Induction and Alternating Current with teacher's notes

Section 22-3
Demonstration 8
Transformers
Purpose Show students how a
transformer works, and reinforce
the idea that a changing current
is required.
Materials iron bar, 9 V battery,
knife switch, flashlight bulb in
holder, two wires (each about
1 m long)
Procedure Set up the primary side
of the transformer before class by
connecting the battery and switch
in series with one of the wires
coiled around the iron bar. The
primary coil should have 50 turns.
Set up the secondary coil with 25
turns, and connect the flashlight
bulb to the secondary coil.
In class, demonstrate this stepdown
transformer by momentarily
closing the switch and then
opening it again. Have students
discuss the transfer of energy that
occurs in this situation.
Close the switch and keep it
closed. Have students note the
behavior of the bulb. Discuss the
brief illumination and fading of
the bulb with students. Lead students
to consider the concept of
changing current. Open the
switch, and note the illumination.
Discuss this effect.
814
22-3 SECTION OBJECTIVES
• Describe how mutual
induction occurs in circuits.
• Calculate the potential
difference from a step-up or
step-down transformer.
• Describe how self-induction
occurs in an electric circuit.
mutual inductance
a measure of the ability of one circuit
carrying a changing current to
induce an emf in a nearby circuit
Figure 22-18
Faraday’s electromagnetic-induction
experiment used a changing current
in one circuit to induce a current in
another circuit.
814
Chapter 22
22-3
Inductance
MUTUAL INDUCTANCE
The basic principle of electromagnetic induction was first demonstrated by
Michael Faraday. His experimental apparatus, which resembled the arrangement
shown in Figure 22-18, used a coil connected to a switch and a battery
instead of a magnet to produce a magnetic field. This coil is called the primary
coil, and its circuit is called the primary circuit. The magnetic field is strengthened
by the magnetic properties of the iron ring around which the primary
coil is wrapped.
A second coil is wrapped around another part of the iron ring and is connected
to a galvanometer. An emf is induced in this coil, called the secondary
coil, when the magnetic field of the primary coil is changed. When the switch in
the primary circuit is closed, the galvanometer in the secondary circuit deflects
in one direction and then returns to zero. When the switch is opened, the galvanometer
deflects in the opposite direction and again returns to zero. When
there is a steady current in the primary circuit, the galvanometer reads zero.
The magnitude of this emf is predicted by Faraday’s law of induction.
However, Faraday’s law can be rewritten so that the induced emf is proportional
to the changing current in the primary coil. This can be done because of
the direct proportionality between the magnetic field produced by a current
in a coil, or solenoid, and the current itself. The form of Faraday’s law in terms
of changing primary current is as follows:
emf =−N⎯ ∆[AB (cos
q)]
⎯ =−M ⎯
∆t
∆I
⎯
∆t
The constant, M, is called the mutual inductance of the two-coil system. The
mutual inductance depends on the geometrical properties of the coils and
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Primary
coil
Iron
ring
Secondary
coil
Galvanometer
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0 +
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