A 4 μF capacitor charged to 200 V is disconnected from its battery and then connected across an identical uncharged 4 μF capacitor. The energy lost in the process is:

zero, since charge is conserved

⚛️

CBSE Class 12 Physics Board Paper 3 — MCQs with Answers

Original paper · CBSE patternWritten by our subject team, not a reprint of an official sample paper.

Questions: 20
Time: 30 min
Marking: +1, no negative marking
Mix: 4 Easy · 8 Medium · 8 Hard

Board Paper 3 — CBSE Class 12 Physics is a full 20-question objective paper for Class 12 Physics, written in the CBSE board pattern and marked the way the board marks: +1 for a correct answer and 0 for a wrong one — no negative marking, exactly like the real board paper. It follows Section A of a real CBSE paper: Q1–Q16 are single-correct MCQs, Q17–Q18 are case-based questions built on a short source, and Q19–Q20 are assertion–reason items using the board's own four option strings.

The difficulty sits deliberately a notch above the real board — 4 Easy · 8 Medium · 8 Hard — because a paper you can clear comfortably tells you nothing on exam day. You get 30 minutes — about 90 seconds a question, the pace the real paper demands. 8 of the questions are solved in full below, with the correct option and a worked explanation for each; the remaining 12 are timed and scored when you sign in free.

Sit it under exam conditions rather than open-book. The score you get on a timed full-length paper is the only honest signal of whether your Physics revision is holding together across chapters — which is precisely what the board tests and what chapter-wise practice cannot tell you.

8 solved questions from this paper

Answer and worked explanation shown for each. The remaining 12 are timed and scored when you sign in free.

Q1Easy
Charges of +2 μC and −2 μC are fixed at the two ends of a 20 cm line. The electric potential at the midpoint of the line is:
A.3.6×10⁵ V
B.1.8×10⁵ V
C.3.6×10⁴ V
D.zero✓ Correct
Answer: D. zero
Explanation: Potential is a scalar and adds with sign, so at the midpoint the two contributions are +kq/0.1 and −kq/0.1, which cancel exactly to zero. Note that the electric FIELD there is not zero — both charges push it in the same direction — so a non-zero value like 3.6×10⁵ V confuses the vector quantity with the scalar one.
Q2Easy
An electric bulb is rated 100 W, 220 V. The resistance of its filament while operating at its rated values is:
A.484 Ω✓ Correct
B.220 Ω
C.2.2 Ω
D.44 Ω
Answer: A. 484 Ω
Explanation: From P = V²/R, R = V²/P = (220)²/100 = 48400/100 = 484 Ω. Dividing V by P instead of V² by P gives 2.2 Ω, a resistance so low it would draw 100 A from the mains — a quick sanity check that catches the slip.
Q3Easy
A lens has a focal length of −20 cm. Its power and nature are:
A.−0.05 D, diverging
B.−5 D, diverging✓ Correct
C.+5 D, converging
D.−20 D, diverging
Answer: B. −5 D, diverging
Explanation: Power is the reciprocal of the focal length expressed in METRES: P = 1/(−0.20) = −5 D. A negative focal length under the Cartesian convention means the lens is concave and diverges the beam. Leaving the focal length in centimetres gives −0.05 D, which is the classic unit error here.
Q4Medium
An electric dipole of dipole moment 4×10⁻⁹ C·m is placed in a uniform electric field of 5×10⁴ N/C, its axis making an angle of 30° with the field. The torque acting on the dipole is:
A.1×10⁻⁴ N·m✓ Correct
B.2×10⁻⁴ N·m
C.1.73×10⁻⁴ N·m
D.1×10⁻⁵ N·m
Answer: A. 1×10⁻⁴ N·m
Explanation: Torque on a dipole is τ = pE sinθ = 4×10⁻⁹ × 5×10⁴ × sin30° = 2×10⁻⁴ × 0.5 = 1×10⁻⁴ N·m. Omitting the sine gives the maximum torque 2×10⁻⁴ N·m, which applies only at 90°, and using cos30° gives 1.73×10⁻⁴ N·m, an expression belonging to the potential energy instead.
Q5Medium
In a metre-bridge experiment an unknown resistance X is placed in the left gap and a 6 Ω standard resistance in the right gap. The balance point is obtained at 40 cm from the left end. The value of X is:
A.9 Ω
B.4 Ω✓ Correct
C.6 Ω
D.2.4 Ω
Answer: B. 4 Ω
Explanation: At balance X/R = l/(100 − l), so X = 6 × 40/60 = 4 Ω. The unknown must be SMALLER than the standard because its balance length is shorter than 50 cm; inverting the ratio to 6 × 60/40 gives 9 Ω and contradicts that check.
Q6Medium
A magnetising field of 1600 A/m produces a magnetisation of 4.8×10⁵ A/m in a certain material. Its magnetic susceptibility and relative permeability are:
A.300 and 301✓ Correct
B.300 and 300
C.3.0×10⁻³ and 1.003
D.480 and 481
Answer: A. 300 and 301
Explanation: Susceptibility is χ = M/H = 4.8×10⁵/1600 = 300, and relative permeability is μ_r = 1 + χ = 301. The large positive susceptibility marks this as a ferromagnetic material. Forgetting the +1 makes χ and μ_r identical, which is only a good approximation when χ is huge, not an exact relation.
Q7Hard
An infinitely long straight wire carries a uniform linear charge density of 2×10⁻⁸ C/m. The electric field at a point 20 cm from the wire is (1/4πε₀ = 9×10⁹ N·m²/C²):
A.900 N/C
B.450 N/C
C.3600 N/C
D.1800 N/C✓ Correct
Answer: D. 1800 N/C
Explanation: A cylindrical Gaussian surface gives E = λ/(2πε₀r) = 2kλ/r = 2 × 9×10⁹ × 2×10⁻⁸ / 0.20 = 360/0.20 = 1800 N/C. Note the field falls as 1/r for a line, not 1/r² as for a point charge; dropping the factor of 2 gives the 900 N/C trap.
Q8Hard
A 4 μF capacitor charged to 200 V is disconnected from its battery and then connected across an identical uncharged 4 μF capacitor. The energy lost in the process is:
A.0.08 J
B.zero, since charge is conserved
C.0.04 J✓ Correct
D.0.02 J
Answer: C. 0.04 J
Explanation: Initially U = ½ × 4×10⁻⁶ × 200² = 0.08 J. Charge is conserved but redistributes, so the common potential is 100 V and the final energy is ½ × 8×10⁻⁶ × 100² = 0.04 J, leaving 0.04 J lost. Conservation of CHARGE does not imply conservation of energy — the difference is dissipated as heat in the connecting wires and as radiation.

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Is this an official CBSE sample paper?

No — and that is deliberate. This is an original paper written in the CBSE board pattern by our subject team, not a copy of CBSE's official Sample Question Paper. It means you get fresh questions you have not already seen on a dozen other sites, with instant scoring and worked solutions rather than a PDF.

How is this Class 12 Physics paper marked?

+1 for a correct answer and 0 for a wrong one — no negative marking, exactly like the real board paper. So attempt every question — there is no penalty for a wrong answer on a board paper, and leaving a question blank can only cost you.

How long should this paper take?

30 minutes for 20 questions, roughly 90 seconds each. The timer runs whether or not you are watching it, which is the point: board marks are lost to pacing at least as often as to gaps in knowledge.

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