Alternating Current (AC): Electrical Current That Reverses Periodically With Time

Do you need detailed information on Alternating Current? Just read through this article that explains what an alternating current (AC) is.

This article has all the necessary information you require on alternating current. Even mathematical expressions are available.

To make this article easy and exciting to read, I will start by giving you an overview of alternating current (AC). After which, I will discuss the terms associated with it.

In the third section, I will be discussing Ohms Law as applicable to AC circuits. Even so, I will analyze the mathematical aspect as regards AC circuits.

Furthermore, I will talk about power in an AC circuit. Then, you will see how to generate AC by referencing Faraday’s Law of electromagnetic induction.

Not only that, but you will also understand the pros and cons of alternating current. Finally, I will give answers to some frequently asked questions and give you my final thought.

Alternating Current (AC); Overview

Alternating Current (AC); Overview

Electric current is the flow of electrical charges through a conductor in an electrical circuit. Furthermore, these electrical charges flow from the positive terminal of the source through the external circuit. These charges flow back to the source through its negative terminal.

In a simple explanation, the continuous flow of this electric current is the electricity we use daily. In addition, most of the appliances in our homes use electricity. Hence, electricity is part of our daily lives.

The SI unit of current is Amperes (A or Amps). An ammeter is an instrument that measures current. More so, smaller levels of currents can be measured with a milliammeter or galvanometer.

However, there are two types of current which are direct current (DC) and alternating current (AC). The direct current, just as the name implies, is direct and unidirectional. In other words, it moves in one direction without changing.

Even so, this direct current is derived from batteries, rectifiers, and so on. It is the current that flows within all electronic devices like TV, phones, radios, computers, etc. The electrical circuit in which direct current flows is called Direct Current Circuit (DC circuit).

On the other hand, The alternating current reverses direction, unlike the direct current. This article focuses on the alternating current.

Therefore, alternating current is a type of electrical current whose direction reverses periodically with time. Unlike the DC, it reverses its direction between a negative and positive cycle periodically in a regular pattern.

Thus, it is represented graphically by a sine wave. Again, though, they could be shown as cosine waves depending on their phase angle – Generally, they are sinusoidal.

The circuit in which alternating current flows is called Alternating Current Circuit (AC circuit). Particularly, voltage is what causes this current to flow.

Basic Terms Associated With Alternating Current

Basic Terms Associated With Alternating Current


This is a graphical representation of a signal. You can obtain it by plotting the instantaneous values of the quantity like current and voltage with the frequency against time or angle. Also, it has the shape of a wave traveling in a physical medium.

Instantaneous Value

This is the numerical value or expression of a quantity at a particular instant. It is the magnitude of a sinusoidal signal at any point. Note that this value can never be greater than the peak value.

More so, the instantaneous value of the alternating current is denoted by a lower case letter “i.”


A sinusoidal ave makes a complete cycle when the waveform reaches a complete set of positive and negative values. In order words, it has gone through both the positive and negative peaks of the wave and is back to the horizontal axis.

One complete cycle is equal to 360 degrees or 2pi.


A period is the time it takes a sinusoidal wave to complete one oscillation or cycle. The letter “T” denotes the period of a wave. Its SI unit is seconds.

Mathematically, T = 2π/w.


The frequency of an alternating quantity refers to the number of cycles it completes in one second. Also, it is the reciprocal of the period. It is denoted by “f” and is measured in hertz (Hz).


f = 1/T (but T = 2π/w)

f = w/2π

Hence, w = 2πf


w = Angular Velocity measured in radians per second (rad/s).

f = frequency measured is Hertz (Hz).

T = Period measured in seconds (s).

Most countries in the world generate electricity at 50Hz or 60Hz.

Peak Value

The peak value is the maximum value of a sinusoidal wave’s positive or negative cycle. You can also refer to it as the amplitude of the signal. However, the peak-to-peak value is the measure between the positive peak value and the negative peak value.

Phase And Phase Difference

Phase is the position of a sinusoidal signal at a fraction of its period. It is expressed in degrees or radians. Also, phase can be referred to as the expression of displacement between two corresponding features of two signals with the same frequency.

On the other hand, the phase difference is the difference between two waves with the same frequency. In addition, this phase difference is determined at the same reference point with time. Its unit is degree or radian.

Phase Angle

In phasors, the phase angle is the angular component of a complex number. However, for sinusoidal signals, it is the angular equivalent of the displacement between two signals, referenced at a point.

In an AC circuit, it is the angular equivalent of the displacement between voltage and current signals. It is the angular component of the impedance of a circuit. Even so, it is called the power factor angle.

