Physics · 2020 · Set 55/3/1

CBSE Class 12 Physics 2020 — Set 55/3/1

CBSE Class XII Board 2020 · Set 55/3/1

Real board examination⚠ Old pattern · pre-2020 syllabus

Sets

About the 2020 exam: The 2020 exam used CBSE's older pattern — 37 questions for 70 marks, with no case studies (Section A was 20 one-mark questions incl. MCQs). All three series (55/1/1, 55/2/1, 55/3/1) are here in full (37/37), each verified against the official CBSE paper. Several topics examined in 2020 were removed from today's syllabus — those questions are badged and filterable above.

This paper has 6 questions on a topic removed in CBSE’s 2023-24 syllabus update (each marked Not in syllabus). It’s kept for historical accuracy — the exam really asked it that year — but isn’t in the current syllabus and doesn’t count toward a concept’s importance. Switch to to focus on what’s still examinable.

About this paper

The real Class-12 board examination held in 2020. Every question below is solved the concept-first way. Sample papers are labelled honestly — never shown as a past exam.

Total marks: 70
Questions: 37
Duration: 180 min
Sections: 4

The marks / questions / duration above are the official exam pattern. We currently have 37 of this paper’s questions (100% of the full paper), with 37 fully solved. Questions we couldn’t yet extract or verify are held — never shown as complete.

Sections & marks

Section	Type	Questions	Marks each	Total
ASection A	Very short answer / MCQ	20	1	20
BSection B	Short answer	7	2	14
CSection C	Long answer	7	3	21
DSection D	Long answer	3	5	15
Total		37		70

The question paper

The questions we hold for this paper, laid out by section. Solutions are on the Answers tab.

Board Examination

Physics

CBSE Class XII Board 2020 · Set 55/3/1

Series/Set: 55/3/1Roll No. ________

Time Allowed: 3 hoursMaximum Marks: 70

General Instructions

This question paper contains 37 questions divided into 4 sections — A, B, C, D.
Section A comprises 20 questions of 1 mark each (Very short answer / MCQ).
Section B comprises 7 questions of 2 marks each (Short answer).
Section C comprises 7 questions of 3 marks each (Long answer).
Section D comprises 3 questions of 5 marks each (Long answer).

Above is the official exam pattern. The questions printed below are those we currently hold for this paper.

Section A

Very short answer / MCQ · 1 mark each · 20 of 20 shown

Q1.

If a positive charge is displaced against the electric field in which it was situated, then (A) work will be done by the electric field on the charge. (B) the intensity of the electric field decreases. (C) energy of the system will decrease. (D) energy will be provided by external source displacing the charge.

[1]

Q2.

The electric flux emerging out from 1 C charge is (A)

\dfrac{1}{\varepsilon_0}

(B)

4\pi

(C)

\dfrac{4\pi}{\varepsilon_0}

(D)

\varepsilon_0

[1]

Q3.

Two capacitors of capacitances

C_1

and

C_2

are connected in parallel. If a charge Q is given to the combination, the ratio of the charge on the capacitor

C_1

to the charge on

C_2

will be (A)

\dfrac{C_1}{C_2}

(B)

\sqrt{\dfrac{C_1}{C_2}}

(C)

\sqrt{\dfrac{C_2}{C_1}}

(D)

\dfrac{C_2}{C_1}

[1]

Q4.

The electrical resistance of a conductor (A) varies directly proportional to its area of cross-section. (B) decreases with increase in its temperature. (C) decreases with increase in its conductivity. (D) is independent of its shape but depends only on its volume.

[1]

Q5.

\mathrm{m^2\,V^{-1}\,s^{-1}}

is the SI unit of which of the following ? (A) Drift velocity (B) Mobility (C) Resistivity (D) Potential gradient

[1]

Q6.

The element of a heater is rated (P, V). If it is connected across a source of voltage

\dfrac{V}{2}

, then the power consumed by it will be (A) P (B) 2P (C)

\dfrac{P}{2}

(D)

\dfrac{P}{4}

[1]

Page 1 of 6

Q7.

In Bohr's model of hydrogen atom, the total energy of the electron in nth discrete orbit is proportional to (A) n (B) (1)/(n) (C) n² (D) (1)/(n²)

[1]

Q8.

A zener diode has (A) heavily doped p-side and lightly doped n-side. (B) heavily doped n-side and lightly doped p-side. (C) heavily doped n-side as well as p-side. (D) lightly doped n-side as well as p-side.

⚠ This question is not in the current syllabus — Zener diode (removed 2023-24)

[1]

Q9.

