Induction and Alternating Current with teacher's notes

As the current increases, the rate of increase lessens and the induced emf
decreases, as shown in Figure 22-22. The decrease in the induced emf results
in a gradual increase in the current. Similarly, when the switch is opened,
the current gradually decreases to zero. This effect is called self-induction
because the changing magnetic field through the circuit arises from the current
in the circuit itself. The induced emf in this case is called self-induced emf.
An example of self-induction is seen in a coil wound on a cylindrical iron
core. (A practical device would have several hundred turns.) When the current
is in the direction shown in Figure 22-23(a), a magnetic field forms
inside the coil. As the current changes with time, the field through the coil
changes and induces an emf in the coil.
Lenz’s law indicates that this induced emf opposes the change in the current.
For increasing current, the induced emf is as pictured in Figure 22-23(b), and
for decreasing current, the induced emf is as shown in Figure 22-23(c).
Faraday’s law of induction takes the same form for self-induction as it does
for mutual induction except that the symbol for mutual inductance, M, is
replaced with the symbol for self-inductance, L.
emf =−N⎯ ∆[AB (cos
q)]
⎯ =−L ⎯
∆t
∆I
⎯
∆t
The self-inductance of a coil depends on the number of turns of wire in the
coil, the coil’s cross-sectional area, and other geometric factors. The SI unit of
inductance is the henry (H), which is equal to 1 volt-second per ampere.
B
I
Lenz’s law emf
for increasing I
− +
(a) (b) (c)
Section Review
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Lenz’s law emf
for decreasing I
+ −
self-induction
1. The centers of two circular loops are separated by a fixed distance. For
what relative orientation of the loops will their mutual inductance be a
maximum? For what orientation will it be a minimum?
2. A step-up transformer has exactly 50 turns in its primary and exactly
7000 turns in its secondary. If the potential difference across the primary
is 120 V, what is the potential difference across the secondary?
3. Does the self-inductance (L) of a coil depend on the current in the coil?
the process by which a changing
current in a circuit induces an
emf in that same circuit
Current
Figure 22-22
The self-induced emf reduces the
rate of increase of the current in
the circuit immediately after the
circuit is closed.
Induction and Alternating Current
Time
Figure 22-23
The polarity of the self-induced emf
is such that it can produce a current
whose direction is opposite the
change in the current in the coil.
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SECTION 22-3
The Language of Physics
The SI unit of inductance, the
henry (H), was named after the
American scientist Joseph Henry.
Henry was a professor of mathematics
and philosophy in Albany,
New York. He discovered electromagnetic
induction while on a
one-month vacation, but he then
returned to work and was not
able to complete his research for
publication. By the time Henry
published his discovery, Faraday
had already discovered the same
phenomenon and published his
results.
Section Review
ANSWERS
1. The planes of the two loops
must be parallel for maximum
mutual inductance. The planes
of the loops must be perpendicular
to each other for minimum
mutual inductance.
2. 1.7 × 10 4 V
3. No, self-inductance (L) only
depends on the number of
turns of wire in the coil, the
coil’s cross-sectional area, and
geometric factors.
819