Magnetism
Natural Attraction without
pheromones
History of Magnets
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More than 2000 years ago, rocks called
lodestones were found in the region of
Magnesia in Greece.
In the 12th century, the Chinese used them
for navigating ships.
What are magnets?
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Most materials
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Have paired up electrons moving in opposite
directions.
The field created by one moving charge is
canceled by the other.
No magnetic field is created.
What are magnets?

Any charges in motion
produce a magnetic field.
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Some materials like Iron, Nickel, or Cobalt
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Have a single electron or paired electrons
spinning in the same directions.
The magnetic field created by one electron is not
canceled by the other.
An atomic sized magnet is created.
Why is Fe magnetic and Al not?
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
What makes a good magnet?
 Every spinning electron is a tiny magnet.
 A pair of electrons spinning in the same direction is a
stronger magnet.
 A pair of electrons spinning in opposite directions work
against one another; the magnetic fields cancel.
Fe has 4 unpaired electrons spinning in the same direction.
 Cobalt has 3.
 Nickel has 2.
 Aluminum has one unpaired electron.
Temporary Magnets
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What happens when you place a magnet
next to a nail?
This is because the magnet causes the nail
to become polarized; the nail becomes a
magnet.
This is temporary; if you pull the magnet
away, the nail loses its magnetism.
Permanent Magnets
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Permanent magnets are produced in the same manner as
the nail; however, due to the microscopic structure of the
material, the magnetism becomes more permanent.
Most permanent magnets are made of ALNICO, an iron
alloy containing 8% Aluminum, 14% Nickel, and 3%
Cobalt.
Some rare earth elements, such as neodymium and
gadolinium, produce strong permanent magnets.
Magnetic Domains
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In magnetic materials, neighboring atoms pair
up to form large groups of atoms whose net
spins are aligned.
These groups are called domains.
When a piece of iron is not a magnet, the
domains point in random directions.
Magnetic Domains
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If the non-magnetized iron is placed in a strong
magnetic field, the domains will line up in the
direction of the field.
In temporary magnets, the domains will return to
their random orientation after the field is removed.
In permanent magnets, the domains will remain
aligned.
1 domain = 1 quadrillion (1015) atoms
Magnetic Poles
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All magnets have two regions “poles” that produce magnetic
forces.
They are named like this because if you take a magnet and
suspend it from the middle (so that it can swing freely), it will
rotate until the north pole of the magnet points north and the
south pole points south.
Like poles repel.
Opposite poles attract.
No less than two.
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North and South
cannot be separated.
If a magnet is broken,
poles aren’t
separated; two
smaller magnets are
formed.
Magnetic Fields
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You have probably noticed that forces between
the magnets (both attraction and repulsion), are
felt not only when the magnets are touching
each other, but also when they are held apart.
In the same way that gravity can be described
by a gravitational field, magnetic forces can be
described by the magnetic fields around
magnets.
Magnetic Fields
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Force lines around a magnet
Always flow from N to S
Circular in path
Magnetic Field Demo
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What kinds of magnetic fields are produced
by pairs of bar magnets?
Magnetic
Field Lines
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The shape of the
magnetic field is
revealed by
magnetic field
lines.
Magnetic Field Lines
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Magnetic field lines are the same as
electric field lines in that both are stronger when
lines are drawn closer together.
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So the magnetic field is stronger at the poles
The magnetic field lines have arrows going
from north to south.
Magnetic field lines do not cross because the
magnetic field cannot go in two directions at
once.
Common Uses of Magnets
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Magnetic recording media: VHS tapes, audio cassettes, floppy disks, hard disks.
Credit, debit, and ATM cards
Common television and computer monitors
Speakers and Microphones
Electric motors and generators
Compasses
Magnets can pick up magnetic items (iron nails, staples, tacks, paper clips) that are
either too small, too hard to reach, or too thin for fingers to hold. Some screwdrivers
are magnetized for this purpose.
