B Understanding the Relationship Between Coulomb and Ampere in Physics

[Elementi]

TL;DR Summary

Coulomb, base units.

Hi!
Why coulomb is not a base unit in Physics, but the Ampere is (and ampere is coulomb/s)?
Isn't it like saying that m/s is a base unit?
Can someone explain it to me?

 

[Baluncore]

Welcome to PF.

amp = coulomb / second ;
The amp is easier to measure at any instant.

coulombs = amps ⋅ seconds.
The coulomb requires integration over a period of time.

 

[Elementi]

Hi Baluncore,
Thank you for your explanation.
So, because it is easy to measure amp, it is a base unit. Correct?

Even if 1coulomb is a specific amount of electrons. And so, it looks like a base unit, but it is not.

 

[vanhees71]

With the revision of the SI the definition of the electromagnetic units indeed has been shift rather from defining the Ampere (as the unit for electric currents) to defining the Coulomb (as the unit of electric charge):

The ampere, symbol A, is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary charge e to be $1.602176634 \cdot 10^{−19}$ when expressed in the unit C, which is equal to A⋅s, where the second is defined in terms of $\Delta \nu_{\text{Cs}}$.

That the base unit is not the Coulomb, C, but the Ampere, A, is due to historical reasons, where the additional electromagnetic base unit was chosen as the unit of currents, because at this time one could measure currents much more accurately than charges.

The new SI is a great progress, because now everything is fixed by fixing natural constants, such that in principle you can reproduce the units precisely everywhere in the universe.

 

[Elementi]

Vanhees71! This makes so much sense! I really appreciate your explanation. Thank you!

 

[Dale]

To be clear, the ampere remains the base unit, not the coulomb. See section 2.3.1 of the SI:

https://www.nist.gov/pml/special-publication-330/sp-330-section-2#2.3.1

However, with the revision of the SI the distinction between base and derived units is unnecessary. See section 2.3

https://www.nist.gov/pml/special-publication-330/sp-330-section-2#2.3

“Defining the SI by fixing the numerical values of seven defining constants has the effect that this distinction is, in principle, not needed, since all units, base as well as derived units, may be constructed directly from the defining constants. Nevertheless, the concept of base and derived units is maintained because it is useful and historically well established”

So the SI still defines the ampere as the base unit but indicates that it is merely a matter of history.

 

[vanhees71]

The history of the SI units is anyway very interesting. In some sense it reflects, of course, the history of physics since the time of the French revolution.

 

[A.T.]

The history of the SI units is anyway very interesting. In some sense it reflects, of course, the history of physics since the time of the French revolution.

Yeah, like having a base unit with a prefix (kilogram).

 

[Baluncore]

Yeah, like having a base unit with a prefix (kilogram).

Or the US customary meter, often assumed to be identical to the SI metre.