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# F95 - Physics 4B FIRST EXAM Chapters 23 25 Fall 1995 Name...

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Physics 4B FIRST EXAM Chapters 23 - 25 Fall 1995 1 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. For full credit you must explain clearly what you are doing especially if your solution involves symmetry arguments or uses Gauss's Law. 1. At some instant the nucleus of a helium atom and its orbiting electrons are oriented as shown. Find (a)the electric field (magnitude and direction) felt by the electron at the bottom right and (b)the electric force felt by this electron. (a)The electric fields due to the point charges are: E 1 = k e r 1 2 = ( 9x10 9 ) 1 . 6x10 - 19 ( 0 . 193x10 - 9 ) 2 = 3 . 87x10 10 N / C E 2 = k 2e r 2 2 = ( 9x10 9 ) 2 ( 1 . 6x10 - 19 ) ( 0 . 141x10 - 9 ) 2 = 14 . 5x10 10 N / C Adding the vector components: E x = E 2 cos30˚ - E 1 = 8.69x10 10 N/ C E y = - E 2 sin30˚ = - 7.25x10 10 N / C

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