S98s(1) - Physics 4B 1998 SECOND EXAM Chapters 25 28 Spring...

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1 Physics 4B SECOND EXAM Chapters 25 - 28 Spring 1998 Name:_________________________________________ Solve the following problems in the space provided. Use the back of the page if needed. Each problem is worth 20 points. You must show your work in a logical fashion starting with the correctly applied physical principles which are on the last page. Your score will be maximized if your work is easy to follow because partial credit will be awarded. 1. The charge -Q shown at the right is a distance y above the origin. Find (a)the electric potential it feels, (b)its potential energy, (c)its potential energy when it reaches the origin. (d)Will it be moving faster or slower when it reaches the origin? Explain in terms of the answers to parts (a), (b) and (c). (a)Adding the potentials due to the point charges, V = k q a 2 + y 2 + k q a 2 + y 2 = 2kq a 2 + y 2 (b)Using the definition of electric potential, ! U " q ! V = # Q 2kq a 2 + y 2 = # 2kqQ a 2 + y 2 . (c)At the origin, y = 0 so, ! U = " 2kqQ a (d)Since the potential energy is now smaller (it is a bigger negative number) the kinetic energy must be larger by the Law of Conservation of Energy. Therefor, the charge must be moving faster.
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