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Unit 12
Magnetism
Magnets

Magnet – a material in which the spinning
electrons of its atom are aligned with one another
Two main types
• Bar Magnet – Straight
piece of metal
• Horse Shoe Magnet –
Curved piece of metal
Poles

Poles of a magnet are the ends where objects
are most strongly attracted


Like poles repel each other and unlike poles
attract each other


Two poles, called north and south
Similar to electric charges
Magnetic poles cannot be isolated


If a permanent magnetic is cut in half repeatedly, you
will still have a north and a south pole
This differs from electric charges
Magnetism

Magnetism – is the force of attraction or
repulsion between magnetic poles

An unmagnetized piece of iron can be
magnetized by stroking it with a magnet


Somewhat like stroking an object to charge an
object
Magnetism can be induced

If a piece of iron, for example, is placed near a
strong permanent magnet, it will become
magnetized
Magnetic Fields (B)

The region where magnetic force exists
around a magnet or any moving
charged object
Poles

North Pole


End of magnet that points to geographic
north
South Pole

End of magnet that points to geographic
south
* So what does that really mean about
the Earth? *
Forces between Poles

Magnetic Force is the force produced by
the motion of charges relative to each
other
Opposites attract (N-S)
 Likes repel (N-N or S-S)

Compass

A compass is a navigational instrument
for determining direction relative to the
Earth's magnetic poles.
• It consists of a magnetized pointer
(usually marked on the North end)
free to align itself with Earth's
magnetic field.
Earth’s Magnetic Field
The Earth’s geographic north pole
corresponds to a magnetic south pole
 The Earth’s geographic south pole
corresponds to a magnetic north pole


Strictly speaking, a north pole should be a
“north-seeking” pole and a south pole a
“south-seeking” pole
Earth’s Magnetic Field

The Earth’s
magnetic field
resembles that
achieved by
burying a huge
bar magnet deep
in the Earth’s
interior
Source of the Earth’s Magnetic
Field
There cannot be large masses of
permanently magnetized materials since
the high temperatures of the core
prevent materials from retaining
permanent magnetization
 The most likely source of the Earth’s
magnetic field is believed to be electric
currents in the liquid part of the core


Magnetic Fields result from moving charges
Magnetic Field Strength

The number of magnetic flux lines per
unit area passing through a plane
perpendicular to the distance of the
lines

The closer to the ends of a magnet the
stronger the magnetic field strength is
Magnetic Flux (Field) Lines

Direction is given by the direction a north
pole of a compass needle points in that
location

Magnetic field lines can be used to show how
the field lines, as traced out by a compass,
would look
1.
2.
3.
4.
Form closed loops
DO NOT CROSS
Exit the (N)orth pole of a magnet and enter the (S)outh pole
Closer the lines the stronger the force
Magnetic Field Lines, sketch
A compass can be used to show the direction
of the magnetic field lines (a)
 A sketch of the magnetic field lines (b)

Magnetic Field Lines, Bar
Magnet
Iron filings are used
to show the pattern
of the electric field
lines
 The direction of the
field is the direction
a north pole would
point

Magnetic Field Lines, Unlike
Poles

Iron filings are used
to show the pattern
of the electric field
lines
Magnetic Field Lines, Like
Poles

Iron filings are used
to show the pattern
of the electric field
lines
Magnetic Field Lines, Horse Shoe
Magnet

Iron filings are used
to show the pattern
of the electric field
lines
Magnetic Field Lines
Where do magnetic fields come from?

The most common causes include:

moving electrical charges


magnetic dipoles


this is how electromagnets work
how most permanent magnets work
changing electrical fields
Domains

Unmagnetized iron


Arrows go in all different
directions
Not Magnetic

Magnetized iron


Arrows align to go in the same
direction
North Pole on right end
Magnetism by Induction

By bringing a magnet near a
Ferromagnetic Material the domains
realign themselves
Current Carrying Wire

Magnetic Field is
counter-clockwise

Magnetic Field is
clockwise
Electromagnets

If a long straight wire is bent into a coil of several closely
spaced loops, the resulting device is called a solenoid


It is also known as an electromagnet since it acts like a magnet only
when it carries a current
The field lines of the solenoid resemble those of a bar magnet
Why does iron core increase the strength?

