Magnets have been known for centuries.
The Chinese and Greeks knew about the “magical”
properties of magnets. The ancient Greeks used a
stone substance called “magnetite.” They
discovered that the stone always pointed in the
same direction. Later, stones of magnetite called
“lodestones” were used in navigation.
William Gilbert, an English physician,
first proposed in 1600 that the earth
itself is a magnet, and he predicted that
the Earth would be found to have
magnetic poles.
What is Magnetism?
Magnetism is the force of
attraction or repulsion of
a magnetic material due
to the arrangement of its
atoms, particularly its
electrons.
All magnetic phenomena
result from forces between
electric charges in motion.
The ends of a magnet are where the
magnetic effect is the strongest.
These are called “poles.” Each
magnet has 2 poles – 1 north, 1
south.
Like repels
like…
Opposites attract!
Poles of a magnet always
Come in pairs! “Law of Poles”
If you cut a
magnet in half,
S
N
S
you get 2 magnets!
N S
N
No Monopoles Allowed
It has not been shown to be possible to end up with a single
North pole or a single South pole, which is a monopole ("mono"
means one or single, thus one pole).
S
N
Note: Some theorists believe that magnetic monopoles may
have been made in the early Universe. So far, none have been
detected.
Magnetic Fields
The region where the magnetic forces
act is called the “magnetic field”
Magnetic fields are vector quantities.
The direction at any location is in the direction
that the north pole of a compass would point if at
that location
Magnetic field lines
represented by iron filings
Field Lines Around a Bar Magnet
Field Lines of Attracting Bars
Field Lines of Repelling Bars
Atoms themselves have magnetic properties due
to the spin of the atom’s electrons.
Groups of atoms join so that their magnetic fields
are all going in the same direction
These areas of atoms are called “domains”
When an unmagnetized substance is placed in a magnetic
field, the substance can become magnetized.
This happens when the spinning electrons line up in the
same direction.
An unmagnetized substance looks
like this…
While a magnetized substance looks
like this…
How to break a magnet:
1. Drop it
2. Heat it
This causes the domains to
become random again!
Magnetic Field Vectors Due to a Bar
Magnet
N
S
N
S
N
S
N
S
N
S
N
S
N
S
N
S
N
S
N
S
• The direction of the magnetic
field at any point is…
– tangent to the magnetic field line at
that point.
– the direction that the north pole
of a compass would point if a
compass were at that location
-defined as the
direction of motion
of a charged
particle on which
the magnetic field
would not create a
force.
Up until 1820 everyone thought that magnetism and
electricity were completely separate. But in that year,
the Danish physicist Hans Oersted (1777-1851)
discovered that a compass needle was deflected by an
electric current.
Magnetic Fields are
quantities! They are
referred to as a B Field and B is the symbol used. These
fields have effects on charged particles.
-If the charge is NOT moving there is NO force acting on
+
the particle.
-If the charge is MOVING ALONG a field line there is NO
force acting on the particle.
-If the charge is MOVING ACROSS
a field line, it feels a FORCE
B
v
F
+
Magnetic fields produce forces on moving charged
particles. The forces are perpendicular to both
the velocity of the particle and the direction of
the magnetic field
The size of the force is proportional to the
intensity of the field and the speed with which the
particle is cutting across
v
F
F
b
v
b
Note: The direction of the field and the velocity
determine a plane. The force is perpendicular to
that plane
The
Hold your right hand with
your index finger straight
out, your middle finger 90o
from the index finger and
your thumb straight up.
Keep this orientation!
Your index finger represents the velocity of the
positively charged particle, your middle finger points
the direction of the magnetic field (from the north
end of a magnet) and your thumbs shows the
direction of the force applied to that positively
charged particle.
Up
Right
Down
Left
Out of Page
x
Into Page
The convention of showing three dimensions on a two dimensional
page.
Examples:
Find the resultant force under the given conditions…
v
x
x
B
F
x
Magnets exert forces on moving particles….and as
Oersted showed, moving charges also created
magnetic fields and that’s what deflected Oersted’s
compass.
To examine the simplest
case, pass a current
carrying wire straight
through a plane covered
with compass needles.
