Notes to a video lecture on http://www.unizor.com
For a short introduction to the International System of units (SI) see the previous lecture "SI Intro & Time" within this topic.
Electric Current Units
In the lecture "Magnetism of Two Parallel Straight Line Currents" of the topic "Magnetism of Electric Current" in the "Electromagnetism" chapter of this course we explained the existence of the force between two parallel wires with electric current running in them.
This force can be either attracting or repelling depending on the relative directions of the currents (the same or opposite).
Until recently, the force between two parallel wires was the base to establish the unit of electric current.
This definition was rejected in 2019 in favor of another, based a physical constant, according to the general approach to definition of units of measurement.
The unit of electric current ampere (A) was redefined based on a physical constant - the amount of electric charge of an electron (e).
The unit of electric charge, a coulomb (C), was, on one hand, defined by postulating that the charge of an electron equals to e=1.602176634·10−19C, where, on the other hand, an electric charge of 1 coulomb is an amount of charge going through a conductor with electric current of 1 ampere during the time of 1 second, that is 1C=1A·1s.
Taking ampere as a base unit and coulomb as derived, we can write
e=1.602176634·10−19A·1s
and, taking an amount of electric charge of an electron e as a given physical constant, we can derive an ampere as such an electric current that delivers e·1019/1.602176634 amount of electric charge per second.
Equivalently, we can say that an electric current of 1 ampere delivers 1019 electrons per 1.602176634 seconds.
Fractions of 1A have the same prefixes as with other units:
1 milliampere (1mA) = 10−3A
1 microampere (1μA) = 10−6A
1 nanoampere (1nA) = 10−9A
1 picoampere (1mA) = 10−12A
Multiples of 1A are
1 kiloampere (1kA) = 103A
1 megaampere (1MA) = 106A
1 gigaampere (1GA) = 109A
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