# HELP on electric field problem

redhawks

## Homework Statement

A 2g plastic ball is suspended by a 2cm string in a uniform electric field of 1.0 x 10^4 N/C (the arrows point to the right)

a) is the balls charge positive of negative?
(B) If the ball is in equilibrium when the string makes a 15 degree angle with the vertical, what is the net charge. (15 degree angle going towards where the E arrows point)

## Homework Equations

E=Ck((q1*q2)/r^2)

## The Attempt at a Solution

a) i know it is positive
b) i did work but i can never get the right answer which is 5.3*10^-7C

## Answers and Replies

Homework Helper
I'm not sure which direction your suspended object is going, but for b) consider the angle of the string. Won't it be the sum of m*g down and the electrostatic force horizontally? So isn't tan15 then the ratio of the 2 forces?

redhawks
its going right

redhawks
i dont understand. help, pleaseee

Homework Helper
i dont understand. help, pleaseee

So isn't tan15 then the ratio of the 2 forces?

F = q*E

and

q*E/(m*g) = tan15

q = m*g*tan15/E

Isn't that all there is to it?

Homework Helper
i dont understand. help, pleaseee

The force on the charged ball due to electric field is Eq, and weight of the ball is mg. The ratio of the forces is Eq/mg = tanθ. Solve for q.