89. (a) The charges are equal and are the same distance from C. We use the Pythagorean theorem to find the distance rddd=+=222bgbg. The electric potential at Cis the sum of the potential due to the individual charges but since they produce the same potential, it is twice that of either one: ()922668.99 10 N m C22 2.0 10 C2.5 10 V.440.020 mqqVddεε−×⋅×====×00ππ(b) As you move the charge into position from far away the potential energy changes from zero to qV, where Vis the electric potential at the final location of the charge. The change in the potential energy equals the work you must do to bring the charge in:
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