alternatingcurrent

Alternating current (AC) is an electric current that periodically reverses direction, in contrast to direct current (DC) which flows in a single direction. In AC systems, voltage and current typically vary with time in a periodic waveform, most commonly a sinusoid. The most familiar frequencies are 50 Hz or 60 Hz, depending on the country. The instantaneous value of the voltage can be described as v(t) = Vmax sin(ωt + φ). The effective or RMS value, Vrms, equals Vmax/√2 and represents the equivalent DC voltage delivering the same power.

AC is advantageous for power transmission because transformers can efficiently raise and lower voltages. Generators in power plants produce AC, and transformers enable distribution over long distances with reduced losses. Most distribution networks use three-phase AC, with three sinusoidal voltages 120 degrees apart, providing smoother power delivery and efficient motor operation.

End users typically receive domestic voltages such as 120 V or 230 V at 50/60 Hz, though actual values vary by region. Electrical devices often require DC, so rectifiers or power adapters convert AC to DC. In circuit analysis, AC behavior is described using impedance, with Z = R + jX and X = ωL − 1/(ωC).

Historically, AC power systems were developed in the late 19th century by inventors such as Nikola Tesla and engineer George Westinghouse, leading to widespread adoption over DC systems in the War of Currents. Safety standards regulate voltage levels, insulation, and frequency to ensure compatibility and protect users.