PHY 131 Exam 3

The units T·m2/s are equivalent to: a. W. b. V. c. N/m. d. webers.

b. V.

In an RLC series circuit, if the AC frequency is increased to a very large value, what value does the phase angle between the current and voltage approach? a. 90° b. 0° c. -90° d. 45°

a. 90°

A straight wire lies along the y-axis initially carrying a current of 10 A in the positive y-direction. The current decreases and reverses to 10 A in the negative y-direction, the change in current happening at a uniform rate. In the 1st quadrant a square conducting coil has 2 sides parallel to the y-axis and the other 2 sides parallel to the x-axis. The side of the coil nearest and parallel to the straight wire is at a distance equal to the length of one of the sides of the square. As the current is going from 10 A in one direction to 10 A in the other, in which direction is the induced current in this side of the square coil nearest to the straight wire? a. It is in the positive y-direction. b. It is in the negative y-direction. c. At first it is in the positive y-direction, but after the current passes through zero, it is in the negative y- direction. d. At first it is in the negative y-direction but after the current passes through zero, it is in the positive y-direction.

a. It is in the positive y-direction.

How is the energy stored in a current-carrying inductor related to the current value, I? a. directly proportional to I 2 b. directly proportional to I 1/2 c. directly proportional to I d. inversely proportional to I

a. directly proportional to I 2

Three loops of wire, one circular, one rectangular, and one square, are made from identical lengths of wire. If the loops are in the same increasing magnetic field perpendicular to the plane of the coils, which loop has the greatest induced emf? a. the circular one b. the rectangular one c. the square one d. All three would have the same emf induced.

a. the circular one

Electricity may be generated by rotating a loop of wire between the poles of a magnet. The induced current is greatest when a. the plane of the loop is parallel to the magnetic field. b. the plane of the loop is perpendicular to the magnetic field. c. the magnetic flux through the loop is a maximum. d. the plane of the loop makes an angle of 45° with the magnetic field.

a. the plane of the loop is parallel to the magnetic field.

In an AC series circuit, the current in a pure resistor differs in phase with the applied voltage by what angle? a. zero b. 45° c. 90° d. 180°

a. zero

In an AC circuit, the ratio of average current to maximum current is: a. zero. b. 0.5. c. 0.707. d. 1.0.

a. zero.

An RLC circuit is connected across an AC power source. How are the maximum and rms voltages from the power source related? a. ΔVmax = square root of 2 x ΔVrms b. ΔVmax = ΔVrms/ square root of 2 c. ΔVmax = 2ΔVrms d. ΔVmax = ΔVrms/2

a. ΔVmax = square root of 2 x ΔVrms

By what factor is the self-inductance of an air solenoid changed if its length and number of coil turns are both multiplied by seven? a. 1/7 b. 7 c. 1 d. 49 e. 1/49

b. 7

By what factor is the self-inductance of an air solenoid changed if only its cross-sectional area, A, multiplied by eight? a. 1/8 b. 8 c. 24 d. 64 e. 1/64

b. 8

Transmitting power at high voltage, e.g., 400,000 V, reduces the power loss for transmission lines. But such a voltage would destroy electrical devices in household use. How then can such 400,000 V electricity be used in a normal house? a. The ground prongs on household plugs reduce the voltage to 120 V. b. Before reaching households, transformers are used to reduce the voltage appropriately. c. No matter the incoming voltage, a 120-V device only lets 120 V in. d. The 400,000 V is just theoretical; long distance transmission lines actually use 120 V.

b. Before reaching households, transformers are used to reduce the voltage appropriately.

The electric field, E, in an electromagnetic wave is oriented in what direction with respect t its associated magnetic field, B? a. E antiparallel to B b. E perpendicular to B c. E at a 45 degree angle to B d. E parallel to B

b. E perpendicular to B

A bar magnet is held above the center of a conducting ring in the horizontal plane. The magnet is dropped so it falls lengthwise toward the center of the ring. Will the falling magnet be attracted toward the ring or be repelled by the ring due to the magnetic interaction of the magnet and the ring? a. It will be attracted. b. It will be repelled. c. It will be attracted only if the north end of the magnet is the leading end as it falls toward the ring. d. It will be attracted only if the south end of the magnet is the leading end as it falls toward the ring.

b. It will be repelled.

A bar magnet is held stationary while a circular loop of wire is moved toward the magnet at constant velocity at position A as in the figure. The loop passes over the magnet's center at position B and moves away from the magnet at position C. Viewing the loop from the left as indicated in the figure, for which position is the induced current zero? a. Position A b. Position B c. Position C d. The induced current is never zero.

b. Position B

A circular loop is suspended in a uniform magnetic field as shown in the figure. If the loop area increases, what affect does this have on the induced current? a. The induced current is clockwise. b. The induced current is counterclockwise. c. There is no induced current. d. We would need to know the magnitude of the magnetic field.

b. The induced current is counterclockwise.

