Question

15. A point charge Q is located a short distance from a point charge 3Q, and no other charges or electric fields are present. If the magnitude of the electric force on Q is F, what is the electric force on 3Q?  

16) the figure below shows two unequal point charges, q and Q, of opposite sign. Charge Q has greater magnitude than charge q. In which of the regions X, Y, Z will there be a point at which the net electric field due to these two charges is zero?

17. The figure shows two tiny 5.0-g spheres suspended from two very thin 1.0-m-long threads. The spheres repel each other after being charged to +91 nC and hang at rest as shown. What is the angle θ?  

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Answer 1

15)

here , as the electric force between the particles is given as

F = k * Q1*Q2/d^2

Now , accroding to Newton's third law of motion

the magnitude of force on both the charged particles is same but the forces are oposite in sign

Now , as the magnitude of force on Q is F

force on the charge 3Q is also F