# HELP on electric field problem

## 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

LowlyPion
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?

its going right

i dont understand. help, pleaseee

LowlyPion
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?

rl.bhat
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.