Which condition of motion must be met with regard to a charged particle if it is in the process of emitting electromagnetic radiation? a. moves at constant velocity b. accelerates c. moves at the speed of light d. moves parallel to a uniform magnetic field

b. accelerates

The rms current is equal to the direct current that: a. produces the same average voltage across a resistor as in an AC circuit. b. dissipates an equal amount of energy in a resistor at the same rate as in an AC circuit. c. provides the same average current in a resistor as in an AC circuit. d. results in the same peak power in a resistor as in an AC circuit.

b. dissipates an equal amount of energy in a resistor at the same rate as in an AC circuit.

The function of the electric motor is which one of the following conversion processes? a. mechanical energy to electrical b. electrical energy to mechanical c. low voltage to high or vice versa d. alternating current to direct

b. electrical energy to mechanical

When an RLC series circuit is in resonance, its impedance is: a. zero. b. equal to its resistance. c. a maximum. d. π/2 Ω.

b. equal to its resistance.

The magnet moving past an object will produce eddy currents in the object if the object: a. is magnetic material only. b. is a conductor. c. is an insulator. d. is a liquid.

b. is a conductor.

A straight wire of length L is oriented east-west and is in a magnetic field B pointing north. The wire is moving downward at a constant speed v. Which end of the rod is positively charged? a. neither b. the east end c. the west end d. both ends

b. the east end

An RLC circuit is connected across an AC power source. Which statement is correct? a. The voltage across the inductor lags the current by 90°. b. The voltage across the capacitor is in phase with the current. c. At resonance, the phase angle f between the current and the voltage is 0. d. None of the above statements are correct.

c. At resonance, the phase angle f between the current and the voltage is 0.

How is the energy stored in a current-carrying inductor related to its self-inductance, L? a. directly proportional to L2 b. directly proportional to L1/2 c. directly proportional to L d. inversely proportional to L

c. directly proportional to L

6. If the induced current in a wire loop were such that the flux it produces were in the same direction as the change in external flux causing the current, which of the following conservation laws would end up being violated? a. momentum b. charge c. energy d. angular momentum

c. energy

An ideal transformer is one that: a. has a turn ratio, N2/N1, equal to 1. b. works with direct current. c. experiences no power loss. d. has an output frequency of 60 Hz.

c. experiences no power loss.

Which of the following combinations of circuit components can be used to make a tuner for a radio, to select the desired frequency? a. fixed inductor, variable resistor b. fixed resistor, variable inductor c. fixed inductor, variable capacitor d. fixed capacitor, variable resistor

c. fixed inductor, variable capacitor

In an LCR series circuit with XL > XC, the phase angle between the current and voltage phasors is 30°. If the AC frequency doubles, what is the magnitude of the resulting phase angle? a. 30° b. less than 30° c. more than 30° d. Additional information is needed to reach a determination.

c. more than 30°

The operation of an electric motor depends on which of the following effects? a. the Doppler effect b. the photoelectric effect c. the force acting on a current-carrying wire in a magnetic field d. current from the motion of a wire in a magnetic field

c. the force acting on a current-carrying wire in a magnetic field

When a voltage is generated by rotating a coil in a magnetic field at a constant rate, the period of the voltage equals the time that it takes for the coil to rotate through ________ radians. a. 1 b. π/2 c. π d. 2π

d. 2π

Consider an AC series circuit containing a coil, capacitor and resistance. Tripling the frequency will change the circuit's impedance by what factor? a. 1/3 b. 1.0 c. 3 d. Information is insufficient to determine answer.

d. Information is insufficient to determine answer.

A changing current in one loop of wire can induce a current in another loop of wire even though the wires do not touch. The induced current appears almost instantaneously for loops close together. How can this energy exchange happen? a. This is one of those things for which science has no adequate explanation yet. b. The second loop is affected by the thermal exchange from the heating of the first loop. c. It is a gravitational effect on the electrons. d. It is done via an electromagnetic wave.

d. It is done via an electromagnetic wave.

In an RLC circuit, the maximum current is 1 amp. What is the average current? a. square root of 2 times A b. (1 divided by square root)A c. (1/2) A d. None of the above.

d. None of the above.

An inductor, battery, resistance, and ammeter and switch are connected in series. If the switch, initially open, is now closed, what is the current's final value? a. zero b. battery voltage divided by inductance c. battery voltage times inductance d. battery voltage divided by resistance

d. battery voltage divided by resistance

An electromagnetic wave is made up of which of the following oscillating quantities? a. electrons only b. electric fields only c. magnetic fields only d. electric and magnetic fields

d. electric and magnetic fields

How is the direction of propagation of an electromagnetic wave oriented relative to the associated E and B fields? a. parallel to B, perpendicular to E b. parallel to E, perpendicular to B c. parallel to both E and B d. perpendicular to both E and B

d. perpendicular to both E and B

In a circuit made up of inductor L, resistance R, ammeter, battery and switch in series, the current is greatest at which of the following times, as measured after the switch is closed? a. zero b. one time constant c. atatimet=L/R d. ten time constants

d. ten time constants

Two loops of wire are arranged so that a changing current in one, the primary, will induce a current in the other, the secondary. The secondary loop has twice as many turns as the primary loop. As long as the current in the primary is steady at 3.0 A, the current in the secondary will be a. 3.0 A. b. 6.0 A. c. 1.5 A. d. zero.

d. zero.