Magnets can be used in scrap and salvage operations to separate magnetic metals
(iron, steel, and nickel) from non-magnetic metals (aluminum, non-ferrous alloys,
etc.).
Magnetic levitation transport, or maglev, is a form of transportation that suspends,
guides and propels vehicles (especially trains). The maximum recorded speed of a
maglev train is 361 mph.
How to demagnetize a magnet
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Heating a magnet past its Curie temperature the molecular motion destroys the alignment of
the magnetic domains.
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768°C for Iron
Hammering or jarring –
the mechanical disturbance tends to randomize
the magnetic domains.
Placing the magnet in an alternating magnetic
field.
Ferrofluid
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a mixture of tiny iron particles covered with a
liquid coating that are then added to water or
oil.
Used in car suspensions, cancer detection,
loud speakers
 video
Earth’s Magnetic Field
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Earth is a huge
magnet.
This is possibly due
to the molten Iron
core.
The magnetic field
around Earth is
called the
Magnetosphere
Earth’s Magnetic Field
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Magnetic north pole is different than geographical north pole.
There is about a 25 ̊difference from geographic north pole to
magnetic north pole, this is called magnetic declination
In addition, the north pole
of a magnet is attracted to
earth’s north pole because that
is the magnetic south pole.
The south pole of a magnet
is attracted to the earth’s
south pole because that
is the magnetic north pole.
Magnetosphere
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Extends several tens of thousands of km into
space.
Protects Earth from solar winds.
Dynamo Theory
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The dynamo theory proposes
a mechanism by which a
celestial body such as the Earth
generates a magnetic field.
In the case of the Earth, the
magnetic field is induced and
constantly maintained by the
convection of liquid iron in the
outer core.
Magnetic field of Earth is not stable
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The magnetic poles of Earth wander up to 15 km
every year.
Based upon the study of lava flows throughout the
world, Earth's magnetic field reverses at intervals,
ranging from tens of thousands to many millions of
years, with an average interval of approximately
250,000 years.
The last reversal is theorized to
have occurred 780,000 years ago.
After a being put into a strong magnetic field a temporary
magnet has the following configuration. If the field if turned off,
which pole is the north pole of the magnet?
A.
B.
C.
D.
Left pole
Right pole
middle
No poles
Auroras
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Charged particles from the sun become trapped
in Earth’s magnetic field.
This occurs near the magnetic poles.
These charged particles collide with electrons of the atoms in
our atmosphere and transfer their energy.
The colors of the lights are determined by the type of gases in
the atmosphere.
 O2 releases green light; N2 releases red light
aurora borealis (northern lights); aurora australis (southern
lights)
Animal Migration
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Some animal species do have the
ability to detect the magnetic field,
& they use it to make their migrations.
Bats and sea turtles use magnetic
information to find their way.
We're not 100 percent sure how animals detect the
magnetic field, but small particles of magnetite have
been found in the brains of some species. Those
particles may be reacting to the magnetic field and
activating nerves in such a way as to send orientation
information to the animal's brain.
Bacteria & Magnets
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Some bacteria have a chain of magnetite as part of their
internal structure
They use this magnetite to find their way in swamps
Bacteria in the northern hemisphere have magnetite that
are opposite in polarity than the bacteria with magnetite
in the southern hemisphere.
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Animal Migration Video
Below show the domains of a magnet, which pole is the north
pole of the magnet?
A.
B.
C.
D.
Left pole
Right pole
middle
No poles
Magnetic field Units
We can determine the magnetic field by measuring the
force on a moving charge:
F
B
qv sin 
B

v
The SI unit of magnetic field is the Tesla (T).
Dimensional analysis: 1 T = 1 N·s / (C·m) = 1 V ·s / m2
Sometimes we use a unit called a Gauss (G):
1 T = 104 G
The earth’s magnetic field is about 0.5 G
Understanding the magnetic force requires us
to work in THREE dimensions.
