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### Transcript of Practice Questions Exam3 Phys 202

Physics 202 Spring 2010 Practice Questions for Exam 3 Note: Answer key is at end 1. For the two solenoids above, if l = 50 cm, N1 = N2 = 200 turns and r1 = 5 cm and r2 = 10 cm, the mutual inductance of the two solenoids is

A) 1.58 mH B) 0.790 mH C) 3.20 mH D) 6.31 mH E) None of these is correct. 2. A region of space contains a magnetic field of 500 G and an electric field of 3 ! 106 N/C. The magnetic energy density in a cubical box of side ! = 20 cm in this region is A) 550 J/m3 B) 670 J/m3 C) 864 J/m3 D) 995 J/m3 E) None of these is correct. 3. An LR circuit has a resistance R = 25 ", an inductance L = 5.4 mH, and a battery of EMF = 9.0 V. How much energy is stored in the inductance of this circuit when a steady current is achieved? A) zero B) 0.35 J C) 0.35 mJ D) 0.70 mJ E) 0.97 mJ

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4. What is the time constant of an RL circuit with a resistance R = 25 " and an inductance L = 5.4 mH? A) 7.4 s B) 4.6 s C) 0.14 s D) 0.22 ms E) 1.5 ms 5. An open switch in an RL circuit is closed at time t = 0, as shown. The curve that best illustrates the variation of current with time is

A) 1 B) 2 C) 3 D) 4 E) 5 6. The growth of current in the inductive circuit in the inset diagram is represented by the curve in the graph. The broken line is tangent to the curve at the origin. The time constant of the circuit is approximately

A) 3.0 ms B) 2/R ms C) 0.40 ms D) 4.0 ms E) 2.0 ms

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7. The current in an RL circuit is zero at time t = 0 and increases to half its final value in 6.0 s. The time constant of this circuit is approximately A) 6.0 s B) 12 s C) 8.7 s D) 3.0 s E) 9.8 s 8. In this circuit, the current I through the battery is approximately

A) 1.7 A B) 4.4 A C) 0.36 A D) 0.60 A E) 3.4 A 9. A 1.2-F 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) 8.0 ! 102 " B) 0.12 " C) 12 k" D) 8.3 k" E) 3.1 k"

10. A 20-F capacitor is charged to 200 V and is then connected across a 1000-" resistor. What is the initial charge on the capacitor? A) 10 mC B) 5.6 mC C) 4.0 mC D) 8.0 mC E) 2.0 mC

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11. A 20-F capacitor is charged to 200 V and is then connected across a 1000-" resistor. The time constant for this circuit is approximately A) 20 ms B) 40 ms C) 10 ms D) 30 ms E) 15 ms

12. A 20-F capacitor is charged to 200 V and is then connected across a 1000-" resistor. The amount of charge on the capacitor after 10 ms is approximately A) 14 mC B) 2.4 mC C) 0.80 mC D) 1.6 mC E) 3.2 mC

13. A 0.120-F capacitor, initially uncharged, is connected in series with a 10.0-k" resistor and a 12.0-V battery of negligible internal resistance. The charge on the capacitor after a very long time is approximately A) 28.8 C B) 14.4 C C) 144 C D) 2.88 C E) 1.44 C

14. An uncharged capacitor and a resistor are connected in series to a battery as shown. If # = 15 V, C = 20 F, and R = 4.0 ! 105 ", the time constant of the circuit is approximately A) 10 s B) 8.0 s C) 18 s D) 4.0 s E) 2.5 s

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15. An uncharged capacitor and a resistor are connected in series to a battery as shown. If # = 15 V, C = 20 F, and R = 4.0 ! 105 ", the current as a function of time for this circuit is A) I(t) = 37.5 e0.250t A B) I(t) = 150 e0.250t A C) I(t) = 37.5 e0.250t A D) I(t) = 37.5 e0.125t A E) I(t) = 300 e0.125t A

16. Lea charges a capacitor and then discharges it through a resistor. She notices that, after three time constants, the voltage across the capacitor has decreased to _____ of its value just prior to the initiation of the discharge. A) 0.368 B) 0.135 C) 0.0498 D) 0.0183 E) 0.00674

17. How much does the maximum EMF produced by a generator (a rotating coil) change if the period of rotation is doubled? A) it is the same B) it is doubled C) it is quadrupled D) it is halved E) it is quartered

18. A motor sometimes burns out when its load is suddenly increased because the resulting sudden decrease in its rotational frequency causes A) an increased back EMF and an increased current flow. B) a decreased back EMF and a decreased current flow. C) a decreased back EMF and zero current flow. D) an increased back EMF and a decreased current flow. E) a decreased back EMF and an increased current flow.

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19. The resistance of the armature windings of DC motor is 5 ". The motor is connected to a 120 V line. When the motor is spinning at full speed, it draws a current of 4 A. The back EMF at full speed is A) 20 V B) 100 V C) 120 V D) 40 V E) 80 V

20. A 200-turn coil rotates in a magnetic field of magnitude 0.25 T at a frequency of 60 Hz. The area of the coil is 5.0 cm2. What is the maximum EMF in the coil? A) 1.5 V B) 4.5 V C) 9.0 V D) 9.4 V E) 24 V

21. A simple generator consists of a single rectangular loop of wire rotating between two magnetic pole faces as shown. Which curve in the graph correctly represents the EMF # as a function of the angle between the normal to the plane of the coil and the direction of the magnetic field?

