Transcript Induced EMF - Edvantage Science

8.1 Induced Emf
Magnetism, EMF, and Electric Current
An Englishman, Michael Faraday (1791-1867) and an American, Joseph Henry (17971878), working independently discovered that magnetism could produce or induce a
current in a circuit.
Inducing an EMF in a Straight Piece of Wire
A current in circuit can be induced if a length of wire, l, is made to move through
a perpendicular magnetic field.
If the wire (assume it is part of circuit that is not shown) is
moved downwards, then according to the right hand rule, free
charges, Q, will be forced to move along the wire due the
magnetic force acting on them.
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8.1 Induced Emf
Inducing an EMF in a Straight Piece of Wire
FB = BQv
Using the right hand motor rule the direction
of the force on the charges in the wire shown
in the diagram would be out of the page.
The amount of work done when a charge, Q, is forced to move
the length l, of the segment of wire:
W = F•d = BQvl
Since there is work done on moving the charges, the charges gain potential energy:
V=
And induced EMF:
Ep
Q
= EMF =
Ɛ = Bvl
BQvl
Q
(Units: Volt)
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8.1 Induced Emf
The Generator (part 1)
Michael Faraday invented the generator. A motor uses a magnetic field , an electric current,
and coils of wire to produce motion (kinetic energy). A generator uses magnetic fields and
coils of wire, and motion (kinetic energy) to produce (induce) a current in a circuit.
This diagram shows the main components of a handoperated generator.
The force acting on the charges in the wire would be a
maximum when the coil is horizontal and a minimum
when the coil is vertical to magnetic field.
The voltage produced by a generator will not be constant. The range will be from zero to a
maximum value called the peak voltage, Ɛmax.
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8.1 Induced Emf
The Generator (part 2)
These series of diagram shows 2 major concepts:
1) The voltage or EMFis not constant and varies from max
+EMF to zero to –EMF.
2) The current direction swicthes direction every ½ turn of
the coil generating an alternating EMF.
A graph of the EMF versus time as the
coil rotates would look like this:
Note the locations of max.
+EMF, 0 EMF and max. –
EMF on the graph.
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8.1 Induced Emf
The Generator (part 3)
At any other position of the coil:
v
T
Where
Ɛ = Ɛmax = Bvl
Ɛ = Bv l
T
When the coil is perpendicular to the lines of magnetic
force the EMF will be a maximum value:
is the speed of the coil at any other position.
Since there are two parts of the coil in the magnetic field, and the wires are not always
perpendicular to the magnetic field and EMF varies from +1 at sin 90o to -1 at sin 270o, the
equation becomes:
Ɛmax = +2NBvlsin Ɵ
Ɛmax = Ɛmax sin Ɵ
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8.1 Induced Emf
Key Questions
In this section, you should understand how to solve the following key questions.
Page 302 – Practice Problems 8.1.1 #2
Page 305 – Quick Check #1, & 3
Page 309 – 310 – Review 7.1 #2,4,6, & 8