5. Shown in each case below are two charged particles that are fixed in place. T

Question

5. Shown in each case below are two charged particles that are fixed in place. The magnitude of the charge Q is greater than the magnitude of the charge q. A third charge is now placed at one of the points A - E. The net force on this charge due to q and Q is zero. Explain your reasoning in each case. i. First Case: Both q and Q are positive. At which point A - E is it possible that the third charge was placed? ii. Second Case: Charge q is positive and charge Q is negative. At which point A - E is it possible that the third charge was placed? iii. Explain why it doesn?t matter whether the third charge is positive or negative.

Explanation / Answer

i.

Both q and Q are positive.

Third charge could have been placed at B.

Since both the charges are positive, force on the third charge due to q is directed to the right and that due to Q is directed to the left. Since, Q > q, the third charge must have been closer to q in order for the net force on it to be zero.

ii.

q positive and Q negative.

Third charge was placed at A.

Force on third charge due to q is directed to the left and that due to Q is directed to the right. Since Q is bigger, distance between Q and the third charge must be grater than that between q and the third charge.

iii.

Force = charge * electric field.

Direction and magnitude of the electric field is decided by q and Q. So, whether the third charge is positive or negative doesn't matter.

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