30 Questions

.

Questions and Answers

- 1.A charged capacitor is being discharged through a resistor. At the end of one time constant the charge has been reduced by (1-1/e)=63% of its initial value. At the end of two time constants the charge has been reduced by what percent of its initial value?
- A.
82%

- B.
100%

- C.
86%

- D.
96%

- 2.The switch S is initially at position a for a long time. It is then switched to position b. Describe what happens to the light bulb as a function of time when the switch is flipped from a to b?
- A.
The light bulb was off. It then lights up but the brightness decreases with time and eventually goes off.

- B.
The light bulb goes off immediately

- C.
The light bulb goes off and stays off

- D.
The light bulb was on and stays on

- 3.A capacitor in a flash camera is charged by a 1.5-V battery. When the camera flashes, this capacitor is discharged through an 8.3 kΩ resistor. The time constant of the circuit is 10-ms. What is the value of the capacitance?
- A.
120 µF

- B.
3.2 µF

- C.
0.83 µF

- D.
1.2 µF

- 4.A device used chiefly for storing energy in a magnetic field is
- A.
Resistor

- B.
Inductor

- C.
Capacitor

- D.
Galvanometer

- 5.A series circuit consists of a 40.0-V battery, a 40.0 Ω resistor, a 2.00 H inductor and an open switch . At t=0 s the switch is closed. At some time later the current through the resistor is 0.400 A. At this time, at what rate is energy being stored in the magnetic field of the inductor?
- A.
9.6 W

- B.
10.2 W

- C.
1.80 W

- D.
13.8 W

- 6.A series circuit consists of a battery, a light bulb and a switch. The time constant for the RL circuit is of the order of a few seconds. What happens to the light bulb when the switch S is closed?
- A.
The light comes on immediately then gradually dims

- B.
The light does not come on at all

- C.
The light comes on in a few seconds then goes off

- D.
The light comes on gradually in a few seconds and stays on

- 7.A charged capacitor and an inductor are connected in series. At time t=0 the current is zero, but the capacitor is charged. If T is the period of the resulting oscillations, the next time, after t=0 that the current is a maximum is:
- A.
T/2

- B.
T/4

- C.
3T/4

- D.
2T

- 8.The rapid decay of charge in just a few cycles on the capacitor in an undriven RLC circuit might be due to:
- A.
A large resistance

- B.
A large capacitance

- C.
A small capacitance

- D.
A small resistance

- 9.Which of the following statements contradicts one of Maxwell's equations?
- A.
A changing magnetic field produces an electric field

- B.
The net magnetic flux through a closed surface depends on the current inside

- C.
The net electric flux through a closed surface depends on the charge inside

- D.
None of these statements contradict Maxwell's equations

- 10.Two of Maxwell's equations contain a path integral on the left side and an area integral on the right. For them:
- A.
The path must pierce the area

- B.
The path must be well-separated from the area

- C.
The path must lie in the area, away from its boundary

- D.
The path must be the boundary of the area

- 11.A magnetic field exists between the plates of a capacitor:
- A.
While the capacitor is being charged or discharged

- B.
Always

- C.
Never

- D.
Only when the capacitor is fully charged

- 12.In each of the following operations, energy is expended. The LEAST percentage of returnable electrical energy will be yielded by:
- A.
Sending a current through an inductor

- B.
Charging a capcaitor

- C.
Charging a battery

- D.
Sending a current through a resistor

- 13.A coil has a resistance of 80 Ω and an impedance of 100 Ω. Its reactance in Ω is:
- A.
60

- B.
40

- C.
117

- D.
20

- 14.In an oscillating LC circuit, the total stored energy is U and the maximum charge on the capacitor is Q. When the charge on the capacitor is Q/2, the energy stored in the inductor is:
- A.
U/4

- B.
3U/4

- C.
U/2

- D.
3U/2

- 15.An RL series circuit is connected to an emf source having angular frequency w. The current:
- A.
Leads the source emf by π/4

- B.
Leads the source by emf by tan

^{-1}(R/wL) - C.
Lags the source emf by tan

^{-1}(R/wL) - D.
Lags the source emf by tan

^{-1 }(wL/R)

- 16.In a sinusoidally driven series RLC circuit leads the applied emf. The rate at which energy is dissipated in the resistor can be increased by:
- A.
Increasing the capacitance and making no other changes

