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Study Union Physics for Engineers 2 ReviewTutor: Carlos Lairet

Charges & Electric Fields

1. Two equal positive point charges q1 = q2 = 2.0 μC are located at x = 0, y = 0.3 m and x = 0, y = -0.3 m, respectively. What are the magnitude and direction of the total (net) electric force that these charges exert on a third point charge Q = 4.0 μC at x = 0.4 m, y = 0?

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2. A disk with radius 0.1 m is oriented with its normal unit vector (n) at an angle 300 to a uniform electric field E with magnitude 2.0 x 103 N/C.

a. What is the electric flux through the disk?b. What is the flux through the disk if it is turned so that its normal is perpendicular

to E?c. What is the flux through the disk if its normal is parallel to E?

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3. A small plastic sphere is charged to -10 nC. It is held 1.0 cm above a small glass bead at rest on a table. The bead has a mass of 15 mg and a charge of +10 nC. Will the glass “leap up” to the plastic sphere?

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4. Under normal circumstances, the earth’s electric field outdoors near ground level is uniform, about 100 N/C, directed down. What is the electric force on a free electron in the atmosphere? What acceleration does this force cause?

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5. A ring-shaped conductor with radius a carries a total charge Q uniformly distributed around it. Find the electric field at a point P that lies on the axis of the ring at a distance x from its center.

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Electric Potential

6. A proton moves through an electric potential created by a number of source charges. Its speed is 2.5 x 105 m/s at a point where the potential is 1500 V. What will be the proton’s speed a short time later when it reaches a point where the potential is -500 V?

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7. The spacing between the plates of a 1.0 F parallel-plate capacitor is 0.070 mm.a. What is the surface are of the plates?b. How much charge is on the plates if this capacitor is attached to a 1.5 V battery?

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8. A parallel-plate capacitor is constructed of two disks spaced 2.00 mm apart. It is charged to a potential difference of 500 V. A proton is shot through a small hole in the negative plate with a speed of 2.0 x 105 m/s. What is the farthest distance from the negative plate that the proton reaches?

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9. How much energy is store in a 220 F camera-flash capacitor that has been charged to 330 V? What is the average power delivered to the flash lamp if this capacitor is discharged in 1.0 ms?

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10. A defibrillator unit contains a 150 F capacitor that is charged to 2000 V. The capacitor plates are separated by a 0.010 mm-thick dielectric with = 300.

c. What is the total area of the capacitor plates?d. What is the energy density stored in the electric field when the capacitor is

charged?

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Current & Resistance

11. Four wires have current as noted below. What are the direction and the magnitude of the current in the fifth wire?

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? A

4 A

3 A

2 A

6 A


12. The glowing element in an incandescent lightbulb is the filament, a long, thin piece of tungsten wire that is heated by the electric current through it. When connected to the 120 V of an electric outlet, a 60 W lightbulb carries a current of 0.5 A.

a. What is the resistance of the filament in the lamp? b. At the operating temperature of the tungsten filament, the resistivity is

approximately 5.0 x 10-7 m. If the wire used to make the filament is 0.04 mm in diameter, how long must the filament be?

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13. A car battery has = 12V. When the car’s starter motor is running, the battery current is 320 A. What power does the battery supply?

14. How much current is drawn by a 75 W lightbulb connected to a 120 V outlet?

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Circuits

15. What is the Current in the circuit? What is the potential Difference across each resistor?

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16. Find the current through and the potential difference across each of the four resistors in the circuit shown

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17. The switch in the circuit has been in position a for a long time, so the capacitor is fully charged. The switch is changed to position b at t=0. What is the current in the circuit immediately after the switch is closed? What is the current in the circuit 25 μs later?

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18. Switch S has been open for a long time. It is then closed. Find:e. The time constant before the switch is closedf. The time constant after the switch is closedg. The current in the switch as a function of time

Magnetic Fields & EM Waves

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19. Two long wires lie parallel to each other, as shown. They each carry a current of 5.0 A, but in opposite directions. What is the magnetic field at Point P?

