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How to measure quantity of electricity without Coulomb law?

  1. Aug 21, 2009 #1
    Or how did Coulomb measure quantity of electricity while doing his experiments that leads him to claim his famous Coulomb's law.

    [tex]F=k\frac{q_1 q_2}{r^2}[/tex]

    You can measure F and r for sure. But how about q1 and q2 ?
  2. jcsd
  3. Aug 22, 2009 #2
    He assumed that quantity...he made a new unit.
  4. Aug 22, 2009 #3
    Let me elaborate a bit. Let's suppose Coulomb assumed:

    [tex]F= k q_1 q_2[/tex] (where r=1)

    He could also decide the quantity q1 is 1c [coulomb] and k=1 as he created the unit of electricity. This way, if we know F, we know q2.

    He would repeat this process using different charges in q2 while keeping q1=1c. Each time, he would measure F to arrive q2.

    This gives him a way to measure quantity of electricity.

    Then, he also supposed:


    and to prove that he would adjust r = r1, r2, r3.. to see how F changes. He could do that with different electric charges and found no inconsistency.

    So, he claimed his law is correct.

    [tex]F= k \frac{q_1 q_2}{r^2}[/tex]


    Does anyone know any book or site that says this was actually what he did ?
    Last edited: Aug 22, 2009
  5. Aug 22, 2009 #4
    According to the below article, Coulomb actually knew how to create equal electric charges without requiring his law.

    http://www.jfinternational.com/ph/coulomb-law.html" [Broken]
    Also, he used cgs unit (http://http://en.wikipedia.org/wiki/Gaussian_units" [Broken]) where k becomes 1.
    Last edited by a moderator: May 4, 2017
  6. Aug 22, 2009 #5
    I think one of the first Coulomb-meters was a gold-leaf in an electrometer inside a Leyden-jar like setup. Equal charge on the two halves of the gold leaf forced them apart, and the force could be calculated by measuring the angle between the two halves. See
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