su09EI - Physics 54 Summer 2009 Exam I Solutions In...

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Physics 54 Summer 2009 1 Exam I Solutions In questions or problems not requiring numerical answers, express the answers in terms of the symbols given and standard constants. If numbers are required, use g = 10 , k = 1/ 4 !" 0 = 9 # 10 9 and μ 0 = 4 ! " 10 # 7 (all in SI units). Part A: Multiple choice questions. Check the best answer. 1. All of the following statements about lines of the electrostatic field are true, but which one follows from the field equation E ! d r ! " = 0 ? Field lines do not cross each other. Field lines start on positive charges and end on negative charges. Field lines do not form closed curves. Field lines do not penetrate a conductor. 2. A small electric dipole is in an E-field due to other charges. The net force on the dipole due to the field: Is zero because the dipole has zero total charge. Is toward the region of stronger field no matter how the dipole is oriented. Is zero if the field is uniform, no matter how the dipole is oriented. Is zero if the dipole moment is aligned with the field, whether the field is uniform or not. 3. Two point charges + q are held at rest at distance a from each other. A third point charge – q is brought in and held at rest at a point on the line between them, at distance a /2 from both. Which of the following is NOT true? If all three charges are simultaneously released and are free to move, the two positive charges will fly away from each other. [They will move toward the negative charge; the system is bound.] The potential energy of the system is negative. The negative charge is at a point of unstable equilibrium. If the negative charge were – q /4 instead of – q , the potential energy would be zero.
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Physics 54 Summer 2009 2 4. Shown are two different cases of hollow spheres of the same dimensions, each with a small conducting sphere of charge + Q suspended in the center of the cavity. In case A the sphere is a grounded conductor; in case B the sphere is a non-conductor with total charge –
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su09EI - Physics 54 Summer 2009 Exam I Solutions In...

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