Fall 2012 Midterm 3 Solutions

# That means that e depends only on the amplitude of

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Unformatted text preview: nds only on the amplitude of the generator emf, not on its frequency. Version 1 Page 10 18. In the circuit below the three resistors are the same at 10 Ω each and the inductor has L = 35 mH. What is the potential difference across the inductor immediately after closing the switch? We do know that the current through the battery is 4.0 A after the switch has been closed for a very long time. L A) B) C) D) E) Two options. 1) calculate the 19. B field of the wire in the middle and multiply by 18. This is NOT an exact answer (it underestimates B by about 20%) but it is close enough to the correct answer to allow you to choose it. 2) integrate the current. This gives a very precise answer, but is more diffecult than option 1. 10 50 V 15 V 30 V 0 In the instant after the switch is closed the current through the inductor is zero. All current goes through loop 1 only. The voltage across the inductor must be the same as the voltage across the second resistor in loop 1. Since the two resistors in loop 1 are identical the voltage across each must be half the battery voltage. That means the inductor also has half of the battery voltage across it, so V_L=30 volts. Eighteen very long, straight conductors with square cross sections carrying current of 5 A straight out of the page are laid side by side to form a current sheet as shown in the diagram. What is the magnitude and direction of the magnetic field at point P, 0.03 m from one edge of the sheet as shown? P 0.03 m 0.12 m A) B) C) D) E) -4 2.8 x 10 T, 1.5 x 10-5 T, 6.9 x 10-5 T, 9.5 x 10-7 T, 3.2 x 10-6 T, down up up left down Version 1 Page 11 20. An LC circuit is composed of an ideal solenoid and a capacitor (no resistance in the circuit). It oscillates at some natural frequency f0. If we now fill the volume of the solenoid with a ferromagnetic material that has permeability Km = 3000 what happens to the natural frequency? A) it is 55 times higher B) it is 3000 times higher C) it is unchanged D) it is 3000 times lower E) it is 55 times lower Version 1 Page 12 Answer Key 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. E D D C B B E E D E D C E D A C A D A E Version 1 Page 13...
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