# Physics Questions and Answers – Forces Between Multiple Charges

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This set of Physics MCQs for Schools focuses on “Forces Between Multiple Charges”.

1. Three equal positive charges are kept at the corners of an equilateral triangle. What will be the vector sum of the forces acting on the particles?
a) Directed towards the center
c) Acts along one of the sides of the triangle
d) Zero

Explanation: The magnitude of the force acting on each of the charges will be the same and they will differ from each other by 120 degrees. Therefore their resultant force will always be zero.

2. A 1C charge is placed at the origin. Other infinite numbers of unit charges are placed at √2, √4, √8, √16,….(up to infinite) distances from the origin in a straight line. What will be the total force acting on the 1st charge?
a) 9*109 N
b) 18*109 N
c) Infinite
d) 1 N

Explanation: The force on 1st charge will be = 9*10-9*($$\frac {1}{\sqrt{2}^2} + \frac {1}{\sqrt{4}^2} + \frac {1}{\sqrt{8}^2}$$ +⋯) = 9*10-9*$$\frac {\frac {1}{2}}{1-\frac {1}{2}}$$ = 9*10-9N [infinite GP formula].

3. Coulomb force is a central force.
a) True
b) False

Explanation: Coulomb force acts along the center-joining-line of the two charges. So this force is called the central force. Gravitational force is also the central force.
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4. Four charges are kept at the corner points of a square. The net force on a charge kept at the center of the square is _________
a) Along diagonal
b) Zero
c) Along one side
d) Depends on the nature of the charges

Explanation: Depending on the nature and quantity of charges, the net force on the central charge may vary its magnitude and direction. If all the charges are the same the net force will be zero.

5. Which among the following is false?
a) Coulomb force is a central force
b) The force between two charges depend on the medium between them
c) Coulomb force is a weak force
d) The net force on a charge is the vector sum of the forces acting on it due to several other charges

Explanation: Coulomb force is a strong force. It can be shown that the Coulomb force between two electrons is 1043 times than gravitational force. All the other options are basic properties of Coulomb’s Law.
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