Spring 2015 - Physics 162 - Exam 2 Review 1. Two very large parallel sheets a distance d apart have their centers directly opposite each other. The sheets carry equal but opposite uniform surface charge densities. A point charge that is placed near the middle of the sheets a distance d/2 from each of them feels an electrical force F due to the sheets. If this charge is now moved closer to one of the sheets so that it is a distance d/4 from that sheet, what force will feel? (a) 4F (b) 2F (c) F (d) F/2 (e) F/4 2. The figure shows four Gaussian surfaces surrounding a distribution of charges. (a) Which Gaussian surfaces have an electric flux of +q/✏0 through them? (b) Which Gaussian surfaces have no electric flux through them? 3. A small glass bead charged to +6.0 nC is 4.0 cm from a thin, uniformly charged, 10-cm-long glass rod. The bead is repelled from the rod with a force of 840 mN. What is the total charge on the rod? You may assume that the bead is closest to the midpoint of the glass rod. 4. 55.3 million excess electrons are inside a closed surface. What is the net electric flux through the surface? 5. An electron with an initial speed of 500,000 m/s is brought to rest by an electric field. (a) Did the electron move into a region of higher potential or lower potential? (b) What was the potential di↵erence that stopped the electron? Answers: 1. C 2. (a) B (b) C 3. 4.0 ⇥ 10 5 C 4. -1.00 N m2 /C 5. (a) Hint: Positively charged particles move “properly”. Like a rock on a hill, they would prefer to fall from higher to lower potential. (b) -0.712 V Full solutions are posted below.
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