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20. An electron is moving to the right in a magnetic field that points upward. What is the direction of the magnetic force?

21. A constant magnetic field of 1.0 T is applied to a rectangular loop of area 4.5 x 10-3 m2. If the area of this loop changes from its original value to a new value of 1.5 x 10-3 m2 in 6.0 s, what is the emf induced in the loop?

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22. A patient having an MRI scan has neglected to remove a copper bracelet. The bracelet is 6.0 cm in diameter and has a resistance of 0.010 Ω. The magnetic field in the MRI solenoid is directed along the person’s body from head to foot; her bracelet is perpendicular to B. As a scan is taken, the magnetic field in the solenoid decreases from 1.00 T to 0.4 T in 1.2s. What are the magnitude and direction of the current induced in the current?

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23. A digital cell phone emits 0.6 W of 1.9 GHz radio waves. What are the amplitudes of the electric and magnetic fields at a distance of 10 cm?

24. A conducting loop of wire has an area of 150 cm2 and a resistance of 12.5 Ω. There is a magnetic field of 100 T perpendicular to the loop. At what rate must this field be reduced to induce a current of 0.100 A in the loop?

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25. A circular coil 0.05 m in radius, with 30 turns of wire, lies in a horizontal plane. It carries a current of 5.0 A in a counterclockwise sense when viewed from above. The coil is in a uniform magnetic field directed toward the right, with magnitude 1.2 T. Find the magnitudes of the magnetic moment and the torque on the coil.

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AC Circuits

26. The hot wire in a toaster dissipates 580 W when plugged into a 120 V outlet.h. What is the wire’s resistance?i. What are the rms and peak currents through the wire?

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27. A book light has a 1.4 W, 4.8 V bulb that is powered by a transformer connected to a 120 V electric outlet. The secondary coil of the transformer has 20 turns of wire. How many turns does the primary coil have? What is the current in the primary coil?

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28. What is the capacitive reactance of a 0.100 μF capacitor at a 100 Hz audio frequency and at a 100 MHz FM-radio frequency?

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29. A 10 μF capacitor is connected to a 1000 Hz oscillator with a peak emf of 5.0. What is the peak current through the capacitor?

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30. A 0.25 μH inductor is used in an FM radio circuit that oscillates at 100 MHz. The current through the inductor reaches a peak value of 2.0 mA at t = 5.0 ns. What is the peak inductor voltage, and when, closest to t=5.0 ns, does it occur?

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31. An LC circuit consists of a 10 μH inductor and a 500 pF capacitor. What is the oscillator’s frequency?

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32. An AM radio antenna picks up a 1000 kHz signal with a peak voltage of 5.0 mV. The tuning circuit consists of a 60 μH inductor in series with a variable capacitor. The inductor coil has a resistance of 0.25 Ω, and the resistance of the rest of the circuit is negligible.

j. To what value should the capacitor be tuned to listen to this radio station?k. What is the peak current through the circuit at resonance?l. A stronger station at 1050 kHz produces a 10mV antenna signal. What is the

current at this frequency when the radio is tuned to 1000 MHz?

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Optics

33. The incident intensity is the 55 W/m2 of the polarized laser light and the transmitted intensity is 8.7 x 10-7 W/m2, find the polarization angle.

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34. A laser beam is aimed at a 1.0-cm-thick sheet of glass at an angle 300 above the glass.m. What is the laser beam’s direction of travel the glass?n. What is its direction in the air on the other side of the glass?

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Sources

Knight, Randall Dewey, et al. College Physics: A Strategic Approach. 3rd ed., Pearson, 2015.

YOUNG, HUGH D. FREEDMAN, ROGER A. UNIVERSITY PHYSICS. Vol. 12, PEARSON, 2019.

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