Alternating Current and Direct Current: Around a hundred and twenty years ago, due to the increasing popularity of electricity, two of the great inventors of all time, Thomas Edison and Nikola Tesla, worked on transmitting electricity. They came up with what they believed was the future of electricity, bringing electricity from the power plants to every household.

Edison believed the direct current to be economical, safe and efficient for transmission of electricity over distances, while Tesla’s belief stayed with the alternating current. Alternating current and Direct current have various sources, and they are measured by different ammeters and voltmeters. In this article, we will study the definition of AC and DC, sources of AC and DC and the advantages of AC over DC.

Electric Current

Definition: The rate of flow of charge through any cross-section is called current. So if through a cross-section, \(\Delta Q\) charge passes in time \(\Delta t\) then \({i_{av}} = \frac{{\Delta Q}}{{\Delta t}},\) and instantaneous current \(i = \mathop {\lim }\limits_{\Delta t \to 0} \frac{{\Delta Q}}{{\Delta t}} = \frac{{dQ}}{{dt}}.\) If the flow is uniform, then, \(i = \frac{Q}{t}.\) Current is a scalar quantity. Its S.I. unit is ampere (A), and C.G.S. unit is emu and is called Biot \(\left({Bi} \right),\) or \(ab\) ampere. \(1A = \left( {\frac{1}{{10}}} \right)Bi\left( {ab\,{\text{amp}}} \right)\)

The direction of current: The conventional direction of current in the direction of flow of positive charge and is opposite to the direction of flow of negative charge as shown below.

A direction is associated with electric current (Opposite to the electron’s motion) conventionally, but it is not a vector. It is because the current can be added algebraically. Electric current obeys the scalar law of addition.

Charge on a current-carrying conductor: The current is caused by electrons (electrons) in a conductor. The number of electrons (negative charge) and protons (positive charge) in a conductor is the same. Hence the net charge in a current-carrying conductor is zero.

Current through a conductor of non-uniform cross-section: Electric current does not change with a change in cross-sectional area for a given conductor. In the following figure \({i_1} = {i_2} = {i_3}\)

Current in Different Situations

1) Due to Translatory Motion of Charge

If \(n\) particles, each having a charge \(q,\) pass through a given area in time, \(t\) then \(i = \frac{{nq}}{t}\)
If \(n\) particles each have a charge \(q\) pass per second per unit area, the current associated with the cross-sectional area \(A\) is \(i = nqA\)
If there are \(n\) particles per unit volume, each having a charge \(q\) and moving with velocity \(v,\) the current through the cross-sectional area \(A\) is \(i = nqvA\)

2) Due to Rotatory Motion of Charge

If a point charge \(q\) is moving in a circle of the radius \(r\) with speed \(v\) (frequency \(v,\) angular speed \(w\) and time \(T\) period ), then corresponding currents \(i = qv = \frac{q}{T} = \frac{{qv}}{{2\pi r}} = \frac{{q\omega }}{{2\pi }}\)

3) When a Voltage V Applied across a Resistance R

The current flowing through the conductor \(i = \frac{V}{R}\) also by definition of power \(i = \frac{P}{V}\)

Current Carriers

Current carriers are the charged particles whose flow in a definite direction constitutes the electric current. In different situations, current carriers are different.

1. Solids: In solid conductors like metals, current carriers are electrons.
2. Liquids: In liquids, current carriers are positive and negative ions.
3. Gases: In gases, current carriers are positive ions and electrons.
4. Semiconductor: In semiconductors, current carriers are holes and electrons.

Types of Electric Current

An electric current is the flow of electric charge through a conductor. Based on the flow of electrons constituting the current, electric current is classified into two types, direct current and alternating current.

Direct Current (DC) 

Direct current abbreviated as DC or dc, is an electric current in which electrons always flow in the same direction. The circuit symbol of a DC source and DC voltmeter is as shown –

DC generators, fuel cells, solar cells, and electric cells are sources of unidirectional current. This is so because DC sources have a constant polarity as their positive and negative terminals are fixed. Here is the graphical representation of a steady direct current. In some cases, even if the magnitude of current changes or pulsates, as long as it moves in one direction only, it is a direct current.

Types of DC Current

Following are the two types of DC current-
1) Pulsating DC

2) Constant DC

Direct current can be obtained from DC sources of a wide range of voltages such as \(1.5~{\text{V}},4.5~{\text{V}},6~{\text{V}},9~{\text{V}},12~{\text{V}}\) etc., for different devices. Most DC circuits are relatively low in voltage. Your car’s battery is approximately \(12~{\text{V,}}\) and that is about as high a DC voltage as most people ever use.

DC can be converted to AC using an inverter, which is used as a power backup device in household works (DC \( \to \) Inverte \( \to \) AC).

