Question

Point charge A is 2 mC, and point charge B is 10 mC, distance between them is 1.0 m. The third charge C, of -5 mC is positioned between A and B so that electric force acting on is zero. We move charge C a little bit along the line connecting charges A and B. The force acting on charge C after we moved it is directed

a. opposite to vector of displacement of C
b. in the same direction as displacement of C
c. perpendicular to displacement of C
d. force is still zero

*I know the answer is b. in the same direction as displacement of C. I just want to know why*

Answer 1

Charge C is -5mC ,so a negative charge.

Charge A is 2mC and charge B is 10m C ,both are positive charges.

We know unlike charges attracts

So both A and B tries to pull C towards them.

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Case 1 :C is placed in a point between A and B ,so that both the pulling forces are equal and acting in opposite direction.

Net force is zero.

Case 2 (a): C is moved towards A

$F=\frac{kq_{1}q_{2}}{d^{2}}$

Now distance between A and C is smaller than used to.Since force increase when distance decreases and vice versa.

$F_{A}>F_{B}$

The force acting on charge C after we moved it is directed in the same direction as displacement of C(towards A)

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Case 2 (b): C is moved towards B

Now distance between B and C is smaller than used to.

$F_{A}<F_{B}$

The force acting on charge C after we moved it is directed in the same direction as displacement of C(towards B)

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ANSWER: b

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