Nevertheless, when two signals reach the maximum, minimum, and zero values simultaneously at the same time, the signals are said to be in phase. If these signals do not reach the maximum, minimum, and zero values at the same time, they are out of phase.

Instantaneous Current, Average Current, And RMS Value Of Current

Instantaneous Current

The Instantaneous current of an AC circuit is the amount of electrical charge or current passing through a conductor at an instant. Furthermore, it has to do with a particular point on the current signal with respect to time. It is denoted by “i.”


i = Io * Sin(wt – ϕ)

Average Current

The average current of an alternating current circuit is the total amount of charge or current flowing through a circuit in an interval of time. It is denoted by “Iav.”


Iav = 0.637 * Io

RMS Value Of Current

The RMS value of an Alternating current is the value of a steady current that will produce the same heat as the alternating current when passed through a conductor. RMS is short for Root Mean Square and is denoted by “Irms.”


Irms = 0.707 * Io


Io = Maximum or peak value of current.

t = time (this is not the same as a period).

ϕ = phase angle or power factor angle.

Ohms Law In An AC Circuit

Ohms Law states that the voltage applied to an electrical circuit is directly proportional to the current passing through the circuit, provided the total resistance and other physical quantities remain constant. Impedance is the total resistance in an AC circuit.


Voltage is the potential difference between both ends of a conductor or circuit. It is the driving force that causes the flow of electrical current in a circuit. More so, its IS unit is volts, and you can measure it with a voltmeter.


Impedance is the total resistance given to the flow of current in an AC circuit. It is the resultant or vector sum of resistance and reactance in a circuit. Also, its SI unit is Ohms and Ohmmeter is the instrument that measures it.

Even so, below is the mathematical expression for impedance as a complex number. Thus, its real part is the resistance, and the imaginary part is reactance.

That is, Z = R + jX


This is the basic opposition to the flow of current in a conductor. It is applicable both in AC and DC circuits. While resistance is the total opposition to the flow of current in a DC circuit, it is only part of the total resistance in an AC circuit.

Resistance is the quantity that causes the heating effect on a conductor. As a result, it brings about heat loss in an AC circuit. Like impedance, its SI unit is Ohms.


This is the resistance to the flow of electric current due to the capacitance and inductance of the circuit. Furthermore, the reactance due to the capacitance of a circuit is known as capacitive reactance. Also, the reactance due to the inductance of a circuit is called inductive reactance.


XC = 1/ 2πƒC

XL = 2πƒL


L = inductance of the circuit.

C = capacitance of the circuit.

f = Frequency of the Alternating Current.

XC = Capacitive Reactance.

XL = Inductive Reactance.

Generally, resistance, inductance, and capacitance are the passive AC circuit components. Therefore, when only resistance and inductance are present in a circuit, it is an RL circuit. Even so, when you have only resistance and capacitance, that is an RC circuit.

However, an RLC circuit comprises resistance, inductance, and capacitance. Note that you can not have only inductance and capacitance. With the presence of a conductor, there is resistance.

In an RL circuit, the load current lags behind the voltage. Hence, the circuit has a lagging power factor.

If it is in an RC circuit, the load current leads to the supply voltage. Thus, the circuit possesses a leading power factor. Read more on the leading and lagging power factors.

Mathematical Resolution Of Ohms Law In AC Circuits

Mathematically, Ohms Law states that V = I * Z

Then, I = V/Z

Writing V and Z with phasor angles, V = V0 and Z = Zϕ

Hence, I = V0 / Zϕ

Therefore, I = (V/Z)(0-ϕ)

Thus, I = I

Writing the voltage and current as an expression:

V = Vm * sin(wt), (where, Vm = maximum voltage)

I = Im * sin(wt-ϕ), (where, Im = maximum current)

However, you can express Z in polar form as Z = Zcosϕ + jZsinϕ

But, R = Zcosϕ, and X = Zsinϕ

So, Z = R + jX

Power In An Alternating Current Circuit

In a circuit, elements dissipate power through the basic P=IV expression. However, not all components of an AC circuit absorb real power. Hence, only resistive components absorb power.

Consequently, the inductive and capacitive elements do not absorb real power. Yet, there is a potential difference (voltage) across them and current flow through them.

Therefore, while active power flows through resistive components, reactive power flow through the capacitive and inductive components.

As a result, there are three types of power in an AC circuit. These are active power, reactive power, and apparent power.

Active Power

Active power is the power that flows through resistive elements in an AC circuit. You can also refer to it as real power. Its SI unit is watts.


