Current Electricity

part 1

Basic Concepts of Current

• Electric current and its definition
• Electric current in conductors
• Ohm's law
• V-I characteristics (linear and non-linear)
• Electrical resistivity and conductivity

part 2

Resistance and EMF

• Drift velocity and mobility
• Temperature dependence of resistance
• Electrical energy and power
• EMF and internal resistance of a cell
• Cells in series and parallel

part 3

Circuits and Measuring Instruments

• Kirchhoff's laws
• Wheatstone bridge
• Metre bridge
• Potentiometer — principle and applications

Ohm's law

$V = IR$

★ board favourite

Resistance from geometry

$R = \frac{\rho L}{A}$

★ board favourite

Resistors in series

$R_{eq} = R_1 + R_2 + R_3$

★ board favourite

Resistors in parallel

$\frac{1}{R_{eq}} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3}$

★ board favourite

EMF and terminal voltage

$V = \varepsilon - Ir$ (during discharge)

★ board favourite

Kirchhoff's Current Law (KCL)

$\Sigma I_{in} = \Sigma I_{out}$ at junction

★ board favourite

Kirchhoff's Voltage Law (KVL)

$\Sigma V = 0$ around closed loop

★ board favourite

Wheatstone bridge balance condition

$\frac{P}{Q} = \frac{R}{S}$

★ board favourite

Electric power

$P = VI = I^2 R = \frac{V^2}{R}$

★ board favourite

Electric current

$I = \frac{Q}{t} = nAev_d$

★ frequently asked

Define the terms EMF and terminal potential difference of a cell. Derive the relation between them. Under what condition is the terminal voltage of a cell equal to its EMF?

Two cells of EMF E₁ and E₂ and internal resistances r₁ and r₂ are connected in parallel. Derive the expression for the equivalent EMF and equivalent internal resistance of the combination.

State Kirchhoff's laws of electrical circuits. Using these laws, find the current through the 4Ω resistor in a Wheatstone bridge configuration.

State the principle of a Wheatstone bridge. Using Kirchhoff's laws, obtain the balance condition of the bridge. How is a metre bridge used to determine the unknown resistance of a wire?

A wire of resistance R is stretched so that its length doubles. What is the new resistance of the wire?

Derive the expression I = neAv_d relating the current in a conductor to the drift velocity of electrons. Hence show that resistivity ρ = m/(ne²τ).

Describe the principle of a potentiometer. With a labelled circuit diagram, explain how a potentiometer can be used to compare the EMFs of two cells.

Two wires A and B of the same material and same length have radii in the ratio 2:1. Compare their (i) resistances and (ii) resistivities.

Define drift velocity and mobility. Establish a relation between drift velocity and electric current. Why is Ohm's law not universally applicable?

Define current density and state whether it is a scalar or a vector quantity. Write its SI unit.

How does the resistivity of a conductor change with temperature? Give reason.

Why is a potentiometer preferred over a voltmeter for measuring the EMF of a cell? Explain how a potentiometer is used to measure the internal resistance of a cell.

Plot a graph showing the variation of resistivity with temperature for (i) a metallic conductor and (ii) a semiconductor. Explain the difference in their behaviour.

How many marks is Current Electricity worth in the Class 12 board exam?

Around 6 marks of the 70-mark CBSE Physics theory paper, based on the official unit-wise weightage. Kirchhoff's laws + Wheatstone bridge — usually a 5-marker.

Is Current Electricity easy or hard?

It's rated hard — one of the tougher chapters in Class 12 Physics. Most students need about 5 hours to cover it well.

What are the important topics in Current Electricity?

The chapter covers 14 NCERT topics in 3 areas: Basic Concepts of Current; Resistance and EMF; Circuits and Measuring Instruments.

What questions come from Current Electricity in board exams?

Between 2018–2024, CBSE board papers asked questions from this chapter on EMF and Internal Resistance, Cells in Combination, Kirchhoff's Laws, Wheatstone Bridge — 1- to 5-mark questions.