# practice_final_answers - Multiple Choice Problems-put...

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Multiple Choice Problems-put correct answer on front page 1) A circular loop of wire carries a current I. If the radius of the loop is increased by a factor of two, the magnetic field at the center of the loop will change by what factor? a) 1/2 b) 1/ 2 c) 1 d) 2 e) 2 2) Which of the following is the best representation of a plot of the magnitude of the magnetic field as a function of distance from the center of a very long cylindrical conductor of radius R with constant current density flowing along the axis of the conductor? a) b) c) d) R R R R 3) Which of the following is a possible electromagnetic plane wave propagating in vacuum? The notation E =E(x,t) k , for example, means that the electric field points in the z direction and depends only on x and t. a) E =E(x,t) k, B =B(x,t) i, propagating in the j direction b) E =E(x,t) k, B =B(y,t) i, propagating in the j direction c) E =E(x,t) k, B =B(x,t) j, propagating in the i direction d) E =E(x,t) i, B =B(y,t) j, propagating in the k direction e) none of the above 4) In a region of space an electric field given by E =E k and some unknown magnetic field exists. A particle of charge q is observed to move at a constant velocity v = v i in this region. Which of the following is a possible value of the magnetic field? Assume that only electric and magnetic forces act on the particle. a) B =-(E/v) j b) B =+(E/v) j c) B =-(E/v) i d) B =-(E/v) k e) None of the above y x z

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5) Two wires are located at (x,y) = (a,0) and (x,y) = (-a,0) and extending in the z direction. Each carries a current given by I = I 0 k. Which of the following is true? It is a right-handed coordinate system ( i x j = k) . a) The magnetic field at the origin is in the positive y direction. b) The wires tend to repel each other. c) The force per unit length on one of the wires is proportional to I 2 . d) Exactly two of a-c. e) None of the above. 6) A particle of charge q moves in the x-y plane in a circular orbit in a region where the magnetic field is uniform with magnitude B. Which of the following statements is not true? a) The time to make one revolution is independent of the velocity. b) The radius of the circle is proportional to the charge. c) The magnitude of the centripetal force is given by qvB. d) The applied magnetic field has only a z component. 7) An externally applied magnetic field is in the +z direction ( B = B k, with B positive ) and the magnitude is increasing with time. A loop of wire in the x-y plane is in this field; it has a total resistance R. Which of the following statements is true? a)
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