Why does one coulomb equal 6.241 × 10^18 electrons?

  • #1
Femme_physics
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Main Question or Discussion Point

Just like the answer to mole is amount of atoms in 12 grams of carbon12. I wonder, is there a reason behind the amount of electrons in a coulomb?
 

Answers and Replies

  • #2
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Everything you can measure as a standard. For instance how do we define 1 gram? Or how do we define 1 second of time?

We define standards for measurements and then everything is based off of those standards. For instance the second is based off of the time it takes for a cesium atom to emit radiation like 9 billion times times.

If you'd like to read about electrical measurement standards:
http://findarticles.com/p/articles/mi_m0IKZ/is_1_106/ai_82777388/
 
  • #3
Femme_physics
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Well, I suppose my question is where this standard number came from... just like in the case of moles where we know it came from carbon-12. Or did someone just say "that seems like a fine huge random number!"
 
  • #4
Nabeshin
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Well, the ampere is define as "the constant current which will produce an attractive force of 2 × 10–7 newton per metre of length between two straight, parallel conductors of infinite length and negligible circular cross section placed one metre apart in a vacuum" (wiki). The coulomb is then defined as the amount 1 ampere * 1 second.
 
  • #5
Femme_physics
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Got it!
 
  • #6
rcgldr
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It's not a reason, but instead a best guess measurment based on how strong the elementary charge of one electron is (compared to a Coulomb). There a proposals to redefine a Coulomb based on the current best guess. If a Couloumb is redefined to be based on elementary charges, then a kilogram effectively becomes a derived unit.
 
  • #7
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What Nabeshin said is correct. Did you know that the ampere is a fundamental base unit in the SI system and the Coulomb is not? Funny huh?
 
  • #8
Femme_physics
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What Nabeshin said is correct. Did you know that the ampere is a fundamental base unit in the SI system and the Coulomb is not? Funny huh?
Well, they don't really represent the same thing if I got it right. Coulomb is an amount, regardless of time. Whereas ampere is an amount with respect to time. So, they're not really convertible I think.
 
  • #9
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Well yes, but that's the whole point. We usually say the A = C/s when actually we should say C = A*s. What I'm saying is that the Coulomb seems to be "more fundamental", but it was actually defined with respect to some current (what Nabeshin said), hence the strange numbers.

And about the mole question, you answered it yourself. Take any element in the periodic table of elements, if you know its atomic mass, then you also know its molar mass in g/mol! How easy is that? Some textbooks I've seen feature pound-moles (lb-mol), so that you know that a pound-mole of carbon atoms weighs 12 pounds (which means that a lb-mol is a larger number than a mole).

EDIT: you might enjoy this article :

http://www.economist.com/node/18007494
 
Last edited:
  • #10
Redbelly98
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Well, they don't really represent the same thing if I got it right. Coulomb is an amount, regardless of time. Whereas ampere is an amount with respect to time. So, they're not really convertible I think.
Amok wasn't trying to say they are the same. The thing is, we are free to choose one quantity, and only one, in electromagnetism for the purpose of defining all electromagnetic units. Much as one defines the meter, second, and kilogram as base units, and then the units for velocity (m/s), force (Newtons = kg*m/s2), etc. are all defined in terms of just the base units.

Likewise, we could choose the Coulomb as a base unit, and then define an Ampere as the current due to 1 Coulomb flowing through a wire per second. But instead, the Ampere is chosen as the base unit, and a Coulomb is defined as the charge produced by a 1 Ampere current in 1 second. (There is actually a good reason for doing it this way.)

And about the mole question, you answered it yourself...
Actually, she wasn't really asking about the mole. Just using it for comparison purposes, asking if there was perhaps a similar relation between the Coulomb and the electron charge. At least I think that was Femme_physics's intent.
 

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