A small 2g plastic ball is suspended by a 20 cm long string in a uniform electric field. If the ball is in equilibrium when the string makes a 15 degree angle with the vertical, what is the net charge on the ball? E=10^3 N/c(answer: 5.25x10^-6)

Work:
I know E=q/r^2
10^3=q/(.2m)(cos15)

3 answers

The answer depends upon the direction of the applied electric field. Is it horizontal?

Let's assume that it is, and that the string tenston force is T. In that case
T cos 15 = M g
T sin 15 = Q E
tan 15 = Q E/(M g) = 0.268
g is the acceleration of gravity and M is the mass.
Solve for Q . Make sure M is in kg

The length of the string does not matter, but the angle does.
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What if the electric field is not horizontal?