Charge on object between two charged plates Problem

[Galileo_Galilei]

Homework Statement

http://aycu16.webshots.com/image/13135/2004772066267823450_rs.jpg

The mass of the ball is 40 milligrams, the applied potential difference is 480 V and the plate separation is 6 cm. If the string hangs at an angle of 20deg to the vertical, determine the charge on the pith ball.

Homework Equations

E = V / d

E = F / q

The Attempt at a Solution

I figured first i should use E = v/d so i did

E = 480/0.06 = 8000

Then i found F using trig and vectors

Cos20 = (40*10^-6) / F

Thus F = 4.26*10^-4 N

Then i plugged in my F and E into the equation q = F / E

q = 4.26*10^-4 N / 8000 = 5.3*10^-9 C

Which is 5.3 nC

But the correct answer is supposed to be 18nC

Forever grateful for assistance.

 

[hage567]

You must consider all forces, including the tension in the string. That will affect where the ball hangs. Draw a free body diagram. Resolve the tension into its components, and sum up the forces in each direction.