Question

A small 2.00 g plastic ball is suspended by a 20.0 cm long string in a uniform electric field. If the ball is in equilibrium when the string makes a 15.0 degree angle with the vertical, what is the net charge on the ball?

Answer 1

From the picture we can write that

$T\cos 15^o=mg$ ......(1)

$T\sin15^o=qE$ ......(2)

From 1 and 2

$\tan 15^o=\frac{qE}{mg}$

$q=0.27\times0.002\times10/E=0.0054/E$