Class – 12, Physics, Module – 02,

Current Electricity

Chapter – 01, Electric Current

The word ” electric ” is derived from the Greek word ” elecktron ” and the word ” Current ” is derived from the Latin word ” currere ” which means ” run”. Hence, we can conclude that the ” electric current ” means the running of an electron.

The phenomenon related to moving of electrons in a particular direction is termed as the current electricity.

Electric Current: The rate of flow of electric charges is called the electric current. It is represented by i. It is a scalar quantity though it has direction. As it does not follow the laws of vector, it is a scalar quantity.

Unit of Electric Current: 

SI Unit: Ampere ( represented by A )

CGS unit: Stat ampere ( represented by stat- A )

Expression of the electric current:

Let us assume that the electric charges pass through the electric conductor in ‘ t ‘ sec is q.

The amount of electric charges flowing in t sec = q

The amount of electric charges flowing in 1 sec = q / t

Electric current ( i ) = q / t 

In differential form,

i = dq / dt 

if number of electron ( electronic charge ‘ e ‘ ) is n then

q = ne

i = ne / t 

Current carrier : The particles that carry the electric current thrtough which the electric current will flow, is called as current carriers.

Here is the table through which we may understand the current carrier in different substance.

Serial No Types of conductor examples current carriers
01 Solid conductor all metals free electrons
02 Liquid Conductor( electrolyte ) the solution of CuSO4 etc ions ( positive and negative )
03 Gas ( low pressure and high potential difference ) gasses free electrons and ions

Types of electric current :

Electric Current  


Steady current                           Varying current                          Direct current                         Altering current 

magnitude = constant         magnitude ≠ constant         direction = constant (w.r.t time)     direction changes                                                                                                                                                                        periodically

The direction of electric current:  The direction of electric current is

i ) the opposite of the direction of flowing of the electron.

ii ) the direction of flowing of a positive charge.

iii ) form a higher potential region to a lower potential region.

iv ) form the positive terminal to the negative terminal of a battery.

Electric Potential: We have already learnt about the electric potential in the module – 01 ( Electrostatics ). It is the work done required to bring a unit positive charge from infinity to a particular point in an electric field.

Potential Difference: It is defined as the amount of work done required to move a unit positive charge from one point to another point in the electric field when the electric circuit is closed. In other words, it is the difference in the electric potentials of two terminals of a closed electric circuit.

SI unit → Volt

CGS unit → stat – volt

Ohm’s Law:

Statement:  It states that ” the electric current between the two ends of a conductor or an electric circuit is proportional to the potential difference between the two ends provided the other physical condition like temperature etc should be kept constant”.

Mathematical Form:

Following the figure,

V = potential difference between two terminals

I = Electric current flowing through the electric circuit.

According to Ohm’s law,

∝ V

I = V / R

Where 1 / R is the proportionality constant and R is termed as the electrical resistance.

Graphical Representation:

In the above graphical representation, we can see that the resistance is the slope of the I – V graph and remains constant whatever be the current and potential difference.

The properties of the graph are given below :

i )  The graph is a linear or straight line passing through the origin.

ii ) The slope of the graph is constant.

iii ) Resistance is independent of the electric current as well as the potential difference.

Types of the electric conductor on the basis of Ohm’s law:

There are two types of electric conductor.

1 ) Ohmic conductor: The conductor that follows the ohm’s law and have a linear I – V graph is termed as the ohmic conductor.

Examples: All metals and a resistor etc.

2 ) Non – ohmic conductor:  The conductor that does not follow the ohm’s law and have a non – linear I – V graph is tremed as the non – ohmic conductor.

Example: semiconductor, thermistor, transistor and diod etc.

Resistance: The resistance is the property of a substance by virtue of which it resists to flow the electric current.

According to Ohm’s law, the resistance is defined as the ratio of the voltage to the electric current.

∴ R = V / I 


Its SI unit is Ohm represented by Ω and its CGS unit is stat – ohm i.e stat – Ω.

Factors on which the resistance depends:

1 ) Length of the conductor:

If cross sectional area of an electric conductor and other physical conditions like temperature etc are kept constant, the resistance is directly proportional to its length.

i.e R ∝ l ———————————————– ( 1 )

2 ) Cross sectional area of the conductor :

If length of an electric conductor and other physical conditions like temperature etc are kept constant, the resistance is inversely proportional to its cross sectional area.

i.e R ∝ 1 / A ———————————————– ( 2 )

3 ) Temperature:

Resistance depends on the temperature of the substance.

For conductor, it increases linearly with small increase in temperature.

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