. ex. A wire 0.50 m long hanging in a magnetic field has a
force of 12 N on it when it carries a current of 2.0 A.
What is the strength of the magnetic field?
F = B I L F
12 N
B = =
( 2.0 A )( 0.50 m )
I L
B = 12 N/Am
= 12 tesla = 12 T
Definition: 1 N/Am = 1 tesla = 1 T

35. Charged Particles Moving in Magnetic Fields
A current-carrying wire in a magnetic field has a
force exerted on it
The charges do not need to be on a wire; a free proton
moving through a magnetic field has a force exerted on it
by the field
The amount of force depends on:
- strength of the field, B
- charge on the particle, q
F = q v B
speed of particle
through the field, v

36. ex. A proton passes through a magnetic field at right angles
to the field at a speed of 2.0 x 106 m/s. The strength of
the field is 4.0 T.
(a)What is the magnitude of the force on the proton?
F = q v B
= ( x C )( 2.0 x 106 m/s )( 4.0 T )
F = 1.3 x N

37. (b) If the field is oriented towards the North and the proton
is passing to the West, what is the direction of the force?
Right-Hand-Rule:
Fingers: North to South
Thumb: in direction of charge
movement
Palm: shows direction of force
Stand facing North with right arm outstretched (fingers pointing to North); turn hand so that thumb is pointing to the West (to the left); palm is facing down
Force is downward, towards Earth’s surface

38. ex. An electron passes to the north at a speed of 4.50 x 105 m/s
through a magnetic field of strength 12.5 T and pointing
straight up (floor to ceiling). Find the magnitude and
direction of the force exerted on the electron.
F = q v B
= ( x C)(4.50 x 105 m/s)(12.5 T)
= x N
Point fingers of right hand up towards the ceiling; thumb
goes in direction of motion, to the north; palm faces
eastward; EXCEPT it’s an electron (negative charge), so
force is opposite, to the west
F = x N to the west

39. ALTERNATE METHOD:
Use right hand for
positive charges
Use left hand for
negative charges

40. ALTERNATE METHOD: Use right hand for positive charges
Use left hand for
negative charges B B
v (north) v
F (east)
F (west)

41. ex. A charged particle enters at right angles to a 0
ex. A charged particle enters at right angles to a 0.24-T magnetic field
at a speed of 3.6 x 106 m/s. It is deflected into a circular path of
radius 0.18 m. If the charge on the particle is 3.20 x C, what is
the mass of the particle?
Hint: use centripetal force
Fcent = Fmag
mv2
= qvB
mv2 = qvB r r
x x x x x x B
qvB r
qBr
m = = v2 v r
( 3.20 x C )( 0.24 T )( 0.18 m ) =
( 3.6 x 106 m/s ) q
m = 3.8 x kg

42. Mass Spectrometer

43. Microwave Oven
Electrons are deflected by magnetic field; accelerated
electrons emit microwaves

44. Inside of the magnetron

53. Charged particles spiral into Earth’s magnetic field; collide with atmospheric atoms and excite electrons into higher energy levels; when the electrons return to ground state, they emit visible light

54. The Aurora Borealis (Northern Lights)

59. Electromagnetic Induction
Electricity produces magnetism (pass current through a
wire and a magnetic field is generated)
Magnetism also produces electricity, but only under
certain conditions
- moving a wire into or out of a magnetic field
moving a bar magnet into or out
of a coil of wire
moving a coil into or out of a
magnetic field

60. * Must have motion in order for current to flow; wire
must cross, or "cut" field lines; if wire is held stationary
in the field, no current flows
* Anytime the magnetic field in the vicinity of a wire
changes, an EMF is induced across the ends of the wire _ N S +

61. _ N S + EMF = B L v Magnitude of EMF depends on:
- strength of mag. field
- length of wire in field
- speed of movement +
B = strength of field
L = length of wire in field
v = speed that wire moves
in field
EMF = B L v

62. ex. A wire 0.50 m long cuts straight up through a magnetic
field of strength 2.0 T at a speed of 25.0 m/s.
(a) Find the EMF induced in the wire.
EMF = B L v
= ( 2.0 T )( 0.50 m )( 25.0 m/s )
EMF = 25 volts = 25 V
units check:
T m m N m m N m m
N m = = = s s s
A m
C m C s J =
= volt C

63. Direction of current can be found from a Right-Hand Rule:+
EMF I
wire N S
Fingers: North to South
Thumb: in direction of v
Palm: Shows direction of I _ v

64. Spin loop of wire in a magnetic field:
Look edge-on at loop and go through one revolution;
determine current direction

65. Field lines cut; max current generated, away from A (shaded end)1. N S I
Loop moving parallel to field lines;
no lines cut, so no current generated;
I = 0 2. N S
Field lines cut; max current generated, toward A (shaded end) 3. N S I
Loop moving parallel to field lines;
no lines cut, so no current generated;
I = 0 4. N S

66. I Draw graph of induced current; designate “away from the
shaded end” as the positive direction
One cycle is shown; period is T; position 1 (on previous page) is
t = 0; position 2 is t = T/4; position 3 = T/2; position 4 is
t = 3T/4; position 5 (same as position 1) is t = T ,5 I t
T/ T/ T/ T

