Class 12 Physics NCERT Solutions

~5 min read

Current Electricity Class 12 Physics NCERT Solutions

The complete NCERT exercise solutions for Chapter 3, Current Electricity — 9 questions from 3.1 to 3.9, each worked through step by step in the CBSE marking pattern. Ohm's law, resistivity and its temperature dependence, combinations of resistors, cells, Kirchhoff's rules and the Wheatstone bridge.

Class:12Subject:PhysicsChapter:3

4 Key Formulas24 Practice MCQs

DWritten byDeep Narayan

UpdatedSeptember 2026

Key Concept Summary

How many questions are in NCERT Class 12 Physics Chapter 3?

Chapter 3 carries 9 exercise questions, numbered 3.1 to 3.9. All of them are solved step by step on this page, along with the chapter's key formulas and exam pointers.

Table of Contents10 sections

Revision & Practice

Chapter Overview

Current Electricity covers Ohm's law, the emf–terminal-voltage connection, resistivity and temperature, and Kirchhoff's rules for networks. Boards test the standard forms hard: I = E/(R + r), V = E – Ir, R = R₀(1 + αΔT), ρ = RA/l, and loop-and-junction analysis of bridges. Every question below is from the NCERT Class 12 textbook (rationalised edition), solved line by line in the board pattern.

Board pattern

Define the current direction on every branch before writing loop equations — the junction rule then fixes the remaining currents. Absolute temperature in kelvin is not needed here, but ΔT must be in the same degree unit as α. When a battery is being charged the current is forced backwards through it, so terminal voltage becomes E + Ir, not E – Ir. And always name the quantity you are conserving (charge at a junction, potential around a loop).

$Ex 3.1 — Maximum current drawn from a car battery$
$Ex 3.2 — Load resistance and terminal voltage from a known current$
$Ex 3.3 — Temperature of a heating element from its resistance rise$
$Ex 3.4 — Resistivity of a wire from R, length and area$
$Ex 3.5 — Temperature coefficient of silver$
$Ex 3.6 — Steady temperature of a nichrome heater$
$Ex 3.7 — Current in every branch of a bridge network (Kirchhoff)$
$Ex 3.8 — Terminal voltage of a storage battery being charged$
$Ex 3.9 — Drift time of an electron across a copper wire$

NCERT Exercise 3.1 — Maximum Current Drawn From a Car Battery

1Exercise question

Step-by-step solution

1The current is maximum when the external resistance is zero (short circuit).
2I_max = E/r = 12/0.4 = 30 A.

Final answer

I_max = 30 A (this is the short-circuit current).

NCERT Exercise 3.2 — Load Resistance and Terminal Voltage From a Known Current

1Exercise question

Step-by-step solution

1By Ohm's law for the whole circuit: I = E/(R + r).
20.5 = 10/(R + 3) → R + 3 = 20 → R = 17 Ω.
3Terminal voltage: V = E – Ir = 10 – (0.5)(3) = 8.5 V.
4Check through the external resistor: V = IR = (0.5)(17) = 8.5 V, the same value.

Final answer

R = 17 Ω; terminal voltage = 8.5 V.

NCERT Exercise 3.3 — Temperature of a Heating Element From Its Resistance Rise

1Exercise question

Step-by-step solution

1Use R = R₀(1 + αΔT), giving ΔT = (R/R₀ – 1)/α.
2ΔT = (117/100 – 1)/(1.70 × 10⁻⁴) = 0.17/1.70 × 10⁻⁴ = 1000 °C.
3Steady temperature = T₀ + ΔT = 27 + 1000 = 1027 °C.

Final answer

Temperature of the element = 1027 °C (≈ 1.0 × 10³ °C above room temperature).

NCERT Exercise 3.4 — Resistivity of a Wire From R, Length and Area

1Exercise question

Step-by-step solution

1R = ρl/A, so ρ = RA/l.
2ρ = (5.0 × 6.0 × 10⁻⁷)/15 = (3.0 × 10⁻⁶)/15 = 2.0 × 10⁻⁷ Ω m.

Final answer

ρ = 2.0 × 10⁻⁷ Ω m.

NCERT Exercise 3.5 — Temperature Coefficient of Silver

1Exercise question

Step-by-step solution

1α = (R₂ – R₁)/[R₁(T₂ – T₁)] for small spreads where R depends linearly on T.
2α = (2.7 – 2.1)/[2.1 × (100 – 27.5)] = 0.6/(2.1 × 72.5).
3α = 0.6/152.25 = 3.9 × 10⁻³ °C⁻¹.

Final answer

α ≈ 3.9 × 10⁻³ °C⁻¹.

NCERT Exercise 3.6 — Steady Temperature of a Nichrome Heater

1Exercise question

Step-by-step solution

1Resistance at room temperature: R₂₇ = 230/3.2 = 71.875 Ω.
2Steady resistance: R = 230/2.8 = 82.14 Ω.
3R = R₀(1 + αΔT) → ΔT = (R/R₀ – 1)/α.
4ΔT = (82.14/71.875 – 1)/(1.70 × 10⁻⁴) = (1.1429 – 1)/1.70 × 10⁻⁴ = 0.1429/1.70 × 10⁻⁴ ≈ 840 °C.
5Steady temperature = 27 + 840 = 867 °C.

Final answer

Steady temperature ≈ 8.4 × 10² °C above room temperature, i.e. about 867 °C.