Due to the fact that as the current runs
through the wires it causes the domains to
realign and become a magnet itself
Left Hand Rule#2

Using your left – start
from the negative end

Think of your first finger
as the wire


Follow the wire around –
like the wire
Which ever direction
your thumb is pointing
is north
Which end is North?
Magnetic Fields - Force

When moving through a magnetic field, a
charged particle experiences a magnetic
force

This is a result of two conflicting magnetic fields


The one that is set up by the moving charges and the
existing field
The force is zero if the motion of the charge is
parallel to the magnetic field lines
Left Hand Rule #3

Used to determine the Force




Hold your left hand open
Place your fingers in the direction
of magnetic field
Place your thumb in the direction
of current or direction of moving
charge
The direction of the force on a
positive charge is directed out
of your palm

If the charge is positive, the force
is opposite that determined by the
left hand rule
Movement of
Charge
Particle
Magnetic
Field
Force
Faraday’s Experiment

A current can be produced by a changing
magnetic field

First shown in an experiment by Michael Faraday


A primary coil is connected to a battery
A secondary coil is connected to an ammeter
– Demo with coils, switch and galvanometer
Faraday’s Conclusions
Magnetic fields don’t produce current
but changing magnetic fields do
 It is customary to say that an induced

current is produced in the secondary
circuit by the changing magnetic field
Magnetic Flux
The induced current is actually induced
by a change in the quantity called the
magnetic flux rather than simply by a
change in the magnetic field
 Magnetic flux is proportional to both the
strength of the magnetic field passing
through the plane of a loop of wire and
the area of the loop

Ways to Change Magnetic Flux
1.
2.
3.
Field - depending on strength
Area – the cross sectional area of the
wire
Orientation – depending on the
position of the wire (angle)
Faraday’s Law

The induced electromotion force (EMF) in a circuit
equals the time rate of change of magnetic flux
through the circuit
or

The EMF generated is proportional to the rate
of change of the magnetic flux.
Electromagnetic Induction –
An Experiment




When a magnet moves
toward a loop of wire, the
ammeter shows the
presence of a current (a)
When the magnet is held
stationary, there is no
current (b)
When the magnet moves
away from the loop, the
ammeter shows a current
in the opposite direction (c)
If the loop is moved
instead of the magnet, a
current is also detected
Lenz’s Law
 An
induced current is always in
such a direction as to oppose
the motion or change causing it
 By
opposing the motion it creates
the same pole on the side the
magnet enters
Motional Emf
Assume the moving bar has zero
resistance
 As the bar is pulled to the right
with velocity v under the
influence of an applied force, F,
the free charges experience a
magnetic force along the length
of the bar
 This force sets up an induced
current because the charges are
free to move in the closed path
 ADAM>COACH

Application of Faraday’s Law –
Motional emf

Electrons will be pointed
in the downward
direction.

Making the bottom of the
bar negative
Left Hand Rule # 4

Only used for
finding the
direction of
induced current

All should be
perpendicular to
each other
Direction of
Conductor
Motion (v)
Magnetic Field
Electron Motion
– Current Flow
Generators

Converts mechanical energy to electrical
energy

Uses a wire loop to rotate

Alternating Current (AC) generator
– Uses a solid ring

Direct Current (DC) generator
– Uses a ring with slits
AC Generators

Basic operation of the
generator




As the loop rotates, the
magnetic flux through it changes
with time
This induces an emf and a
current in the external circuit
The ends of the loop are
connected to slip rings that
rotate with the loop
Connections to the external
circuit are made by stationary
brushed in contact with the slip
rings
DC Generators
Components are
essentially the same
as that of an ac
generator
 The major difference
is the contacts to
the rotating loop are
made by a split ring,
or commutator

Motors

Motors are devices that convert
electrical energy into mechanical energy
A motor is a generator run in reverse
 A motor can perform useful mechanical
work when a shaft connected to its
rotating coil is attached to some external
device
