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Current Electricity Class 12 Notes for Boards, JEE and NEET: Formulas, Examples and Handwritten Notes

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eSaral Academic and Editorial Team

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Summary

Current Electricity Class 12 notes cover Chapter 3 of NCERT Physics: drift velocity, Ohm’s law, resistivity, combinations of resistors, EMF and internal resistance, Kirchhoff’s laws and the Wheatstone bridge, with potentiometer notes for competitive exams. This page gives key formulas, solved examples, handwritten notes and a guide to what each exam needs.

Current Electricity Class 12 Notes for Boards, JEE and NEET Exams

Current Electricity Class 12 Notes: Chapter Overview

In these Current Electricity Class 12 notes, the chapter is Chapter 3 of NCERT Class 12 Physics (Part 1). It builds on the Electrostatic Potential and Capacitance chapter and studies charges in motion through conductors. Later chapters such as Moving Charges and Magnetism and Electromagnetic Induction assume that you are comfortable with circuits, resistance and EMF.

The chapter covers:

  • Electric current, drift velocity and mobility
  • Ohm’s law, resistance, resistivity and conductivity, and the limits of Ohm’s law
  • Temperature dependence of resistance
  • Series and parallel combinations of resistors, and electrical power
  • EMF, potential difference, internal resistance and combination of cells
  • Kirchhoff’s rules
  • Wheatstone bridge and metre bridge
  • Potentiometer (for competitive exams: see the note below)

Tip from the eSaral Physics team: For circuit questions, redraw the circuit neatly, mark the current directions and then write the equations. Most sign mistakes in Kirchhoff’s laws come from skipping this step.

Key Formulas in Current Electricity Class 12 Notes

Concept

Formula

Notes

Electric current

I = dq/dt

SI unit: ampere (A)

Drift velocity

v_d = eEτ/m

τ is the relaxation time

Current and drift velocity

I = n e A v_d

n is the number density of free electrons

Mobility

μ = v_d/E

SI unit: m² V⁻¹ s⁻¹

Ohm’s law

V = IR

For ohmic conductors at constant temperature

Resistance

R = ρl/A

ρ is the resistivity

Resistivity

ρ = m/(n e² τ)

Conductivity σ = 1/ρ

Temperature dependence

R_T = R₀[1 + α(T − T₀)]

α is the temperature coefficient

Series resistors

R = R₁ + R₂ + …

Same current

Parallel resistors

1/R = 1/R₁ + 1/R₂ + …

Same potential difference

Electrical power

P = VI = I²R = V²/R

Energy = P × t

Current in a circuit with a cell

I = E/(R + r)

r is the internal resistance

Terminal voltage

V = E − Ir (discharging); V = E + Ir (charging)


Kirchhoff’s junction rule

ΣI (in) = ΣI (out)

Conservation of charge

Kirchhoff’s loop rule

ΣV = 0 around a closed loop

Conservation of energy

Wheatstone bridge (balanced)

P/Q = R/S

No current through the galvanometer

Metre bridge

S = R(100 − l)/l

R in the left gap, l is the balancing length in cm

Potentiometer: comparing EMFs

E₁/E₂ = l₁/l₂

For JEE and NEET

Potentiometer: internal resistance

r = R(l₁ − l₂)/l₂

l₁ with the cell open, l₂ with R connected

India's Best Exam Preparation for Class 12th - Download Now

Current Electricity Class 12 Handwritten Notes

Current Electricity Class 12 Notes

India's Best Exam Preparation for Class 12th - Download Now

ncert current electricity

India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now

Potentiometer

India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now

India's Best Exam Preparation for Class 12th - Download Now

Potentiometer notes: the potentiometer was removed from the CBSE board syllabus in the earlier rationalisation, so the Potentiometer pages are not needed for CBSE boards. Check the latest NTA and NMC syllabi for JEE Main and NEET before you study them.

Solved Examples from Current Electricity Class 12 Notes

These solved examples use the formulas from the Current Electricity Class 12 notes above.

Example 1 (drift velocity): A copper wire of cross-section 1 mm² carries a current of 2 A. If n = 8.5 × 10²⁸ m⁻³ and e = 1.6 × 10⁻¹⁹ C, find the drift velocity.

v_d = I/(nAe) = 2 / (8.5 × 10²⁸ × 10⁻⁶ × 1.6 × 10⁻¹⁹) ≈ 1.5 × 10⁻⁴ m/s (about 0.15 mm/s).

Example 2 (combination of resistors): A 6 Ω and a 3 Ω resistor are in parallel, and this combination is in series with a 4 Ω resistor and a 12 V battery. Find the current from the battery.

Parallel part = (6 × 3)/(6 + 3) = 2 Ω. Total resistance = 2 + 4 = 6 Ω. Current = 12/6 = 2 A.

Example 3 (internal resistance): A cell of EMF 1.5 V and internal resistance 0.5 Ω is connected to a 2.5 Ω resistor. Find the current and the terminal voltage.

I = E/(R + r) = 1.5/(2.5 + 0.5) = 0.5 A. V = E − Ir = 1.5 − 0.5 × 0.5 = 1.25 V.

