Deck 20: Electromagnetic Induction and Waves

Question 1

Examine Figure 20-1.

When the switch is closed, current is induced to flow through the Galvanometer in what direction?

Answer 1

toward the left

Question 2

Consider an outer loop of wire surrounding an inner loop of wire, as shown in Figure 20-2.

Suppose the current in the outer loop is counterclockwise and decreasing. What is the direction of the induced current in the inner loop?

Answer 2

counterclockwise

Question 3

Why would one not expect to observe an induced magnetic field aiding (not opposing) the original field change?

Answer 3

Such a situation would signify non-conservation of energy.

Question 4

Why won't a transformer work (stepup or stepdown potential) if a battery is connected across the input?

Question 5

To transmit power over great distances, why is HIGH VOLTAGE chosen for long distance power lines?

Question 6

Why are transformer cores made of laminated, rather than solid, magnetic material?

Question 7

A popular novelty contains 4 vanes balanced on a needle in an evacuated globe (see Figure 20-3) The front side of each vane is a shiny mirror and the back side is blackened.

Since electromagnetic waves exert a "radiation pressure," which way will the vanes rotate when a bright light shines on them?

Question 8

Early AM radios required very long antennae wires. FM antennae are often approximately a meter in size. Can you surmise why sensitive FM aerials are much smaller than those AM wires?

Question 9

An induced emf is produced in a loop or complete circuit whenever the number of magnetic field lines passing through the plane of the loop or circuit changes.

Question 10

An electric generator is a device that converts electrical energy into mechanical energy.

Question 11

A time-varying magnetic field produces a time-varying electric field.

Question 12

A time-varying electric field produces a time-varying magnetic field.

Question 13

In a vacuum, all electromagnetic waves travel at the same speed.

Question 14

Faraday's law of induction states that the emf induced in a loop of wire is proportional to

A) current divided by time.
B) the magnetic flux density times the loop's area.
C) the time variation of the magnetic flux.
D) the magnetic flux.

Question 15

Lenz's Law is a consequence of the law of conservation of

A) momentum.
B) energy.
C) mass.
D) charge.
E) electric field.

Question 16

A bar magnet is dropped, N pole down, so it falls through a horizontal loop of wire. As it approaches the loop, the induced current (viewed from above) is:

A) clockwise
B) counterclockwise
C) zero

Question 17

A bar magnet is dropped, N pole down, so it falls through a horizontal loop of wire. As it leaves the loop, the induced current (viewed from above) is:

A) clockwise
B) counterclockwise
C) zero

Question 18

A coil lies flat on a table top in a region where the magnetic field vector points straight up. The magnetic field vanishes suddenly. When viewed from above, what is the sense of the induced current in this coil as the field fades?

A) The current flows clockwise initially, and then it flows counterclockwise before stopping.
B) There is no induced current in this coil.
C) The induced current flows counterclockwise.
D) The induced current flows clockwise.

Question 19

A circular loop of wire is rotated at constant angular speed about an axis whose direction can be varied. In a region where a uniform magnetic field points straight down, what must be the orientation of the loop's axis of rotation if the induced emf is to be a maximum?

A) It must be vertical.
B) It must make an angle of 90° to the direction North.
C) It must make an angle of 45° to the direction North.
D) Any horizontal orientation will do.
E) It must make an angle of 45° to the vertical.

Question 20

Doubling the number of loops of wire in a coil produces what kind of change on the induced emf, assuming all other factors remain constant?

A) The induced emf is times as much.
B) The induced emf is 4 times as much.
C) The induced emf is 3 times as much.
D) There is no change in the induced emf.
E) The induced emf is twice as much.

Question 21

A loop of wire is rotated about a diameter (which is perpendicular to a given magnetic field.) In one revolution the induced current in the loop reverses direction how many times?

A) 4
B) 1
C) 0
D) 3
E) 2

Question 22

A horizontal metal bar rotates at a constant angular velocity w about a vertical axis through one of its ends while in a constant magnetic field B that is directed down. The emf induced between the two ends of the bar is

A) constant and proportional to the product B https://d2lvgg3v3hfg70.cloudfront.net/TB9720/https://d2lvgg3v3hfg70.cloudfront.net/TB9720/.
B) variable with a maximum of B ².
C) constant and proportional to the product B²11eea40d_1a70_2022_97f4_6f417475d6e1_TB9720_11².
D) variable with a maximum of B 11eea40c_fed2_1e41_97f4_31d4f10cfd73_TB9720_11.
E) constant and proportional to the product B 11eea40d_1a70_2022_97f4_6f417475d6e1_TB9720_11².

Question 23

A lightbulb is plugged into a household outlet, and one of the wires leading to it is wound into a coil. Now a slug of iron is slid into the coil. What effect will this have?

A) The lamp will get brighter because there will be an induced emf that will drive more current through the lamp.
B) The lamp will dim because a back emf will be produced by the coil, and this will reduce the current flowing in the coil and the lamp.
C) This will have no effect on the brightness of the lamp.
D) The lamp will dim because energy will now be dissipated in the iron.
E) This will not affect the lamp's brightness because the coil will shift the phase, but not the magnitude, of the current through the lamp.

Question 24

A transformer is a device that

A) increases the power level of a circuit.
B) operates only on AC.
C) operates on either DC or AC.
D) operates only on DC.

Question 25

Alternating current and voltage (AC), rather than direct current and voltage (DC), are used for domestic electrical power. The reason for this is based on one of the following physical principles. Which one?

