Exam 1-solutions physics

# Exam 1-solutions physics - Version 066 Exam 1 chiu(57460...

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Version 066 – Exam 1 – chiu – (57460) 1 This print-out should have 19 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0points A lightweight metal ball hangs from a thread, to the right of an plastic rod. (See Figure 1.) Both are initially uncharged. The plastic (insulator) case: You rub the left end of the plastic rod with wool, deposit- ing charged molecular fragments whose total negative charge is that of 1 × 10 9 electrons. The metal (conductor) case: You perform a similar experiment with a conducting metal rod. You touch the left end of the rod with a charged metal object, depositing the same excess electrons to the left end. You then remove the object. Statement I : Consider two cases below: (i). There are polarized dipoles inside of the rod. (ii). There are electrons at the surface. Ia. (i) is applicable for the insulator. (ii) is applicable for the conductor. Ib. (ii) is applicable for the insulator. (i) is applicable for the conductor. Statement II : Compare the attraction be- tween the rod and the ball IIa. The insulator rod has more attraction. IIb. The conductor rod has more attraction. Statement III : IIIa. The induced dipole moment for a con- ductor occurs over a macroscopic length scale. IIIb. The induced dipole moment for a con- ductor occurs over an atomic length scale. 1. Ib, IIa, IIIb 2. Ia, IIb, IIIa correct 3. Ia, IIa, IIIb 4. Ib, IIa, IIIa 5. Ia, IIa, IIIa 6. Ia, IIb, IIIb 7. Ib, IIb, IIIa 8. Ib, IIb, IIIb Explanation: Answer to statement I: The excess negative charge at the left end leads to the alignment of induced dipoles in the interior of the insulator rod. For the metal rod case, free electrons are on the surface of the rod. Ia is correct. Answer to statement II: Since electrons are free to move along the surface of a conductor, the average location of the free electrons in the conductor is closer to the average location of the induced negative charges in in the ball than is the case for the insulator. So the former gives a stronger attraction. IIb is correct. Answer to statement III: The induced dipole moment for the conductor is dis- tributed throughout the conductor. Hence, the dipole moment induced is across a macro- scopic length scale. IIIa is correct. 002(part1of2)10.0points Suppose you want to create an electric field = ( 2 , 2 , 0 ) N/C at the origin located at ( 0 , 0 , 0 ) . Where would you place an electron to create this field? Denote the coordinate vector of the electron by vectorr = r ˆ r Determine ˆ r

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Version 066 – Exam 1 – chiu – (57460) 2 1. ( 1 , 1 , 0 ) 2. (− 1 , 1 , 0 ) 2 3. (− 1 , 1 , 0 ) 2 4. ( 1 , 1 , 0 ) 2 correct 5. (− 1 , 1 , 0 ) 6. 2 (− 1 , 1 , 0 ) 7. 2 ( 1 , 1 , 0 ) 8. ( 1 , 1 , 0 ) 2 Explanation: The electron has a negative charge. The direction of the vector should along the direc- tion of the electric field. ˆ r = ˆ E = vector E E ˆ r = 1 8 ( 2 , 2 , 0 ) ˆ r = 1 2 ( 1 , 1 , 0 ) 003(part2of2)10.0points Find the magnitude r (in units of m).
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