Example 4 (Wheatstone bridge): In a balanced Wheatstone bridge, P = 10 Ω, Q = 20 Ω and R = 15 Ω. Find S.

P/Q = R/S gives S = R × Q/P = 15 × 20/10 = 30 Ω.

What Do Boards, JEE and NEET Need from Current Electricity Class 12 Notes?

Topic

CBSE boards

JEE Main

NEET

Drift velocity, Ohm’s law, resistivity

Yes

Yes

Yes

Combination of resistors, power

Yes

Yes

Yes

EMF and internal resistance, combination of cells

Yes

Yes

Yes

Kirchhoff’s rules

Yes

Yes

Yes

Wheatstone bridge, metre bridge

Yes

Yes

Yes

Potentiometer

Removed from the board syllabus

Check the NTA syllabus

Check the NMC syllabus

The number of questions from this chapter changes every year, so check the official papers. eSaral’s NEET Physics weightage page shows how the chapter has been weighted in NEET.

Common Mistakes in Current Electricity Class 12 Notes Problems

Mistake

Correct approach

Applying Ohm’s law to non-ohmic devices

Use V = IR only for ohmic conductors, at constant temperature

Confusing electron flow with conventional current

Conventional current is opposite to the electron drift

Wrong signs in Kirchhoff’s loop equations

Fix a loop direction and treat a potential rise as + and a drop as −

Using EMF instead of terminal voltage

Terminal voltage is E − Ir when the cell supplies current

Using the Wheatstone condition when the bridge is not balanced

Use Kirchhoff’s rules for an unbalanced bridge

How to Study Current Electricity Class 12 Notes

  1. Read the NCERT chapter first, and solve the examples and exercises. eSaral’s NCERT Solutions for Class 12 Physics can help you check your answers.
  2. Revise from the formula table and the handwritten notes above.
  3. Solve the circuit problems on Kirchhoff’s rules and the Wheatstone bridge until the signs become a habit.
  4. Revise these Current Electricity Class 12 notes with eSaral’s Class 12 Physics revision notes before the exam.

Frequently Asked Questions

What are the most important topics in Current Electricity Class 12 notes for JEE Main?

Kirchhoff’s rules, combinations of resistors, the Wheatstone bridge and metre bridge, EMF and internal resistance, and the potentiometer are the most commonly asked topics. Check the NTA syllabus for the exact list, and practise problems from each of these areas.

How many questions come from Current Electricity in NEET?

In Current Electricity Class 12 notes, the exam focus is clear: NEET asks questions from this chapter every year, mostly on Ohm’s law, combinations of resistors, Kirchhoff’s rules and the EMF and internal resistance of a cell. The count varies from year to year, so check previous years’ papers and eSaral’s NEET Physics weightage page.

Are Current Electricity Class 12 notes from NCERT enough for the boards?

NCERT is the main resource for CBSE boards. Read the theory, solve all examples and exercises, and practise previous year questions. Use eSaral’s NCERT Solutions for Class 12 Physics to check your working.

What is the difference between EMF and terminal voltage?

EMF (E) is the potential difference between the terminals of a cell when no current is drawn from it. Terminal voltage (V) is the potential difference across the terminals when current flows, V = E − Ir while the cell supplies current, where r is its internal resistance. When the cell is being charged, V = E + Ir.

How is a potentiometer better than a voltmeter?

At the balance point, a potentiometer draws no current from the cell, so it measures the EMF without the error caused by the cell’s internal resistance. A voltmeter draws some current, so it reads a value slightly lower than the EMF. The potentiometer is asked in JEE, but it was removed from the CBSE board syllabus.

What is drift velocity in Current Electricity Class 12 notes, and why is it so small?

Drift velocity is the average velocity that free electrons gain in the direction opposite to the applied electric field. It is small (of the order of 10⁻⁴ m/s in a typical copper wire) because the electrons collide frequently with the lattice ions, even though the electric effect travels through the circuit almost instantly.

What are Kirchhoff’s rules?

In the Current Electricity Class 12 notes, the junction rule says that the total current entering a junction equals the total current leaving it, which follows from conservation of charge. The loop rule says that the sum of potential changes around a closed loop is zero, which follows from conservation of energy.

What is the condition for a balanced Wheatstone bridge?

In Current Electricity Class 12 notes, a Wheatstone bridge is balanced when P/Q = R/S, where P and Q are in one arm and R and S in the other. In that case, no current flows through the galvanometer.

Where can I find more Class 12 Physics notes on eSaral?

eSaral has chapter-wise Class 12 Physics revision notes, NCERT solutions and NCERT books, linked with these Current Electricity Class 12 notes, so you can study the chapter and revise it in one place.

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Team eSaral

eSaral Academic and Editorial Team

Team eSaral is the collective author profile for educational content created by eSaral’s teachers and academic contributors. The team draws on expertise from IIT graduates, doctors, experienced educators and subject specialists to develop resources for JEE, NEET and school students. Our articles aim to explain concepts clearly and help students study with confidence.

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