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Basic charge and coulombs

  1. Jun 28, 2015 #1
    why isn't the number of e or p making a Coulomb the same as the reciprocal of the 'basic' charge if the basic charge is defined as a fraction of a Coulomb?

    basic charge = 1.6 x e-19C, but number of p or e constituting a C is 6.25 x e18
     
    Last edited by a moderator: Jun 28, 2015
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  3. Jun 28, 2015 #2

    Orodruin

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    Yes? 1/1.6e-19 = 6.25e18
     
  4. Jun 28, 2015 #3
    thank you. i may be getting stupid, but why isn't the number of e or p 1.6 e19, if each e or p, basic charge, has a charge

    there's something i'm not getting. if a quantity x was 1/10th of y, it would take 10 x's to make y. why not 1.6 e19 e or p to make 1 C?
     
  5. Jun 28, 2015 #4

    Orodruin

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    Because 1/1.6 is not equal to 1.6. Imagine instead that 1 C was the charge of 20 = 2e1 protons. This would make the basic charge 1/20 = 0.05 = 5e-2 C. Obviously, this is not equal to 2e-1, which it would be if you applied the same logic as the one you just applied.

    Because 10*1/10 = 1 while 1.6e19 * 1.6e-19 = 1.6^2, which is not equal to one.
     
  6. Jun 28, 2015 #5

    Drakkith

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    If quantity X is 0.2 of Y, then 1/0.2 = 5, meaning it takes 5X to equal Y. If X is 15 millionths of Y, then 1/0.000015 = 66,666, so it takes 66,666 X to equal Y.
     
  7. Jun 28, 2015 #6
    still, .2 is 1/5. and 5x = y so if a quantity is 1/ 15 millionth of another it would take 15 million of that quantity to equal that other, as it requires 5 of the amount that is 1/5 (.2) of another to equal that other amount
     
  8. Jun 28, 2015 #7

    Drakkith

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    No it doesn't. I just showed you the math in my post.
     
  9. Jun 28, 2015 #8
    i know, the math makes sense, but i can't see my way around the logical demand that a fractional part of a quantity is that fraction because it takes the amount denominated to equal the whole quantity. the example of .2 follows this logic, five .2s equal the whole, .2 is the fraction 1/5
     
  10. Jun 28, 2015 #9

    SteamKing

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    I have no idea what you mean here.

    That's because 0.2 = (2/10) = (1/5) in its simplest terms, and 5 * (1/5) = 1.

    However,
    1 / 1.6 = 1 / (16 / 10) = (10 / 16) = 5 / 8 = 0.625
     
  11. Jun 28, 2015 #10

    Drakkith

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    Yes, but look at your original numbers.
    basic charge (c)= 1.6 x 10-19C
    C = 6.25 x 1018c

    1.6 x 10-19 is already a fraction equal to 1/6.25x1018, just like 0.2 is a fraction equal to 1/5. You multiply 0.2 times 5 to get 1, and you multiply 1.6x10-19 by 6.25x1018 to get 1.

    If you're getting confused over the fact that 1.6x10-19 is not 1/1.6x1019, then the only way I know of understanding this is to just do the math.
    1.6x10-19 = 1/X
    1.6x10-19X = 1
    X = 1/1.6x10-19
    X = 6.25 x 1018

    Similarly: 0.2 = 1/X
    0.2X = 1
    X = 1/0.2
    X = 5
     
  12. Jun 29, 2015 #11

    andrevdh

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    Let's say it takes N electrons to make up one coulomb of charge then

    N x e = 1 C

    so

    N = 1 coulomb / 1.6 x 10-19 coulomb

    Historycally the coulomb, a certain amount of charge or electrons, was defined
    via the ampere - the amount of current in 2 parallel wires 1 meter apart in a vacuum
    when the magnetic force on one meter of these wires is 2 x 10-7 newton.
     
    Last edited: Jun 29, 2015
  13. Jun 29, 2015 #12

    A.T.

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    Is math not logical enough?
     
  14. Jun 29, 2015 #13

    Orodruin

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    I do not think this is where the OP's confusion lies, but rather in the fact that the reciprocal of 1.6e-19 is not 1.6e19, see post #4.
     
  15. Jun 29, 2015 #14

    andrevdh

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    Yes, that is why I started to mention the definition of the ampere.
    Maybe that might clear it up.
     
  16. Jun 29, 2015 #15
    Thanks, you explained this beautifully
     
  17. Jul 5, 2015 #16
    yeh, math, logical, mind, not so much, sometimes
     
  18. Jul 7, 2015 #17

    andrevdh

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    The mind is much like a muscle.
    The more you use it the stronger it gets.
     
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