⚡ CBSE · Class 12 · Physics · Chapter 3

Current
Electricity

Complete chapter resources for CBSE Class 12 Physics — topic breakdown, key formulas, sample questions, previous year board questions, and instant AI question paper generation.

5Topics
7–10Board marks
8Sample questions
3PYQ included
Sign Up Free & Generate → See sample questions ↓

Free for independent teachers · No credit card required

Key Formulas — Chapter 3
  • Ohm's Law: V = IR
  • Drift velocity: I = nAev_d
  • Resistivity: R = ρL / A
  • EMF & terminal V: V = ε − Ir
  • Wheatstone bridge: P/Q = R/S
  • Metre bridge: R/S = L / (100−L)
Chapter Overview

What this chapter covers

Current Electricity is one of the highest-weightage chapters in CBSE Class 12 Physics, building on the electrostatics concepts from Chapter 1 and 2. The chapter begins with the microscopic picture of current — the ordered drift of free electrons driven by an electric field — leading to the concept of drift velocity and the relation I = nAevd. From here, Ohm's law (V = IR) is derived and its limitations discussed, alongside the concepts of resistivity and conductivity and their temperature dependence.

The chapter then builds toward circuit analysis using Kirchhoff's laws: the Current Law (KCL — sum of currents at a junction is zero) and the Voltage Law (KVL — algebraic sum of EMF equals sum of IR drops in a closed loop). These tools are applied to derive conditions for the Wheatstone bridge and the practical metre bridge. The concept of EMF, internal resistance, and terminal voltage is central to understanding real-world battery behavior: V = ε − Ir under load.

Board questions test a wide range — from 1-mark MCQs on the nature of resistivity-temperature graphs to 5-mark numericals involving Kirchhoff's equations in multi-loop circuits. Potentiometer-based questions (comparing EMF, finding internal resistance) appear reliably in every CBSE paper and require both conceptual clarity and numerical accuracy to score full marks.

Topics

What's inside Chapter 3

As per NCERT Class 12 Physics Part I (CBSE syllabus)

Topic 1
Electric Current, Drift Velocity & Ohm's Law
Definition of current as charge flow per unit time. Drift velocity vd = eEτ/m. Derivation of I = nAevd. Ohm's law and its microscopic form J = σE. Limitations of Ohm's law.
Topic 2
Resistance, Resistivity & Temperature Dependence
Factors affecting resistance: length, cross-section, material. Resistivity ρ and conductivity σ. Temperature variation: ρT = ρ0(1 + αT). Distinction between conductors, semiconductors, and alloys like Nichrome and Manganin.
Topic 3
EMF, Internal Resistance & Cell Combinations
EMF versus terminal voltage: V = ε − Ir. Cells in series and parallel — equivalent EMF and internal resistance. Conditions for maximum current and maximum power transfer.
Topic 4
Kirchhoff's Laws & Wheatstone Bridge
KCL (junction rule) and KVL (loop rule). Sign conventions for traversing a loop. Application to derive the Wheatstone bridge balance condition P/Q = R/S. Metre bridge as practical implementation.
Topic 5
Potentiometer — Principle & Applications
Principle of potentiometer: potential drop proportional to length. Two applications: (1) comparing EMF of two cells, (2) measuring internal resistance of a cell. Advantage over voltmeter — draws no current at balance.
Connections

How this chapter fits in

Useful for setting question difficulty and cross-chapter papers.

Builds on
Ch 1–2 · Electrostatics
Electric field, potential, capacitors — the static charge foundation for moving charges
Class 11 · Mechanics
Newton's second law applied to electron motion under electric field (drift derivation)
Chapter 3 Current
Electricity
Leads to
Ch 4 · Moving Charges & Magnetism
Current-carrying conductors in magnetic fields — force, torque, Biot-Savart law
Ch 6–7 · Electromagnetic Induction & AC
Induced EMF, impedance, and resonance circuits extend Ohm's law to AC
Board Blueprint

Marks & question-type breakdown

Typical pattern based on CBSE Class 12 Physics board papers from the last five years.

Question type Marks Typical count What's usually tested
MCQ / Assertion–Reason 1 1–2 Drift velocity formula, nature of resistivity-temperature graph, or Ohm's law limitations
Very Short Answer 2 1 Define EMF vs terminal voltage, state Kirchhoff's laws, or find equivalent resistance
Short Answer / Numerical 3 1 Wheatstone bridge balance, metre bridge calculation, or cell combination problem
Long Answer / Numerical 5 1 Kirchhoff's laws on a multi-loop circuit, potentiometer derivation + numerical, or resistivity experiment
Total (approximate) 7–10 4–5 Weightage varies across paper sets and years
Sample Questions

8 sample questions — generated by MarksZen AI

Aligned to CBSE Class 12 Physics Chapter 3. Covers all question types across Easy, Medium, and Hard difficulty.

