Why did Coulomb choose 1.6x10^-19 for the charge of an electron?

[kjamha]

Does anyone know why Coulomb chose 1.6x10^-19 for the charge of an electron? (or why he chose 6.25x10^18 electrons for 1 coulomb)

 

[FroChro]

I think your question is ill-posed. I believe that in times of Coulomb they hadn't known that there is some ''atom'' of charge, much less what is its magnitude.

 

[Borek]

Coulomb didn't choose anything. Unit of charge (defined as charge transferred by 1 ampere current in 1 second) was named Coulomb in 1881, and Coulomb died in 1806. At the time Coulomb (unit of charge) was defined, electron was yet to be discovered.

 

[kjamha]

defined as charge transferred by 1 ampere current in 1 second) was named Coulomb .

Thanks for the history!

But isn't 1 Ampere equal to 1 coulomb per second - so are you saying 1 coulomb per sec in one sec? Can you clarify this for me?

 

[cepheid]

Thanks for the history!

But isn't 1 Ampere equal to 1 coulomb per second - so are you saying 1 coulomb per sec in one sec? Can you clarify this for me?

You are concerned, because the definition seems circular (a coulomb is the charge transferred by a current of one ampere in one second, and an ampere is the current produced when one coulomb of charge flows past in one second).

But the definition is not circular, because there is a separate and independent definition of what an ampere is. See my post in this thread:

https://www.physicsforums.com/showthread.php?t=580819

EDIT: Why don't I just quote my post from that thread here:

The coulomb is the fundamental unit of electric charge, so in that sense, it has a somewhat arbitrary definition, just like other base units do. (The kilogram is just defined as the mass of some platinum cylinder in a lab in France).

Actually, however, the coulomb is a derived unit in the SI system, not the base unit. The ampere is actually the base unit (along with the metre, kilogram, and the second), and a coulomb is defined as 1 ampere*1 second. In other words, the coulomb is the amount of electric charge that flows past in one second when you have an electric current of 1 ampere flowing.

So what is the definition of 1 ampere? Since it is a base unit, it should just have an arbitrary definition that is standard and universally adopted (just like for the kilogram). In this case, I think the ampere was chosen as the amount of electric current that is measured to produce an attractive force of 1 Newton between two current-carrying wires whose currents are in opposite directions.

(Anytime you have an electric current, you have a magnetic field. For two parallel, current carrying wires whose currents are moving in opposite directions, the opposing magnetic fields generated for each wire will produce an attractive force between the wires).

EDIT: I just checked, and the definition is that the two wires have to be separated by 1 metre, and the force produced is supposed to be 2e-7 N per metre of length (not 1 Newton as I stated before)

 

[Dickfore]

The real question is why the fine structure constant:
$$\alpha \equiv \frac{k \, e^2}{\hbar \, c} = \frac{1}{137.0}$$
where k is Coulomb's constant has the numerical value that it does.

 

[kjamha]

Thanks Cepheid, but now I am wondering why they chose 2e-7 N - seems odd.

 

[Borek]

To quote wikipedia:

The ampere was originally defined as one tenth of the CGS system electromagnetic unit of current (now known as the abampere), the amount of current that generates a force of two dynes per centimetre of length between two wires one centimetre apart. The size of the unit was chosen so that the units derived from it in the MKSA system would be conveniently sized.