Physics 202 Midterm Exam 3 Nov 30th, 2011 Name: Student ID: Section: TA (please circle): Daniel Crow Scott Douglas Yutao Gong Taylor Klaus Aaron Levine Andrew Loveridge Jason Milhone Hojin Yoo Instructions: 1. Don't forget to write down your name, student ID#, and section number. You need to do this on this page of your test book and on your Scantron sheet as well, where you should fill in your student ID number under Identification Number and your three digit section number starting with "3" under special codes. 2. Answer all multiple questions in this test book by indicating the best answer among choices. You must do this both on your test book and on your Scantron sheet. Follow instructions on the Scantron sheet on how to mark valid answers. 3. When you finish, you need to turn in both this test book and the Scantron sheet. 4. Use the blank sides of question pages as additional draft spaces. An extra bank sheet is provided at the end of the test book. 5. Only one answer is allowed per problem/question/ All problems have equal weight. Constants: k e = 8.99 x 10 9 Nm 2 /C 2 electric vacuum permittivity ε 0 = 8.85x10-12 C 2 /(Nm 2 ) magnetic vacuum permeability µ 0 = 4πx10-7 Tm/A Area of a sphere: 4πr 2, Volume of a sphere: (4/3)πr 3 Speed of light: c = 3.00x10 8 m/s Page 1
1. For the current in a stationary circuit to induce a current in an independent stationary circuit, it is necessary for the first circuit to have A) a steady current. B) a large current. C) no current. D) a changing current. E) None of these is correct. 2. After you measure the self-inductance of a coil, you unwind it and then rewind half the length of wire into a coil with the same diameter and length but half the number of turns. How does this change the self-inductance? A) it is the same B) it is doubled C) it is quadrupled D) it is halved E) it is quartered 3. For the two solenoids above, if l = 50.0 cm, N 1 = 250 turns and r 1 = 5.0 cm and r 2 = 10.0 cm, the mutual inductance of the two solenoids M = 0.7896 mh. What is the number of turns N 2, in the second solonoid. A) 250 turns B) 160 turns C) 40 turns D) 200 turns E) None of these is correct. Page 2
4. A region of space contains a magnetic field of 500.0 G and an electric field of 3.00 10 6 N/ C. The total energy density in a cubical box of side = 0.200 m in this region is A) 19.4 kj/m 3 B) 12.2 kj/m 3 C) 1.03 kj/m 3 D) 66.9 J/m 3 E) 24.6 J/m 3 5. An LR circuit has a resistance R = 25 Ω, 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 6. What is the time constant of an RL circuit with a resistance R = 25 Ω 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 7. An exactly 200-turn coil rotates in a magnetic field of magnitude 0.25 T at a frequency of 60.0 Hz. The area of the coil is 5.0 cm 2. 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 Page 3
8. As the frequency in this simple AC circuit increases, the rms power dissipated by the resistor A) increases. B) does not change. C) may increase or decrease depending on the magnitude of the original frequency. D) may increase or decrease depending on the magnitude of the resistance. E) decreases. 9. In the U.K. the power companies supply an rms voltage of 240 V to most home outlets. If you were to try and use your standard U.S. rated 1200-W (at 120-V) hair dryer in the U.K. without using a transformer, what would be the average power supplied to the hair dryer? A) 1200 W B) 2400 W C) 4800 W D) 600 W E) None of the above 10. Which of the following statements about an inductor that is connected to an AC voltage is true? A) The average power dissipated by the inductor is zero. B) At low frequencies the inductor behaves like a short circuit. C) At high frequencies the inductor behaves like an open short circuit. D) The back EMF produced by the inductor is the maximum when the voltage is at zero part of the cycle. E) All the above statements are true. 11. A 5.00-µF capacitor is charged to 30.0 V and is then connected across a 10.0-µH inductor. The frequency of oscillation of the energy stored in the electric field of the capacitor and the magnetic field of the inductor is A) 1.41 10 5 Hz B) 7.07 10 4 Hz C) 2.25 10 4 Hz D) 3.18 10 9 Hz E) 2.00 10 10 Hz Page 4
12. The differential equation for this circuit is L di/dt + Q/C = 0 Because I = dq/dt, the solution is A) a wave. B) a constant current. C) a simple harmonic oscillation. D) an exponential decay process. E) an exponential growth process. 13. A series RLC circuit is driven by a 1.00-kHz oscillator. The circuit parameters are V rms = 12.0 V, L = 5.00 mh, C = 4.00 µf, and R = 10.0 Ω. 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 Page 5
14. Maxwell generalized Ampère's law to read from which the displacement current is defined as A) B) C) D) E) 15. 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) A changing electric field produces a magnetic field. D) The net electric flux through a closed surface depends on the charge inside. E) None of these statements contradict any of Maxwell's equations. 16. The electric field of an electro-magnetic wave is given by E y = 25 sin(2.4 10 6 π (x 3.0 10 8 t)). What is the wavelength of the wave? A) 4.8 10 7 m B) 2.1 10 8 m C) 2.1 10 7 m D) 8.3 10 7 m E) 7.5 10 6 m 17. What is the frequency of 555-nm light? A) 16.7 khz B) 5.40 10 14 Hz C) 5.40 10 15 Hz D) 1.70 10 7 Hz E) 5.40 10 17 Hz Page 6
18. The detection of radio waves can be accomplished with either a dipole antenna or a loop antenna. The dipole antenna detects the of the wave, and the loop antenna detects the field of the wave. A) electric field; electric B) electric field; magnetic C) magnetic field; magnetic D) magnetic field; electric E) electric and magnetic fields; electric 19. A 60.0-W light bulb emits spherical electromagnetic waves uniformly in all directions. If 50.0% of the power input to such a light bulb is emitted as electromagnetic radiation, what is the radiation intensity at a distance of 2.00 m from the light bulb? A) 15 W/m 2 B) 4.8 W/m 2 C) 2.4 W/m 2 D) 0.60 W/m 2 E) 1.2 W/m 2 20. The rms value of the electric field in an electromagnetic wave is E rms = 0.600 kv/m. The average energy density of this wave is approximately A) 0.796 µj/m 3 B) 1.59 µj/m 3 C) 3.18 µj/m 3 D) 6.78 µj/m 3 E) 5.21 µj/m 3 Page 7
Answer Key - Midterm3_draft_exam 1. D 2. E 3. B 4. C 5. C 6. D 7. D 8. B 9. C 10. A 11. C 12. C 13. B 14. D 15. B 16. D 17. B 18. B 19. D 20. C Page 8