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

  • Context: High School 
  • Thread starter Thread starter Femme_physics
  • Start date Start date
  • Tags Tags
    Coulomb Electrons
Click For Summary

Discussion Overview

The discussion revolves around the definition of a coulomb and its relationship to the number of electrons it represents, exploring the standards of measurement in physics, particularly in the context of electrical units. Participants question the origins of these definitions and how they relate to fundamental constants.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants inquire about the reasoning behind the specific number of electrons in a coulomb, comparing it to the definition of a mole based on carbon-12.
  • Others explain that measurement standards, such as the definitions of grams and seconds, are arbitrary but based on physical constants.
  • A participant notes that the coulomb is defined in terms of the ampere and time, suggesting a relationship between these units that may not be intuitive.
  • There is mention of proposals to redefine the coulomb based on the elementary charge, which could affect the definitions of other units like the kilogram.
  • Some participants discuss the distinction between the coulomb as a measure of charge and the ampere as a measure of current, emphasizing their different roles in electromagnetism.
  • A participant highlights the potential for choosing different base units in electromagnetism, suggesting that the choice of the ampere as a base unit leads to the current definition of the coulomb.

Areas of Agreement / Disagreement

Participants express various viewpoints regarding the definitions and relationships between the coulomb, ampere, and electron charge, with no consensus reached on the best way to conceptualize these units or their origins.

Contextual Notes

Some discussions involve assumptions about the nature of charge and current that may not be universally accepted. The relationship between the coulomb and the elementary charge is also noted as a point of potential redefinition.

Femme_physics
Gold Member
Messages
2,548
Reaction score
1
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?
 
Physics news on Phys.org
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/
 
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!"
 
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.
 
Got it!
 
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.
 
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?
 
Amok said:
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.
 
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
Femme_physics said:
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.)

Amok said:
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.
 

Similar threads

High School How many electrons does one Ohm prevent/resist?
  • · Replies 12 ·
Replies
12
Views
4K
Undergrad Why Is Electric Charge Measured in Coulombs Instead of Newtons?
  • · Replies 7 ·
Replies
7
Views
2K
Graduate How do we know the mass of an electron?
  • · Replies 28 ·
Replies
28
Views
5K
Graduate Why dielectrics get polarized instead of directly ionized?
  • · Replies 13 ·
Replies
13
Views
2K
High School Does the Electric Field Change When One Charge Moves Relative to Another?
  • · Replies 7 ·
Replies
7
Views
2K
Undergrad Are Electrons in Different Energy States Responsible for Variations in Power Output?
  • · Replies 5 ·
Replies
5
Views
3K
Undergrad Origins of the Coulomb - Understanding How the Unit of Charge Came to Be
  • · Replies 1 ·
Replies
1
Views
5K
Graduate Coulomb's force in atoms and effect on dipoles
  • · Replies 1 ·
Replies
1
Views
2K
High School Help needed with concept of electricity, magnetism and EM
  • · Replies 8 ·
Replies
8
Views
2K
Undergrad Why does an electron shell further away from nucleus has higher energy?
  • · Replies 13 ·
Replies
13
Views
5K