Download Electric Currents

Magnetism & Induction
Physics
Objectives:
 Describe the magnetic field produced by a current carrying wire.
 Compare and contrast magnetic poles and electric charges.
 Describe how voltage is induced in a coil of wire.
 Describe how a transformer works.
The Relationship between Electricity & Magnetism
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Electricity and magnetism were regarded as unrelated phenomenon into the early
19th century.
In 1820, Hans Christian Oersted discovered a relationship between the two.
While demonstrating electric currents, Oersted and his students noticed that the
needle on a nearby compass moved when a current passed through the wire.
This discovery showed that a moving electric charge is accompanied by a magnetic
force!
The Source of Magnetism
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In most atoms, there are pairs of electrons that spin in opposite directions and
their magnetic affect is canceled.
Iron, however, has four electrons whose spin is uncanceled.
Clusters of aligned atoms are called magnetic __________________.
Unmagnetized Material
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Magnetized Material
The alignment of domains causes a material to become magnetized.
Two magnetic poles result from this alignment.
Like electric charges, opposites poles attract and like poles repel.
Unlike electric charges, magnetic poles cannot be isolated. If you break a bar magnet in
half, you get two short bar magnets, each with a north and south pole.
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Magnetic Field Lines
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As with electric fields, magnetic fields are
illustrated with lines.
It is customary for the lines to originate at a
north pole and terminate at a south pole.
Where lines are closer together, the
magnetic field is stronger.
Magnetic Field around a Current-Carrying Conductor
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As Oersted discovered, a magnetic field
surrounds a wire which is carrying an
electric current.
The direction of the magnetic field is
given by
________________________________
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Arrange your right hand so that the thumb
points in the direction of the current.
Your fingers will now curl in the direction
of the magnetic field.
Arranging the wire in a coil
causes the magnetic field to
be concentrated at the
center of the coil.
The electrical device used
to store energy in this
magnetic field is called an
____________________ .
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Induction
Much as a moving electric charge creates magnetism, a changing magnetic field produces
a current in a wire. This is known as induction.
Faraday’s Law may be used to calculate the voltage produced by a changing magnetic
field.
Transformers
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Constructed of two coils and usually connected by a common iron core.
The input voltage is applied to the primary coil. The current produces a magnetic
field.
The iron core directs much of this magnetic field to the secondary (output) coil.
The changing magnetic field in the iron core produces a voltage at the secondary
coil.
Because the secondary coil depends on the changing magnetic field to induce a voltage,
the input voltage must be from an alternating current (ac) source.
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The input and output voltages of a transformer are directly related to the number of turns
in the primary and secondary coils:
𝑉𝑠 𝑁𝑠
=
𝑉𝑝 𝑁𝑝
Vs = secondary voltage
Vp = primary voltage
Ns = number turns on secondary coil
Np = number of turn on primary coil
Example: A transformer is constructed with 1600 turns on the primary coil and 4000
turns on the secondary coil. If a potential of 40 volts is applied to the primary, what
voltage will appear on the secondary?
Q1) When 120 volts is applied to the input (primary) coil of a transformer, the output
voltage on the secondary is 6 volts. If the primary coil consists of 1000 turns, how many
turns are in the secondary coil?
Q2) A transformer is designed to increase its input voltage with 250 turns on the primary
coil and 1400 turns on the secondary coil. If we measure 40 volts on the output
(secondary) of the transformer, what is the input (primary) voltage?
Q3) A transformer has a primary input voltage of 18 volts. The output voltage is 4.95
volts. If the primary coil has 800 turns, how many turns are on the secondary?
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