NCERT Exercise 3.7 — Current in Every Branch of a Bridge Network (Kirchhoff)

1Exercise question

Step-by-step solution

1Label the currents: I₁ = current supplied by the battery (through the 10 Ω feed resistor), I₂ = current in AB, I₃ = current in AD, I₄ = current in the diagonal BD (labelled from B to D).
2Then by the junction rule: current in BC = I₂ – I₄ and current in CD = I₃ + I₄.
3Loop ABDA: 10I₂ + 5I₄ – 5I₃ = 0 → I₃ = 2I₂ + I₄. ... (1)
4Loop BCDB: 5(I₂ – I₄) – 10(I₃ + I₄) – 5I₄ = 0 → I₂ = 2I₃ + 4I₄. ... (2)
5From (1) and (2): I₃ = 2(2I₃ + 4I₄) + I₄ → –3I₃ = 9I₄ → I₃ = –3I₄.
6Using (1): –3I₄ = 2I₂ + I₄ → I₂ = –2I₄.
7Outer loop A–B–C–feed–A: –10 + 10I₁ + 10I₂ + 5(I₂ – I₄) = 0 and I₁ = I₂ + I₃, giving 5I₂ + 2I₃ – I₄ = 2.
8Substitute I₂ = –2I₄ and I₃ = –3I₄: 5(–2I₄) + 2(–3I₄) – I₄ = 2 → –17I₄ = 2 → I₄ = –2/17 A.
9Hence I₃ = 6/17 A, I₂ = 4/17 A, and I₁ = I₂ + I₃ = 10/17 A.
10Branch currents: AB = I₂ = 4/17 A; AD = I₃ = 6/17 A; BC = I₂ – I₄ = 4/17 + 2/17 = 6/17 A; CD = I₃ + I₄ = 6/17 – 2/17 = 4/17 A; diagonal BD = |I₄| = 2/17 A (flowing opposite to the assumed B→D label, i.e. from D to B).

Final answer

Supply current 10/17 A; AB = 4/17 A, AD = 6/17 A, BC = 6/17 A, CD = 4/17 A, BD = 2/17 A (directed from D to B against the label).

NCERT Exercise 3.8 — Terminal Voltage of a Storage Battery Being Charged

1Exercise question

Step-by-step solution

1During charging the current is forced into the battery against its emf: I = (120 – 8.0)/(15.5 + 0.5) = 112/16 = 7 A.
2Terminal voltage = E + Ir = 8.0 + (7)(0.5) = 11.5 V.
3Cross-check across the series resistor: 120 – (7)(15.5) = 120 – 108.5 = 11.5 V, the same value.
4The series resistor limits the charging current — without it the initial surge would be (120 – 8)/0.5 = 224 A, far too large for the battery.

Final answer

Terminal voltage during charging = 11.5 V; the series resistor limits (damps) the charging current to a safe value.

NCERT Exercise 3.9 — Drift Time of an Electron Across a Copper Wire

1Exercise question

Step-by-step solution

1Drift speed: v_d = I/(n e A).
2v_d = 3.0/(8.5 × 10²⁸ × 1.6 × 10⁻¹⁹ × 2.0 × 10⁻⁶) = 3.0/(2.72 × 10⁴) = 1.10 × 10⁻⁴ m s⁻¹.
3Time to drift 3.0 m: t = l/v_d = 3.0/(1.10 × 10⁻⁴) = 2.7 × 10⁴ s.

Final answer

t ≈ 2.7 × 10⁴ s (about 7.6 hours) — a striking illustration of how slow the actual drift of electrons is.

Quick Revision

Key formulas at a glance

Memorise these equations — direct application numericals and derivations in CBSE & JEE frequently hinge on these.

Ohm's law

V = I R

Resistance of a wire

R = ρ \frac{L}{A}

Temperature dependence of resistance

R_{T} = R_{0} (1 + α Δ T)

Terminal potential of a cell

I = \frac{ε}{R + r}

Exam Strategy

How this chapter is asked

High-yield question patterns observed across CBSE boards, JEE Main & Advanced, and NEET.

Kirchhoff's junction rule conserves charge and the loop rule conserves energy — set up the junction rule first, since it fixes the sign convention for the loop equations.
In a balanced Wheatstone bridge no current flows through the galvanometer, so the four resistances satisfy R1/R2 = R3/R4 and the bridge can be treated as two separate loops.

FAQ

Frequently asked questions

How many questions are in NCERT Class 12 Physics Chapter 3 (Current Electricity)?

There are 9 exercise questions in this chapter, numbered 3.1 to 3.9. Every one is solved step by step on this page in the official NCERT numbering.

Which formulas come up in Current Electricity Class 12 Physics?

The formulas this chapter's questions actually turn on are: Ohm's law, Resistance of a wire, Temperature dependence of resistance, Terminal potential of a cell. They are listed with their expressions in the key formulas section below, and the solved questions show where each one is used.

Is Current Electricity important for JEE Main and NEET?

Very important — current electricity carries the heaviest numerical weight in Class 12 boards, and potential-divider, combination and Wheatstone-bridge problems appear in every JEE Main and NEET paper.

Next Chapters

Continue learning

Previous: Electrostatic Potential and Capacitance Next: Moving Charges and Magnetism Current Electricity revision notes All Class 12 Physics NCERT Solutions

Interactive Quiz

Chapter MCQ practice test

Instant scoring with complete solutions — test your mastery in under 15 minutes.

Active Recall Practice

Chapter MCQ Mock Test

Evaluate how well you have retained the concepts, formulas, and reaction mechanisms from this chapter. Questions adhere strictly to latest CBSE, JEE & NEET trends.

15 questions (of 24)~23 minutesInstant Score & Solutions

Same solutions, live doubt-clearing help

Reading a solution is step one — getting a doubt resolved in real time is what clears it. ClassApna runs small-batch CBSE, JEE & NEET coaching with daily doubt sessions and mock tests.

Book a Free Demo Class Explore Plans & Batches

Small batches · 1-on-1 personal mentorship · Live online & offline centre