fall_07_exam1_sol_phy2049

# fall_07_exam1_sol_phy2049 - PHY2049 Fall 2007 Prof Yasu...

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PHY2049 Fall 2007 1 Prof. Yasu Takano Prof. Paul Avery Sep. 24, 2007 Exam 1 Solutions 1. ( SAT type problem ) A positively charged insulating rod is brought close to an object that is suspended by a string. If the object is attracted toward the rod we can conclude: (1) none of these (2) the object is positively charged (3) the object is an insulator (4) the object is a conductor (5) the object is negatively charged Since either (4) or (5) could explain the phenomenon, none of the answers are conclusive. 2. ( Similar to Webassign and class problem ) Charges +3 Q and 9 Q are held in place at positions x = 0 m and x = 2 m, respectively. At what position in x (in m) should a third charge be placed so that it is in equilibrium? (1) 2.73 (2) +1.33 (3) +3.73 (4) 1.33 (5) 1.86 We see immediately that the equilibrium point should be to the left of the +3Q charge, i.e. x < 0. Put a charge q 3 at position x. If L is the distance between the first two charges, then the total force on the third charge at equilibrium is ( ) 2 2 3 3 3 / 9 / 0 x F kQq x kQq L x = − + = . Solving yields x = 2.73 m.

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