Two idential point charges, Q 1 and Q 2 are separated by a distance r = 0.30 m .

Question

Two idential point charges, Q1 and Q2 are separated by a distance r = 0.30 m.

Part A

At what distance from Q1 is the electric field zero? Mark in the point P where this occurs in the figure above.

Part B

The electric potential due to the two charges at the point P where the electric field is zero is -100 V. Determine the charge on Q1.

Part C

What if a positive test charge were placed at the point P? Would the net force on the charge point to the left, point to the right or be zero? Briefly explain your answer.

Explanation / Answer

(a) Electric field is zero exactly midway between the line joining the two charges.

(b) V = 2kq/(r/2) = 2*9*109*q/0.15 = -100

q = -100*0.15/2*9*109 = -0.833nC

(c) Since electric field at P is zero and F = qE so Force on positive test charge is also zero.

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