# CBSE Class 12 Physics Sample Question Paper with Marks Distribution 2019-20 | Code 042

## Physics (Code 042)

Time allowed: 3 Hrs.

Maximum Marks : 70

General Instructions:
1. All questions are compulsory. There are 37 questions in all.
2. This question paper has four sections: Section A, Section B, Section C and Section D.
3. Section A contains twenty questions of one mark each, Section B contains seven questions of two marks each, Section C contains seven questions of three marks each,and Section D contains three questions of five marks each.
4. There is no overall choice. However, internal choices have been provided in two questions of one mark each, two questions of two marks, one question of three marks and three questions of five marks weightage. You have to attempt only one of the choices in suchquestions.

5. You may use the following values of physical constants where ever necessary.
Section - A

Q1.  A charge q is placed at the point of intersection of body diagonals of a cube. The
electric flux passing through any one of its face is
1 Q2. The electric potential of earth is taken to be zero because earth is a good
1
(a) Insulator               (b) conductor  (c) semiconductor       (d) dielectric

Q3. If the ammeter in the given circuit shown in the diagram reads 2A, the resistance
R is
1
(a) 1Ω       (b) 2Ω      (c) 3Ω      (d) 4Ω

Q4. The heat produced by 100W heater in 2 minutes is equal to
1
(a) 10.5kJ      (b) 16.3kJ      (c) 12.0kJ      (d) 14.2kJ

Q5.Time period of a charged particle undergoing a circular motion in a uniform
magnetic field is independent of
1
(a) speed of the particle      (b) mass of the particle
(c) charge of the particle     (d) magnetic field

Q6.The final image formed in an astronomical refracting telescope with respect to the
object is
1
(a) Real inverted     (b) Real erect     (c) Virtual erect     (d) Virtual inverted

Q7. The shape of the interference fringes in Young’s double slit experiment when D
(distance between slit and screen) is very large as compared to fringe width is
nearly
1
(a) straight line        (b) parabolic       (c) circular         (d) hyperbolic

Q8. Unpolarized light is incident on a plane glass surface having refractive index .
The angle of incidence at which reflected and refracted rays would become
perpendicular to each other is :
1
(a) 15°     (b) 30°     (c) 45°     (d) 60°

Q9. Photoelectric emission from a given surface of metal can take place when the
value of a ‘physical quantity’ is less than the energy of incident photon. The
physical quantity is :
1
(a) Threshold frequency     (b) Work function of surface
(c) Threshold wave length  (d) Stopping Potential

Q10. A photon beam of energy 12.1eV is incident on a hydrogen atom. The orbit to
which electron of H-atom be excited is
1
(a) 2nd     (b) 3rd     (c) 4th     (d) 5th

Directions (Q11 –Q15) Fill in the blanks with appropriate answer.

Q11. Horizontal and vertical components of earth’s magnetic field at a place are equal.
1
The angle of dip at that place is ________.

OR

A free floating magnetic needle at North pole is __________to the surface of earth.

Ans. 45° or vertical

Q12. The magnetic flux linked with a coil changes by 2×10-2Wb when the current
changes by 0.01A. The self inductance of the coil is_______.
1
Ans. 2 H

Q13. If the angular speed of the armature of a dynamo is doubled then the amplitude of the induced e.m.f will become_______.
1
Ans. double

Q14. An electron is accelerated through a potential difference of 100 V , then de-Broglie wavelength associated with it is approximately ____________ A°
1
Ans. 1.227 A°

Q15. An equilateral prism is made up of material of refractive index √3. The angle of
minimum deviation of light passing through the prism is_________.
1
Ans. 60°

Directions (Q16 –Q20) Answer the following

Q16. Which physical quantity in a nuclear reaction is considered equivalent to the Q-value of the reaction?
1
Ans.Difference in initial mass energy and energy associated with mass of products

Or
Total Kinetic energy gained in the process

Q17. Zener diode is used in reverse bias. When its reverse bias is increased, how does the thickness of the depletion layer change?
1
Ans. Increases

Q18. The initial concentration of a radioactive substance is No and its half life is 12
hours. What will be its concentration after 36 hours?
1
Ans. No/8

Q19. Work function of Sodium is 2.75eV. What will be KE of emitted electron when photon of energy 3.54eV is incident on the surface of sodium?
1
Ans.0.79 eV

Q20. From the information of energy band gaps of diodes, how do you decide which
can be light emitting diodes?
OR

Give any one advantage of LEDs over conventional incandescent low power lamps
1
Ans.Diodes with band gap energy in the visible spectrum range can function as LED

OR
Any one use

Section - B

Q21. Derive the expression for drift velocity of free electron in terms of relaxation time and electric field applied across a conductor.
2
Ans.When electric field E is applied on conductor force acting on free electrons

Average thermal velocity of electron in conductor is zero
(ut)av= 0
Average velocity of electron in conductors in τ (relaxation time) = vd (drift velocity)

Q22.Find total energy stored in capacitors given in the circuit
2 Ans.