So let’s use a new notation to depict
the forces in the TWO dimensional world.
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
B into the page
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
B out of the page
What should you use-- dots or x’s?
The tail of
an arrow.
The tip of
an arrow.
x
.
The x’s
The dots
Magnetic Forces
1. Forces on moving charges
2. Forces on currents in wires or fluids
• A charged particle in a static (not changing with time)
magnetic field will experience a magnetic force only if
the particle is moving.
• q is charge,v is velocity, B is the magnetic field B,
angle , F is the magnitude of the force on the charge
is:
|F| = | q v B sin | = q vB
Or
B = F magnetic
qv
DON’T FORGET:
Forces
have
directions!
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F = qv  B = q v B sin 
WARNING: cross-product
(NOT simple multiplication!)
This force acts in the direction perpendicular
to the plane defined by the vectors v and B
as indicated by the right-hand rule!
A particle with a charge of 8 C is moving at 2,000
m/s when it enters a magnetic field perpendicular
to its direction of motion that has a magnitude of 4
Tesla. What is the magnitude of the force exerted
on the particle?
A. 64,000 N
B. 1000 N
C. 62.5 N
D. 4000 N
Right Hand Rule
• Draw vectors v and B
with their tails at the
location of the charge q.
• Point fingers of right hand
along velocity vector v.
• Curl fingers towards
Magnetic field vector B.
• Thumb points in direction
of magnetic force F on q,
perpendicular to both v
and B.

 
F = qv  B
A current-carrying wire is placed in a magnetic field, as
shown below. Indicate the direction of the force
exerted on the wire.
A. up
B. down
C. right
D. left
Force on parallel wires
I1
b
I2
• Each of two parallel wires with current I, experiences
an attractive magnetic force that diminishes as one
over the distance separating the wires: F  I1 I2 L / b.
– L = length
– We use this proportionality to define the unit of current
• The force on wire 2 is equal to current in wire 2 times
magnetic field from wire 1 times length of wire 2.
– Magnetic field generated by a current diminishes as one
over distance from wire (1/d)
Magnetic Field from a Wire
•The magnetic field lines from a current form circles around a
straight wire with the direction given by another “right hand
rule” (thumb in direction of current, finger curl around
current indicating direction of magnetic field).
Fmagnetic
B1 =
Il
= B-field a distancer from current I1
I2
I1
r
B
The magnetic force of a straight 1.5 m segment of
wire carrying a current of 9 A is 3.0 N. What is the
magnitude of the component of the magnetic field
that is perpendicular to the wire?
A. 40.5 T
B. 2 T
C. .22 T
D. 4.5 T
Example
A loudspeaker is an electromechanical device
which converts an electrical signal into sound.
Microphone
Audio Circuit
Speaker
Loudspeakers are used in numerous applications
from hearing aids to air raid sirens
Loudspeakers are the most variable elements in
any audio system, and may be responsible for
marked audible differences between otherwise
identical systems.
The Main Components
Magnetic Forces due to Electric Current
• Current is charges in motion
• Causes force on magnet
• Example: Compass near wire with current
current
wire
Side View
Top View
B Fields of Current Distributions
By winding wires in various geometries, we can
produce different magnetic fields.
For example, a current loop
( to plane, radius r, current emerging from plane at top of loop):
Magnetic field at center of loop
B = m0 I / (2r)
I
Magnetic field far from loop:
B  I·(Area of loop) / r3
Solenoids
If we stack several current loops together we end up with
a solenoid:
In the limit of a very long solenoid, the magnetic field
inside is very uniform:
B=m0 n I
n = number of windings per unit length,
I = current in windings
B  0 outside windings
Galvanometer
•Current in coil is
finite, due to nonzero resistance of coil
•Magnetic field
produces torque on
current in coil.