Q1 Easy 1 mark MCQ
The drift velocity of electrons in a conductor is of the order of: (a) 10⁸ m/s (b) 10³ m/s (c) 10⁻³ m/s (d) 10⁻⁸ m/s
Q2 Easy 2 marks Short Answer
A battery of EMF 12 V and internal resistance 2 Ω is connected to an external resistance of 10 Ω. Calculate the terminal voltage of the battery.
Q3 Medium 2 marks Short Answer
State Kirchhoff's Current Law (KCL) and Voltage Law (KVL). On what conservation principles are they based?
Q4 Medium 3 marks Numerical
In a Wheatstone bridge, P = 100 Ω, Q = 200 Ω, and R = 150 Ω. Find the value of S for which the bridge is balanced. If R is increased by 10%, find the new value of S required to balance the bridge.
Q5 Medium 3 marks Short Answer
Derive the expression for the equivalent resistance of two resistors connected in parallel. Hence show that the equivalent resistance is always less than either individual resistance.
Q6 Hard 4 marks Numerical
Use Kirchhoff's laws to find the currents I₁, I₂, and I₃ in the following circuit: — Branch AB: EMF = 6 V (internal resistance 1 Ω), connected to a 2 Ω external resistor. — Branch CD: EMF = 4 V (internal resistance 1 Ω), connected to a 3 Ω external resistor. — Both branches share a common 4 Ω resistor between nodes B and D. Write loop equations clearly before solving.
Q7 Hard 5 marks Long Answer
With a neat circuit diagram, explain the principle of a potentiometer. Describe how it can be used to compare the EMF of two cells. Derive the formula used. State one advantage of a potentiometer over a voltmeter for measuring EMF.
Q8 Hard 5 marks Word Problem
A nichrome wire of resistivity 1.0 × 10⁻⁶ Ω·m has a length of 2 m and cross-sectional area 0.5 mm². (i) Calculate its resistance at room temperature. (ii) If the temperature coefficient of resistance of nichrome is 0.0004 /°C, find the resistance at 300 °C. (iii) Compare this with a copper wire of the same dimensions (ρ_Cu = 1.7 × 10⁻⁸ Ω·m) and explain which is more suitable for a heating element.
Generate a full paper with answer key →

MarksZen AI creates a complete question paper with answer key in under 2 minutes.

Previous Year Questions

From CBSE board examinations

Actual questions from past Class 12 Physics board papers — Current Electricity chapter.

Board 20223 marks
Define the term 'drift velocity' of charge carriers in a conductor. Obtain the expression for the current flowing in a conductor of cross-sectional area A in terms of drift velocity. (CBSE All India 2022)
Board 20232 marks
In a metre bridge, the null point is found at a distance of 40 cm from end A when a resistance of 12 Ω is connected in the left gap. Find the value of the unknown resistance in the right gap. (CBSE Delhi 2023)
Board 20205 marks
Using Kirchhoff's laws, find the current through each branch of the following network: a circuit with two batteries (ε₁ = 10 V, r₁ = 1 Ω; ε₂ = 8 V, r₂ = 2 Ω) and three resistors (R₁ = 2 Ω, R₂ = 4 Ω, R₃ = 6 Ω) arranged in a two-loop configuration. (CBSE 2020)
For Teachers

Create a board-aligned
question paper in 2 minutes.

Pick chapter, set the question-type mix and total marks — MarksZen AI generates the full paper with answer key. CBSE, ICSE, and all State Boards supported.

  • All 5 topics of this chapter
  • MCQ + short answer + numericals
  • Answer key included
  • PDF export ready
Sign Up Free & Generate →
FAQ

Questions teachers ask

How many marks does Current Electricity carry in the CBSE Class 12 Physics board exam? +
Current Electricity typically carries 7–10 marks in the CBSE Class 12 Physics board exam. Questions are spread across MCQs (1 mark), short-answer questions (2–3 marks), and one long-answer or numerical problem (5 marks). Kirchhoff's laws, Wheatstone bridge, and drift velocity are the most frequently tested concepts.
What is the difference between EMF and terminal voltage, and why do board questions focus on it? +
EMF (electromotive force, ε) is the maximum potential difference delivered by a cell on open circuit. Terminal voltage (V) is the actual voltage across the cell's terminals when current flows: V = ε − Ir, where r is the internal resistance and I is the current. Board questions frequently ask students to distinguish the two, calculate internal resistance from given EMF and terminal voltage, or explain why terminal voltage is less than EMF under load.
How do I apply Kirchhoff's laws correctly in board exams to avoid losing marks? +
State both laws before applying them: KCL (sum of currents at a junction = 0) and KVL (sum of EMFs = sum of IR drops around a closed loop). Always assign current directions first — if your answer comes out negative, the actual direction is opposite. Write separate equations for each loop and junction. CBSE examiners award step marks, so showing the loop equations clearly is as important as the final answer.
What is the Wheatstone bridge condition and how is it applied in board numericals? +
A Wheatstone bridge is balanced when P/Q = R/S, making the galvanometer current zero. Board numericals typically give three known resistances and ask for the fourth, or ask whether the bridge is balanced. The metre bridge is the practical application of this principle — L/(100−L) = R/S — where L is the length at which the galvanometer reads zero.
How do I generate a custom question paper for Current Electricity using MarksZen? +
Sign up for a free MarksZen account, choose CBSE Class 12 Physics, select Chapter 3 (Current Electricity), set your preferred question-type mix (MCQ, short answer, numerical) and total marks — the AI generates a complete board-aligned paper with answer key in under 2 minutes, ready for PDF export.