The
needles
line up
in circles
around
the wire
The magnetic field of a current is circular centered
on the wire and lying on a plane perpendicular to the
current.
You can find the direction of the
magnetic field in a current
carrying wire by pointing your
thumb of your right hand along the
direction of the flow of positive
charges. Your fingers curl in the
direction of the magnetic field.
If you look at the negative
charges flowing than use
the left hand rule.
Two parallel currents attract each other.
The magnetic field circling each wire
causes forces on the current in the other
wire, pulling it closer.
Andrea-Marie Ampere,
discovered the force
between parallel wires
If a current carrying wire is bent into a circle, a magnetic field is
produced.
Notice that one
side looks just like
a
–
the field lines are
coming out.
By winding many turns, the magnetic field
is made proportionally larger. By winding
turns along a cylinder, a
is
produced, with a magnetic field just like a
bar magnet
The other side
looks like a
–
with field lines
going in.
Inserting an iron bar into
the coil concentrates and
strengthens the magnetic
field, the result is an
For 12 years after Oersted’s
discovery “electricians” looked
for the complimentary effect.
How to make a magnetic field
produce a current?
In 1832 it was Michael Faraday that suggested moving
the magnet!
Thrusting a magnet through a
loop of wire connected to a
sensitive ammeter, a
, deflects the
galvanometer needle. Thus
showing a current being induced.
When the magnet is held still,
the meter registers no current.
Faraday described this effect by saying that
(EMF) are generated
in the wire whenever magnetic field lines cut
across the wire. This is actually not a force but a
potential difference measured in volts
It does not matter whether the
magnetic field moves or the
wire moves with respect to the
magnet.
When the magnet is thrust
into the loop. It’s field lines
cut across the wire,
generating an EMF that
produces a current.
The same is
true when the
loop is moved
over the
magnet
Although Faraday’s discovery was at first received
with indifference, today nearly all our electrical
power is generated by moving giant coils of wire
near magnets.
Another way to induce a
current in a wire is to place a
second loop of wire nearby the
first and energize it with a
power source.
When a current in the second
loop is switched on or off, a
current pulse is induced in the
first.
But when the current in the
second loop is steady, no current
is induced in the first loop
In the case of the two wire loops, when the
current is first turned on in one loop, magnetic
field lines build up, cutting across the other loop
and producing an EMF.
When the current is
switched off, the field
collapses, again cutting
across the loop.
The induced emf creates a current that itself
creates a secondary magnetic field. This secondary
magnetic field also changes with time and thus
creates a changing secondary magnetic flux. The
secondary flux changes in such a way to opposes the
change in flux creating the emf.
Normally this means that the secondary magnetic
field increases or decreases in such a way as to
oppose the change in the magnetic field creating the
induced emf.
F => Force (N)
q => Charge (c)
v => Velocity of Charge particle (m/s)
B => Magnetic Field (N/Am = T =Tesla)
q => Angle between v and B
F
F
B
F => Force (N)
I => Current (A)
I
v
L
L = vt (m) => Length of wire a charge would
move in a given time
B => Magnetic Field (T =Tesla)
q=> Angle between v and B
v
B => Magnetic Field (T =Tesla)
m => 4p x 10 -7 (Tm/A)
n => Linear Turn Density (N/L)
# of turns per meter
I => Current (A)
– A relative
measure of the number of field
lines passing through an area
F => Magnetic Flux (Tm2 = wb (weber))
B => Magnetic Field (T =Tesla)
A => Area Vector (m2)
q => Angle between A and B
Axis of
Rotation
I
II
A
A
III
IV
A
A
q
B
I & II If B and A are parallel, q = 0o or 180o then the Magnetic
Flux is at maximum.
=> maximum # of lines through the loop
III
If B and A are perpendicular, q = 90o than the Magnetic
Flux is at minimum or zero.
=> No lines pass through the loop
IV
If B and A are between parallel and perpendicular then
there is a partial Magnetic Flux.
=> maximum # of lines >
>0
The EMF induced in a coil of
N loops depends on the time
rate change of the number of
filed lines through the loop.
