CBSE CLASS 12TH PHYSICS LATEST SAMPLE PAPER 2019 20

CBSE CLASS 12TH PHYSICS LATEST SAMPLE PAPER 2019-20

PHYSICS

Time Allowed:3hr                                                                                           Max 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 20 questions of one mark, Section B consists of 7 questions of 2 marks , Section C consists of 7 questions of 3 marks , Sections D consists of 3 questions of five marks of each.
4. Section A comprises of MCQs, Very Short Answer Questions.These questions are to be answered in one word,one sentence or as per the exact requirement of the question.
5. There is no overall choice.However, an internal choice(s) has been provided in 4 questions of one marks , 3 questions of two marks,3 questions of three marks.You have to attempt only one of choices in such questions.
6. Use of calculators are not permitted. You may ask for logarithmic tables, if required.
7. You may use the following values of physical constants wherever necessary :

c = 3 x10⁸ m/s , _{h    = 6}._{63 x 10}^–34 _{Js ,   e = 1}._{6 x 10}^–19 _{C ,} µ_{0 =} 4 x 10^–7 T m A^{–1 ,} ɛ₀ = 8.854 x  10^–12 C² N^–1 m^–2

Mass of electron (m_e) = 9^.1  x  10^–31 kg

Mass of neutron = 1^.675  x 10^–27 kg

Mass of proton = 1^.673  x  10^–27 kg

Avogadro’s number = 6^.023  x   10²³ per gram mole

Boltzmann constant = 1^.38 x   10^–23 JK^–1

SECTION A

Choose and write a correct option in the following questions.

1. A parallel plate capacitor C has a charge Q. The actual charges on the plates are

(a) Q, Q 

(b) Q/ 2, Q/ 2

(c) Q, -Q 

(d)Q/2,-Q/2    

OR

Electric potential due to a point charge –q at distance x from it is given by:

(A) Kq/x^-2

(B) Kq/x

(C) -Kq/x^-2 

(D) -Kq/x     

2. In a Whetstone’s bridge, all the four arms have equal resistance R. If resistance of the

galvanometer arm is also R, then equivalent resistance of the combination is

(a) R 

(b) 2R

(c) R/2

(d)R/4    

3. To convert a moving coil galvanometer into an ammeter of given range, we must connect:

(a) A suitable low resistance in series 

(b) A suitable low resistance in parallel

(c) A suitable high resistance in parallel

(d) A suitable high resistance in series     

4. One 60 V, 100 W bulb is to be connected to 100 V, 50 Hz ac- source. The potential drop across

the inductor is (f = 50 Hz)

(a) 80 V 

(b) 40V

(c) 10 V 

(d) 20V

OR

When AC source is connected across series R-C combination, the ac- current may lead ac voltage

by

(a) 0° 

(b) 180°

(c) 30° 

(d) 90°

5. In EM spectrum minimum wavelength is of:

(a) gamma rays 

(b) radio waves

(c) visible rays 

(d) microwave.

6. A glass lens is immersed in water. What will be the effect on the power of lens? 

(a) increase 

(b) decrease 

(c) constant 

(d) not depends

7.Identify the correct relation for the given diagram for frequency

(a) υ₁= υ₂ = υ₃ 

(b) υ₁> υ₂> υ₃

(c) υ₁< υ₂< υ₃ 

(d) υ₁=2υ₂ =3υ₃

_8.In gamma rays emission from a nucleus

(a) only the proton number changes

(b) both the proton no and neutron no changes

(c) there is no change in the proton number and neutron number

(d) only the neutron no changes

9.The depletion layer in the P-N junction region is caused by

(a) Drift of holes

(b) Diffusion of charge carriers

(c) Migration of impurity ions

(d) Drift of electrons

10.In the Young double slit experiment, the fringe pattern as seen on the screen is:

(a) parabola 

(b) Hyperbola

(c) Ellipse 

(d) Spiral

Fill in the blanks:

11.When an electron jumps from 2nd stationary orbit of hydrogen atom to 1st stationary orbit, the

energy emitted is _______________.

12.Magnetic susceptibility is slightly negative for ....................... type substances.

Answer the following question:

13. Why is the electrostatic potential inside a charged conducting shell constant throughout the volume of the conductor ?

14.Under what condition will the current in a wire be the same when connected in series and in parallel of n identical cells each having internal resistance r and external resistance R ?

15.A proton is accelerated through a potential difference V, subjected to a uniform magnetic field acting normal to the velocity of the proton. If the potential difference is doubled, how will the radius of the circular path described by the proton in the magnetic field change ?

OR

How does dip angle vary as one moves from magnetic equator to the pole ?

16. What is the work done in moving a charge of 10 nC between two points on an equipotential surface?

OR 

Calculate the charge carried by 12.5 x 10⁸ electrons.

17. What is the angle between the plane of the polariser and that of an analyser , in order that the intensity of light reduces to half.

18. What is the condition for first minima in case of diffraction due to a single slit?

19. Write the reaction of β-decay.

20. Write the truth table for 

SECTION B

21.Apply Gauss’s law to show that for a charged spherical shell, the electric field outside the shell is, as if the entire charge were concentrated at the center.

