**(a)**

The second charge floats if it is repelled by the first charge.

According to the basic law of electrostatics, like charges repel while unlike charges attract.

A positively charged particle Q1 = +45 nC is held fixed at the origin. A second charge Q2 of mass m = 7.5 μg is floating a distance d = 25 cm above charge Q1. The net force on Q2 is equal to zero. You may assume this system is close to the surface of the Earth.

(a) What is the sign of the charge on Q2?

(b) Write an equation for the magnitude of charge Q2 in terms of the given variables.

(c) Calculate the magnitude of Q2 in units of nanocoulombs.

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