CBSE Class 12 Physics Notes Chapter 3 Current Electricity

For students preparing for the exam tomorrow, CBSE Class 12 Physics Notes Chapter 3 Current Electricity is a must for quick and effective revision. With the exam scheduled on 20 February 2026, these notes help you focus on important concepts like Ohm’s law, resistance, resistors in series and parallel, electric power, and internal resistance. They are designed to make last-minute revision easy, clear doubts, and boost your confidence before entering the exam hall.

CBSE Class 12 Physics Notes Chapter 3 PDF

CBSE Class 12 Physics Notes Chapter 3 Current Electricity PDF

CBSE Class 12 Physics Notes Chapter 3 Current Electricity

Current electricity

Electric Current

Electric current is the flow of electric charge through a conductor. In most cases, this charge is carried by electrons moving through a wire or other conductive material. The amount of electric charge that flows through a conductor per unit time is defined as the current which is represented by the symbol $I$ .

Current Carriers

Current carriers are the charged particles that move through a conductor to create an electric current. Depending on the type of material (solid, liquid, or gas), the nature of these current carriers can vary:

In Solids :

• The most common current carriers in solids, especially metals, are electrons . In metallic conductors, free electrons move through the lattice of positive ions when an electric field is applied, resulting in a current.
• Metals have a large number of free electrons that are not bound to any particular atom, allowing them to move easily through the material.

In Liquids :

• In liquids, the current is carried by ions . When an electric field is applied, positive ions (cations) move towards the negative electrode (cathode), and negative ions (anions) move towards the positive electrode (anode).
• This movement of ions constitutes an electric current. An example of this is the conduction of electricity in saltwater or any other electrolyte solution.

In Gases :

• In gases, current carriers are a mix of electrons and ions . Normally, gases are poor conductors, but under certain conditions (like high voltage), they can become ionized.
• In ionized gases, free electrons and ions move under the influence of an electric field, resulting in a current. This phenomenon is commonly seen in neon signs or lightning.

Drift Velocity

Drift velocity is the average velocity that a charged particle, such as an electron, attains due to an electric field applied across a conductor. It represents the net motion of charge carriers in a conductor when an electric field is applied. Drift velocity is different from the random thermal velocity of the particles, which is much higher.

Ohm’s Law

Ohm’s Law is a fundamental principle in electrical engineering and physics that relates the voltage, current, and resistance in an electrical circuit. It states that the current through a conductor between two points is directly proportional to the voltage across the two points, provided the temperature remains constant.

Drawbacks of Ohm’s Law

Non-Linear V-I Relationship : Ohm's Law assumes a linear relationship between voltage and current. If the relationship between V and I is non-linear, Ohm's Law does not apply. This non-linearity can occur in devices like diodes and transistors, where the resistance changes with the voltage or current.

Dependence on Voltage Sign : In some materials and devices, the relationship between current and voltage can depend on the sign of the voltage. For example, certain semiconductor devices may exhibit different behaviors when the polarity of the applied voltage changes, leading to a non-Ohmic behavior.

Non-Unique Relation : Ohm's Law implies a unique resistance for a given conductor. However, in practical scenarios, the resistance can vary with factors like temperature, frequency of the applied voltage, and the presence of impurities. This makes the relation between current and voltage non-unique in some cases.

Electrical Resistance

Electrical Resistance is a measure of how much a material opposes the flow of electric current. It quantifies how difficult it is for electric charge to pass through a conductor or component. Understanding electrical resistance is crucial for analyzing and designing electrical circuits.

Colour Code for Carbon Resistors

Colour Code for Carbon Resistors is a system used to identify the value of a resistor through a series of coloured bands printed on its body. Each colour represents a specific digit or multiplier, and the combination of these colours gives the resistance value of the resistor. This code system simplifies the process of reading resistor values without needing to measure them directly.

First Band : Represents the first significant digit of the resistor value.

Second Band : Represents the second significant digit.

Third Band : Represents the multiplier (which tells you the power of ten by which the combined digits should be multiplied).

Fourth Band (if present): Represents the tolerance, which indicates how much the actual resistance might vary from the stated value.

What Are Kirchhoff’s Rules?

1. Kirchhoff’s Voltage Law (KVL)

Statement : The algebraic sum of all the voltages around any closed loop in a circuit is zero.

Explanation : KVL is based on the conservation of energy. In a closed circuit loop, the total amount of energy gained from voltage sources (like batteries) must be equal to the total amount of energy lost in resistive elements (like resistors). Therefore, when you sum up all the voltage rises and drops around a loop, they must add up to zero.

2. Kirchhoff’s Current Law (KCL)

Statement : The sum of currents entering a junction (or node) in a circuit is equal to the sum of currents leaving the junction.

Explanation : KCL is based on the principle of conservation of electric charge. At any electrical junction, the amount of current flowing into the junction must equal the amount of current flowing out. This ensures that charge is neither created nor destroyed at the junction.

Potentiometer

Tolerance percentage

Combination of resistors

• In series
• In parallel

Resistors in parallel

Some important properties

• Equivalent resistance R is

1/R = 1/R1 + 1/R2 + 1/R3 + 1/R4

• The potential difference across each resistor is the same.
• The electric current ‘I’ drawn from the source equals the sum of currents flowing through individual currents.

I = I1 + I2 + I3 + I4

Preparation Strategy for CBSE Class 12 Physics Notes Chapter 3 Current Electricity

To score well in tomorrow exam, 20 February 2026, follow this focused preparation strategy for effective last-minute revision:

Understand the Key Concepts:
Make sure you clearly understand the main ideas like current, voltage, resistance, Ohm’s law, and how electric circuits behave. Questions often test your conceptual clarity.

Revise Series and Parallel Combinations of Resistors:
Focus on how resistors combine in series and parallel and how it affects total resistance, current, and voltage. Be ready to answer numerical and theory-based questions.

Go Through Internal Resistance and Power:
Review how internal resistance affects a cell and the concept of electric power and energy in circuits. Understanding these helps in solving both numerical and practical questions.

Practice Important Diagrams:
Revise key diagrams such as simple circuits, series-parallel resistor arrangements, and cells with internal resistance. Diagrams often fetch marks in theory answers.

Use Notes for Quick Revision:
Go through your CBSE Class 12 Physics Notes Chapter 3 Current Electricity for a rapid last-minute revision. Focus on the highlighted points and examples to clear doubts quickly.

Stay Calm and Confident:
Keep your mind relaxed, revise smartly, and step into the exam hall with full confidence.