OR

Two large parallel plane sheets have uniform charge densities + σ and -σ. Determine the electric field (i) between the sheets, and (ii) outside the sheets.
22.Two bulbs are rated (P₁, V) and (P₂, V). If they are connected (i) in series and (ii) in parallel across a supply V, find the power dissipated in the two combinations in terms of P₁ and P₂.
23.State Bohr’s quantization condition of angular momentum. Calculate the shortest wavelength of the Bracket series and state to which part of the electromagnetic spectrum does it belong.
OR
Calculate the orbital period of the electron in the first excited state of hydrogen atom.
24.Plot a graph showing the variation of undecayed nuclei N versus time t. From the graph, find out how one can determine the half-life and average life of the radioactive nuclei.
25.Calculate the radius of curvature of an equi-concave lens of refractive index 1.5, when it is kept in a medium of refractive index 1.4, to have a power of –5D ?
OR
An equilateral glass prism has a refractive index 1.6 in air. Calculate the angle of minimum deviation of the prism, when kept in a medium of refractive index 4√2/5.

26.Plot a graph showing variation of de Broglie wavelength (λ) associated with a charged particle of mass m, versus 1/ V , where V is the potential difference through which the particle is accelerated. How does this graph give us the information regarding the magnitude of the charge of the particle ?
27. State two properties of electromagnetic waves.How can we show that em waves carry momentum.

SECTION C

28.(a) Describe briefly the functions of the three segments of n-p-n transistor.

(b) Draw the circuit arrangement for studying the output characteristics of n-p-n transistor in CE configuration. Explain how the output characteristics is obtained.
OR
Draw the circuit diagram of a full wave rectifier and explain its working. Also, give the input and output waveforms.
29.A student uses the circuit diagram of a potentiometer as shown in the figure (a) For a steady current I passing through the potentiometer wire, he gets a null point for the cell ɛ₁ and not for ɛ₂. Give reason for this observation and suggest how this difficulty can be resolved. (b) What is the function of resistance R used in the circuit ? How will the change in its value affect the null point ? (c) How can the sensitivity of the potentiometer be increased ?

30. (i) How are eddy currents generated in a conductor which is subjected to a magnetic field ? 
(ii) Write two examples of their useful applications.
(iii) How can the disadvantages of eddy currents be minimized ?
31.Draw a labelled diagram of cyclotron. Explain its working principle. Show that cyclotron frequency is independent of the speed and radius of the orbit.
OR
(a) Derive, with the help of a diagram, the expression for the magnetic field inside a very long solenoid having n turns per unit length carrying a current I.
(b) How is a toroid different from a solenoid ?
32. (i) Define a wavefront. Using Huygens’ geometrical construction, explain with the help of a diagram how the plane wavefront travels from the instant t₁ to t₂  in air.
(ii)A plane wavefront is incident on a convex lens. Explain, with the help of the diagram, the shape of the refracted wavefront formed.
33. (i) How does one explain the emission of electrons from a photosensitive surface with the help of Einstein’s photoelectric equation ?
(ii) Work function of aluminium is 4·2 eV. If two photons each of energy 2·5 eV are incident on its surface, will the emission of electrons take place ? Justify your answer.
(iii) The stopping potential in an experiment on photoelectric effect is 1·5 V. What is the maximum kinetic energy of the photo electrons emitted ? Calculate in Joules.
34. Write three points of differences between para-, dia- and ferro- magnetic materials, giving one example for each.

SECTION D

35. (a) In a series LCR circuit connected across an ac source of variable frequency, obtain the expression for its impedance and draw a plot showing its variation with frequency of the ac source.
(b) What is the phase difference between the voltages across inductor and the capacitor at resonance in the LCR circuit ?
(c) When an inductor is connected to a 200 V dc voltage, a current of 1A flows through it. When the same inductor is connected to a 200 V, 50 Hz ac source, only 0.5 A current flows. Explain, why ? Also, calculate the self inductance of the inductor.
OR
(a) Draw the diagram of a device which is used to decrease high ac voltage into a low ac voltage and state its working principle. Write four sources of energy loss in this device.
(b) A small town with a demand of 1200 kW of electric power at 220 V is situated 20 km away from an electric plant generating power at 440 V. The resistance of the two wire line carrying power is 0.5 Ω per km. The town gets the power from the line through a 4000-220 V step-down transformer at a sub-station in the town. Estimate the line power loss in the form of heat.
36. (a) Describe any two characteristic features which distinguish between interference and diffraction phenomena. Derive the expression for the intensity at a point of the interference pattern in Young’s double slit experiment.
(b) In the diffraction due to a single slit experiment, the aperture of the slit is 3 mm. If monochromatic light of wavelength 620 nm is incident normally on the slit, calculate the separation between the first order minima and the 3rd order maxima on one side of the screen. The distance between the slit and the screen is 1.5 m.
OR
(a) Under what conditions is the phenomenon of total internal reflection of light observed ? Obtain the relation between the critical angle of incidence and the refractive index of the medium.
(b) Three lenses of focal lengths +10 cm, –10 cm and +30 cm are arranged coaxially as in the figure given below. Find the position of the final image formed by the combination.

37.(a) When a parallel plate capacitor is connected across a dc battery, explain briefly how the capacitor gets charged.
(b) A parallel plate capacitor of capacitance ‘C’ is charged to ‘V’ volt by a battery. After some time the battery is disconnected and the distance between the plates is doubled. Now a slab of dielectric
constant 1 < k < 2 is introduced to fill the space between the plates. How will the following be affected ? (i) The electric field between the plates of the capacitor. (ii) The energy stored in the capacitor. Justify your answer in each case.
(c) The electric potential as a function of distance ‘x’ is shown in the figure. Draw a graph of the electric field E as a function of x.

OR

(a) Derive an expression for the potential energy of an electric dipole in a uniform electric field. Explain conditions for stable and unstable equilibrium.

(b) Is the electrostatic potential necessarily zero at a point where the electric field is zero ? Give an example to support your answer.