P = (I^2) * R, or P = V * I * cosϕ, or P = S * cosϕ.

Reactive Power

Reactive power is the phantom power that flows through reactive elements. Another name for it is Imaginary Power. Its SI unit is volt-ampere reactive (VAR).


Q = V * I * sinϕ, or Q = S * sinϕ.

Apparent Power

This is the vector sum or resultant of both active and reactive powers. Its SI unit is volt-ampere (VA).


S = P + jQ.


S = Apparent Power.

Q = Reactive Power.

P = Real Power.

Generation Of Alternating Current

The voltage source should also be alternating for an alternating current to flow through a circuit. However, AC voltage is a term that describes the potential difference between both ends of a conductor where AC flows.

Therefore, the periodical switching of the potential between two terminals produces an alternating current. It takes the shape of a sinusoidal wave on a graph.

Furthermore, this AC voltage is generated through Electromagnetic Induction. It is a result of Michael Faraday’s experiment. However, this led to the formulation of Faraday’s Law Of Electromagnetic Induction.

The production of AC generators and alternators follows Faraday’s Laws. This involves rotating conductors across a magnetic field. Hence, leads to the current flowing in one direction.

Thus, this current continues to flow in that direction and reaches its maximum when the conductor is at 90 degrees to the field. When the conductor is parallel to the field, the current equals zero. Hence, the current changes direction when the conductor rotates in the opposite direction.

Alternating Current (AC); Pros And Cons


  • It aids in efficient electrical power generation and transmission.
  • Also, it makes power consumption possible.
  • Furthermore, it supplies better lighting.
  • However, it is easier to generate when comparing it with the mechanical generation of DC.
  • Lastly, AC is very much available.


  • It is expensive to utilize.
  • Also, it requires insulation.
  • AC causes heat and spark, which might lead to fire outbreaks or shock.

Alternating Current (AC); Frequently Asked Questions

Frequently Asked Questions
1. What is alternating current in simple words?

Alternating Current is a type of electrical current that switches its direction back and forth at regular intervals.

2. What is the difference between DC and AC?

DC plofws in one direction. That is, it is unidirectional. In comparison, AC keeps changing directions periodically.

3. What is an alternating current circuit?

It is a circuit through which alternating current flows. An AC source always powers the AC circuit.

4. Why DC is not used in homes?

Direct current (DC) is not used in homes because DC does not go through zero. Hence, for an equal valve of voltage, DC is more harmful and dangerous than AC.

5. Why is AC used over DC?

The most outstanding reason why AC is used over DC is that the AC voltages can be easily transformed. You can transform it to higher or lower voltage levels.

6. Where is DC used?

Direct current (DC) is useful in electronic circuits. In order words, all electronic devices use DC with a battery or rectifier for a source.

7. Are solar panels AC or DC?

Solar panels convert the heat energy shining on them from the sun to electrical energy—the charges that the sun’s heat stimulates flow in one direction.
Hence, the solar panel is DC.

8. Is a TV AC or DC?

Alternating current is what is available in the outlets of our homes. Thus, the TV has a design that enables it to accept AC. However, the internal circuit components of your TV use DC. Hence, the TV has a rectifier circuit that converts AC to DC.

9. Is a house AC or DC power?

Domestic buildings and offices use AC power.

What’s an inverter do?

The Inverter is a device that converts direct current (DC) to alternating current (AC).

Alternating Current (AC); Wrap Up

Alternating Current (AC); Wrap Up

The fact that we use alternating current in our homes shows how beneficial it is to humans. Virtually all domestic equipment and appliances use AC.

However, this current type is easy to generate at a high voltage. Thus, making it easy to utilize despite the high cost in some parts of the world.

Therefore, this PowerVersity article has every information you need on Alternating Current. It covers the generation of AC, its uses and setbacks, etc.

Furthermore, it gives mathematical details of every subheading and law, especially Ohms Law. Even so, it gives an analysis of how to express circuit parameters that involve AC.

Thus, you are good to go.

I hope you found this article helpful.

In that case, go ahead and fill off the “Leave A Reply” form at the end of the article to share your thoughts.

Meanwhile, if you find this guide interesting and would wish to read more about its kind, kindly read this:

What Is An Atom?: The Smallest Unit of Matter

What Does Current Mean

Christian Jerome
Christian Jerome
Akan Christian writes for He holds an HND in Electrical/Electronic Engineering. Christian enjoys reading, listening to solemn music, singing, and playing sports. For, Christian writes reviews, buying guides, and best-pick articles.


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