Alternating current, commonly abbreviated AC, is electric current whose direction and usually whose magnitude change periodically with time. In an ordinary AC circuit, electric charge does not simply move continuously in one direction from a source to a load as it does in an idealised direct current circuit. Instead, the voltage driving the current reverses polarity periodically, causing the conventional current direction to reverse as well.
The most familiar waveform is a sinusoidal wave, described mathematically by a voltage such as V equals V peak sine omega t, where V peak is the maximum voltage and omega is angular frequency. The current through a suitable load can similarly vary approximately sinusoidally, although real circuits can produce distorted, pulsed, triangular or otherwise non sinusoidal alternating waveforms. The defining characteristic is therefore periodic reversal or variation rather than the particular sine wave shape. AC is fundamental to modern electrical power systems because it can be generated efficiently by rotating electrical machines, transformed conveniently between voltage levels and distributed over large distances.
The frequency of alternating current specifies how many complete cycles occur each second and is measured in hertz. A frequency of 50 hertz means that the waveform completes 50 cycles per second, while 60 hertz means 60 cycles per second. The period is the reciprocal of frequency, so a 50 hertz waveform has a period of 0.02 seconds. In a sinusoidal AC waveform, the current and voltage may also have a phase difference.
In a purely resistive load, voltage and current are in phase, whereas inductors and capacitors introduce phase shifts. This phase relationship is important because the instantaneous product of voltage and current represents instantaneous power. AC systems are therefore described using quantities such as peak voltage, peak to peak voltage, root mean square voltage, frequency, phase angle and impedance.
Root mean square voltage is especially important in power engineering because it gives the equivalent heating effect of a corresponding DC voltage in a resistor. A domestic mains supply specified as 230 volts AC refers to an RMS value rather than the instantaneous peak, which is substantially higher for an ideal sinusoidal waveform.
AC generation is closely associated with electromagnetic induction. In a typical alternator, mechanical rotation causes magnetic flux through coils to change continuously, inducing an electromotive force according to Faraday's law of electromagnetic induction. As the rotating magnetic arrangement moves through successive orientations, the induced voltage reverses direction periodically, naturally producing alternating voltage. Power stations exploit this principle using turbines driven by steam, water, wind or other energy sources.
The turbine rotates a generator, and electromagnetic induction converts mechanical energy into electrical energy. The resulting AC can then be increased to a high transmission voltage using transformers. High voltage reduces current for a given transmitted power, and because resistive transmission losses depend on the square of current, reducing current substantially reduces heating losses in transmission conductors. At substations, transformers reduce voltage again for distribution and eventual consumer use. This ability to transform voltage efficiently is one of the historical reasons AC became dominant for large scale electrical distribution.
Transformers are especially important to AC systems because electromagnetic induction in a transformer requires changing magnetic flux. An alternating current in the primary winding creates a changing magnetic field in the transformer core, which induces an alternating voltage in the secondary winding. The voltage ratio approximately follows the turns ratio, allowing electrical energy to be transferred between circuits at different voltage levels.
A transformer can therefore step voltage up for efficient transmission or step it down for safer and more practical distribution. An ideal transformer does not create energy, so increasing voltage corresponds to a reduction in current for approximately conserved power, apart from losses. Direct current cannot be transformed between voltage levels using an ordinary stationary transformer because a constant current produces essentially constant magnetic flux after the transient period. Modern power electronics can convert DC between voltage levels, but that requires electronic switching rather than the simple transformer mechanism. AC therefore has an important historical and technological relationship with transformers, even though modern electrical systems increasingly combine AC and DC technologies.
Alternating current behaves differently in circuits containing inductance and capacitance because these components respond to changing voltage and current. An inductor opposes changes in current by producing an induced voltage, while a capacitor stores electric charge and opposes changes in voltage. In sinusoidal steady state, inductive and capacitive effects produce phase differences between voltage and current. Engineers describe these effects using impedance, which generalises resistance to AC circuits.
Resistance dissipates electrical energy as heat, whereas ideal inductance and capacitance store and return energy during each cycle rather than continuously dissipating it. The resulting exchange of energy between electric and magnetic fields is important in motors, filters, oscillators, radio circuits and power systems. Real inductors and capacitors are not perfectly lossless, so practical components also contain resistance and other parasitic effects. At different frequencies, the same circuit can behave very differently because impedance depends on frequency. This makes frequency a fundamental design parameter in AC electronics and electrical engineering.
AC is used extensively in motors because alternating magnetic fields can produce rotating magnetic fields in suitable machine designs. Induction motors are a major example. In a three phase induction motor, three AC currents displaced in phase create a rotating magnetic field in the stator. This changing magnetic environment induces currents in the rotor, producing electromagnetic torque. Three phase AC is particularly valuable for industrial power because it provides efficient power transmission and naturally supports rotating machinery.
Single phase AC is common in domestic applications, while three phase systems are widely used for industrial equipment and electrical grids. AC motors exist in many forms, including synchronous, induction and various electronically controlled designs. Power electronics can also convert grid AC into controlled electrical waveforms for variable speed drives. Thus the modern use of AC extends beyond simply supplying alternating voltage to appliances; it forms part of complex systems involving generation, transmission, distribution, control and conversion.
AC should not be understood as meaning that electrons travel from a power station to a household and then reverse direction throughout the entire transmission network as a single stream. In metallic conductors, electrical energy is transferred through electromagnetic fields and circuit interactions, while the individual conduction electrons undergo relatively small net drift motions superimposed on their oscillatory response to the alternating electric field.
The speed at which electrical signals propagate through a circuit is much greater than the average drift speed of individual electrons. This distinction is important when explaining why a lamp can respond almost immediately after a circuit is energised even though individual electrons drift comparatively slowly. The exact microscopic behaviour depends on material, frequency and electromagnetic conditions, but the macroscopic circuit description treats current as the rate of charge flow through a defined cross section. AC therefore describes the temporal behaviour of current and voltage, not the simple idea of electrons travelling long distances back and forth as a rigid stream.
Alternating current is ultimately a description of time varying electrical motion and energy transfer, particularly important in power engineering and electrical technology. Its defining feature is that current changes with time and ordinarily reverses direction periodically. Frequency, waveform, amplitude, RMS value and phase provide the main quantities used to characterise it. AC can be generated naturally by rotating electromagnetic machines, transformed efficiently using transformers, transmitted economically at high voltage and converted into many other electrical forms using power electronics.
It is used in domestic electricity, industrial machinery, telecommunications, instrumentation and countless other applications. Modern systems also combine AC and DC extensively, with rectifiers, inverters, converters, batteries and renewable energy systems allowing energy to move between different electrical forms. The distinction between AC and DC is therefore fundamental, but practical electrical engineering frequently involves controlled conversion between the two rather than treating them as completely separate technologies.
