Download 1 Electromagnetic Induction

Electromagnetic Induction Date _______
When we studied circuits a battery or power supply was needed to produce a potential difference. The electric field associated with the potential difference causes charges to move through the circuit and create a current. Now we will study how to INDUCE a current in a circuit.
We know that a current in a circuit is the source of a magnetic field.
In 1831, Michael Faraday from England and Joseph Henry from the US discovered that electric
current can be produced by moving a magnet
in and out of a coil of wire.
Electromagnetic induction is the process of inducing
a current in a circuit by changing the magnetic
field passing through the circuit.
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A potential difference, ___________, is induced by the relative motion between a wire and a magnetic field.
This happens whether the magnetic field of the magnet moves near the stationary conductor or the conductor moves in a stationary magnetic field.
A changing magnetic field produces an electric field and a changing electric field produces a magnetic field.
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In the last lecture we learned that a moving charge can be deflected by a magnetic field.
The picture below shows a conductive wire moving through a magnetic field.
(The magnetic field is going into the smartboard).
The negative charges in the wire will experience a magnetic force.
As a result of the magnetic force, electrons
will start to accumulate at the top of the rod.
The top will have an excess of electrons
(negative charge). The bottom will have
a deficit of electrons (positive charge).
This charge distribution will produce an electric field in the rod.
This induced electric field will create a potential difference, or voltage, in the rod.
The potential difference will induce a current in a circuit.
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Faraday's Law
As the number of loops of wire increases, the amount of induced voltage also increases.
Twice as many loops results in twice the induced current.
If the magnet is moved through the coils faster, the amount of induced current increases.
Faraday's Law: The induced voltage in a coil is proportional to the product of the number of loops and the rate at which the magnetic field changes within those loops.
When a magnet is pushed through
a coil of wire a current is induced
in the coil. This induced current
produces its own magnetic field.
The induced magnetic field opposes
the direction of the magnetic field
from the magnet. It is more difficult to push a magnet into a coil with more loops.
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When a circuit loop is moved in a magnetic field, a current is induced in the circuit.
As the loop rotates the magnitude of the magnetic field changes. The magnetic force on the
current is at the maximum when the charged particles are moving perpendicular to the magnetic field as shown above.
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A generator converts the mechanical energy needed to turn a wire loop in an magnetic field into electrical energy.
When the area of the loop is perpendicular to the
magnetic field (pictures a and c), every segment of the wire is moving parallel to the magnetic field so there is NO induced potential difference and NO induced current.
As the loop rotates, segments a and c cross
the magnetic field lines so the induced
voltage and current increase.
The greatest magnetic force on the charges in the
wire occurs when segments a and c move
perpendicularly to the magnetic field as
pictures b and d. This happens when the area
of the loop is parallel to the magnetic field
lines.
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As the coil rotates between the stationary magnetic field, the magnetic force on the charges particles changes because angle between the wire loop and the magnetic field changes.
Where the potential difference and induced current is at the maximum or minimum the loop is
______________ to the magnetic field. Where the potential difference is zero and there is no induced current, the loop is ______________ to the magnetic field.
Since the potential difference alternates from positive to negative, the output current changes its direction which is alternating current (ac).
The frequency of rotation of the determines the frequency of the alternating current. For the US, the frequency is 60 Hz and for Europe it is 50 Hz.
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Generators need a source of energy to move the coil within the magnetic field.
What are some sources of energy?
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Motors are machines that convert electrical energy to mechanical energy.
This picture shows a motor in its
most basic form.
The nail wrapped with wire is an electromagnet.
When current flows through the wire, the north
end of the electromagnet is repelled by the
north end of the magnet.
The key to a motor is that the moment that
the electromagnet flips, the current flowing
through the electromagnet reverses so the
magnetic field of the electromagnet switches.
This results in another half spin of the electromagnet. After that half spin, the direction of the current switches again and the electromagnet takes another half spin.
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Changing the electric field so that the motor continues to rotate
is done with a commutator (green) and brushes (red).
The brushes make contact with the commutator.
The commutator alternates the current in the coil as the wire coil
passes through the horizontal position.
This alteration of the current causes the magnetic field on the
wire coil to reverse and thus always be repelled by the fixed
magnetic field.
In this picture of a motor you can't see the
wire coil, but it is on the part of the motor
that is rotating.
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Applications for electromagnetic induction
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