Chapter 3 Current Electricity

Class 12 Physics: Chapter 3 - Current Electricity

Introduction

This chapter covers the principles of current electricity, exploring the concepts of electric current, resistance, electromotive force (emf), and the laws governing electric circuits. Understanding these fundamentals is crucial for further studies in electronics and electrical engineering.

Topics Covered

1. Electric Current
2. Ohm's Law
3. Drift of Electrons and the Origin of Resistivity
4. Resistance and Resistivity
5. Temperature Dependence of Resistivity
6. Electrical Energy and Power
7. Combination of Resistors - Series and Parallel
8. Cells, emf, Internal Resistance
9. Kirchhoff’s Laws
10. Wheatstone Bridge
11. Meter Bridge
12. Potentiometer

Electric Current

Electric current is the flow of electric charge. The current I is defined as the charge Q flowing through a conductor per unit time t:

I = Q / t

It is measured in amperes (A).

Ohm's Law

Ohm's Law states that the current flowing through a conductor between two points is directly proportional to the voltage across the two points. It is given by:

V = I * R

Where V is the voltage, I is the current, and R is the resistance.

Drift of Electrons and the Origin of Resistivity

In a conductor, electrons move randomly, but when an electric field is applied, they drift with a velocity known as drift velocity. Resistivity is the property of a material that quantifies how strongly it resists electric current. It depends on the nature of the material and temperature.

Resistance and Resistivity

Resistance R is the opposition to the flow of current in a conductor. It is given by:

R = ρ * (L / A)

Where ρ is the resistivity of the material, L is the length, and A is the cross-sectional area.

Temperature Dependence of Resistivity

The resistivity of most materials changes with temperature. For metals, resistivity increases with temperature, while for semiconductors, it decreases.

Electrical Energy and Power

Electrical energy is the work done by an electric current. Power P is the rate at which electrical energy is consumed or produced:

P = V * I

For a resistor, it can also be given by:

P = I2 * R or P = V2 / R

Combination of Resistors - Series and Parallel

Series Combination

In series, resistors are connected end-to-end. The total resistance R_eq is the sum of individual resistances:

Req = R1 + R2 + ... + Rn

Parallel Combination

In parallel, resistors are connected across the same two points. The reciprocal of the total resistance R_eq is the sum of the reciprocals of individual resistances:

1/Req = 1/R1 + 1/R2 + ... + 1/Rn

Cells, emf, Internal Resistance

A cell provides the electromotive force (emf) needed to drive the current. The internal resistance of a cell is the opposition to the flow of current within the cell itself.

Kirchhoff’s Laws

Kirchhoff’s Current Law (KCL)

The sum of currents entering a junction is equal to the sum of currents leaving the junction.

Kirchhoff’s Voltage Law (KVL)

The sum of the electromotive forces and potential differences in any closed loop is zero.

Wheatstone Bridge

A Wheatstone bridge is used to measure unknown resistance by balancing two legs of a bridge circuit.

Meter Bridge

A meter bridge, a practical application of the Wheatstone bridge, is used to measure unknown resistance by comparing it with a known resistance.

Potentiometer

A potentiometer measures the emf of a cell or the potential difference between two points in a circuit without drawing any current from the circuit.

Important Terms and Meanings

Electric Current

Electric Current: The flow of electric charge, measured in amperes (A).

Resistance

Resistance: The opposition to the flow of electric current, measured in ohms (Ω).

Resistivity

Resistivity: A material's inherent opposition to the flow of electric current, dependent on its nature and temperature.

Electromotive Force (emf)

Electromotive Force (emf): The energy provided by a cell or battery per coulomb of charge.

Internal Resistance

Internal Resistance: The resistance within a cell or battery that opposes the flow of current.

Ohm's Law

Ohm's Law: The relationship V = I * R, stating that current is directly proportional to voltage and inversely proportional to resistance.

Drift Velocity

Drift Velocity: The average velocity of electrons moving through a conductor under the influence of an electric field.

Series Circuit

Series Circuit: A circuit in which resistors are connected end-to-end, resulting in the same current flowing through each resistor.

Parallel Circuit

Parallel Circuit: A circuit in which resistors are connected across the same two points, resulting in the same voltage across each resistor.

Power

Power: The rate at which electrical energy is consumed or produced, measured in watts (W).

Wheatstone Bridge

Wheatstone Bridge: A circuit used to measure unknown resistance by balancing two legs of a bridge circuit.

Meter Bridge

Meter Bridge: A practical application of the Wheatstone bridge, used to measure unknown resistance by comparing it with a known resistance.

Potentiometer

Potentiometer: A device used to measure the emf of a cell or the potential difference between two points in a circuit without drawing current.

Kirchhoff’s Current Law (KCL)

Kirchhoff’s Current Law (KCL): The sum of currents entering a junction equals the sum of currents leaving the junction.

Kirchhoff’s Voltage Law (KVL)

Kirchhoff’s Voltage Law (KVL): The sum of electromotive forces and potential differences in any closed loop is zero.

Electrical Energy

Electrical Energy: The work done by an electric current, measured in joules (J).

Drift of Electrons

Drift of Electrons: The slow movement of electrons in a conductor when an electric field is applied.