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Takeaway Highlights

Learn what AC voltage is.

Understand how AC voltage is generated.

Explore the behavior of resistors, capacitors, and inductors in AC circuits.

So what is AC voltage?

AC stands for alternating current and refers to the way electrons alternate in a conductor. In electronics, electrons move from a negative potential to a positive potential. Alternating current is produced by switching the potential between two terminals in a fixed time interval, the frequency.

The potential difference between the positive and negative terminals is expressed in volts. Therefore, the term alternating voltage is used to determine the value of the potential difference between the terminals through which the alternating current passes.

An AC source that powers a load.

When plotted on a graph, AC voltage takes the form of a sine wave. In one cycle, the AC voltage starts at 0V, rises to its peak, returns through 0V to its negative peak, and returns to 0V.

Since the value of the alternating voltage varies during the cycle, it is expressed in its peak (Vpeak) and root mean square (Vrms) values.

Vpeak refers to the maximum amplitude of the sine waveform, while Vrms is derived using the following formula:

Vrms = Vpeak x 0, 7071

Vrms is also identified as Vac. Represents the equivalent voltage supplied by DC. In the USA the grid provides 120 VAC while the UK uses 230 VAC.

How is AC voltage generated?

A simple AC generator that powers a lamp.

Alternating voltage is made possible by Faraday's law of induction. The law specifies how electric currents can be induced in a voice coil as it passes through the magnetic flux at right angles. The actual change is proportional to the rate of change of the magnetic flux.

Alternators or generators are components built on the basis of Faraday's law. They involve the rotation of a circuit of conductors through a magnetic field. When the circuit breaks the magnetic field, the current begins to flow in one direction and reaches its maximum when the circuit is perpendicular to the magnetic field.

The circuit continues to rotate until the conductor is in parallel with the magnetic flux, resulting in zero current. Current begins to flow in the opposite direction when the circuit begins to cut off the magnetic flux but in an opposite direction.

How resistors, capacitors and inductors work with AC voltage

AC in inductors and capacitors.

Just like the difference in resolution of one side versus the six sides of the Rubik's cube, circuit analysis involving resistors, capacitors, and inductors becomes more complicated with alternating current. Unlike DC voltages, the behaviors of these components are no longer straightforward when used with AC voltages.

Resistance measurement is expressed as impedance (Z) in AC circuits, rather than resistance (R) for DC circuits.

There is no difference with the resistive value, regardless of the magnitude or frequency of the AC voltage. The difference in terminology exists because of how phase difference is taken into account when expressing resistance as a function of voltage and current.

What is most interesting is the behavior of capacitors and inductors when AC voltage is applied. These components behave like an open circuit and a short circuit respectively with an aDC source, but everything changes with AC. Capacitors store and release charge as the AC voltage rises and falls from its peaks.

This behavior causes the voltage to lag 90 degrees relative to the current.

When operated with AC voltage, the resistive property of a capacitor is defined as capacitive reactance, which has the formula:

XC = 1/2πƒC

Meanwhile, with an alternating voltage, the behavior of an inductor is dictated by Lenz's law and begins to oppose the direction of the current which changes the magnetic flux. Therefore, current through an inductor lags the AC voltage by 90 degrees. The behavior is characterized by an inductive reactance, which has the following formula:

XL = 2πƒL

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