CBSE Class 12 Physics Notes Chapter 3 Current Electricity

CBSE Class 12 Physics Chapter 3 Notes: Current Electricity

List of Content
What is Current?
Ohm’s Law
Drawback of Ohm’s Law
What are Kirchhoff’s rules?
Important Questions

What is Current?

Current is referred to the passing of charge through an area per unit time. For the maintenance of a steady flow of current, it is to be ensured that the circuit is closed and the electric charge is influenced by an external source which moves them from a lower to higher potential energy.

The work done to per unit charge to move the charge is known as electromotive force or emf.

Ohm’s Law

According to this law, the electric current flowing through a substance is proportional to the voltage across its ends. In most of the cases, the carriers of current are the electrons and the rest types are ionic crystals, negative ions, electrolytic liquid etc.

Drawbacks of Ohm’s Law

Every homogeneous conductor containing impurities obeys ohm’s law but there are certain drawbacks to it.

  • If V depends on I in a non- linear manner then the law fails.
  • The relation between I and V depends upon the sign of V for the same absolute value of V.
  • The relation between I and V is non- unique

What are Kirchhoff’s rules?

Kirchoff has suggested two rules, which are:

  • The Algebraic sum of all the changes in potential around the closed loop is zero.
  • In circuit elements junction, the sum of currents leaving the circuit is equal to the sum of currents entering it.


It is a device to compare the potential difference. It can also be used to measure the internal resistance of a cell and in the comparison of two sources

Important Question

  1. Calculate the maximum current if the emf of a storage battery of a car is 25 V and the battery’s internal resistance is 1 Ω
  2. What is the resistance of the resistor? What is the terminal voltage of the battery when the circuit is closed if a battery of internal resistance 10 Ω and emf 20 V and is connected to the resistor and the current in the circuit is 0.2 A
  3. Find the total resistance? If Three resistors 1 Ω, 2 Ω, and 3 Ω are combined in series.

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Practise This Question

Name the device which is used to measure the distance travelled by a vehicle.