June 10, 2006
Problem 1 (two points).
The diagram shows two charges +Q and -Q.
The electric field at point P on the
perpendicular bisector of the line joining them:
Positive charge is producing the electric field in the bottom right direction, while negative charge is
producing the field in the bottom left direction. The resulting field is to the bottom. Answer B
Problem 2. (two points).
An electron goes from one equipotential surface to another along one of the four
paths shown below.
Rank the paths according to the work done by the electric field, from least to greatest.
4, 3, 2, 1
1, 3, 4 and 2 tie
4, 3, 1, 2
4 and 2 tie, then 3, then 1
1, 2, 3, 4
The work done by electric field in cases 1,2,3,4 is W1=|e|*30 V, W2=|e|*10 V, W3=|e|*20 V,
W4=|e|*10 V, where |e| is the absolute value of the charge of the electron. Therefore, W2=W4 is the least work,
then W3, and W1. Answer D.