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Homework Help: General charge question

  1. Feb 1, 2009 #1
    1. The problem statement, all variables and given/known data
    If I have a 24 volt power supply.The positive lead is attached to a disk and the negative lead is attached to another. The disks are separated by about 24 cm. The disks are on a sheet of carbon impregnated paper. Is the charge on the disk 24 volts?

    I am trying to determine the electric field strength at a point directly in the middle of the two disks.

    E= 2*8.99x10^9*q/r^2

    but I am not sure what the charge is on the sphere.
  2. jcsd
  3. Feb 1, 2009 #2


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    Homework Helper

    You've got me confused, or at least the question has! Am I correct in picturing the two disks sitting flat on carbon paper with 24 cm between their outer edges?
    Under these circumstances, I don't think it is possible to determine the charges on the disks, if any. Where is the sphere?

    How could their be a single point in the middle of both disks?
  4. Feb 1, 2009 #3
    Thanks for responding.
    Sorry for the confusion.
    I am supposed to chart the electric field between two point charges.
    And determine the excess charge on each sphere.

    The carbon impregnated paper is used to model the electric field in a plane that passes through the center of two spheres. In this case, pennies.

    So basically, I attached the positive lead from the power source 24v to one disc and the negative lead to the other.

    I attached the negative lead of a voltmeter to the negative disc and used the positive to find the 4v,8v,12v,16v, and 20v equipotential lines on the paper. These locations I plotted on to a graph paper.

    I need to calculate the electric field strenght at a point b which is the intersection of a straight line that runs through the center of the discs and the 12 v equipotential line.

    I know the distance between the discs but I am not sure what the charges on the discs would be.
  5. Feb 1, 2009 #4


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    Oh, interesting! You can actually measure the potential anywhere and find the lines. E field lines are perpendicula to equipotential lines. Neat.

    With all that information, you could use E = V/d to find the potential. Here "V" should be a delta V, the difference in potential between two points at the location you are interested in. Measure the potential difference and the distance between the two points - then you can calculate the Electric field strength.
  6. Feb 1, 2009 #5
    Thank you, I appreciate your response. It helped.
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