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Homework Help: Magnitude of electric charge on a glass rod

  1. Dec 4, 2014 #1
    1. The problem statement, all variables and given/known data
    I am a physics tutor. I want to show the relationship between electric charges and forces and gravitational forces. I am relating an example as follows.

    If we have a hollow metal sphere the size of the earth with a charge of 1.63E15 C and a piece of tinfoil the surface with a charge of only 2.72E-9 C , the tinfoil would weigh the same as a 250 pound (986 N) person. So I am wondering..

    How much charge can you get if you rubbed a glass rod with fur, i.e. how many coulombs. If I rubbed the glass rod with fur, would it be to heavy to hold?

    Just wondering.

    2. Relevant equations

    3. The attempt at a solution
    Last edited by a moderator: Dec 4, 2014
  2. jcsd
  3. Dec 4, 2014 #2


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    Staff: Mentor

    A glass rod charged by rubbing with fur is capable of lifting small bits of paper against gravity over a few centimeters of distance. So you might be able to estimate the charge from the force it can produce.

    Can you think of anything that might limit the amount of charge that can be deposited on or removed from the surface of the rod?

    By the way, 250 lbs is about 113 kg, weighing about 1112 N at the Earth's surface.
  4. Dec 4, 2014 #3
    Estimating charge by lifting paper might be hard since you don't know the induced charge on the paper. I am now thinking that if I conceptually made a capacitor from two 0.1 X 0.1m plates 1 mm apart and charged the plates with 30 volts I would have the correct charge on the plates. Then I could cut the wire on one of them to isolate the charge. Now a plate would have 2.72E-9 C of charge and weigh about 946 N = 211 pounds.

    Anyway, just trying to relate electrical force to gravitational force.
  5. Dec 4, 2014 #4


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    Staff: Mentor

    I'm not sure where you're getting this "weight" from. If it's an electric force, you'd need a heck of a big charge nearby to attract (or repel) a plate by that amount with that tiny charge.
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