Electric Current
Electric current represents the directed flow of electric charge carriers, typically electrons or ions, through a conductive medium. It is an SI base quantity denoted by the symbol $I$, forming the quantitative basis for the entirety of electrical and electronic engineering disciplines.
The SI base unit for electric current is the ampere (A). The redefinition of the SI in 2019 revolutionized how the ampere is realized, linking it directly to the fundamental elementary charge.
The Ampere (A)
The ampere is defined by taking the fixed numerical value of the elementary charge, $e$, to be exactly $1.602176634 \times 10^{-19}$ when expressed in the unit C, which is equal to A s. This definition relies on the definition of the second in terms of the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom.
This means that one ampere of current is equivalent to the passage of approximately $6.241509 \times 10^{18}$ elementary charges flowing past a specific boundary per second.
Realization of the Ampere
Because the elementary charge is a discrete, invariant constant, modern metrology realizes the ampere through quantum phenomena rather than macroscopic force measurements. The practical realization involves combining the Josephson effect (quantized voltage) and the quantum Hall effect (quantized resistance).
- Josephson Effect: Generates a highly accurate voltage $V$ related to the Josephson constant $K_J = \frac{2e}{h}$.
- Quantum Hall Effect: Produces a highly accurate resistance $R$ related to the von Klitzing constant $R_K = \frac{h}{e^2}$.
By applying Ohm's law ($I = \frac{V}{R}$), the current $I$ can be determined with exceptional precision. Advanced research also involves single-electron transport (SET) devices, which directly pump individual electrons at a known frequency, effectively generating a current precisely defined by $I = ef$, where $f$ is the frequency of the transport cycle.