Test bank chapter 21- Electromagnetic induction and Faraday's Law

1) All of the following are units of magnetic flux except A) T * m2 B) T/V*m C) weber D) V*S

B

List the three ways that an emf can be induced in a loop of wire

1. by changing the magnetic field 2. by changing the area of the loop in the field 3. by changing the loop's orientation with respect to the field

A circular coil lies flat on a horizontal table. A bar magnet is held above its center with its north pole pointing down. The stationary magnet induces (when viewed from above) A) no current in the coil. B) a clockwise current in the coil. C) a counterclockwise current in the coil. D) a current whose direction cannot be determined from the information given

A

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) Any horizontal orientation will do. B) It must make an angle of 45e to the vertical. C) It must be vertical. D) none of the given answers

A

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

A

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 w. B) constant and proportional to the product B w2. C) constant and proportional to the product B2 w2. D) none of the given answers

A

A horizontal rod (oriented in the east-west direction) is moved northward at constant velocity through a magnetic field that points straight down. Make a statement concerning the potential induced across the rod. A) The west end of the rod is at higher potential than the east end. B) The east end of the rod is at higher potential than the west end. C) The top surface of the rod is at higher potential than the bottom surface. D) The bottom surface of the rod is at higher potential than the top surface.

A

Doubling the diameter of a loop of wire produces what kind of change on the induced emf, assuming all other factors remain constant? A) The induced emf is 4 times as much. B) The induced emf is twice times as much. C) The induced emf is half as much. D) There is no change in the induced emf

A

In a transformer, if the secondary coil contains more loops than the primary coil then it is a A) step-up transformer. B) step-down transformer.

A

State Faraday's law

A changing magnetic field induces an emf.

A coil lies flat on a horizontal table top in a region where the magnetic field points straight down. The magnetic field disappears suddenly. When viewed from above, what is the direction of the induced current in this coil as the field disappears? A) counterclockwise B) clockwise C) clockwise initially, then counterclockwise before stopping D) There is no induced current in this coil.

B

A coil lies flat on a level table top in a region where the magnetic field vector points straight up. The magnetic field suddenly grows stronger. When viewed from above, what is the direction of the induced current in this coil as the field increases? A) counterclockwise B) clockwise C) clockwise initially, then counterclockwise before stopping D) There is no induced current in this coil.

B

A generator coil rotates through 60 revolutions each second. The frequency of the emf is A) 30 Hz. B) 60 Hz. C) 120 Hz. D) cannot be determined from the information given

B

A resistor and an inductor are connected in series to a battery. The battery is suddenly removed from the circuit. The time constant for of the circuit represents the time required for the current to decrease to A) 25% of the original value. B) 37% of the original value. C) 63% of the original value. D) 75% of the original value.

B

A transformer is a device that A) operates on either DC or AC. B) operates only on AC. C) operates only on DC.

B

An electric generator transforms A) electrical energy into mechanical energy. B) mechanical energy into electrical energy. C) direct current into alternating current. D) alternating current into direct current.

B

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

B

Doubling the strength of the magnetic field through a loop of wire produces what kind of change on the induced emf, assuming all other factors remain constant? A) The induced emf is 4 times as much. B) The induced emf is twice as much. C) The induced emf is half as much. D) There is no change in the induced emf

B

In a transformer, if the primary coil contains more loops than the secondary coil then it is a A) step-up transformer. B) step-down transformer

B

In a transformer, the power input A) is larger than the power output. B) is equal to the power output. C) is smaller than the power output. D) can be either larger or smaller than the power output.

B

A bar magnet falls through a loop of wire with the north pole entering first. As the north pole enters the wire, the induced current will be (as viewed from above) A) zero. B) clockwise. C) counterclockwise. D) to top of loop.

C

A circular coil lies flat on a horizontal table. A bar magnet is held above its center with its north pole pointing down, and released. As it approaches the coil, the falling magnet induces (when viewed from above) A) no current in the coil. B) a clockwise current in the coil. C) a counterclockwise current in the coil. D) a current whose direction cannot be determined from the information provided

C

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 zero? A) Any horizontal orientation will do. B) It must make an angle of 45e to the vertical. C) It must be vertical. D) none of the given answers

C

A resistor and an inductor are connected in series to a battery. The time constant for the circuit represents the time required for the current to reach A) 25% of the maximum current. B) 37% of the maximum current. C) 63% of the maximum current. D) 75% of the maximum current.

C

A resistor is connected to an AC power supply. On this circuit, the current A) leads the voltage by 90e. B) lags the voltage by 90e. C) is in phase with the voltage. D) none of the given answers

C

A transformer is a device used to A) transform an alternating current into a direct current. B) transform a direct current into an alternating current. C) increase or decrease an ac voltage. D) increase or decrease a dc voltage

C

Faraday's law of induction states that the emf induced in a loop of wire is proportional to A) the magnetic flux. B) the magnetic flux density times the loop's area. C) the time variation of the magnetic flux. D) current divided by time.

C

A long straight wire lies on a horizontal table and carries an ever-increasing current northward. Two coils of wire lie flat on the table, one on either side of the wire. When viewed from above, the induced current circles A) clockwise in both coils. B) counterclockwise in both coils. C) clockwise in the east coil and counterclockwise in the west coil. D) counterclockwise in the east coil and clockwise in the west coil

D

A wire moves across a magnetic field. The emf produced in the wire depends on A) the strength of the magnetic field. B) the length of the wire. C) the orientation of the wire with respect to the magnetic field vector. D) all of the given answers

D

According to Lenz's law, the direction of an induced current in a conductor will be that which tends to produce which of the following effects? A) enhance the effect which produces it B) produce a greater heating effect C) produce the greatest voltage D) oppose the effect which produces it

D

As a coil is removed from a magnetic field an emf is induced in the coil causing a current to flow within the coil. This current interacts with the magnetic field producing a force which A) acts at right angles to the coil's motion. B) acts in the direction of the coil's motion. C) causes the coil to tend to flip over. D) acts in the direction opposite to the coil's motion

D

State Lenz's Law

The magnetic field generated by the induced current will be in the direction opposite the change in magnetic flux

Define magnetic flux

measure of the strength of the magnetic field running through a loop of wire

Define emf

what causes electrons to move and produce a current (if you produce and emf, you produce a current)