A) 1 B) 2 C) 3 D) 4 E) None of these is correct.

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22. A square coil with 12 turns and side 5 cm is rotated in a uniform magnetic field of 1.2 T at an angular frequency of 90 radians per second. What is the maximum voltage induced in the coil? A) 10 V B) 3.2 V C) 0.51 V D) 0.27 V E) none of the above

23. A circular coil with 20 turns and radius 5 cm is rotated in a uniform magnetic field of 3.2 T. If the maximum voltage induced in the coil is 5 V, calculate the number of rotations of the coil per minute. A) 600 revs per minute B) 15 revs per minute C) 5 revs per minute D) 95 revs per minute E) 200 revs per minute

24. If you were to plug a 1200-W heater and a 600-W toaster into a 110-V electrical outlet, what total rms current would these two appliances draw if the wiring is parallel? A) 5.45 A B) 10.9 A C) 11.6 A D) 16.4 A E) 23.1 A

25. Two heaters are plugged into the same 120-V AC outlet. If one heater is rated at 1100 W, then what can be the maximum rating of the second heater in order not to exceed the 20 A trip rating on the circuit? A) 1100 W B) 1300 W C) 1200 W D) 2400 W E) 920 W

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26. If you connect a 12-V rms AC power supply that operates at 5.0 kHz to a 20-F capacitor, what is the rms current in the circuit? (neglect any resistance) A) 1.2 A B) 7.5 A C) 19 A D) 8.4 A E) 11 A 27. An inductor with inductance L = 10 mH is connected to a power supply which provides a current I = 5 mA sin \$t. The maximum back EMF is A) 5 mV B) 10 mV C) 25 V D) 50 V E) 0 V 28. At what frequency would the reactance of a 100-pF capacitor be 1000 "? A) 3.18 MHz B) 15.9 MHz C) 1.59 MHz D) 3.98 MHz E) 2.54 Mhz 29. At what frequency would the reactance of a 1.0-mH inductor be twice that of a 10-F capacitor? A) 10 kHz B) 3.2 kHz C) 2.2 kHz D) 1.6 kHz E) 1.1 kHz 30. An inductor is placed across an AC generator that has a maximum EMF of 170 V. For a frequency of 3000 Hz, the rms current through the inductor is 5.0 A. Find the value of the inductor. A) 8.0 ! 103 H B) 9.2 ! 104 H C) 1.8 ! 103 H D) 1.3 ! 103 H E) 5.6 ! 103 H

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31. A 5-!F capacitor is charged to 30 V and is then connected across a 10-!H inductor. The maximum value of the current in this circuit is A) 1.41 A B) 2.25 A C) 8.73 A D) 15.4 A E) 21.2 A 32. The battery has a voltage V = 6 V, the inductance of the inductor L = 12 mH and the capacitance of the capacitor C = 8 F. Assume that the inductor and the wires have zero resistance.

The switch S was at position a for a long time when it is switched to b. What is the charge on the capacitor at t = 0 s? A) 0.75 C B) 1.33 C C) 48 C D) 72 C E) zero 33. The battery has a voltage V = 6 V, the inductance of the inductor L = 12 mH and the capacitance of the capacitor C = 8 F. Assume that the inductor and the wires have zero resistance.

The switch S was at position a for a long time when it is switched to b. What is the charge on the capacitor at t = 0.487 ms? A) 0.75 C B) 1.33 C C) 48 C D) 72 C E) zero

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34. A 10-mH inductor has a resistance of 10 ". Its impedance in an AC circuit is A) 10 " B) 14 " C) 0.10 k" D) 5.0 " E) impossible to tell from the given information.

35. You have a 30-!H inductor and a 50-!F capacitor with which you form a parallel, resonant circuit. The frequency of this circuit will be A) 4.11 kHz B) 13.7 kHz C) 25.8 kHz D) 31.7 kHz E) 39.1 kHz

36. The approximate resonant frequency of the circuit shown is

A) 1.5 kHz B) 0.25 kHz C) 2.4 MHz D) 25 kHz E) None of these is correct.

37. A series RLC circuit is driven by a 1.0-kHz oscillator. The circuit parameters are L = 5.0 mH, C = 4.0 F, R = 10 ". If you adjust the frequency of the oscillator so that it is at the resonance frequency of the RLC circuit, and if it supplies 3.0 V rms, what is the rms current in the circuit? A) 0.23 A B) 0.36 A C) 0.21 A D) 0.30 A E) 0.42 A

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38. A series RLC circuit is driven by a 1.0-kHz oscillator. The circuit parameters are Vrms = 12 V, L = 5.0 mH, C = 4.0 F, and R = 10 ". Under steady-state conditions, the rms potential difference across the resistor will be A) 5.13 V B) 9.19 V C) 12.3 V D) 14.7 V E) 18.8 V

39. You connect a 100-" resistor, a 800-mH inductor, and a 10.0-F capacitor in series across a 60.0-Hz, 120-V (peak) source. The impedance of your circuit is approximately A) 377 " B) 302 " C) 265 " D) 100 " E) 107 "

40. You connect a 250-" resistor, a 1.20-mH inductor, and a 1.80-F capacitor in series across a 60.0-Hz, 120-V (peak) source. The approximate impedance of your circuit is A) 1.49 k" B) 1.47 k" C) 0.452 " D) 250 " E) None of these is correct.

41. You connect a 250-" resistor, a 1.20-mH inductor, and a 1.80-F c