Download Magnetism

Magnetism
Just the basics
Magnetic Poles
• Magnetic poles are LIKE electric charges
• Like poles repel and opposite poles attract.
• They are caused by the movement of
electrons in the atoms of elements
Magnetic Poles
• The two poles are called NORTH and SOUTH
• All magnets are dipolar
• This means that EVERY magnet, NO MATTER
HOW SMALL, has both a north pole and a
south pole
Magnetic monopoles do not exist!
• If I have a magnet, it has a north and south
pole
• If I cut the magnet in half…
Each half will STILL have both a north and a
south pole
Magnetic Fields
• Magnetic fields are VERY similar to electric
fields and we draw them the same way.
Magnetic Fields
• Magnetic field lines are drawn to point away from
north and toward south
• The symbol for magnetic field strength is  B
• The unit magnetic field strength is measured in is
 the Tesla (T)
Electricity and Magnetism
• Hans Christian Oersted discovered that ANY
time there is an electric current (a moving
electron) there is a magnetic field produced.
• That magnetic field is perpendicular to the
current.
First Right Hand Rule
• This rule helps us find the direction of the
magnetic field around a wire with current
flowing through it.
• Use your right hand
– Point your thumb in the
Direction of the CONVENTIONAL
CURRENT.
– The magnetic field wraps around
The wire in the direction your
Fingers curl.
Example
Second Right Hand Rule
• This rule is used to find the polarity of an
electromagnet
• Using your Right Hand
– Curl your fingers in the direction
of the conventional current and
your thumb will be pointing in the
direction of the north pole
Michael Faraday
• Faraday discovered that when a current
carrying wire is placed inside a magnetic
field, the magnetic field will cause a force to
be applied to the wire.
• The direction of this force can be found by
yet another right hand rule
Third Right Hand Rule
• Used to find the direction of the FORCE
acting on a current carrying wire in a
magnetic field
• Using the right hand
– Point your thumb in the direction of the conventional
current and your index finger in the direction of magnetic
field. The force will be in a direction OUT of your palm.
Electromagnetic Induction
• Oersted discovered that an electric current
causes a magnetic field.
• Faraday discovered that if a wire is moved
while in a magnetic field, then the magnetic
field will cause (induce) an electric current.
• This is how electric generators work
– Most of the time however, we move the magnet
instead of the wire
– It is also how microphones and speakers work
Alternating Current
• When a magnet is spun inside of a coil of
wire in a generator, the direction of the north
and south pole is continuously changing.
• This causes the direction of the current
induced in the wire to change
• This is called alternating current (A/C)
Electric Transformers
• Transformers are devices
that use electromagnetic
induction to increase or
decrease the voltage in a
circuit.
• How much the voltage is
stepped up or down depends
on the number of coils on
each side
Calculating Voltage of a Transformer
• The voltage in the primary and secondary
coils have the same ratio as the number of
turns in the coils
• Use the following equation
Example 1
• What is the resulting voltage in the
secondary coil of 40 turns if the primary coil
has 20 turns and a voltage of 60V?
Example 2
• I have a circuit with a voltage of 120 V. If I
wrap this wire around an iron core 3000
times, how many times will I need to wrap a
second wire to produce a voltage of only 6 V?