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cy19861126

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## Homework Statement

A small 2.00-g plastic ball is suspended by a 20.0-cm-long string in a uniform electric field, as shown in the below diagram. If the ball is in equilibrium when the string makes a 15.0 degree angle with the vertical as indicated, what is the net charge on the ball? If you can't see the diagram below, please go to http://students.washington.edu/cy1126/15.38.jpg [Broken]

## Homework Equations

F = Eq

F = mg

## The Attempt at a Solution

I drew the free body diagram and identified three forces, weight, tension and electric force. Direction of the weight points down, electric force to the right and tension to the northwest. Since the electric force is parallel to the x-axis, we would not need to worry about the y direction. So my equation came out to be:

Fx = 1.00^3N/C*q - (0.002kg)(9.8N/kg)(sin 15)

0 = 1.00^3N/C*q - 6.70^-5N

q = 6.7*10^-8C

Okay, the number looks okay, but I feel like it's wrong. This is supposed to be a difficult problem, as marked in the book, but I solved it in one step. Moreover, I did not use the 20cm from the problem. Can anyone take a look at my calculation and see what I did wrong?

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