# Definitions of the ampere and coulomb.

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#1
I cannot make sense of these two definitions as they cross reference one another.
0
5 years ago
#2
(Original post by jamesgates1)
I cannot make sense of these two definitions as they cross reference one another.
What part exactly are you unsure of? The fact you can define one ampere in terms of charge, and can alternatively define one coulomb in terms of current?

Obviously the definition of an ampere is slightly more complex in reality, but the above is just using definitions and SI units.
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5 years ago
#3
The definition of the SI amp is in terms of the SI base units of force and distance (Newton and meter)

the coulomb is a derived unit from the SI base units for current and time (Amp and second)

it wouldn't be a very sensible thing if the SI units really had circular definitions for obvious reasons
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5 years ago
#4
(Original post by jamesgates1)
I cannot make sense of these two definitions as they cross reference one another.
The ampere is a base SI unit and is not defined in terms of the coulomb. It's defined in terms of force.
The coulomb (not a base SI unit) is then defined in terms of this current and the time of 1 second. The second is also a base SI unit.
There is no cross reference in terms of the basic SI definition of the two.
They are related via time in
current = charge / time.
Is this what you mean?
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#5
Ok, I think I get it. The ampere is first defined in terms of a force and the coulomb is defined from the ampere.
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5 years ago
#6
(Original post by jamesgates1)
Ok, I think I get it. The ampere is first defined in terms of a force and the coulomb is defined from the ampere.
That's it really. It's probably more natural to think of coulomb as a fundamental quantity but historically SI didn't use this because of difficulties in accurately measuring charge. It was easier to define and measure the ampere accurately and then the coulomb followed from that as the amount of charge passing a point in one second when a current of 1 ampere flows.
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