32
CHAPTER OUTLINE
32.1
SelfInductance
32.2
RL
32.3
Energy in a Magnetic Field
32.4
Mutual Inductance
32.5
Oscillations in an LC Circuit
32.6
The
Circuits
Circuit
RLC
Inductance
ANSWERS TO QUESTIONS
Q32.1
The emf induced in an inductor is opposite to the direction of the
changing current. For example, in a simple
RL
circuit with current
flowing clockwise, if the current in the circuit increases, the
inductor will generate an emf to oppose the increasing current.
Q32.2
The coil has an inductance regardless of the nature of the current
in the circuit. Inductance depends only on the coil geometry and
its construction. Since the current is constant, the selfinduced
emf in the coil is zero, and the coil does not affect the steadystate
current. (We assume the resistance of the coil is negligible.)
Q32.3
The inductance of a coil is determined by (a) the geometry of the
coil and (b) the “contents” of the coil. This is similar to the
parameters that determine the capacitance of a capacitor and the
resistance of a resistor. With an inductor, the most important
factor in the geometry is the number of turns of wire, or turns per
unit length. By the “contents” we refer to the material in which the
inductor establishes a magnetic field, notably the magnetic
properties of the core around which the wire is wrapped.
Q32.4
If the first set of turns is wrapped clockwise around a spool, wrap the second set counterclockwise,
so that the coil produces negligible magnetic field. Then the inductance of each set of turns
effectively negates the inductive effects of the other set.
Q32.5
After the switch is closed, the back emf will not exceed that of the battery. If this were the case, then
the current in the circuit would change direction to counterclockwise. Just after the switch is opened,
the back emf can be much larger than the battery emf, to temporarily maintain the clockwise current
in a spark.
Q32.6
The current decreases not instantaneously but over some span of time. The faster the decrease in the
current, the larger will be the emf generated in the inductor. A spark can appear at the switch as it is
opened because the selfinduced voltage is a maximum at this instant. The voltage can therefore
briefly cause dielectric breakdown of the air between the contacts.
Q32.7
When it is being opened. When the switch is initially standing open, there is no current in the
circuit. Just after the switch is then closed, the inductor tends to maintain the zerocurrent condition,
and there is very little chance of sparking. When the switch is standing closed, there is current in the
circuit. When the switch is then opened, the current rapidly decreases. The induced emf is created in
the inductor, and this emf tends to maintain the original current. Sparking occurs as the current
bridges the air gap between the contacts of the switch.
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 Spring '08
 Shannon
 Inductance, Energy, Magnetic Field, Inductor, dt, DI

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