Current Electricity

Notes

1. Chapter Introduction

Current Electricity deals with the flow of electric charges through conductors. Unlike Electrostatics, where charges remain at rest, current electricity studies charges in motion. This chapter introduces electric current, drift velocity, Ohm's Law, resistance, resistivity, electrical energy, power, cells, Kirchhoff's Laws, Wheatstone Bridge, Meter Bridge, and Potentiometer.

This is one of the highest-weightage chapters in the JAC Board Class 12 Physics Examination because it contains many numericals, derivations, and practical applications.

2. Learning Objectives

After studying this chapter, students will be able to:

Define electric current and current density.

Understand drift velocity and mobility of electrons.

Apply Ohm's Law to solve problems.

Differentiate between resistance and resistivity.

Solve series and parallel resistor problems.

Calculate electrical energy and power.

Apply Kirchhoff's Laws in electrical circuits.

Understand Wheatstone Bridge and Meter Bridge.

Explain the principle and applications of a Potentiometer.

3. All Important Concepts

Electric Current

Electric current is the rate of flow of electric charge through a conductor.

Formula

I=

t

Q

​

where

I = Current

Q = Charge

t = Time

SI Unit: Ampere (A)

Conventional Current

The direction of current is taken from positive terminal to negative terminal, although electrons move in the opposite direction.

Drift Velocity

The average velocity with which free electrons drift under an applied electric field is called drift velocity.

Formula

I=nAeV

d

​

where

n = Number density of electrons

A = Cross-sectional area

e = Electronic charge

V_d = Drift velocity

Current Density

Current flowing per unit cross-sectional area.

Formula

J=

A

I

​

SI Unit

A/m²

Resistance

Resistance is the opposition offered by a conductor to the flow of electric current.

SI Unit

Ohm (Ω)

Resistivity

Resistivity is the resistance of a conductor having unit length and unit cross-sectional area.

Formula

ρ=

L

RA

​

SI Unit

Ω·m

Conductivity

Conductivity is the reciprocal of resistivity.

σ=

ρ

1

​

Ohm's Law

At constant temperature, the current through…