What is current?

  • Electric current is the flow of electrons in a conductor.
  • Electric current (I) can also be defined as time rate (t) of change of charge (q).

                                                      I = q/t    

Concept

Initially, in a conductor, electrons randomly drift in all direction as shown in fig below,

The electric current is always produced due to the potential difference between the two charges. In order to make the electrons move, a small potential difference must be applied across the conductor. So the potential difference (or voltage) is connected across the ends of a conductor as shown in next fig. This applied voltage forces the electrons to move, which produces an electric current.

 Unit

 

  • The unit of current is Ampere (A), named after the famous French scientist Andre- Marie Ampere.
  • Ampere is the measure of the amount of charges moving a given point per unit time.
  • One ampere(A) is the amount of current which causes one coulomb of charge(equal to 6.25 × 1018 electrons) to pass a given point in one second.

     1 Ampere = 1 coulomb / 1 second = 6.25 × 1018 electrons / 1 second

How current will flow?

  • Due to negative charge, electrons move from the negative terminal to the positive terminal, through the conductor. This direction of charge flow is known as the electron current flow.
  • If the positive charge flows out of the positive terminal through the conductor to the negative terminal, it is known as conventional current flow.
  • The electron flow is in a direction opposite to that of a conventional flow.
  • In the early days, it was thought that current is flow of positive charge and hence current always comes out from the positive terminal.
  • Later the concept is well established that an electric current in a conductor actually results due to movement of negative charges from the negative terminal to the positive terminal.
  • Even now many people think that current is movement of positive charges from the positive terminal to the negative terminal.
  • The electron flow is used only when it is necessary to explain effects (as in semiconductor devices like diodes, transistors etc)
  • In basic electrical circuits and devices, we use conventional flow of current.

Types of current

 

  • There are two types of electric current, Direct current (DC) and Alternating current (AC).
  • Alternating current is defined as flow of charge that often changes its direction. As shown in the fig below AC starts from zero covers the positive values, reaches zero and again covers the negative values, reaches zero in the form of a wave called sinusoidal waveform. The AC output obtained even in other forms of waves such as square wave, triangular wave.
  • How many times the direction of AC changes each second is called the frequency, expressed in Hz.
  • Direct current is unidirectional electric current, means that charges does not alter its direction during its flow.
  • AC is better than DC. Because AC can provide any voltage required but DC can provide only a particular voltage which becomes risky to handle during a higher voltage supply.
  • DC is very expensive to transmit over large distances compared to AC.

Applications of AC and DC

  • AC is used in industries, houses, office buildings etc. AC is also used in home appliances such as heaters, refrigerators, toasters etc.
  • DC is popular in applications that contain batteries, example in flashlights, cell phones, etc (these contain an adapter which converts the AC to DC).
  • In India, 50Hz AC frequency is used whereas in some foreign countries, 60Hz AC frequency is used.

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