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Thread: Ball in parallel plates

  1. #1 Ball in parallel plates 
    Forum Junior DivideByZero's Avatar
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    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???

    I'm so lost in this problem please help guys!


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  3. #2  
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    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


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  4. #3  
    Forum Junior DivideByZero's Avatar
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    oooo thanks
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  5. #4  
    Forum Junior DivideByZero's Avatar
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    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?????
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  6. #5  
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    In this case E is not the voltage, it's the electric field. E=V/d
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  7. #6  
    Forum Junior DivideByZero's Avatar
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    Quote 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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