CBSE Class 12 Physics Notes Chapter 3 Current Electricity

Have you ever flipped on a light switch and instantly expelled darkness, or felt the rhythmic hum of your morning coffee maker? From the flicker of a phone screen to the zoom of a train, they are all powered by the captivating flow of current electricity.

Electric current represents the dynamic movement of charge carriers, typically electrons, through a conductive medium. Think of it like water rushing through a pipe. The more forceful the flow, the brighter your lightbulb shines. The wider the pipe, the faster your phone charges. This intricate interplay between current, voltage (think of it as water pressure), and resistance (obstacles in the pipe) governs every electrical whirr and click in our modern world.

The CBSE Class 12 Physics Current Electricity chapter lays the foundation for understanding electric circuits. It delves into resistor combinations, temperature effects, and Wheatstone Bridge, offering a comprehensive overview of the chapter's essential ideas and their relationships. The current Electricity chapter is a fundamental building block in Class 12 Physics.

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Class 12 Physics Chapter 3 Notes - Current Electricity

Electric Current

• Electric current is the flow of electric charge.
• Symbol: Represented by the symbol 'I.'
• Unit: Measured in Amperes (A), where 1 Ampere equals 1 Coulomb of charge flowing per second.

Formula of Electric Current:

I = Q/t

Where Q is the charge and t is the time

I = ne/t

Where n is the number of electrons, e is the charge of an electron, and t is the time.

Electric Current

Electric Current in Conductors

• Electric current in conductors refers to the flow of electric charge carriers, typically electrons, within a conductive material.
• The direction of current flow is opposite to the direction of electron flow.
• Conductivity: The ability of a material to allow the easy flow of electric current characterizes conductors. 
• Metals are excellent conductors due to their free electron availability.

Ohm’s Law

• Ohm's Law states a direct proportionality between voltage (V) across a conductor, current (I) flowing through it, and resistance (R) within the conductor.
• Mathematical Expression: V=IR, Where, V is the voltage, I is the current, and R is the resistance.
• The current flowing through a conductor is directly proportional to the voltage across it, provided the temperature remains constant.
• Ohm's Law is fundamental for understanding and calculating the behaviour of electric circuits.

Ohm's Law

Drift Of Electrons And The Origin Of Resistivity

• Electrons in a conductor experience collisions with atoms, leading to resistance.
• Drift Velocity: Average velocity attained by electrons in the presence of an electric field.
• Electrons collide with atoms as they drift, leading to resistance. 
• The collective effect of these collisions results in the resistivity of the material.
• Resistivity arises from factors like the density of charge carriers, their mobility, and collisions with lattice vibrations. 

Formula of Drift velocity:

v_d = eE\(\tau\)/m

Where

• E is the electric field
• e is the charge of an electron
• \(\tau\) is relaxation time or mean free time
• m is the mass of the electron

Mobility

• Definition: Mobility is the ability of charge carriers to move in response to an electric field.
• Mobility is directly related to drift velocity; materials with higher mobility allow charge carriers to attain higher drift velocities for a given electric field.
• Relation: Mobility (μ) is inversely proportional to resistivity (ρ): μ= 1/ρ

Limitations of Ohm’s Law

Some limitations of Ohm’s law include - 

• Temperature Dependency: Ohm's Law assumes a constant resistance, but in reality, the resistance of conductors can vary with temperature. 
  • As the temperature increases, the resistance may also change.
• Non-Ohmic Materials: Ohm's Law does not apply to all materials. 
  • Some materials, especially semiconductors and insulators, may exhibit non-ohmic behavior, where the relationship between current and voltage is not linear.
• Validity Under Limited Conditions: Ohm's Law is most accurate and applicable to conductors under steady-state, linear conditions. 
  • It may not hold in situations involving rapidly changing currents, high frequencies, or complex circuit components.

Resistivity Of Various Materials

• Resistivity quantifies the inherent resistance of a material to the flow of electric current.
• Different materials have different resistivities (ρ).
• Metals generally have low resistivity, while insulators have high resistivity.

Resistivity Of Various Materials

Temperature Dependence Of Resistivity

• Resistivity is temperature-dependent. 
• In some materials, resistivity increases with temperature (e.g., conductors), while in others, it decreases (e.g., semiconductors).

The variation of resistivity with temperature is given by

ρ = ρ_o( 1 + αΔT)

Where

• ρ_o is original resistivity
• ρ is final resistivity
• α is temperature coefficient of resistivity
• ΔT is the change in temperature

Electrical Energy and Power

• Electrical Power (P): Power is the rate at which electrical energy is consumed or produced in a circuit. 
  • It is given by the product of voltage (V) and current (I) and is measured in watts (W). 
  • The formula is P = VI or P = I²R or P = V²/R
  • The SI unit is watt (W).

• Electrical Energy: Electrical energy is the amount of work done or energy transferred when an electric current flows through a circuit. 
  • It is measured in watt-hours (Wh) or kilowatt-hours (kWh).
  • Given by E = Pt, where 't' is time.

Electrical energy

Combination Of Resistors – Series And Parallel

• Series Combination: Total resistance (Rtotal) is the sum of individual resistances.
  • Series: R_total = R₁ + R₂ + R₃ + … (sum of individual resistances)
• Parallel Combination: The reciprocal of total resistance is the sum of reciprocals of individual resistances.
  • Parallel: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃ + … (reciprocal sum of individual resistances)

Combination Of Resistors – Series And Parallel

Cells, Emf, Internal Resistance

• Cell: Device converting chemical energy into electrical energy.
• Electromotive Force (EMF): Maximum potential difference across a cell when no current is flowing.
• Internal Resistance (r): Resistance within the cell affecting its ability to provide current.

Cells In Series And In Parallel

• Series Connection: EMF adds up, internal resistances add up.
• Parallel Connection: EMFs remain the same, and reciprocals of internal resistances add up.
• Series connections are used in devices requiring higher voltage, like flashlights. 
• Parallel connections are used in applications demanding more current, such as electric vehicles.

Wheatstone Bridge

• Used for precise resistance measurements.
• Principle: The Wheatstone Bridge is a circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit.
• Balancing Condition: The bridge is considered balanced when the ratio of resistances in one leg equals the ratio in the other, resulting in no current flow through the detector.

Wheatstone Bridge

There are Some important List Of Top Physics Questions On Current Electricity Asked In CBSE CLASS XII

Current Electricity, along with the first two chapters of Class 12 Physics have a weightage of 16 marks in CBSE Class 12 Physics Exam 2024.