=> Electromotive Force (v)
=> Number of loops in the wire
=> Change in the flux through one loop (Tm2 = wb)
=> Time (s)
The induced electromotive force
(EMF) in any closed circuit is
equal to the time rate change of
the magnetic flux through the
circuit.
Or alternatively,
The EMF generated is
proportional to the rate of
change of the magnetic flux.
The induced electromotive force
(EMF) is not actually a force
but a measure of potential
difference. It is measured in
volts (V)
• Generator: converts mechanical
energy into electrical energy (AC
current)
• A loop of wire is rotated between
the poles of a magnet by a power
source (in this case by the water)
and the loop moves through the
field of the magnet
• Thus there is a change in the
magnetic field resulting in an
induced current through the wire
A transformer is a static device that
transfers electrical energy from one
circuit to another through inductively
coupled conductors—the transformer's
coils. A varying current in the first or
primary winding creates a varying
magnetic flux in the transformer's
core and thus a varying magnetic field
through the secondary winding. This
varying magnetic field induces a
varying electromotive force (EMF) or
"voltage" in the secondary winding.
This effect is called mutual induction.
• If there are more loops in
the secondary coil the
voltage in the second coil is
greater
• This increases the voltage,
making it a step-up
transformer
• Used by power companies
to transmit high-voltage
electricity as well as
fluorescent light and X-rays
• In a step-down transformer,
there are more loops in the
first coil than the second coil
• The voltage in the second coil
is less than the first coil
• Used to lower the voltage of
electricity before it can be
used in homes or offices as
well as doorbells, small
radios, and calculators
An electric motor, is a
machine which
converts electrical
energy into mechanical
(rotational or kinetic)
energy.
A current is passed
through a loop which is
immersed in a magnetic
field. A force exists on
the top leg of the loop
which pulls the loop,
while a force on the
bottom leg of the loop
pushes the loop.
The net effect of these forces
is to rotate the loop.
DC motors are in many ways the simples electric
motors. All DC "brushed" motors operate in the
same way. There is a stator (a larger stationary
part) and a rotor (a smaller part spinning on an
axis within the stator). There are magnets on the
stator and a coil on the rotor which is
magnetically charged by supplying current to it.
Brushes are responsible for transferring current
from the stationary DC voltage source to the
spinning rotor. Depending on the position of the
rotor its magnetic charge will change and
produce motion in the motor. The animation
below further explains the basic operation of a
DC motor. Utilizing a DC power source, very few
controls are needed. To control speed an inline
variable resistance can be utilized to change the
amount of current reaching the coils.
The animation to the shows a DC motor in operation. The motor shown is a simplified "two-pole" motor which
uses just two magnets in the stator. In this case the magnets in the stator are permanent magnets for the sake of
simplicity. The brushes deliver current from a DC voltage source which supplies a magnetic field to that end of the
rotor. The polarity of the field depends on the flow of the current. As the rotor turns the brushes make contact
with one side of the DC source, then briefly do not make contact with anything, then continue making contact with
the other side of the DC source effectively changing the polarity of the rotor. The timing of this change is
determined by the geometrical setup of the brushes and leads to the DC source. The animation helps to illustrate
how at the moment of maximum attraction the current will change direction and thus change the polarity of the
rotor. At this moment the maximum attraction suddenly shifts to maximum repulsion which puts a torque on the
rotor's shaft and causes the motor to spin.
1. Magnets make modern life possible.
2. There are North Poles and South Poles.
3. Like poles repel, unlike poles attract.
4. Magnetic forces attract or repel only magnetic
materials.
5. Magnetic forces act at a distance.
6. While magnetized, temporary magnets act like
permanent magnets.
7. A charged particle experiences no magnetic
force when moving parallel to a magnetic field, but
when it is moving across a field it experiences a
force perpendicular to both the field and the
direction of motion.
8. A current-carrying wire in a perpendicular magnetic field experiences a force in a
direction perpendicular to both the wire and the field.
9. Magnetic Flux is the relative number of magnetic field lines passing through an area.
10. The EMF induced in a coil of N loops depends on the time rate change of the
number of filed lines through the loop.
“ Mr. McMullen, may I be excused? My brain is full”