67. Wind Power Wind turns blades
Gears take rotation underground to generators
Renewable; no emissions
Large-scale wind farms could supply the energy needs for much of the nation

68. Hydroelectric Power River is dammed up with a concrete barrier
Path through is created (penstock)
Turbine is placed in the penstock
Water is allowed through the penstock, which turns the turbine
Turbine spins the generator, making AC electricity

69. Can have as many as 10 generators in a single dam
Creates large amounts of electricity
Renewable
Large-scale plants limited to few regions of US
Not very viable for Wisconsin

70. Coal-Fired Plant Coal is conveyed to boiler
In primary water loop, water is boiled into steam
Steam is sent through turbine
Turbine spins the generator, producing AC electricity
Steam is condensed back into water by secondary water loop; secondary water gets hot

71. Secondary water is sent to cooling pond. Water releases heat to the air
Carbon dioxide and other emissions released to air through stack
Ash is dumped in landfill

72. Nuclear Power Plant
Nuclear reactions convert mass into energy by E = mc2, releasing tremendous amounts of heat
Reaction controlled by control rods, which control rate of reaction
Reactor cooled by water in the primary cycle; this water becomes radioactive

73. Primary cycle and reactor are in containment building, made of reinforced concrete
Reactor water pumped to steam generator, which boils secondary water into steam
Steam taken out of containment building to turbine, which spins the generator, making AC

74. Steam is condensed back into water with third (tertiary)loop of water; secondary water is then pumped back to steam generator
Tertiary water is sent to
cooling towers, which
releases its heat to the air

75. No emission of CO2 or any other gases; only steam from the cooling towers
Radioactive products and water are kept in containment building
Problem of radioactive waste products, which must be kept stable for many years, until they are no longer radioactive

76. Alternating current (AC) : current that alternates in direction
Generated by spinning a loop in a magnetic field
US: 60 Hz , 120 V
Europe: Hz , 220 V V
Vmax
Vrms = 120
black wire
white wire t
- 120
red wire
Vrms = ( 2 / 2 ) Vmax = Vmax

77. Lenz’s Law As a magnet is brought near a coil of wire, current is
induced in the coil; what is the direction of the
induced current? N S

78. the direction of the induced current is such
Lenz’s Law:
the direction of the induced current is such
that it opposes the change that produced it
A field is generated that has a north pole at the right, to
oppose the N pole of the bar magnet S N N S

79. the direction of the induced current is such
Lenz’s Law:
the direction of the induced current is such
that it opposes the change that produced it
A field is generated that has a north pole at the right, to
oppose the N pole of the bar magnet S N N S

80. Transformers

81. Transformers Core takes magnetic field to secondary coil
Electricity causes
magnetism in
primary coil
Magnetism induces
electricity in
secondary coil

82. Vs Ns = Vp Np Ns = number of turns in secondary
Np = number of turns in primary = Vp Np

83. In the above transformer, there are 1000 turns on the
primary coil and 50 on the secondary coil.
(a) If the primary voltage is 120 V, what is the secondary
voltage? Vs Ns Ns Vp = Vs = Vp Np Np

84. In the above transformer, there are 1000 turns on the
primary coil and 50 on the secondary coil.
(a) If the primary voltage is 120 V, what is the secondary
voltage? Vs Ns 50
(120 V) = Vs = Vp Np
1000
step-down transformer Vs
= 6.0 V

85. Pp = Ps
By Cons. of Energy
P = IV
Ip Vp = Is Vs

86. (b) If the current in the primary is 2.0 A, what is the
current in the secondary?
(2.0 A)(120 V)
Ip Vp
Ip Vp = Is Vs
Is = = Vs
6.0 V
Is = 40 A

87. Transformer in a portable radio, which can operate on both
120 V AC and 12 V DC; but only has circuitry for
12 V DC; so must transform 120 V AC to 12 V DC
rectifier, which
changes AC
to DC
primary
secondary
step-down transformer, with ten times
fewer turns on secondary coil,
stepping down 120 V to 12 V

88. Transmission Lines
Electricity is generated in power plants;
sent to customers by transmission lines; not paid for until it enters homes
As electricity passes through the transmission lines,
the wires heat up
Heat is given off to atmosphere; wasted power
Power lost depends on P = IV = I2R ; referred to
as “I2R losses”

89. P = 250 MW
= W
V = V
I = A
P = 250 MW
= W
V = V
I = 1000 A
step-up
transformer
reduces I2R losses
by factor of 100

90. step-down transformers to reduce voltage and produce enough current for each customer
step-up transformers to reduce I2R losses
enters house with
V = 240 V ,
I = 100 A available

91. Electricity vs. Magnetism
Similarities Differences
Opposites attract,
likes repel
Can charge anything, but only
certain things can be
magnetized
Both are non-contact
forces, and have fields
Earth is a magnet, but
Earth is not charged
Field lines look the same
for likes, unlikes
Can easily separate charges;
can never separate poles
Two kinds of charge, two
kinds of polarity
Can easily take away charge;
hard to take away magnetism
Can shield electrically, but
not magnetically
Both attract “neutral”
substances

92. Electricity vs. Magnetism
Similarities Differences
Each can produce
the other
Electricity produces magnetism
always, but magnetism only
produces electricity under
certain conditions