Types of Electric Current - Direct and Alternating Current

Electricity is an invisible energy that constitutes the flow of electrons to perform various operations. Electricity can be converted into any other form such as lighting, heating, traction, and many other forms. In this article let us learn about what is an electric current and various types of electric current.

What is an Electric Current?

Electric current is defined as the flow of electric charge through an electrical conductor. When a potential difference is applied across a conductor, the loosely bound electrons start moving from the negative terminal of the source to the positive terminal of the source as shown below. This movement of electrons in the conductor constitutes the electric current.

Electric Current

Conventional Current :

Prior to electron theory, it was assumed that the current flows from positive terminal to negative terminal i.e., from higher potential to lower potential. This convention is used now although it is known that the actual direction of current is due to the flow of electrons which will be from the negative terminal to the positive terminal of the source.

The material which conducts electric current is called a conductor. The strength of the current will depend upon the flow of charge per unit time. Mathematically it is expressed as,

Current I = Q/t

In the above equation, Q is the charge measured in Coulombs, and t is the time in seconds. Thus the unit of electric current will be Coulomb per second or Ampere.

Types of Electric Current :

Depending upon how the current flow in the circuit, the electric current is classified into,
  • Direct current,
  • Alternating current,
  • Steady current,
  • Varying current.

Direct Current (DC) :

Direct Current, abbreviated as DC, is an electric current where the flow of electric charge is unidirectional. The magnitude of a direct current can vary with time, but the direction of current flow in the circuit remains the same at every time.

In direct current, the electrons move from the negative terminal of the source to the positive terminal of the source. However, the direction of the conventional current flow is taken from the positive to the negative terminal. The below shows the waveform of direct current.

Direct Current

Since the direction of a direct current remains the same, the polarity always remains constant i.e., one pole is always negative and the other pole is always positive. The frequency of direct current is zero as the current only flows in one direction.

Production of Direct Current :

Most of the direct current is produced by sources such as batteries, solar cells, dc generators, and rectifiers.
  • A battery is the best example of a direct current source. A battery is a device in which chemical reactions cause a build-up of electrical energy.
  • A dc generator is an electrical machine that converts mechanical energy into direct current electricity.
  • A photovoltaic cell also known as a solar cell generates direct current electricity from solar energy.
  • A rectifier is an electronic device that converts an alternating current into a direct current using one or more PN junction diodes.

Applications of Direct Current :

Direct current is generally used in low-voltage applications. It is used in a wide range of applications such as electronic devices like flashlights, laptops, cell phones, etc, in electric vehicles, motors, telecommunication systems, and power supplies.

Traction systems equipped with dc series motors use direct current. DC current is also used for high-voltage power transmission in HVDC systems. Compare to high-voltage AC transmission, high-voltage DC transmission has more advantages such as better voltage regulation, less power losses, and high efficiency.

Steady Current :

A steady current is a type of direct current that flows only in one direction and at a constant rate rather than with fluctuating or changing magnitude as shown in the above figure with pure direct current. The current provided by the battery is almost a steady direct current.

Varying Current :

An electric current whose magnitude changes along with variation in time is called a varying current. The direction of the varying current doesn't change but the magnitude changes over time. A varying current can be produced by an electric generator. The below shows the graph between the current and time.

Varying Current

Alternating Current (AC) :

An alternating current is a type of electric current whose magnitude changes continuously with time and polarity changes periodically. Unlike direct current (dc) where the flow of electric charge is unidirectional, the direction of alternating current changes at regular intervals of time.

Generally, a sinusoidal, triangular, or square waveform is used for alternating current. Due to technical and economical reasons, in most of the circuits, the waveform of alternating current will be sinusoidal (or cosine) in nature as shown in the below figure.

Alternating Current

In the above sinusoidal waveform, it can be seen that the alternating current increases steadily from zero with a positive magnitude. The current increases to a positive maximum value and then starts decreasing back to zero completing the positive half cycle.

Now the current changes its direction (i.e., the magnitude becomes negative) and increases to a negative maximum value. Once the current reaches a negative maximum value, again it decreases to zero completing the negative half cycle. In this way, the same cycle is repeated again and again.

Below are some important terms of alternating current waveform,
  • One set of positive and negative half-cycles completes one cycle.
  • The number of cycles completed in one second by the alternating waveform is called frequency (f).
  • The time taken to complete one cycle by the alternating waveform is called the time period (T).
  • The maximum value of current in either direction is called amplitude.

From time period 0 to T/2, the current flows in one direction, and from T/2 to T, the current flows in the opposite direction. Since the current flows in an alternate direction in the circuit it is called alternating current.

Production of Alternating Current :

An alternator is a type of electrical generator designed to produce alternating current. The alternator consists of two main parts, the rotor, and the stator. The rotor rotates inside the stator driven by either a wind turbine, steam turbine, water turbine, etc.

The rotor creates a magnetic field while rotating that passes through the stator. Due to the rotation of the rotor, its magnetic field also rotates and this rotating magnetic field when intersects with the stator winding induces an alternating current.

Applications of Alternating Current :

Alternating current is the most widely used type of current in domestic and industrial applications. In ac system, the power can be generated at high voltages and can be transmitted across long distances easily. The voltage levels in ac systems can be easily changed using transformers.

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