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Magnetism
What are magnets?
• Let’s first start off with what causes an
magnetic field…
• A magnetic field is created around any
moving charged object.
What is charged that is moving
within an atom?
• Electrons (e-)
• The atoms within most
materials have paired up
electrons spinning in
opposite directions so
the magnetic field that is
created by one is
cancelled out by the
other.
• Some materials like
iron, nickel, and
colbalt have a
single electron or
pair of electrons
that spin in the
same direction
creating a magnetic
field or a small
atomic magnet.
The atomic sized magnets line up to create
domains within the material.
The individual domains line up to form a magnet.
A magnet has two ends called poles
(dipoles), where the magnetic force is the
strongest.
• No matter how
many times a
magnet is broken,
each piece always
has a north pole
and a south pole.
• There is no such
thing as a
monopole.
A magnetic field exists around a magnet or
any moving charged object.
Magnetic fields are like electric fields or
gravitational fields in that they allow
magnets to interact without touching.
Imaginary lines that map out the magnetic
field (B) around a magnet are known as
magnetic field lines or magnetic flux lines.
Rules for drawing magnetic field lines
(same as an electric field):
1. Magnetic field lines always go from the north pole to the
south pole (outside a magnet).
The direction of the magnetic field is
determined by using a compass. The
needle of the compass points in the direction
of the magnetic field.
Rules for drawing magnetic field lines:
2. Magnetic field lines are closed loops and never cross
or intersect.
Rules for drawing magnetic field lines:
3. Where the magnetic field lines are closer the magnetic
field is stronger.
Draw magnetic flux lines around
the magnets below:
S
S
N
N
S
N
N
S
Magnetic Field Strength (B)
B=F
qv
The number of magnetic lines of flux per unit area
passing through a plane perpendicular to the direction
of the lines is called the magnetic field strength (B).
The magnetic field strength is a vector like electric field strength.
The weber (Wb) is the unit used for
measuring the number of lines of flux.
1 Wb = 1 Tesla (T)
m2
Tesla’s are used as the unit for magnetic field strength (magnetic flux density).
Feeling the strongest magnetic force…
An object that enters this magnetic field
will feel a maximum force
only if it is moving
perpendicular to the flux lines.
F=0
Fmax
B
It will feel no magnetic force
if is is moving
in line, or parallel, with the flux lines.
Magnetism
• Magnetism is
the force of
attraction or
repulsion
between
magnetic poles.
Magnetic Field around the Earth
What we do know…
The north magnetic pole and the geographic North
Pole do not coincide. The magnetic pole is about 1500 km
(930 mi) south of the geographic North Pole and it wanders.
A compass actually indicates the direction of magnetic north,
not true north. Therefore a navigator must need to know the
magnetic declination for a specific area. This is the angular
difference between magnetic and true north.
The details and mechanisms of
how and why the Earth has a
magnetic field are unclear.
They do believe that it may
have something to do with
motions in the liquid outer core.
Charges moving through a wire:
Remember what creates
magnetic fieldsa charged object in motion.
Current is the flow of electrons. A moving charged object.
Current in a wire produces a magnetic field around the wire.
A compass can be used to detect a
magnetic field around a wire that
had current flowing through it.
LEFT HAND RULE: Helps us to analyze the path of a
charged object in a magnetic field.
Which way will the magnetic field below push this electron traveling through the field?
B
N
e-
S
Thumb is aligned with the
direction of current (I) or movement.
We use the LHR for electron flow.
Your palm indicates
the direction the
object would move
do to the magnetic force.
Fingers are aligned
with the direction of
the magnetic field (B).
LHR Symbols:
Sometimes the magnetic field or current is going into or out of the page.
For this we us the following symbols:
Coming out of page
WIRE:
FIELD:
Going into page
LHR with wires:
Effects of two wires:
1. Two wires next to each other that both have currents in the same direction.
WIRES WILL ATTRACT EACH OTHER
Effects of two wires:
2. Two wires next to each other that have currents in opposite directions.
WIRES WILL REPEL EACH OTHER
Effects of two wires:
3. Two wires with currents flowing perpendicular to each other.
NO INTERACTION, F=0
Practice…
The diagram below shows an end view of a current carrying wire
between the poles of a magnet. The wire is perpendicular to the
magnetic field.
F
If the direction of the electron flow is into the page, what is the
direction of the magnetic force (F) acting on the wire?
Practice…
The wire below is moved toward the right through a magnetic field.
In which direction will the magnetic force push the wire?
Looped wire
What’s the magnetic field around a loop of wire?
Where’s the
north pole
created?
e-
The north pole
is located where
your finger tips
end up.
Behind the wire.
+
-
What’s the magnetic field around a loop of wire?
+
e-
Where’s the
north pole
created?
Above the wire.
Looping a wire around several times into a coil will produce an…
…Electromagnet
An electromagnet is a type of magnet whose
magnetic field is produced by the flow of electric
current. The magnetic field disappears when the
current ceases.
An electromagnet is most commonly made by coiling wire
around a piece of iron. This electromagnet is called a
solenoid. The shape of the magnetic field is the same as
a bar magnet.
As electrons move through the coil of wire, the magnetic field of one electron
adds to the field of any others moving in the same direction.
Factors affecting the magnetic field of a solenoid:
1. The magnetic field around a solenoid is directly
related to the current through the coil.
Factors affecting the magnetic field of a solenoid:
2. The magnetic field around a solenoid is directly
related to the number of turns or coils around
the solenoid.
Factors affecting the magnetic field of a solenoid:
3. The magnetic field around a solenoid is directly
related to the material around which the wire is
coiled. The soft iron is more permeable to the
magnetic field than the air is.
Determining the poles of a
solenoid:
LHR Modified:
1. Have palm away from you.
2. Point fingers of left hand in
the direction of the current
(many coils, many fingers).
3. Stick thumb out and that’s
where the north pole is.
Determine the poles of this
solenoid:
S
N
Electromagnetic Induction
This is the process of generating a potential difference
(voltage) in a conductor (wire) due to the motion
of the conductor in a magnetic field.
Generating a voltage in a wire would mean generating
areas of uneven charge within a wire.
How can this be done?
What can be forced to move within a wire that will create a potential difference?
Moving a wire through a magnetic field will cause a magnetic force
which will move electrons towards one end of the wire
creating a potential difference.
Let’s take a closer look…
Motion of wire
B
- -
- wire
If this wire is moved up or down (perpendicular to the field), a voltage
will be generated and if the wire is part of a complete
circuit then current is induced.
If this wire is moved left or right (parallel to the field),
no current will be generated because no force is generated to
move electrons and create a voltage.
Other ways to induce voltage…
This is how Niagara Falls was used to
produce electricity…
Water flow is used to turn a
turbine through a magnetic field
inducing a potential difference and
producing electrical current.
This is how an electric motor works…
An electric motor utilizes the
property of electromagnetic
induction to convert electricity
into mechanical energy to make
things move. The conductor itself,
a coiled wire, will move to oppose
the magnetic field. Just when it
gets into position the current is
reversed, and the coil spins round
and round and round full of
mechanical energy.
A Generator…
A generator is simply the same
process in reverse, converting
mechanical energy into electricity.
Almost all of the electrical energy
we use in our daily lives is
supplied by electric generators.
http://www.stmary.ws/highschool/physics/home/java/fendt/ph11e/generator_e.htm
• http://www.physics.sjsu.edu/becker/physic
s51/mag_field.htm
• http://www.regentsprep.org/Regents/physi
cs/phys03/cintromag/sld001.htm