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Homework Help: How to determine the magnitude of electric force

  1. Jan 10, 2017 #1
    1. The problem statement, all variables and given/known data
    A piece of tape is pulled from a spool and lowered toward a 170-mg scrap of paper. Only when the tape comes within 8.0 mm is the electric force magnitude great enough to overcome the gravitational force exerted by Earth on the scrap and lift it.

    Determine the magnitude and direction of the electric force exerted by the tape on the paper at this distance

    2. Relevant equations
    Coulomb's Law:
    Force (electric)= k(Q1*Q2)/(r^2)

    3. The attempt at a solution

    I was not sure how to approach this problem.
    But I converted 170mg=.17g and 8mm=.008m

    The force is attractive, directed upward

    Since k is 9*10^(9) N*(m^2)/(C^2)
    and we know that the distance between the tape and the paper is .008m
    I assume that we need to find the charges of the tape and the paper.

    I know an electron charge is -1.602*10^(19) C

    I thought that needed to see how many electrons there are in the .17g of tape. But I do not know the chemical makeup of the tape or the paper. So I don't know how many electrons there are. In other words, I am not sure how to find the charges Q1 and Q2
  2. jcsd
  3. Jan 10, 2017 #2


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    Welcome to PF!

    Suppose a calculator of mass 100 g rests on a table top. How much force would be required to lift the calculator off the table?
  4. Jan 10, 2017 #3
    100g= .1kg
    (.1kg)(9.8m/s^2)= .98N?
  5. Jan 10, 2017 #4
  6. Jan 10, 2017 #5


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    Yes. Can you apply the same idea to your question about the electrical force of the tape?
  7. Jan 11, 2017 #6
    I am not quite sure. The calculator scenario is F=ma.
    The tape and paper scenario is an electrical scenario.
    I am just not quite sure where the example is getting at.
    I know that F(mag. of E.field)= q*E
    But I am unsure of how to put these pieces together
  8. Jan 11, 2017 #7


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    The calculator had a mass of 100gm.
    The paper has a mass of 170mg. How much force is required to pull the paper up? Which force is pulling the paper up?
  9. Jan 11, 2017 #8


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    Note that TSny asked you to calculate the magnitude of the force required to lift the calculator. He didn't specify what the source of that force was. Does the source of the force (eg crane, hand, rocket, magnetic, electrostatic) make a difference to the magnitude and direction?
  10. Jan 11, 2017 #9
    Thank you
  11. Jan 11, 2017 #10


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    Good. All you need to do is add the units and the direction of the force.
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