A positive charge Q is placed at the origin. Four other positive charges are placed on a circle in the x-y plane centered on Q as shown. The top charge is located on the y axis and has a magnitude of 3q. The three bottom charges are symmetric about the y axis as shown and each has magnitude q.


1) Which of the following statements best describes FX, the x component of the net force on the charge at the origin

FX > 0

FX = 0

FX < 0


3) For the collection of charges shown in the above problem, which of the following statements best describes FY, the y component of the net force on the charge at the origin

FY > 0

FY = 0

FY < 0

Verified Answer :

  1. FX = 0 since the bottom charges are symmetrical about the y axis and the top charge is aligned with the y-axis
    3. 
    FY > 0 since the bottom charges do not equate the top charge. The bottom charges have horizontal components which is less than q and adding them all, will not result to 3q.

\(F_{x}=0\) is correct explanation for the net horizontal force on the charge at origin.  

\(F_{y}>0\) is correct explanation for the net vertical force on the charge at origin.

Given data:

Magnitude of charge placed at origin is, Q.

The magnitude of charge at y-axis is, 3q.

(1)

  • Since each charge at y-axis are positive and have magnitude of 3q. Therefore, the force due to charges are at equilibrium. And the horizontal force cannot be greater than zero. Thus, the statement \(F_{x}>0\) is not valid.
  • The force due to charges 3q and q are at equilibrium. Therefore, the net horizontal force is zero. Thus, \(F_{x}=0\)  is the best explanation for the net horizontal force.
  • Similarly, symmetricity at the bottom also maintains the equilibrium of forces between q and Q. Thus, the statement \(F_{x}<0\) is not valid.

Thus, we conclude that \(F_{x}=0\) is correct explanation for the net horizontal force on the charge at origin.  

(3)

  • Since, the charges at bottom are not equal to the charges at top. The bottom charges have horizontal components of force which is less than q and adding them all, will not result to 3q. Therefore, the net vertical force  \(F_{y}\)  on Q will be greater than zero. Hence, \(F_{y}>0\)  is the best explanation for force on charge at origin.
  • \(F_{y}=0\) resembles that charges at bottom are equal to charges at top. Which is not true. Hence, the statement is not valid.
  • Similarly as per the previous explanations, the net force on the charge at origin is greater than zero. But  \(F_{y}<0\)  resembles the opposite case. Hence, the statement is not valid.

Thus, we conclude that \(F_{y}>0\) is correct explanation for the net vertical force on the charge at origin.

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