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ive tried a lot of methods but im wrong. even my tutor couldnt help me?
Identical point charges of +6.1 μC are fixed to diagonally opposite corners of a square. A third charge is then fixed at the center of the square, such that it causes the potentials at the empty corners to change signs without changing magnitudes. Find the sign and magnitude of the third charge.
1 Answer
- oldprofLv 76 years ago
Here's what I think you have:
+ ------ ? ------ +
..........+/-
Clearly the field at the +/- empty corner is + without the third charge at ? So ? has to be negative charge - Q to offset the two + q charges in the occupied corners.
So in an empty corner we have E = kq/s^2 + kq/s^2 and we add Q in the middle to have E' = 2kq/s^2 - kQ/r^2; where s^2 = 2r^2 so r^2 = s^2/2 and E' = 2kq/s^2 - 2kQ/s^2. s is the side of the square. We set E' = 0 to find the critical value at which the sign changes and have q = Q.
So Q = - 6.1 μC or greater. ANS.
