3.2C Electromagnetic Induction

3.2C Electromagnetic Induction & Waves

QLD SYllabus

Multiple Choice

A conducting bar slides with a velocity v to the right on some conducting rails as shown. A uniform magnetic field is directed perpendicular to the bar. Answer the following two questions: (1) Will the light bulb “turn on” in this case? and (2) How is the magnetic flux through the conducting loop changing, if at all, as the bar slides to the right?

• no, the magnetic flux remains constant

• no, the magnetic flux decreases

• no, the magnetic flux increases

• yes, the magnetic flux increases

• yes, the magnetic flux remains constant

Multiple Choice

Which of the following choices is the SI unit for magnetic flux?

• gauss (G)

• tesla (T)

• weber (Wb)

• Lumen (L)

• Fluxon (Fl)

Multiple Choice

Which of the flowing expressions equals the magnetic flux?

• $BA\cos\phi$

• $BA\sin\phi$

• $B\cos^2\phi$

• $\frac{B\cos\phi}{A}$

Multiple Choice

Which of the following phrases best describe the term magnetic flux?

• the direction of the magnetic field relative to a surface

• the amount of magnetic field that passes through a surface

• the number of magnetic dipoles moving through a wire

• Choice (a) and (b) are both correct.

Multiple Choice

A coil of wire with N turns and area A is placed into a magnetic field of magnitude B.  The angle of the normal to the plane of the coil is at an angle  $\phi$  with respect to the magnetic field.  According the Faraday’s law, which of the following changes will produce an emf in a coil of wire?

• B is decreased

• A is increased

•  $\phi$  is decreased

• any of the above choices

Multiple Choice

Complete the following sentence: In a coil of wire, the direction of the induced current caused by an increasing magnetic flux is

• directed in the direction of the original magnetic field.

• directed opposite to the original magnetic field.

• such that the induced magnetic field decreases the magnetic flux.

• such that the induced magnetic field increases the magnetic flux.

Multiple Choice

Complete the following statement: Faraday’s law indicates that a changing magnetic field produces

• an electric field.

• an induced magnetic field.

• a force field.

• light.

Lenz's Law

States that the direction of the electric current which is induced in a conductor by a changing magnetic field is such that the magnetic field created by the induced current opposes the initial changing magnetic field.

Multiple Choice

Lenz’s law is a consequence of what other physical law?

• Newton’s first law

• conservation of energy

• Newton’s third law

• Gauss’ law

Multiple Choice

A permanent magnet is moved toward and away from a solenoid with a frequency of 60 Hz. The ends of the solenoid are connected in series with a light bulb. Which one of the following statements concerning an induced current, if any, in the loop is true?

• An alternating current will be induced in the

  circuit that is clockwise when the magnet

  moves to the right and counterclockwise when

  the magnet moves to the left.

• An alternating current will be induced

  in the circuit that is clockwise when the

  magnet moves to the left and counterclockwise when the magnet moves to the right.

• An direct current will be induced in the circuit that is clockwise as the magnet oscillates to the left and to the right.

• No current will be induced in the circuit by using this method.

Multiple Choice

Consider the situation shown. A triangular, aluminum loop is slowly moving to the right. Eventually, it will enter and pass through the uniform magnetic field region represented by the tails of arrows directed away from you. Initially, there is no current in the loop. When the loop is entering the magnetic field, what will be the direction of any induced current present in the loop?

• clockwise

• counterclockwise

• No current is induced.

Multiple Choice

Consider the situation shown. A triangular, aluminum loop is slowly moving to the right. Eventually, it will enter and pass through the uniform magnetic field region represented by the tails of arrows directed away from you. Initially, there is no current in the loop. When the loop is exiting the magnetic field, what will be the direction of any induced current present in the loop?

• clockwise

• counterclockwise

• No current is induced.

Multiple Choice

A rigid, circular metal loop begins at rest in a uniform magnetic field directed away from you as shown. The loop is then pulled through the field toward the right, but does not exit the field. What is the direction of any induced current within the loop?

• clockwise

• counterclockwise

• No current is induced.

Multiple Choice

The ac adapter for a laptop computer contains a transformer. The input of the adapter is the 120 volts from the ac wall outlet. The output from the transformer is 20 volts. What is the turns ratio of the transformer?

• 0.17

• 6

• 100

• This cannot be determined without knowing how many turns one of the coils in the transformer has.

Multiple Choice

Transformer A has a primary coil with 400 turns and a secondary coil with 200 turns. Transformer B has a primary coil with 400 turns and a secondary coil with 800 turns. The same current and voltage are delivered to the primary coil of both transformers. The secondary coils are connected to identical circuits. How does the power delivered by secondary coil A compare to the power delivered by secondary coil B?

• Secondary coil A delivers one fourth the power delivered by secondary coil B

• Secondary coil A delivers one half the power delivered by secondary coil B

• Secondary coil A delivers the same power as delivered by secondary coil B

• Secondary coil A delivers twice the power delivered by secondary coil B

Multiple Choice

In a well-designed transformer with an iron core, the secondary coil has twice as many turns as the primary coil. Which one of the following statements concerning this transformer is true?

• The flux that passes through each turn of the primary coil is twice the flux that passes through the secondary coil.

• The effect of the iron core is to reduce the magnetic field passing through the coils.

• The induced emf generated in the secondary coil is twice as large as that generated in the primary coil.

• This is a step down transformer because the current in the secondary coil is less than that in the primary coil.

Multiple Choice

The drawing shows an x, y, z coordinate system. A circular loop of wire lies in the z, x plane when used with an AC circuit, produces an electromagnetic wave. Which one of the following statements is correct?

• The wave travels along the x axis, and its electric field oscillates along the y axis.

• The wave travels along the z axis, and its electric field oscillates along the x axis.

• The wave travels along the z axis, and its electric field oscillates along the y axis.

• The wave travels along the y axis, and its electric field oscillates along the x axis.

Multiple Choice

An electromagnetic wave travels in a vacuum. The wavelength of the wave is tripled. How is this accomplished?

• By tripling the frequency of the wave

• By tripling the speed of the wave

• By reducing the frequency of the wave by a factor of three

• By reducing the speed of the wave by a factor of three

Multiple Choice

Which one of the following will not generate electromagnetic waves or pulses?

• a steady direct current

• an accelerating electron

• an alternating current

• charged particles traveling in a circular path in a mass spectrometer

Multiple Choice

Which one of the following statements concerning the wavelength of an electromagnetic wave in a vacuum is true?

• The wavelength is independent of the speed of the wave for a fixed frequency.

• The wavelength is inversely proportional to the speed of the wave.

• The wavelength is the same for all types of electromagnetic waves.

• The wavelength is inversely proportional to the frequency of the wave.

Multiple Choice

Note the different types of electromagnetic radiation:

(1) X-rays (4) visible light (2) radio waves (5) infrared radiation (3) gamma rays (6) ultraviolet radiation.

Which list correctly ranks the electromagnetic waves in order of increasing frequency?

• 2, 3, 4, 5, 6, 1

• 2, 5, 4, 1, 6, 3

• 2, 5, 4, 6, 1, 3

• 3, 1, 6, 4, 5, 2