What is the electric charge on the charged polystyrene ball?

[SimpleBurrito]

Homework Statement

A charged polystyrene ball of mass 0.14g is suspended by a nylon thread from a fine glass spring. In the absence of any electric field the spring extends by 30mm. The polystyrene ball is then placed in an electric field that acts vertically upwards, of strength 200kV m^-1, and the spring extends by a further 6mm. What is the electric charge on the ball.

Homework Equations

The Attempt at a Solution

change in Ep = Eqd, so 200kV = 200000 Nc^-1 so 200000*q(how di I get q) * 30mm??

 

[Orodruin]

You need to use the information you are given on how much the spring extends in the absence of an electric field (supposedly by the action of gravity alone). What does this tell you about the spring? How can you use this to determine the electric force on the ball using the other information that you are given?