A region has a uniform magnetic field in it. A proton enters into the region with velocity making an angle of 45° with the direction of the magnetic field. In this region the proton will move on a path having the shape of a (A) straight line (B) circle (C) spiral (D) helix

[1]

Q10.

An isosceles right angled current carrying loop PQR is placed in a uniform magnetic field vecB pointing along PR. If the magnetic force acting on the arm PQ is F, then the magnetic force which acts on the arm QR will be (A) F (B) dfracF√(2) (C) √(2)F (D) -F

[1]

Q11.

The shape of the wavefront originating from a line source is _____ .

[1]

Q12.

The refractive index of the material of a converging lens is 1·5. If air is replaced by a medium of refractive index 1·6, then the lens will now behave as a _____ lens.

[1]

Q13.

In Young's double slit experiment, the separation between the two slits is halved. The new fringe width will be _____ times its initial value.

[1]

Q14.

The value of Brewster's angle for air-glass interface is (π)/(3), hence the refractive index of glass is _____ .

⚠ This question is not in the current syllabus — Brewster's law (removed 2023-24)

[1]

Q15.

In photoelectric effect, the number of emitted photoelectrons is proportional to _____ of incident light. OR Light of frequency nu is incident on a photosensitive surface of threshold frequency nu₀ (nu > nu₀). The value of kinetic energy of the emitted photoelectrons will be _____ .

[1]

Q16.

An ac is passed through a series LCR circuit. What is the impedance of the circuit at resonance ?

[1]

Page 2 of 6

Q17.

Two identical coils, one of copper and the other of aluminium are rotated with the same angular speed in an external magnetic field. In which of the two coils will the induced current be more ?

[1]

Q18.

In an ac circuit, the applied voltage and flowing current are E = E₀ sin ω t and I = I₀ sin(ω t + (π)/(2)) respectively. What is the average power consumed in one cycle in this circuit ? OR What happens when a block of metal is kept in a varying magnetic field ?

[1]

Q19.

Mention the contribution of Indian physicist J.C. Bose in the production of electromagnetic waves.

[1]

Q20.

Write one use of the electromagnetic waves of frequency range from 10¹⁶ Hz to 10²⁰ Hz.

[1]

Section B

Short answer · 2 marks each · 7 of 7 shown

Q1.

Two cells of emf E₁ and E₂ (E₁ > E₂) are connected as shown in the figure below. When a potentiometer is used to measure potential difference between the points A and B, the balancing length of the potentiometer wire is 300 cm. But the same potentiometer for the potential difference between points A and C, gives the balancing length 100 cm. Find (E₁)/(E₂).

⚠ This question is not in the current syllabus — potentiometer (comparing EMFs of cells) (removed 2023-24)

[2]

Q2.

Two identical bars, one of paramagnetic material and other of diamagnetic material are kept in a uniform external magnetic field parallel to it. Draw diagrammatically the modifications in the magnetic field pattern in each case.

⚠ This question is not in the current syllabus — para-/dia-/ferromagnetic substances with examples (removed 2023-24)

[2]

Q3.

Two coplanar and concentric coils 1 and 2 have respectively the number of turns N₁ and N₂ and radii r₁ and r₂ (r₂ gg r₁). Deduce the expression for mutual inductance of this system.

[2]

Q4.

How does an oscillating charge radiate an electromagnetic wave ? Give the relation between the frequency of radiated wave and the frequency of oscillating charge. OR (a) Explain briefly the fact that electromagnetic waves carry energy. (b) Why do we not feel the pressure due to sunshine ?

[2]

Page 3 of 6

Q5.

A converging lens of focal length f₁ is placed coaxially in contact with a diverging lens of focal length f₂ (f₁ > f₂). Determine the power and nature of the combination in terms of f₁ and f₂. OR How is the resolving power of a compound microscope affected if (a) wavelength of light used is decreased, and (b) the diameter of its objective lens is increased ? Justify your answers.

[2]

Q6.

Define the terms (a) threshold frequency, and (b) stopping potential. How were these terms incorporated in Einstein's photoelectric equation ?

[2]

Q7.

A hydrogen atom is in its third excited state. (a) How many spectral lines can be emitted by it before coming to the ground state ? Show these transitions in the energy level diagram. (b) In which of the above transitions will the spectral line of shortest wavelength be emitted ?

[2]

Section C

Long answer · 3 marks each · 7 of 7 shown

Q1.

