# Thread: Ball in parallel plates

1. This is a hw problem so please explain your process instead of telling me the whole answer, thanks!

There is a conductive ball hanging from a string in a electric plate.

As you can see its slightly tilted toward the negative end.

mass of ball: 0.04g
potential difference: 480V
separation of plates: 0.06m
angle of string hanging: 20 degrees

the question asks: What is the charge of the ball?

I drew a free body diagram of three forces.
one: force of tension
two: gravity/weight
three: force of charge pointing towards the right

I think I'm suppose to add up the three forces to equal zero.
F1 + F2 + F3 = 0.

I think F3 should use the formula (C)(q1)(q2)/(r^2). Am I right?
If so then what is r???

2.

3. The E-field between the parallel plates of a capacitor is constant, not a function of r. Check out this.
http://www.ac.wwu.edu/~vawter/Physic...ParallCap.html

To find the force, you would use F=E*q
http://en.wikipedia.org/wiki/Electric_field

4. oooo thanks

5. OK so here is what I've done so far:

F_g = m*g = (0.00004kg)*(9.8) = 3.92e-4

F_e = E*q = (480)*(q)

F_t = m*g*cos(20) = 3.684e-4

F_g + F_e + F_t = 0

...

q = 1.58e-6

is this right?????

6. In this case E is not the voltage, it's the electric field. E=V/d

7. Originally Posted by Harold14370
In this case E is not the voltage, it's the electric field. E=V/d
oh woops! thanks again!

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