Chapter 10

download report

Transcript Chapter 10

Chapter 25
Electromagnetic Induction
Voltage will be induced in a wire loop
when a magnetic field within that loop
a.
b.
c.
d.
changes.
aligns with the electric field.
is at right angles to the electric field.
converts to magnetic energy.
Voltage will be induced in a wire loop
when a magnetic field within that loop
a.
b.
c.
d.
changes.
aligns with the electric field.
is at right angles to the electric field.
converts to magnetic energy.
When you thrust a bar magnet to and
fro into a coil of wire, you induce
a.
b.
c.
d.
direct current.
alternating current.
neither dc nor ac.
alternating voltage only, not current.
When you thrust a bar magnet to and
fro into a coil of wire, you induce
a.
b.
c.
d.
direct current.
alternating current.
neither dc nor ac.
alternating voltage only, not current.
Explanation: Indeed alternating voltage is induced, which
produces alternating current, so we can say that
current as well as voltage is induced in a metal coil.
Hence b is correct.
When a magnet is moved into a wire
coil, an induced current in the coil
produces a magnetic field that
a.
b.
c.
d.
resists motion of the magnet.
attracts the magnet.
has negligible effect on the magnet.
interferes with the electric field.
When a magnet is moved into a wire
coil, an induced current in the coil
produces a magnetic field that
a.
b.
c.
d.
resists motion of the magnet.
attracts the magnet.
has negligible effect on the magnet.
interferes with the electric field.
Comment: This is shown in Figure 25.4.
Faraday’s law is the law
a.
b.
c.
d.
of energy conservation.
of charge conservation.
of electromagnetic induction.
that connects voltage and resistance to
magnetic fields.
Faraday’s law is the law
a.
b.
c.
d.
of energy conservation.
of charge conservation.
of electromagnetic induction.
that connects voltage and resistance to
magnetic fields.
Comment: Answer a has merit, but answer c is central.
Answer d is gibberish.
The underlying physics of an electric
motor is that
a. electric and magnetic fields repel each other.
b. a current-carrying wire experiences force in a
magnetic field.
c. like magnetic poles repel each other.
d. ac voltage is induced by a changing magnetic
field.
The underlying physics of an electric
motor is that
a. electric and magnetic fields repel each other.
b. a current-carrying wire experiences force in a
magnetic field.
c. like magnetic poles repel each other.
d. ac voltage is induced by a changing magnetic
field.
Comment: Answer c is indirect, but answer b is more direct.
The essential physics concept in an
electric generator is
a.
b.
c.
d.
Coulomb’s law.
Ohm’s law.
Faraday’s law.
Newton’s second law.
The essential physics concept in an
electric generator is
a.
b.
c.
d.
Coulomb’s law.
Ohm’s law.
Faraday’s law.
Newton’s second law.
Within both a conventional ammeter
and voltmeter you will find a
a.
b.
c.
d.
diode.
capacitor.
galvanometer.
tiny motor.
Within both a conventional ammeter
and voltmeter you will find a
a.
b.
c.
d.
diode.
capacitor.
galvanometer.
tiny motor.
Both a motor and a generator
operate via
a.
b.
c.
d.
similar concepts.
quite different concepts.
idealized transformers.
independent energy sources.
Both a motor and a generator
operate via
a.
b.
c.
d.
similar concepts.
quite different concepts.
idealized transformers.
independent energy sources.
The major difference between a
motor and generator is
a.
b.
c.
d.
input and output.
direction of windings of coils.
that one uses ac, and the other dc.
primarily cosmetic.
The major difference between a
motor and generator is
a.
b.
c.
d.
input and output.
direction of windings of coils.
that one uses ac, and the other dc.
primarily cosmetic.
Explanation: The main difference between a motor and a
generator is the roles of input and output, which is
opposite for each.
When a generator is used to light a
lamp, the energy of the lit lamp
originates in the
a.
b.
c.
d.
coils of wire in the generator.
magnet inside the generator.
lamp itself.
work done to turn the coils in the generator.
When a generator is used to light a
lamp, the energy of the lit lamp