(a) Differentiate between the random velocity and the drift velocity of electrons in an electrical conductor. Give their order of magnitudes. (b) A conductor of uniform cross-sectional area is connected across a dc source of variable voltage. Draw a graph showing variation of drift velocity of electrons (vd) as a function of current density (J) in it.

[3]

Q2.

A series LCR ac circuit has L = 2·0 H, C = 32μF and R = 10Ω. (a) At what angular frequency of ac will it resonate ? (b) Calculate the Q value of the circuit. OR An ideal inductor of (5)/(π) H inductance is connected to a 200 V, 50 Hz ac supply. (a) Calculate the rms and peak value of current in the inductor. (b) What is the phase difference between current through the inductor and the applied voltage ? How will it change if a small resistance is connected in series with this inductor in the circuit ?

[3]

Q3.

(a) Using the necessary ray diagram, derive the mirror formula for a concave mirror. (b) In the magnified image of a measuring scale (with equidistant markings) lying along the principal axis of a concave mirror, the markings are not equidistant. Explain.

[3]

Page 4 of 6

Q4.

(a) The density of the nuclear matter is tremendously larger than the physical density of the material. Explain. (b) The nuclear forces are not coulomb forces between nucleons. Explain. (c) Draw a plot of the potential energy between a pair of nucleons as a function of distance between them inside a nucleus.

[3]

Q5.

What do you mean by wave nature of an electron ? How was quantisation of angular momentum of the orbiting electron in Bohr's model of hydrogen atom explained by de Broglie hypothesis ?

[3]

Q6.

Name the diode which can act as a voltage regulator. Explain its working with the help of its labelled circuit diagram. Draw its V – I characteristic.

⚠ This question is not in the current syllabus — Zener diode and its use as a voltage regulator (removed 2023-24)

[3]

Q7.

(a) Why is an intrinsic semiconductor deliberately converted into an extrinsic semiconductor by adding impurity atoms ? (b) Explain briefly the two processes that occur in p-n junction region to create a potential barrier.

[3]

Section D

Long answer · 5 marks each · 3 of 3 shown

Q1.

(a) An electric dipole of dipole moment vecp is placed in a uniform electric field vecE at an angle θ with it. Derive the expression for torque (τ) acting on it. Find the orientation of the dipole relative to the electric field for which torque on it is (i) maximum, and (ii) half of maximum. (b) Two point charges q₁ = +1μC and q₂ = +4μC are placed 2 m apart in air. At what distance from q₁ along the line joining the two charges, will the net electric field be zero ? OR (a) Derive an expression for the energy stored in a parallel plate capacitor of capacitance C when charged up to voltage V. How is this energy stored in the capacitor ? (b) A capacitor of capacitance 1 μF is charged by connecting a battery of negligible internal resistance and emf 10 V across it. Calculate the amount of charge supplied by the battery in charging the capacitor fully.

[5]

Page 5 of 6

Q2.

(a) Derive the expression for the force acting per unit length between two long straight parallel current carrying conductors. Hence define one ampere. (b) Two long parallel straight conductors are placed 12 cm apart in air. They carry equal currents of 3 A each. Find the magnitude and direction of the magnetic field at a point midway between them (drawing a figure) when the currents in them flow in opposite directions. OR (a) Draw the schematic sketch of a cyclotron. Explain the shape of the path on which charged particle moves when the particle is accelerated by it. (b) To convert a given galvanometer into a voltmeter of ranges 2 V, V and (V)/(2) volt, resistances R₁, R₂ and R₃ ohm respectively, are required to be connected in series with the galvanometer. Obtain the relationship between R₁, R₂ and R₃.

[5]

Q3.

(a) What is meant by plane polarised light ? An unpolarised light is incident at an angle θ on the surface of glass of refractive index μ. If the reflected and refracted rays are perpendicular to each other, then obtain the relationship between μ and θ. (b) Two polaroids P₁ and P₂ are placed in a crossed position. Unpolarised light of intensity I₀ is incident on P₁. If P₂ is rotated through an angle θ about the direction of propagation of light, keeping P₁ fixed, plot the graph of intensity of light for 0 < θ < 360° which is (i) transmitted by P₁, and (ii) transmitted by P₂. OR (a) Briefly describe the Young's double slit experiment of interference of light. Derive the expression for fringe width in the pattern. (b) Monochromatic light of wavelength 588 nm is incident from air to water interface. Find the wavelength and speed of the refracted light. The refractive index of water is (4)/(3).

⚠ This question is not in the current syllabus — polarisation (removed 2023-24)

[5]

Page 6 of 6