•Needle swings until
magnetic torque is
balanced by torsion
of spring
A current-carrying wire is placed in a magnetic
field, as in the diagram below. Find the
direction of the Force applied to the wire.
A. up
B. down
C. right
D. Out of the page
A current-carrying wire is placed in a magnetic
field, as in the diagram below. Find the
direction of the Force applied to the wire.
A. up
B. down
C. right
D. Out of the page
As current goes through a wire below, it
produces the magnetic field shown below.
The red arrows show the direction a
compass point around the wire. Which
direction is the current flowing?
A. Up
B. Down
C. No current
D. Not enough info
wire
Side View
Top View
Electromagnets
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Use current to “make” a
magnet
Usually add a piece of iron to
increase the field
Higher the current and higher
the number of loops increases
the strength of the magnet
Electric motor
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A magnet and an electromagnet are set up
to rotate around an axle.
Poles attract – electricity is converted into
mechanical energy
Induction
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Just as a moving charge can create a
magnetic field, a moving B can create a
moving charge – current
Induced EMF and Current
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The changing magnetic field causes an
induced current. Since a source of emf
(voltage) is needed to produce a current
(remember Ohm’s Law), the moving
magnet acts like a source of emf. So we
would say the moving magnet induces
an emf in the coil, producing an induced
current.
Magnetic Flux
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Magnetic Flux deals with the magnetic
field and the surface through which it
passes. It is a measure of the quantity
of magnetism that goes through a
surface.
Magnetic Flux
The flux only depends on the portion of the
magnetic field that is perpendicular to the area in
which is passes through. So when the magnetic
field direction is not perpendicular to the area the
equation for flux becomes
Φ = B A cos θ
The SI unit of magnetic flux is the Weber (Wb)

What is the magnetic flux of a 0.230 T uniform magnetic field through a
rectangular area of 0.5x0.2 meters that is at a 45 degree angel?
A.
B.
C.
D.
.023 Wb
.016 Wb
.099 Wb
.039 Wb
EMF and Flux
So, an EMF [ε] is created due to the rate change of flux
[Φ]. How can the flux change?
 The coil can spin (Δθ)
 The coil can change area (ΔA)
 The coil can move out of the field
 The magnetic field can change strength (ΔB)
Since flux depends on B, A and θ, that means that any
of them can be changing to produced an induced emf
ε.
Faraday’s Law of Induction
The induced electromotive force
(EMF) in any closed circuit is equal
to the time rate of change of the
magnetic flux through the circuit.
The SI unit for EMF is Volts (V)
EMF, potential difference and
voltage are the same things. They
all produce current in a closed
circuit.
 B
=
t
A single loop of wire with an area of 0.09 m2 enters a magnetic
field of 0.186 T in a time of 0.09 seconds perpendicularly. What
emf is induced in the loop?
A.
B.
C.
D.
.01674 V
.00150 V
.186 V
Huh?
Electric Generator
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Opposite of electric
motor
Move wire through a B
field and generate
electricity
Converts mechanical
energy into electric
energy
Transformer
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Allows us to increase current or voltage at
the expense of the other.
When we need high voltage we step it up.
When we need high current we step it
down.
Using current from
B field
Transformer Formula

N1 V1
=
N 2 V2
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N1 = Number of turns in
the primary coil
N2 = Number of turns in
the secondary coil
V1 = Voltage in the
primary coil
V2 = Voltage in the
secondary coil
A step-up transformer has a primary voltage of 12-Volts with 2
loops around the primary terminal. If the secondary terminal has
500 loops, what voltage is produced by the transformer?
A.
B.
C.
D.
.048 V
3000 V
83 V
21 V
A loop of wire that has a cross-sectional area of 0.75m2 is
brought completely into a magnetic field (B=1.1 T) in a
time of 0.1 seconds. What is the induced emf?
A.
B.
C.
D.
.825 V
.0825 V
8.25 V
.147 V