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Physics9C-A_Final%20Exam

# Physics9C-A_Final%20Exam - Physics 9C-A 1 Final Exam Winter...

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Physics 9C-A Final Exam Winter of 2010 1 . A small sphere with mass m carries a positive charge q and is attached to one end of a silk fiber of length L . The other end of the fiber is attached to a large vertical insulating sheet that has a positive surface charge density σ . (a) (10 points) Find the magnitude and direction of the electric force on the sphere; (b) (10 points) Show that when the sphere is in equilibrium, the fiber makes an angle θ equal to arctan ( q σ /2mg ε 0 ) with the vertical sheet. g is the acceleration due to gravity. 2 . A small conducting spherical shell with inner radius a and outer radius b is concentric with a larger conducting spherical shell with inner radius c and outer radius d . The inner shell has total charge + 2q , and the outer shell has total charge + 4q . (a) (20 points) Find the charges on all four surfaces on these two conducting shells; (b) (10 points) Find the magnitude and direction of the electric field for r < a and for b < r < c ; (c) (10 points) Find the electric potential at r = d relative to that at the infinity.

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