Q23. An α - particle and a proton are accelerated through same potential difference.
Find the ratio (vα/ vp)of velocities acquired by two particles.
2
Ans: Q24. What is Brewster’s angle? Derive relation between Brewster angle and refractive
index of medium which produces Plane Polarized light.
2
Ans: “The angle of incidence at which the reflected light is completely plane
polarized, is called as Brewster’s angle (iB) Q25. The work function of Cs is 2.14eV.Find
2
(a) threshold frequency for Cs (b) Wavelength of incident light if the photo current is brought to zero by stopping
potential of 0.6 V.

Ans:

wave function    ω = 2.14eV Q26. Derive an expression for the radius of nth Bohr’s orbit in Hydrogen atom.
2
Ans: OR

Energy of electron in first excited state in Hydrogen atom is -3.4eV. Find KE and
PE of electron in the ground state.

Ans:

Energy of electron in n = 2 is -3.4eV ∴ energy in ground state = -13.6eV

kE = -TE = +13.6eV

energy in ground state x = - 13.6eV.

PE = 2TE = -2×13.6eV = -27.2eV

Q27. Draw energy band diagram of p & n type semiconductors. Also write two
differences between p and n type semiconductors.
2
Ans: OR

Energy gap in a p – n photodiode is 2.8 eV. Can it detect a wavelength of 6000

Ans: Section - C

Q28. State working principle of potentiometer. Explain how the balance point shifts
when value of resistor R increases in the circuit of potentiometer, given below.
3 Ans: Principle of potentiometer, when a constant current flows through a wire of uniform
area of cross-section, the potential drop across any length of the wire is directly
proportional to the length.
Let resistance of wire AB be R1 and its length be ‘l’ then current drawn from
driving cell –
Where R increases, current and potential difference across wire AB will be decreased and hence potential gradient ‘k’ will also be decreased. Thus the null point or balance point will shift to right (towards, B) side.

Q29.  Using Biot-Savart’s law, derive an expression for magnetic field at any point on
axial line of a current carrying circular loop. Hence, find magnitude of magnetic
field intensity at the centre of circular coil.
3
Ans:

Q30. Obtain the resonant frequency and Q – factor of a series LCR circuit with L = 3H,
C = 27𝛍F, R = 7.4Ω. It is desired to improve the sharpness of resonance of circuit by reducing its full width at half maximum by a factor of 2. Suggest a suitable way.
3
Ans:

Q31. State the conditions of total internal reflection. Refractive indices of the given
prism material for Red, Blue and Green colors are respectively 1.39, 1.48 and 1.42 respectively. Trace the path of rays through the prism.
3 Ans:

Q32. Define resolving power of an astronomical refracting telescope and write
expression for it in normal adjustment.Assume that light of wave length 6000Å is
coming from a star, what is the limit of resolution of a telescope whose objective
has a diameter of 2.54m?
3
Ans:Resolving power (R.P) of an astronomical telescope is its ability to form separate
images of two neighboring
astronomical objects like stars etc.
1 OR

Write the basic assumptions used in the derivation of lens – maker’s formula and
hence derive this expression.
3
Ans:

Q33. 3
Ans:

Q34. Suggest an idea to convert a full wave bridge rectifier to a half wave rectifier by
3

Section - D

Q35.

(a) Using Gauss’s law, derive expression for intensity of electric field at any point near the infinitely long straight uniformly charged wire.
5
(b)The electric field components in the following figure are Ex = αx, Ey = 0, Ez= 0;
in which α = 400 N/C m. Calculate

(i) the electric flux through the cube, and

(ii) the charge within the cube assume that a = 0.1m. Ans:(a)

ANS (b)

OR

a) Define electrostatic potential at a point. Write its SI unit.
5
Three charges q1 , q2 and q3 are kept respectively at points A, B and C as shown in figures. Write the expression for electrostatic potential energy of the system.

b) Depict the equipotential surfaces due to

(i) an electric dipole

(ii) two identical negative charges separated by a small distance.

Ans:(a)

Ans:(b)

Q36. In the following diagram, the arm PQ of the rectangular conductor is moved from x = 0; outwards.
5

The uniform magnetic field is perpendicular to the plane and extends from x = 0 to
x = b and is zero for x >b. Only the arm PQ possesses substantial resistance ‘r’.
consider the situation when the arm PQ is pulled outwards from x = 0 to x = 2b,
and is then moved back to x = 0 with constant speed ‘ѵ’. Obtain expressions for
the (i) electric flux,
(ii) the induced emf,
(iii)the force necessary to pull the arm and
(iv) the power dissipated as Joule heat.

Sketch the variation of these quantities with distance.

Ans:

OR

Write working principle of cyclotron and with a suitable diagram explain its

working. Give any two applications of cyclotron.
5
Ans:

Working principle of cyclotron
1
Diagram
1
Working of cyclotron with explanation
2

Any two appliations
1

Q37. Derive mirror equation for a convex mirror. Using it, show that a convex mirror
always produces a virtual image, independent of the location of object.

5
Ans:

OR

(a) Draw a ray diagram for final image formed at distance of distinct vision (D) by a
compound microscope and write expression for its magnifying power.
5
(b) An angular magnification (magnifying power) of 30x is desired for a compound
microscope using as objective of focal length 1.25cm and eye piece of focal length 5cm. How will you set up the compound microscope?

Ans:

(a) Ray Diagram : (with proper labeling)
1 1

(b)

Source- cbse.nic.in