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I Working with Coulomb's Law

  1. May 12, 2016 #1
    Hello, I am pretty new to physics. I have no formal education on physics, and I have been studying it on my own out of curiousity and fascination. Please correct me if anything that I say is wrong and assume I know nothing. Math is also not my strongest skill, but I am trying to become better.

    For the past couple of days, I have been working with Coulomb's Law. From my understanding the formula used is : (k) q1q2/r^2. I feel pretty comfortable with the formula and I have been doing some practice problems and getting them correct. In all of the questions though, q1 and q2 are given to me. I would like to know how to find the value of "q" without it being given to me.

    If I were to construct my own experiment and put 2 charges at a certain distance from eachother, how would I go about finding the value of the charges?

  2. jcsd
  3. May 12, 2016 #2
    ##F=\frac{k_E \cdot q_1 \cdot q_2}{r^2}⇔q_1=\frac{F\cdot r^2}{k_E\cdot q_2}##
  4. May 12, 2016 #3
    Thanks! What is that called so I can look more into it?
  5. May 12, 2016 #4
    I do not think this equation ##q_1=\frac{F\cdot r^2}{k_E\cdot q_2}## has any name. Just another form of Coulomb law.
  6. May 12, 2016 #5
    From Coulomb's Law, if you measure the separation of the charges, r, and the force on either one, you could only find the product of the two charges, q1q2. If you knew the value of one of the charges then you could use the above formula to find the other. Also, if the two charges were equal, q1 = q2 = q, then Coulomb's Law would give you q^2 so you could calculate q.
  7. May 12, 2016 #6
    How do I measure the force of the individual charge though?
  8. May 13, 2016 #7
    you can measure net force with newton meter.
    Or consider that Newton II law ##F_{net}=a*m## and measure acceleration and mass.
  9. May 13, 2016 #8
    This would get complicated if there is acceleration, i.e. the charges are not at rest, since then the separation distance, r, would be changing and the force would not be constant.

    One possible experiment would be to suspend two charged objects on strings. The Coulomb force would cause the objects to attract or repel, depending on the charges, and the strings would deviate from the vertical. Knowing the mass of the objects and measuring the angle of deviation from the vertical, the Coulomb force can be calculated.
  10. May 14, 2016 #9
    As far are the 2 charged objects are concerned, what material would be the best to use? My knowledge on the subject is extremely limited.
  11. May 14, 2016 #10


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