ampere
Electric currentexact by definition
The ampere is the SI base unit of electric current, defined since 2019 by fixing the elementary charge at exactly 1.602176634e-19 coulombs. One ampere is a flow of about 6.24 quintillion elementary charges per second past a point in a circuit. Because the definition fixes a constant rather than describing an experiment, the ampere is exact by definition and no longer depends on the old force-between-two-wires realisation.
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Named for André-Marie Ampère, who in the 1820s worked out the force between current-carrying conductors and founded what he called electrodynamics.
About the ampere
Current is a rate of charge flow, \(I = Q/t\), so one ampere is one coulomb per second. Until 2019 the ampere was defined mechanically, as the current that would produce a force of \(2 \ imes 10^{-7}\) newtons per metre between two infinitely long parallel wires one metre apart. That definition had the awkward consequence of fixing the magnetic constant \(\mu_0\) exactly while leaving the elementary charge to be measured. The 2019 revision swapped the two: \(e\) is now exact, and \(\mu_0\) is a measured quantity that happens to sit very close to its old value.
In practice almost nobody realises an ampere by counting electrons. Current is usually derived from Ohm's law by measuring a voltage across a known resistance, with both the volt and the ohm realised from quantum effects. The everyday intuition worth keeping is that amperes describe how much is flowing while volts describe how hard it is pushed, and that a conductor's temperature rise depends on \(I^2R\), which is why wire gauge tables are written in amperes and why doubling the current quadruples the heating.