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Magnetism
Lodestone : Naturally Magnetic Rock
The early Greeks found natural rock magnets in an area of
Greece called Magnesia. They called this magnetic stone,
lodestone. Our term, magnet derives from Magnesia.
Lodestone Used For Navigation From 1200s
Lodestone needles were placed on cork which was floated in
a water dish. The needle always pointed in a north/south
direction which helped ships to navigate their way.
William Gilbert 1600s
William Gilbert, Queen Elizabeth’s physician, experimented
with lodestone and concluded that the earth itself is a
lodestone to be able to cause compasses to align
themselves along a north/south axis.
The Poles of A
Magnet
A magnet, when
suspended from a
string will align
itself with one pole
pointing north and
the other pole
pointing south.
The north-seeking
pole is labelled N
while the southseeking pole is
labelled S.
The Poles of a Magnet Can Not be Separated
Unlike electric charges which can be separated, magnetic
poles can not be separated. When a magnet is broken,
each piece is found to have a north and south pole.
The Laws of Magnetic Forces
Like poles of magnets repel each other while opposite poles
attract each other.
The Nature of Earth’s North and South Magnetic Poles
Since a magnet’s north pole is attracted to Earth’s Magnetic
North Pole, the Magnetic North Pole must actually be a
south pole, since only opposite poles attract each other.
Likewise the Magnetic South Pole must be an actual north
pole.
Earth’s Geographic and Magnetic Poles Differ
Earth’s Magnetic poles
are aligned on an angle
to Earth’s Geographic
poles (true north and
south pole).
Earth’s Wandering Magnetic Poles
The magnetic poles of the Earth are created by its moving
molten iron and nickel core. The internal fluid nature of the
Earth causes its magnetic poles to move yearly. Over the
past 100 years the pole has moved over 1000 km.
Presently it is moving at 40 km per year.
Adjusting Compass Readings
Since the Magnetic pole is not aligned with the Geographic
pole, compass readings must be adjusted in most regions.
These adjustments are referred to as declinations and are
printed on maps of various regions in Canada.
Magnetic Field
The region surrounding a
magnet in which
magnetic forces can be
exerted is called a
magnetic field. Iron
filing sprinkled around
a magnet reveal the
nature of the invisible
magnetic field.
The Vector Nature of a Magnetic Field
When compasses are set in a magnetic field, they show a
direction for the magnetic field. Magnetic fields like
electric fields are vector quantities, showing both
magnitude and direction.
Magnetic Field Line Direction
Magnetic field lines are drawn from north to south poles in
the same direction as a compass would point in the region
outside the magnet. (The field lines continue inside the
magnet from from south to north).
Fields Between Attracting And Repelling Poles
Field lines between attracting poles link together while field
lines between repelling poles stay apart.
Comparing Electric and Magnetic Fields
Electric and Magnetic Fields show similar field lines for
attracting and repelling fields.
Uniform Magnetic Fields
The region between two opposite poles of two bar magnets is
uniform except that it “fringes” out at the edges. In Fig.
12.4 below, The magnetic field lines in (a) are from left to
right. The magnetic field lines in (b) are perpendicular into
the page, away from you. The magnetic field lines in (c) are
perpendicular and out of the page, towards you.
Magnetic Field Strength
The number of magnetic field lines, referred to as magnetic
flux, indicates the strength of the magnetic field and is
measured in teslas (T - SI unit). A 1 tesla magnetic field
exerts a force of 1 N on a charge of 1 coulomb moving at 1
m/s. Earth’s magnetic field is 5 x 10-5 T, a fridge magnet
produces 5 mT. Strong neodymium magnets produce 1-2
T. Bar magnets produce from .001 to .01 T.
Strengths of Some Magnetic Materials
Smallest value in a magnetically
shielded room
10^-14 Tesla
10^-10 Gauss
Interstellar space
10^-10 Tesla
10^-6 Gauss
Earth's magnetic field
0.00005 Tesla
0.5 Gauss
Small bar magnet
0.01 Tesla
100 Gauss
Within a sunspot
0.15 Tesla
1500 Gauss
Small NIB magnet
0.2 Tesla
2000 Gauss
Big electromagnet
1.5 Tesla
15,000 Gauss
Strong lab magnet
10 Tesla
100,000 Gauss
Surface of neutron star
100,000,000 Tesla
10^12 Gauss
Magstar
100,000,000,000 Tesla
10^15 Gauss
The Origin of Magnetic Fields
The spin and angular momentum of an electron generate a
magnetic field. In most atoms, reverse spins and motions
cancel the magnetic fields generated.
Magnetic Classification of Materials
In terms of magnetism, materials are classified as paramagnetic,
diamagnetic or ferromagnetic.
Paramagnetic materials are weakly attracted to magnetic fields.
Diamagnetic materials are weakly repelled by magnetic fields.
Ferromagnetic materials (like iron and nickel) respond strongly to
magnetic fields.
