# Exam 2 + Solutions Winter 2002

## Physics 126 with Riles at University of Michigan - Ann Arbor *

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- Exam 2 + Solutions Winter 2002

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Page 1 "r 0/ A rectangular loop of wire (0.10 m by 0.20 m) carries a current of 5.0 A in a counterclockwise direction. The loop is oriented as shown in a uniform magnetic field of 1.5 T. The force acting on the upper 0.10-m side of the loop is A) l.s N* B) 0.75 N c) o.sO N D) 0.15 N E) zero The force on a wire is F = I L B sin 0. 2. A circular, 2O-turn coil of radius 5.0 cm is oriented in such a way that its axis makes a 30" angle with a uniform magnetic field of 0.15 T. What is the torque on the coil when it carries a current of 2.5 A? A) 1.5 x 10-3 N'm B) 9.4 x 10-3 N'm * C) 2.9 x lDa N'm D) 5.1 x10-2N'm E) 0.59 N 'm The torque on a loop is r = NIABsind htto: I lheo I ohvsic s 1 26 w02lexams/EXAM2. htm 1U2U2002 Exam2 ,{rr# ,t*,Q sl* ,,$* '*S G*, S-* g * ..p {rl ta}, '1+l t{l $), Two very long, parallel conducting wires carry equal currents in the same direction, as shown. The numbered diagrams show end views of the wires and the resultant force vectors due to current flow in each wire. Which diagram best represents the direction of the forces? A)1* B)2c)3 D)4E)s Wires with current in the same direction attract. pase2"t, Q7/ 4, The magnetic flux through a certain coil is given by 0,n = (1/50m) cos 100nr where the units are SI. The coil has 100 turns. The magnitude of the induced emf when t = 1/200 s is A) 100 v B) 200 v C) zero D) 2lnY E) l/50n V This is the equation for the flux through a generator coil, which in general is Pr= BAcosalf The EMF from a generator is EMF = .IIBAalsin alf which in this case is EMF - loo'1oorr ,i* f 1oo" I50rr |. 200 J 5. According to Faraday's law, a necessary and sufficient condition for an electromotive force to be induced in a closed circuit loop is the presence in the loop of A) a magnetic field. B) magnetic materials. IU2lt2002htn:l lheplshvsics 1 26 wO2lexamslEXAM2.htm Exam2 C) an electric current. * D) a time-varying magnetic flux. E) a time-varying magnetic field. i^ EMF - _.fAf Page 3 "t, 67 6. The self-inductance of a wire coil is a proportionality constant that relates A) electric field to current. B) electric flux to current. * C) magnetic flux to current. D) magnetic field to current. E) voltage to current. This is the definition of self inductance, .NpL= - T 7. As you increase the frequency in this circuit from zero, A) the impedance increases to a maximum and then decreases. * B) the impedance decreases to a minimum and then increases. C) the impedance will decrease continuously. D) the impedance does not change. E) None of these is correct. The impedance of the capacitor decreases, the impedance of the inductor increases, the impedance of the resistor is constant. The total impedance is minimum at resonance. 8. You connect a 250-Q resistor, a1.20-Hinductor, and a 3.60-pF capacitor in series across a 60.0- Hz,I20-V (RMS) source. When you connect a voltmeter across the inductor in this circuit, the meter reads approximately 25.7 mY 83.6 V A) B) http : | | hep / ph y s i c s 1 2 6-w 02 | exams IEXAM 2. htm ty2U2002 Exam2 c) r20 Y* D) l43V E) 84.9 V Find the current through the circuit by finding the total impedance. the current times the impedance of the inductor. V=IZ Xr= Vt= Page4"r6( The voltage across the inductor is rrc Rt+(x"-Xr)t I 2n fC znf L Ixr 9. The 8000-turn primary of a transformer is connected to a 50-kV transmission line and the secondary has 19 turns. What is the voltage output of this transformer? A) 21 MV* B) 119V c) 110 v D) 21 kV E) 220Y J\T V. - V^., S " - I,/o L0. The detection of radio waves can be accomplished with either a dipole (straight wire) 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 The wire has a voltage due to the electric field, the loop has a voltage due to the changing magnetic field. 11. The rms value of the electric field in an electromagnetic wave is E_^ = 200 V/m. The average intensity of this wave is approximately A) 53.1Wm2 * B) 106 Wm2 C) 1.59 x 1o1o wm2 D) 3.18 x 10ro Wm2 htto://heo/phvsics 126 w02lexamslEXAM2.htm It/21/2002 -l Exam2 E) 1.70 kWm2 ,S - rocEt Page5 "t, 05 A) *B) c) D) E) 12. Light that has a wavelength of 600 nm has a frequency of 5.0 x 1015 Hz 5.0 x 10ra Hz 5.0 x 106 Hz l.2x I}s Hz I.2 x l0r4 Hz As light passes from one medium into another, the angle of refraction is - in the medium with the index of refraction and speed of light. A) smaller; larger; lower B) smaller; larger; higher C) larger; smaller; lower D) larger; larger; lower E) smaller; smaller; higher 14. A glass block with index of refraction 1.50 is immersed in water whose index of refraction is 1.33. The critical angle at the glass to water interface is A) non-existent; there is no critical angle for this situation. B) 41..9" c) 48.8o D\ 27.s"* E) 62.5" 9c = sin 15. From directly above, you're watching a fish swim 1.83 m beneath the surface of a clear lake (ru = 1.33). How far beneath the surface does the fish seem to be? A) 0.914 m B) 1.37 m C) 1.83 m D\ 2.44 m E) 293 m "['#J httn://hen/nhvsi cs I 26 wO2/ exams/EXAM2.htm tU2U2002 Exam 2 d, =don Page6 "186 16. * L)1 B)2 c)3 D)4E)s Light travels in a medium of index of refraction n', passes into a medium of index n",whete n" > n', and then into air, where fl^,-1ft'

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