Alternating Current: Definition, Properties, and Applications

What is Alternating Current?

An alternating current (AC) is an electric current whose direction and magnitude change periodically, where direct current (DC) keeps one direction. That reversal lets transformers raise and lower voltage cheaply, which is why AC moves bulk power across long distances with less loss. Most household appliances draw AC from a wall socket.

The AC waveform’s shape depends on the source and the load. The most common form is a sine wave with its smooth, symmetrical shape. Square, triangular and sawtooth waves also appear, each with its own characteristics and uses.

How is Alternating Current Measured?

One way to measure AC is through its root mean square (RMS) value. The RMS value of an AC waveform equals the DC value that produces the same heat in a resistor. It comes from squaring the instantaneous values, averaging them over one cycle and taking the square root.

The RMS value of a sinusoidal AC waveform is equal to its peak value divided by the square root of two:

The RMS value of a square AC waveform is equal to its peak value:

The RMS value of a triangular AC waveform is equal to its peak value divided by the square root of three:

The RMS value of a sawtooth AC waveform is equal to its peak value divided by the square root of three, the same result as for a triangular wave because both ramp linearly:

Frequency offers another measure of AC. It counts the cycles completed in one second, with the hertz (Hz) as the unit, so a 60 Hz waveform finishes 60 cycles every second.

The frequency of an AC waveform is related to its period, which is the time it takes for one cycle to complete. The period can be calculated by dividing one second by the frequency:

The frequency can be calculated by dividing one second by the period:

Why is Alternating Current Used?

AC holds several practical advantages over DC for moving power around. They include:

• Ease of generation: Spinning a coil in a magnetic field, or a magnet past a coil, generates AC directly, with no commutators or rectifiers of the kind DC generation needs.
• Voltage transformation: AC can be stepped up or stepped down easily by using transformers, which are devices that use electromagnetic induction to change the voltage and current levels of an AC circuit. This allows AC to be transmitted at high voltages and low currents, which reduces the power loss due to the resistance of the wires. It also allows AC to be delivered at different voltages and currents for different applications and devices.
• Power factor correction: AC circuits can be tuned for a better power factor, the measure of how efficiently they use power. Power factor is the ratio of real power (the part doing useful work) to apparent power (the product of voltage and current). A low power factor means power sloshes uselessly as reactive power between source and load, bringing voltage fluctuations, overheating and extra losses. Adding capacitors or inductors cancels part of that reactive component.
• Information transmission: Modulating an AC signal’s amplitude, frequency or phase lets it carry sound or images. Radio, television and cellular transmissions all work this way, over wires or through the air.

What are Some Applications of Alternating Current?

AC powers work across many industries. Examples include:

• Power generation: Most power plants use AC generators to produce electricity from various sources such as coal, natural gas, nuclear, hydroelectric, wind, solar, etc.
• Power transmission: High-voltage transmission lines use AC to transport electricity from power plants to substations across long distances with minimal losses.
• Power distribution: Substations use transformers to step down the voltage and distribute electricity to local areas through low-voltage distribution lines. These lines use AC to supply power to homes, businesses, factories, etc.
• Power conversion: Some devices use rectifiers to convert AC to DC for their operation, such as computers, chargers, LED lights, etc. Some devices use inverters to convert DC to AC for their operation, such as solar panels, electric vehicles, etc.
• Electric motors: Many types of electric motors use AC to create rotational motion from electrical energy. These motors are used in appliances, machines, vehicles, etc.
• Electric heating: Some devices use resistive heating elements or induction coils to convert electrical energy into heat using AC. These devices include electric stoves, ovens, water heaters, furnaces, etc.
• Electric lighting: Some types of electric lamps use filaments or gas discharge tubes to produce light from electrical energy using AC. These lamps include incandescent bulbs, fluorescent tubes, neon signs, etc.
• Communication: Some devices use modulated AC signals to transmit or receive information such as sound or images. These devices include radios, televisions, cellular phones, etc.

Conclusion

Alternating current owes its grip on the grid to one property: periodic reversal. Reversal makes transformation cheap. Transformation makes long-distance transmission efficient. RMS and frequency measurements then make AC easy to specify. From generators through transmission lines to the motors and heaters at the destination, the same reversing waveform does the work.