su11FinalS - Physics 54 Summer 2011 Final Exam Solutions If...

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Final Exam Solutions If numbers are required, use g = 10, k = (4 πε 0 ) 1 = 9 × 10 9 , μ 0 / 4 π = 10 7 and c = 3 × 10 8 (all in SI units). Part A: Multiple choice questions, 4 points each. Check the best answer. 1. The left sides of the four Maxwell equations are (1) E d A (2) E d r (3) B d A (4) B d r and the right sides are (a) 0 (b) Q enc / ε 0 (c) μ 0 I linked + ε 0 E t d A (d) B t d A The correct matches of the two sides are: (1) = (a), (2) = (b), (3) = (c), (4) = (d). (1) = (b), (2) = (a), (3) = (c), (4) = (d). (1) = (d), (2) = (a), (3) = (c), (4) = (b). (1) = (b), (2) =(d), (3) = (a), (4) = (c). 2. Two point charges are on the x -axis. Taking V to vanish at infinite distance, there is no point on the axis between the charges where the total potential is zero. The total potential is zero at some point(s) on the finite x -axis. The total E-field vanishes at a point on the axis between the charges. The potential energy stored in the system is negative. None of the above is true. [Charges have the same sign.] Physics 54 Summer 2011 1
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3. Shown is a thundercloud, a tall building, and a tree. Lightning from the cloud is more likely to strike: The building because it is closer to the cloud and therefore the potential difference between them is greater. [Same.] The building because it is closer to the cloud, and therefore the E-field is greater between them. Either is equally likely because they are at the same potential. Either is equally likely because the E-field between them and the cloud is the same. 4. A beam of electrons and protons of the same speed traveling parallel to each other enter a region of uniform magnetic field perpendicular to their initial velocity. What occurs? They move in identical circular arcs. They move along circular arcs of the same radius, but in opposite directions (i.e., one clockwise, the other counter-clockwise). They move in circular arcs, but in opposite directions, and the radius of the electrons’ arc is larger. They move in circular arcs, but in opposite directions, and the radius of the electrons’ arc is smaller. [ r = mv / qB .] 5. A ray of unpolarized light is incident on a block of glass at an angle such that the reflected ray is completely linearly polarized. The arrow that best represents the refracted ray is 1. 2. [ θ 1 + θ 2 = π /2 .] 3. None of the above. 1 2 3 Physics 54 Summer 2011 2
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6. A flat horizontal glass plate ( n = 1.6 ) has a film of water ( n = 1.33 ) on it, and white light reflected by this system is viewed from above. As the water film becomes very thin just before it evaporates: The reflected intensity is unusually high for all wavelengths. [ δ 0 .] The reflected light is bluish violet in color. The reflected light is reddish orange in color. The reflected intensity is unusually small for all wavelengths. 7. An astronomical telescope uses a mirror of diameter D as its objective. This telescope can make use of infrared, visible or ultraviolet light. To produce a better resolved image of a distant double star, which of the following will work?
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