Induction and Alternating Current with teacher's notes

Section 22-1
Teaching Tip
Remind students that, as discussed
in Chapter 21, electric
charges that move through a
magnetic field experience a magnetic
force. The magnitude of this
force is qvB. When charges are at
rest with respect to the magnetic
field (v = 0 m/s), they do not
experience a magnetic force.
Visual Strategy
Figure 22-1
Be sure students understand the
conditions required to induce a
current in a circuit.
Why is a current generated in
Q a moving conductor in a
magnetic field but not in a moving
insulator in the same magnetic
field?
Charges experience a force in
A both cases. In conductors, the
electrons are free to move under
the influence of the magnetic
force. Electrons in an insulator
feel the same force but are not
free to move.
Q
What is the condition
required for current to exist in
the circuit shown in this figure?
There must be relative
A motion between the circuit
and the magnetic field; when the
two are at rest relative to one
another, charges do not experience
a magnetic force, so they
remain at rest.
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22-1 SECTION OBJECTIVES
• Describe how the change in
the number of magnetic field
lines through a circuit loop
affects the magnitude and
direction of the induced
current.
• Apply Lenz’s law to
determine the direction of an
induced current.
• Calculate the induced emf
and current using Faraday’s
law of induction.
electromagnetic induction
the production of an emf in a
conducting circuit by a change in
the strength, position, or orientation
of an external magnetic field
Figure 22-1
When the circuit loop
crosses the lines of the
magnetic field, a current
is induced in the circuit,
as indicated by the
movement of the
galvanometer needle.
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22-1
Induced current
MAGNETIC FIELDS AND INDUCED EMFS
Recall that in Chapter 20 you were asked if it was possible to produce an electric
current using only wires and no battery. So far, all electric circuits that you
have studied have used a battery or an electrical power supply to create a
potential difference within a circuit. In both of these cases, an emf increases
the electrical potential energy of charges in the circuit, causing them to move
through the circuit and create a current.
It is also possible to induce a current in a circuit without the use of a battery
or an electrical power supply. Just as a magnetic field can be formed by a current
in a circuit, a current can be formed by moving a portion of a closed electric
circuit through an external magnetic field, as indicated in Figure 22-1.
The process of inducing a current in a circuit with a changing magnetic field
is called electromagnetic induction.
Consider a circuit consisting of only a resistor in the vicinity of a magnet.
There is no battery to supply a current. If neither the magnet nor the circuit is
moving with respect to the other, no current will be present in the circuit. But
if the circuit moves toward or away from the magnet or the magnet moves
toward or away from the circuit, a current is induced. As long as there is relative
motion between the two, a current can form in the circuit.
The separation of charges by the magnetic force induces an emf
It may seem strange that there can be an induced emf and a corresponding
induced current without a battery or similar source of electrical energy. Recall
that in the previous chapter a moving charge can be deflected by a magnetic
field. This deflection can be used to explain how an emf occurs in a wire that
moves through a magnetic field.
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Galvanometer
S
Current
N
Magnetic field
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