magnitude of charges

[Unlearner]

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Q1, Q2, Q3 are all charges of equal magnitude and sign. If the sign of Q3 is reversed, what happens to the net force vector of Q1?
(note: ignore the black lines)

_____________ Q3
_____________ l
_____________ l 1cm
_____________ l
Q1----------- -l
_____________ l
_____________ l 1cm
_____________ l
_____________ Q2

the answer was "the net force vector will rotate 90 degree clockwise." i'm really confused as to how this works?

 

[kobe200LATE]

Since all of the charges are the same, the charges will repel one another. This means that the net force on Q1 from Q2 and Q3 will point towards the left. However, when the sign of Q3 is reversed, Q1 is now attracted to Q3 and it instead is pulled towards Q3. The overall vector in this case points in the upward direction, which signifies a 90 degree rotation in vector direction.

 

[Unlearner]

thanks!