originates in the
a.
b.
c.
d.
coils of wire in the generator.
magnet inside the generator.
lamp itself.
work done to turn the coils in the generator.
The purpose of a transformer is
to transform
a. and create energy.
b. power at one voltage to the same power at
another voltage.
c. current from one place to another.
d. voltage to useful applications.
The purpose of a transformer is
to transform
a. and create energy.
b. power at one voltage to the same power at
another voltage.
c. current from one place to another.
d. voltage to useful applications.
Explanation: Answers c or d are not the best choices, as is
b, the more direct answer. If you answered a, OUCH!
… no device creates energy!
A transformer works by way of
a.
b.
c.
d.
Coulomb’s law.
Ohm’s law.
Faraday’s law.
Newton’s second law.
A transformer works by way of
a.
b.
c.
d.
Coulomb’s law.
Ohm’s law.
Faraday’s law.
Newton’s second law.
A step-up transformer in an
electrical circuit can step up
a.
b.
c.
d.
voltage.
energy.
Both of these.
None of these.
A step-up transformer in an
electrical circuit can step up
a.
b.
c.
d.
voltage.
energy.
Both of these.
None of these.
Explanation: Stepping up energy is a big no-no in
energy conservation!
A step-down transformer has a
greater number of coils on the
a.
b.
c.
d.
input side.
output side.
side with lower power.
None of the above.
A step-down transformer has a
greater number of coils on the
a.
b.
c.
d.
input side.
output side.
side with lower power.
None of the above.
Comment: The fewer number of coils on the output side
steps voltage down.
The workings of a transformer are
consistent mainly with
a.
b.
c.
d.
Newton’s second law.
Coulomb’s law.
the conservation of momentum.
the conservation of energy.
The workings of a transformer are
consistent mainly with
a.
b.
c.
d.
Newton’s second law.
Coulomb’s law.
the conservation of momentum.
the conservation of energy.
To minimize heat losses for power
transported across the countryside, it
is best that current in the wires is
a.
b.
c.
d.
low.
high.
not too low and not too high.
replaced with voltage.
To minimize heat losses for power
transported across the countryside, it
is best that current in the wires is
a.
b.
c.
d.
low.
high.
not too low and not too high.
replaced with voltage.
Comment: High amperage produces large heat losses. So
power at low current (and correspondingly high
voltage) means less heat loss.
Lighting a lamp via
electromagnetic induction
a. bypasses the need of work input.
b. requires work input.
c. may or may not require work input depending
on efficiency.
d. produces a low-heat lamp.
Lighting a lamp via
electromagnetic induction
a. bypasses the need of work input.
b. requires work input.
c. may or may not require work input depending
on efficiency.
d. produces a low-heat lamp.
Comment: Never forget an important fundamental of physics:
Work is necessary to transform energy, whether the
means of doing so is electromagnetic induction or
otherwise.
A changing electric field can induce a
changing
a.
b.
c.
d.
current loop.
voltage.
resonance that produces radio.
magnetic field.
A changing electric field can induce a
changing
a.
b.
c.
d.
current loop.
voltage.
resonance that produces radio.
magnetic field.
Comment: This was Maxwell’s generalization of
electromagnetic induction.
If you change the magnetic field in
a closed loop of wire, you induce in
the loop a
a.
b.
c.
d.
current.
voltage.
electric field.
All of these.
If you change the magnetic field in
a closed loop of wire, you induce in
the loop a
a.
b.
c.
d.
current.
voltage.
electric field.
All of these.
Explanation: Recall how Maxwell generalized Faraday’s
law to include induction of an electric field.
The fact that electric and magnetic
fields regenerate each other is
important in
a.
b.
c.
d.
burglar alarms.
radio broadcasting.
metal detectors.
All of these.
The fact that electric and magnetic
fields regenerate each other is
important in
a.
b.
c.
d.
burglar alarms.
radio broadcasting.
metal detectors.
All of these.
Explanation: A general answer would be electromagnetic
waves, of which only radio broadcasting qualifies.