A) Opposite electric charges attract each other, and like charges repel each other.
B) When the number of magnetic field lines passing through a circuit changes, a voltage is generated in the circuit.
C) When a current flows through a wire, a magnetic field is created.
D) When a current flows through a resistance, heat is generated.
E) When a current-carrying wire is placed in a magnetic field, it may experience a force if properly oriented.

Question 26

James Clerk Maxwell presented the "complete" theory of electromagnetism as how many equations?

A) 1
B) 4
C) 3
D) 2
E) 5

Question 27

The tail of a comet is primarily caused by

A) eddy currents.
B) nuclear magnetism.
C) radiation pressure.
D) Coulomb repulsion.
E) electromagnetic induction.

Question 28

Electromagnetic radiation of wavelength 3.5 meters is which type?

A) FM-radio
B) visible
C) ultraviolet
D) microwaves
E) AM-radio

Question 29

Electromagnetic radiation of wavelength 0.3 pico-meters is which type?

A) gamma rays
B) ultraviolet
C) FM-radio
D) visible
E) X-rays

Question 30

Electromagnetic radiation of wavelength 2.5 cm is which type?

A) microwaves
B) AM-radio
C) FM-radio
D) ultraviolet
E) visible

Question 31

Electromagnetic radiation of wavelength 200. nano-meters is which type?

A) ultraviolet
B) microwaves
C) AM radio
D) visible
E) X-rays

Question 32

A flux of 4 × 10^-5 Wb is maintained through a coil for 0.5 s. What emf is induced in this coil by this flux?

A) No emf is induced in this coil.
B) 4 × 10^-5 V
C) 6 × 10^-5 V
D) 2 × 10^-5 V
E) 8 × 10^-5 V

Question 33

The cross-sectional area of an adjustable single loop is reduced from 1.0 m² to 0.25 m² in 0.10 s. What is the average emf that is induced in this coil if it is in a region where B = 2.0 T upward, and the coil's plane is perpendicular to B?

A) 0 V
B) 15 V
C) 12 V
D) 21 V
E) 18 V

Question 34

The flux through a coil changes from 4 × 10^-5 Wb to 5 × 10^-5 Wb in 0.1 s. What emf is induced in this coil?

A) 3 × 10^-4 V
B) 5 × 10^-4 V
C) 7 × 10^-4 V
D) 1 × 10^-4 V
E) 4 × 10^-4 V

Question 35

A very long solenoid has 1.0 × 10⁶ turns per meter and a cross-sectional area of 2.0 × 10⁶ m². A short, secondary coil with 1.0 × 10³ turns is wound over the solenoid's midpoint. What emf is induced in the secondary coil if the current in the solenoid is changing at a rate of 4.0 A/s?

A) 1.6 V
B) 1.0 V
C) 4.4 V
D) 2.7 V
E) 0.52 V

Question 36

Suppose that the maximum rate of change of current in a "high power" transmission line is 10⁴ A/s. How large a coil (area) placed 10. meters from the line would have 120. volts (max) induced in it?

A) 2. × 10³ m²
B) 6. × 10³ m²
C) 6. × 10⁴ m²
D) 2. × 105³ m²
E) 6. × 10⁵ m²

Question 37

A generator coil rotates through one revolution 60 times each second. The frequency of the emf is

A) 30 Hz.
B) 60 Hz.
C) (60/ ) Hz.
D) 120 Hz.
E) 60 Hz.

Question 38

The coil of a generator has 50 loops and a cross-sectional area of 0.25 m². What is the maximum emf generated by this generator if it is spinning with an angular velocity of 4.0 rad/s in a 2.0 T magnetic field?

A) 400. V
B) 0. V
C) 50. V
D) 100. V
E) 75. V

Question 39

The coil of a generator rotates through one complete turn in a uniform magnetic field 50. times every second. When this generator is connected to an external load, the current through the load reverses directions

A) 100 times every second.
B) 50 times every second.
C) 200 times every second.
D) 25 times every second.
E) 75 times every second.

Question 40

An AC generator has 80. rectangular loops on its armature. Each loop is 12. cm long and 8.0 cm wide. The armature rotates at 1200. rpm about an axis parallel to the long side. If the loop rotates in a uniform magnetic field of 0.30 T, which is perpendicular to the axis of rotation, what will be the maximum output voltage of this generator?

A) 220 V
B) 124 V
C) 167 V
D) 120 V
E) 182 V

Question 41

A horizontal metal bar that is 2.0 m long rotates at a constant angular velocity of 2.0 rad/s about a vertical axis through one of its ends while in a constant magnetic field of 5 × 10^-5 T. If the magnetic field vector points straight down, what emf is induced between the two ends of the bar?

A) 2.0 × 10^-4 V
B) 3.6 × 10^-3 V
C) 5.6 × 10^-3 V
D) 7.0 × 10^-4 V
E) 1.6 × 10^-4 V

Question 42

The output of a generator is 440. V at 20. A. It is to be transmitted on a line with resistance of 0.60 ?. To what voltage must the generator output be stepped up with a transformer if the power loss in transmission is not to exceed 0.01% of the original power?

A) 4.4 × 10⁵ V
B) 4.4 kV
C) 44.5 kV
D) 22. kV
E) 72.7 kV

Question 43

The secondary coil of a neon sign transformer provides 7500. V at 0.010 A. The primary coil operates on 120. V. What current does the primary draw?

A) 1.66 A
B) 0.625 A
C) 0.16 A
D) 6.25 × 10^-4 A
E) 333. mA