Ferromagnetic Materials: Domains
In ferromagnetic substances like iron and nickel, their atoms
have a number of unpaired electrons whose magnetic fields
are NOT cancelled by opposing motions. Atoms in
ferromagnetic substances cooperate with 1015 – 1020 nearby
atoms to create small microscopic regions (10-6 m) called
domains in which the atoms’ magnetic fields are all aligned.
Each domain acts as if it were a small magnet with a north
and south pole.
Temporary and Permanent Magnets
Ferromagnetic materials can be made into permanent or temporary
magnets. If, as liquid iron or nickel is cooled, a magnetic field
lines up the domains while the metal solidifies, the domains may
remain more or less lined up throughout the substance, forming
a permanent magnet. If the domains are not lined up in the solid
state, an external magnetic field may be applied to line up the
domains, forming a temporary magnet that demagnitizes when
the magnetic field is removed.
Magnetic Induction
When a magnetic field is applied to a non-magnetic,
ferromagnetic substance, the magnetic field causes or
“induces” the ferromagnetic substance to become a
temporary magnet by aligning its domains.
Electric Current and Magnetism
Hans Christian Oersted in 1820 accidentally discovered that an
electric current produces a magnetic field. He noticed that a
magnet near a wire moved when an electric current was
passed through the wire. This suggested that the current in
the wire was producing a magnetic field that affected the
compass.
Attracting and Repelling Magnetic Fields
Magnetic fields in the same direction repel each other while
magnetic fields in the opposite direction attract each other.
The Magnetic Field Around A Current-Carrying Wire
The current in a wire creates a circular magnetic field around
a wire.
Right Hand Rule for a Current-Carrying Wire
Using conventional current flow, grasp the wire with the
thumb of the right hand in the direction of the current flow
(+ to -). The direction of the magnetic field is shown by the
direction fo the right hand fingers.
Right and Left Hand Rules
When working with conventional current, the right hand wire
rule gives the correct relation of current and magnetic field
directions. When working with actual flow, the left hand
rule gives the correct relation of current and field
directions.
Diagram Conventions for Wire On-End Views
In on-end views of wires, an X denotes current flowing into
the page while a dot denotes current flowing out of the
page. Use the right hand rule with the thumb in the
direction of current flow to find the direction of the
magnetic field (direction of right hand fingers).
Magnetic Field Around a Wire Loop
Current flowing through a loop generates a somewhat linear
magnetic field inside the loop.
Electromagnets or Solenoids
When a wire is coiled with many loops, the magnetic field
within and around the coil is intensified and resembles the
field of a bar magnet.
Electromagnets or Solenoids
If the core of the coil (space inside) is occupied by a
ferromagnetic material, the electromagnet thus formed
becomes very strong.
Right Hand Coil Rule
If a coil is grasped with a right hand with the fingers pointing
in the direction of the conventional current flow, the thumb
will point in the direction of the magnetic field or in the
direction of the north pole.
Finding Magnetic Strength Inside an Electromagnet
Inside a coil or solenoid with an air core, the Magnetic Field
Strength, B, (in teslas –T) is found by the formula, B = µ0In ,
where µ0 is a constant, I = the current in amperes and n =
the number of loops per metre. µ0 = 4π x 10-7 Tm/A . µ0 is a
constant called the permeability of free space .
Interactions of Magnetic Fields 1
Magnetic fields in the same direction repel each other while
magnetic fields in the opposite direction attract each other.
Interactions of Magnetic Fields 2
Parallel current-carrying wires close together, experience a
force between them of repulsion or attraction, depending
on the direction of the current in the two wires.
no current
current in
opposite
direction
current in
same
direction
Predicting Compass Positioning
A compass needle (itself a magnet with a field) will move
itself to form an attraction between its field and any nearby
field.
N
Compass
Needle
X
Wire with
current into the page
Predicting Compass Positioning
In the diagram below, the compass is placed above the wire.
In (A), a larger current is produced (with lower resistor). In
(B) the current is reduced by ½ (2X resistor) so that the
magnetic field in the wire is only slightly larger than the
Earth’s field, causing an intermediate movement in which
the Earth’s field (dashed arrow) opposes a full movement.
Predict the Compass or Current Direction
Predicting Coil Current and Poles
Below, which side is the north pole and what is the field
direction if current flows from A to B?
Below, which side is the north pole and in what direction is
the current flowing?
Uses for Solenoids or
Electromagnets
Electric meters make use of
electromagnets placed
between permanent
magnets.
Uses for Solenoids or Electromagnets
Speakers for stereos, radios and broadcast systems use
electromagnets placed within a permanent magnetic field.
As the electromagnet receives pulses of current, it sets up
a changing magnetic field that pushes it back and forth
within the permanent magnetic field.
Uses for Solenoids or Electromagnets
Electromagnets are used to lift and move ferromagnetic
materials.
Uses for Solenoids or Electromagnets
A relay switch uses an electromagnet in one circuit (usually a
low current circuit) to open and close a second circuit
(usually a high current circuit).
Uses for Solenoids or Electromagnets
Bells and buzzers use electromagnets. In the circuit below, a
single tone is made when the switch is pressed. In other
circuits, the hammer can be made to hit continuously.