What point charges, all having the same magnitude, would you place at the corners of a square (one charge per corner), so that both the electric field and the electric potential (assuming a zero reference value at infinity) are zero at the center of the square?
A. negative on top, positive on bottom
B. positive on top, negative on bottom
C. positive on the left side, negative on the right
D. positive on one diagonal, negative on the other
E. negative on the left side, positive on the right

A) No

B) No

C) No

D) Si

E) No

Explanation:

Imagine a square A,B,C,D ( counterclock view ) then A and C are opposite corners and so are B and D.

To get E = 0 in the center of the square, with charges of equal magnitude we need to have a configuration of same sign charges in opposite cornes (that is, same sign charges  in A and C ; and same sign charges in  B and D )

If we have E = 0 then V = 0

wavelength variability is the right one

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hope this : d