- B.
Decreasing the capacitance and making no other changes

- C.
Decreasing the inductance and making no other changes

- D.
Decreasing the driving frequency and making no other changes

- 17.An electromagnetic wave is generated by:
- A.
Any moving charge

- B.
Only a charge with changing direction

- C.
Only a charge with a changing acceleration

- D.
Any accelerating charge

- 18.An electromagnetic wave is transporting energy in the positive y direction. At one point and one instant the magnetic field is in the positive x direction. The electric field at that point and instant is:
- A.
Positive z direction

- B.
Negative y direction

- C.
Positive y direction

- D.
Negative x direction

- 19.The maximum E-component of an electromagnetic wave is 600 V/m, what is the maximum B-component?
- A.
1.4 T

- B.
2.0*10

^{-6}T - C.
1.0*10

^{-3 }T - D.
1.6*10

^{-10 }T

- 20.A 1.2-m radius cylindrical region contains a uniform electric field that is increasing uniformly with time. At t=0 the field is 0 and at t=5.0 seconds the field is 200 V/m. The total displacement current through a cross section of the region is:
- A.
4.5*10

^{-16}A - B.
3.5*10

^{-10}A - C.
8.0*10

^{-9}A - D.
1.6*10

^{-9}A

- 21.A capacitor in an LC oscillator has a maximum potential difference of 16-V and a maximum energy of 360 µJ. At that instant what is the emf induced in the inductor?
- A.
10 V

- B.
15 V

- C.
20 V

- D.
0 V

- 22.You connect a 100 Ω resistor, an 800 mH inductor, and a 10.0 µF capacitor in series across a 60.0 Hz, 120 V (peak) source. The maximum current in the circuit is approximately
- A.
1.59 A

- B.
1.20 A

- C.
1.12 A

- D.
1.69 A

- 23.You connect a 100 Ω resistor,, an 800 mH inductor, and a 10.0 µF capacitor in series across 60.0 Hz, 120 V (peak) source. The resonant frequency of your circuit is approximately
- A.
56 Hz

- B.
37 Hz

- C.
354 Hz

- D.
60.0 Hz

- 24.The primary of an ideal transformer has 100 turns and the secondary has 500 turns. Then:
- A.
The power in the primary circuit is less than that in the secondary circuit

- B.
The primary current is five times the secondary current

- C.
The primary voltage is five times the secondary voltage

- D.
The frequency in the secondary circuit is five times that in the primary current

- 25.Radio receivers are usually tuned by adjusting the capacitance of an LC circuit. If C=C
_{1}allows you to receive a 1600 kHz station then for a 800 kHz station you must adjust C to:- A.
4C

_{1} - B.
2C

_{1} - C.
C

_{1}/4 - D.
C

_{1}/2

- 26.A battery is connected to a series combination of a switch, a resistor, and an initially uncharged capacitor. The switch is closed at t=0. Which of the following statements is true?
- A.
As the charge on the capacitor increases, the voltage drop across the resistor decreases

- B.
As the charge on the capacitor increases, the current increases

- C.
As the charge on the capacitor increases, the voltage drop across the resistor increases

- D.
As the charge on the capacitor increases, the current remains constant

- 27.The switch S is initially at b for a long time. It is then moved to a. Describe what happens to the light bulb.
- A.
The light was on but goes out immediatley

- B.
The light was out and stays out

- C.
The light was on but goes out gradually

- D.
The light was out. It goes on but decreases in brightness gradually and goes out

- 28.An uncharged capacitor and a resistor are connected in series to a battery. If ε=15 V, C= 20µF, and R=4.0*10
^{5}Ω, the time constant of the circuit is approximately- A.
10 seconds

- B.
8 seconds

- C.
18 seconds

- D.
2.5 seconds

- 29.A 1.2 µF capacitor in a flash camera is charged by a 1.5 V battery. When the camera flashes, this capacitor is discharged through a resistor. The time constant of the circuit is 10 ms. What is the value of the resistance?
- A.
0.12 kΩ

- B.
8.0*10

^{-2}kΩ - C.
3.1 kΩ

- D.
8.3 kΩ

- 30.A series circuit constant of a 40.0 V battery, a 20.0 Ω resistor, a 2.00 H inductor and an open switch. At t=0 seconds the switch is closed. At some time later the current through the resistor is 0.300 A. At this time, at what rate is energy being stored in the magnetic field of the inductor?
- A.
10.2 W

- B.
0.090 W

- C.
1.80 W

- D.
13.8 W