Concept of Electricity – What is Current?

What is Current?

Electric Current is defined as the rate of flow of net charge through the conductor with respect to time. The direction of current is conventionally is in the direction of flow of positive charge. It is the flow of charge per unit time.

If Q current is flowing through any cross sectional area of conductor in time t then electric current

I = Q/t

The SI unit of charge is Columb. Columb is denoted as C. SI unit of time is second, therefore the SI unit of current is Columb/second. Columb / second is also known as Ampere.

Definition of Ampere

Thus 1 Ampere of current is defined as the flow of 1 columb of charge in 1 second. Ampere is SI unit of current.

1 Ampere = 1 Columb / 1 Second

Concept of Electricity:

As we know that, matter is made up of atoms. Atom consists of electron, proton and neutron. Proton is positively charged, electron is negatively charged and neutron is neural. The value of charge on electron is -1.6×10-19 C and that on the proton is +1.6×10-19 C. As the number of proton and electron is equal in an atom, therefore atom is neutral.

If we look at the structure of an atom, atom consists of Nucleus and Circular Orbits (also called cell). Proton and neutron lie in the Nucleus of atom while electrons revolve around the Nucleus in the circular orbits. Mind here that, centripetal force required for the circular motion is provided by the columb force of attraction between the nucleus and electron.  There is infinite number of orbits around the nucleus.  

When a conductor is connected across the terminals of a battery, the conductor terminals faces a potential difference. Due to this potential difference, an electric filed is set up in the conductor as shown in figure below.


Due to this electric field, the loosely bound electrons in the outermost cell get detached from the influence of Nucleus and become free to move. Thus the atom becomes positively charged due to detachment of electron. Note that theses positive ions are so heavy that they cannot move under the influence of electric field. It is the free electrons which drift by experiencing a force opposite to the direction of electric field as per the Columb’s Law. Due to this force, the free or mobile electrons drifts toward the positive terminal of the battery. Mind the word drift. As under room temperature, the free electrons of the conductor are in random motion, therefore on experiencing an electric field, the electrons starts to drift toward the high potential terminal from the low potential terminal while having some random behavior. The drift of electrons from high potential to low potential actually follows a zig zag path. During the drift of electron, it suffers many collisions with the positively charged ions.

Thus due to the drift of electron, there is a net flow of electrons per unit time. Therefore from the definition of current, a net current flows through the conductor.

Though flow of electron is only responsible for the flow of current, still we take the direction of electric current in a direction opposite to the flow of electron. This is explained as below.

Suppose the electrons moves from left to right. Therefore we can say that, positive ions are moving in opposite direction i.e. from right to left. Due to this, there is virtual movement of positive ions (though positive ions do not move). Hence as per the convention, current flow is in the direction of flow of positive ions or charge.

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