#### Physics Test 2 - Conceptual Chapter 21

All of the following are units of magnetic flux except

T/V∙m.

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

the time variation of the magnetic flux.

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?

The induced emf is twice times as much.

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?

The induced emf is twice as much.

Doubling the diameter of a loop of wire produces what kind of change on the induced emf, assuming all other factors remain constant?

The induced emf is 4 times as much.

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

acts in the direction opposite to the coil's motion.

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?

oppose the effect which produces it

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)

no current in the coil.

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 counterclockwise current in the coil.

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?

The induced current flows counterclockwise.

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?

clockwise

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?

clockwise

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

counterclockwise in the east coil and clockwise in the west coil.

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)

counterclockwise.

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?

It must be vertical.

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?

Any horizontal orientation will do.

A changing magnetic field induces an emf.

State Lenz's law

A current produced by an induced emf moves in a direction so that its magnetic field opposes the original change in flux.

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

1. by changing the magnetic field2. by changing the area of the loop in the field3. by changing the loop's orientation with respect to the field

A wire moves across a magnetic field. The emf produced in the wire depends on

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.

The west end of the rod is at higher potential than the east end.

A horizontal metal bar rotates at a constant angular velocity ω 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

constant and proportional to the product B ω.

An electric generator transforms

mechanical energy into electrical energy.

A generator coil rotates through 60 revolutions each second. The frequency of the emf is

60 Hz.

A transformer is a device used to

increase or decrease an ac voltage.

A transformer is a device that

operates only on AC.

In a transformer, if the secondary coil contains more loops than the primary coil then it is a

step-up transformer.

In a transformer, if the primary coil contains more loops than the secondary coil then it is a

step-down transformer.

In a transformer, the power input

is equal to the power output.

In a given LC resonant circuit

all of the given answers are possible.

A resistor and an inductor are connected in series to an ideal battery of constant terminal voltage. At the moment contact is made with the battery, the voltage across the resistor is

zero.

A resistor and an inductor are connected in series to an ideal battery of constant terminal voltage. At the moment contact is made with the battery, the voltage across the inductor is

equal to the battery's terminal voltage.

A series RL circuit with inductance L and resistance R is connected to an emf V. After a period of time, the current reaches a final value of 2.0 A. A second series circuit is identical except that the inductance is 2L. When it is connected to the same emf V, what will be the final value of the current?

2.0 A

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

63% of the maximum current.

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

37% of the original value.

All of the following have the same units except:

inductance.

A resistor is connected to an AC power supply. On this circuit, the current

is in phase with the voltage.

A pure inductor is connected to an AC power supply. In this circuit, the current

lags the voltage by 90°.

The inductive reactance in an ac circuit changes by what factor when the frequency is tripled?

3

If the frequency of the AC voltage across an inductor is doubled, the inductive reactance of that inductor

increases to twice its original value.

As the frequency of the AC voltage across an inductor approaches zero, the inductive reactance of that coil

approaches zero.

A pure capacitor is connected to an AC power supply. In this circuit, the current

The capacitive reactance in an ac circuit changes by what factor when the frequency is tripled?

1/3

As the frequency of the AC voltage across a capacitor approaches zero, the capacitive reactance of that capacitor

approaches infinity.

What is the phase angle between the voltages of the inductor and capacitor in a RLC series circuit?

180°

The power factor of an RLC circuit is defined as cos ∅ = R/Z, but the phase angle can also be calculated from

sin ∅ = (XL - XC)/Z

Consider an RLC circuit. The impedance of the circuit increases if R increases. When is this statement true?

always true

Consider an RLC circuit. The impedance of the circuit increases if XL increases. When is this statement true?

true only if XL is greater than or equal to XC

Consider an RLC series circuit. The impedance of the circuit increases if XC increases. When is this statement true?

true only if XL is less than or equal to XC

If the inductance and the capacitance both double in an LRC series circuit, the resonant frequency of that circuit will

decrease to one-half its original value.

Consider an RLC circuit that is driven by an AC applied voltage. At resonance,

the current is in phase with the driving voltage.

Resonance in a series RLC circuit occurs when

XC equals XL.