Applications of Direct Current

Direct current is used in portable devices such as a flashlight, pocket calculator or CD player. Direct current is also used in computers, television sets and processes like electroplating.

Note:

1. The first commercial electric power transmission was done by Thomas Alva Edison in the late nineteenth century using direct current. 
2. Electric power distribution is nearly all alternating current because of the significant advantages of alternating current over direct current in transformation and transmission.

Alternating Current

An alternating current, abbreviated as AC or ac, is an electric current in which the direction of the flow of electrons reverses at regular intervals of time. They switch back and forth about relatively fixed positions and never get around the circuit. The circuit symbol of an AC source is shown as-

AC has two well established standard voltages that are used around the world: \(220\,{\text{V}}\) and \(110\,{\text{V}}.\)

An AC generator, originally designed by Nikola Tesla, produces an alternating current.

Here is the graphical representation of an alternating current as a function of time. The current is considered to be positive when the electrons flow in one direction and negative when they flow in the opposite direction.

Simple direct current circuits are generally described only by their voltage. But, alternating current circuits require one more characteristic, its frequency, for their description. In most parts of the world, it is \(50\,{\text{Hz}},\) i.e. \(50\) cycles per second. This means that the electrons go forward and back again \(50\) times per second. In most areas of the United States and Canada, the frequency is \(60\,{\text{Hz}}.\)

The current flowing in power lines that reaches wall outlets in homes, schools, and industries is alternating throughout the world.

Electrical devices such as light bulbs and heaters that convert electricity directly into other forms of energy, such as heat, can operate effectively using both alternating and direct currents. However, nearly all modern electronic devices such as laptops, mobile etc., require direct current for their operation. The converters of such devices are designed to convert AC from the household outlet into DC

Conversion of AC to DC can be done using a rectifier diode (AC\( \to \)Rectifie \( \to \)DC)

A question then naturally arises, “Why not dispense entirely with AC and just use DC for all purposes?” It is not advisable because AC has several advantages over DC

Advantages of AC over DC

There are many reasons why electric power is mostly distributed using alternating current throughout the world. They are-

1. The generation of AC is more economical than that of DC.
2. The alternating voltages and hence, current can be easily increased or decreased using a transformer.
3. The alternating voltages can be transmitted economically to distant places at a very small loss of electric power. Using a step-up transformer, AC power, generated at a remote place, is raised to a very high voltage for long-distance transmission. It is then reduced to safe, low voltage in stages using step-down transformers for distribution to individual homes and industries.
4. Further, AC can be easily converted to dc using rectifiers wherever needed.

Summary

1. Direct current is an electric current in which electrons always flow in the same direction.
2. The first commercial electric power transmission was done by Thomas Edison in the late nineteenth century using direct current.
3. Direct current is used in portable devices and in nearly all modern electronic devices and the processes like electroplating etc.
4. An alternating current (AC) is an electric current in which the direction of the flow of electrons reverses at regular intervals of time.
5. The generation of AC is more economical than that of DC.
6. The alternating voltages and hence, current can be easily increased or decreased using a transformer.
7. The alternating voltages can be transmitted economically to distant places at a minimal loss of electric power. 

FAQ’s on Alternating Current and Direct Current

Q.1. What happens if you mix AC and DC current?
Ans: If accidentally AC line touches the DC line connected to the electronic circuit and components, the result may be a bad smell, smoke, flames or even worse. The DC devices are designed to be operated at specific and pure DC voltage and current.

Q.2. Why can’t we use DC in homes?
Ans: Direct current is not used at home because, for the same voltage value, DC is more lethal than AC. Electrolytic corrosion is a disadvantage of direct current(DC).

Q.3. Can you charge the DC battery with AC directly?
Ans: No. We can’t charge the DC battery with AC directly. However, AC can be used to charge a DC battery by using a rectifier.

Q.4. Which is safer, AC or DC?
Ans: An electric shock can induce ventricular fibrillation, which can lead to heart failure and death. Avoiding any form of electric shock is preferable, but DC is considered safer in these circumstances as the human body’s threshold to DC is considerably higher than to AC.

Q.5. Why can we not store AC current?
Ans: The batteries convert the chemical energy to electrical energy. At that same time, we cannot store alternating current in batteries because AC changes its polarity periodically, which means the conventional AC supply has a frequency of 50Hz or 60Hz (50 to 60 times in a second).

Q.6. What are the sources of AC and DC voltage?
Ans: Direct current can come from multiple sources, including batteries, solar cells, fuel cells. Alternating current is derived from Generators and modified alternators. DC power can also be “made” from AC power by using a rectifier that converts AC to DC.

Learn Everything About Electric Generator Here

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