UniversityPhysicsVolume2-Ch14.  Old Dominion University PHYS 232N

University Physics Volume II
Unit 2: Electricity and Magnetism
Chapter 14: Inductance
Conceptual Questions
1. Show that N I m / and  / ( / ), dI dt which are both expressions for self-inductance, have the
same units.
Solution

2. A 10-H inductor carries a current of 20 A. Describe how a 50-V emf can be induced across it.
Solution
The current must change 5 A/s to induce a 50-V emf.
3. The ignition circuit of an automobile is powered by a 12-V battery. How are we able to
generate large voltages with this power source?
Solution
The induced current from the 12-V battery goes through an inductor, generating a large voltage.
4. When the current through a large inductor is interrupted with a switch, an arc appears across
the open terminals of the switch. Explain.
Solution
The arc is the current caused by the induced emf from the changing current. If this induced emf
is larger than the dielectric breakdown of air, a spark occurs, along with a loud noise.
5. Does self-inductance depend on the value of the magnetic flux? Does it depend on the current
through the wire? Correlate your answers with the equation Nm LI.
Solution
Self-inductance is proportional to the magnetic flux and inversely proportional to the current.
However, since the magnetic flux depends on the current I, these effects cancel out. This means
that the self-inductance does not depend on the current. If the emf is induced across an element,
it does depend on how the current changes with time.
6. Would the self-inductance of a 1.0 m long, tightly wound solenoid differ from the selfinductance per meter of an infinite, but otherwise identical, solenoid?
Solution
The loops at the end of a solenoid have less magnetic field lines, thereby less flux and selfinductance than an infinite solenoid.
7. Discuss how you might determine the self-inductance per unit length of a long, straight wire.
Solution
Consider the ends of a wire a part of an RL circuit and determine the self-inductance from this
circuit.
8. The self-inductance of a coil is zero if there is no current passing through the windings. True
or false?
Solution
False, self-inductance is a property of the circuit element regardless of current flow.
9. How does the self-inductance per unit length near the center of a solenoid (away from the
ends) compare with its value near the end of the solenoid?
Solution
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