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I 3 basic question about Coulomb's Law

  1. Dec 3, 2016 #1
    Hello everyone!

    I'm currently trying to calculate electric forces between two objects, so for that I of course need Coulomb's Law.
    I understood its equation (etc.), but there are 3 things that I haven't found out yet:

    -How exactly can you find out or calculate what amount of charge an object has?
    -Does the amount of charge of an object have any unit?
    -In the equation of Coulomb's law F12 = F21 = k*q1*q2/r² ,what exactly does the "k" stand for? I mean, what exactly is the number?
  2. jcsd
  3. Dec 3, 2016 #2


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    Google !

    17 digits :smile: of k
    the dimension of k tells you the units.
    Is a little harder to answer. Measure ?
  4. Dec 3, 2016 #3


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    This is actually rather vague. Are you asking about finding the charge experimentally (i.e. someone gives you an object and asks you to find the charge it has), or are you asking for a theoretical calculations (i.e,. I give you the E-field that the object emanates and ask you to find the charge, or I say that it exerts a certain force on another charged object at some distance and ask you to find the charge)? Unless you are more specific than what you have written, there's no way to answer.

    This is trivial enough to find out simply via googling.

    Again, this can be found via googling. "k" is a constant IF you are dealing with charges in air/vacuum, because it contains the permitivity and permeability of free space. This value may be different if you are doing this in a medium with different properties.

  5. Dec 3, 2016 #4


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    I'm just curious... you've already got the answer to this, but where did you learn about Coulomb's Law that doesn't tell you the value of the constant? Every introductory textbook that I've used tells you its value right off the bat (or the value of ##\varepsilon_0## if it uses ##\frac 1 {4 \pi \varepsilon_0}## instead of ##k##). So do online references like Hyperphysics and the Wikipedia[/PLAIN] [Broken] article about Coulomb's law.
    Last edited by a moderator: May 8, 2017
  6. Dec 4, 2016 #5
    I've learned it on this website : http://study.com/academy/lesson/electric-force-definition-equation.html
    If you scroll down a bit then you will find the headline :

    Calculating Electric Force Using Coulomb's Law

    and in its text you can find the explanation and the equation of coulombs law, but not the value of the constant "k".
  7. Dec 4, 2016 #6
    I was asking about how it's possible to find out (is there an equation or is it just an experimental thing?) what charge exactly a certain object has. So, if you for example have an atom (just as an example!) with 5 protons, 5 neutrons and 20 electrons, then how can I say what charge the atom has?
  8. Dec 4, 2016 #7


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    Thanks for sharing that. I understand your question a lot better now. Watched the video until they wanted me to sign up and was dazzled by the enormous amount of information they wanted to transfer - most of it raising more questions than answers and all of it very casual. And nothing to lay a sturdy and convincing scientific basis to underpin the whole thing.

    What follows now is my personal impression: You are inquiring and critical and you are looking for depth; and that's not what the video is aiming at. Witness their leaving out any further info on k.

    If you have a good relationship with your physics teacher (which isn't impossible), you could discuss this with him/her and ask for some guidance finding better ways to satisfy your curiosity in a pleasant way. If that doesn't work out, try to find some paper version: a so-called book. Some schools and most universities still have a library and if you're lucky there's a choice of introductory physics textbooks so you can pick one you like. College or first-year university is the level you should be looking for. Don't work through from cover to cover but pick subjects you like (I can guess which) and then check where you have to go back to in order to understand more. Then you can still use google to dig up more and/or find answers to your inevitable questions.

    Keep asking those questions. And build a bottle rocket.
  9. Dec 4, 2016 #8


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    That's easy: proton is +1, neutron is 0 and electron is -1, so you end up with -15. What units ? |electron charges| !

    However, no such atom can be put together. Why not ? For that you need to research the periodic system and isotopes.
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