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midterm 3

# midterm 3 - Version 042 midterm 03 chiu(56565 This...

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Version 042 – midterm 03 – chiu – (56565) 1 This print-out should have 17 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0points A unipolar generator consists of a copper disk of radius R rotating in a uniform, steady magnetic field B perpendicular to the disk, out of the page. (The magnetic field is pro- duced by large coils carrying constant current, not shown in the diagram.) Sliding contacts are made at the center (on the axle) and at the rim of the disk, and the wires are connected to a voltmeter. If the outer rim travels coun- terclockwise at a speed v , choose a correct answer for each of following questions. The direction of the instantaneous average velocity of the electrons at any point along AB Ia) upward Ib) downward The direction of the magnetic force on the electrons at any point along AB IIa) towards A IIb) towards B The direction of the conventional current flow IIIa) into the + terminal of the voltmeter IIIb) out of the + terminal of the voltmeter 1. Ib, IIb, IIIb 2. Ia, IIb, IIIb 3. Ia, IIa, IIIb correct 4. Ia, IIa, IIIa 5. Ib, IIa, IIIa 6. Ib, IIa, IIIb 7. Ia, IIb, IIIa 8. Ib, IIb, IIIa Explanation: Counterclockwise rotation of the disk im- plies an upward instantaneous velocity at any point along AB. Hence the answer is Ia. The magnetic force vector F = qvectorv × vector B is pointing radially inward. Hence, the answer is IIa. Negative charge is accumulated at the cen- ter of the disk A, or the conventional current flows out from the + terminal. Hence, the answer is IIIb. 002 10.0points Consider the set up shown in the figure where a solenoid has a steadily increasing magnetic flux which generates identical induced emf s for the two cases illustrated. A B #1 #2 (1) i Case 1: Two identical light bulbs are in series; the electrical power consumed by bulb 1 and bulb 2 is P 1 and P 2 , respectively. A B #1 #2 (2) D F C Case 2: Bulb 2 is shorted by a wire which is connected between the two points C and F ; the electrical power consumed by bulb 1 and bulb 2 is P 1 and P 2 , respectively. What is the ratio P 1 P 1 ?

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Version 042 – midterm 03 – chiu – (56565) 2 1. P 1 P 1 = 0 2. P 1 P 1 = 3 3. P 1 P 1 = 1 4 4. P 1 P 1 = 1 8 5. P 1 P 1 = 1 6. P 1 P 1 = 1 3 7. P 1 P 1 = 2 8. P 1 P 1 = 8 9. P 1 P 1 = 1 2 10. P 1 P 1 = 4 correct Explanation: Let E and R be the induced emf and resis- tance of the light bulbs, respectively. For case 1, since the two bulbs are in series, the equivalent resistance is simply R eq = R + R = 2 R and the current through the bulbs is I = E 2 R . Thus for case 1, the power consumed by bulb 1 is P 1 = parenleftbigg E 2 R parenrightbigg 2 R = E 2 4 R . For case 2, since bulb 2 is shorted, the current through bulb 1 is now I = E R . so the power consumed by bulb 1 is P 1 = parenleftbigg E R parenrightbigg 2 R = E 2 R and the ratio is P 1 P 1 = 4 .
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midterm 3 - Version 042 midterm 03 chiu(56565 This...

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