CBSE Sample Papers for Class 12 Physics Paper 8

CBSE Sample Papers for Class 12 Physics Paper 8 are part of CBSE Sample Papers for Class 12 Physics. Here we have given CBSE Sample Papers for Class 12 Physics Paper 8

CBSE Sample Papers for Class 12 Physics Paper 8

Board	CBSE
Class	XII
Subject	Physics
Sample Paper Set	Paper 8
Category	CBSE Sample Papers

Students who are going to appear for CBSE Class 12 Examinations are advised to practice the CBSE sample papers given here which is designed as per the latest Syllabus and marking scheme as prescribed by the CBSE is given here. Paper 8 of Solved CBSE Sample Paper for Class 12 Physics is given below with free PDF download solutions.

Time Allowed: 3 hours
Maximum Marks: 80

General Instructions

Question 1.
Nichrome and copper wires of same length and same radius are connected in series. Current I is passed through them. Which wire gets heated up more ? Justify your answer.
Ans.
(i)  Nichrome
(ii) R_Ni> RR_Cu (or Resistivity_Ni >Resistivity_Cu )  

Question 2.
Do electromagnetic waves carry energy and momentum?
Answer : Yes

Question 3.
How does the angle of minimum deviation of a glass prism vary, if the incident violet light is replaced by red light ? Give reason.
Answer :
(i) Decreases
(ii) n_violet > n_Red
(Also accept if the student writes λ_v<λ_R)

Question 4.
Name the phenomenon which shows the quantum nature of electromagnetic radiation.
Answer :
Photoelectric Effect (/Raman Effect/ Compton Effect)

Question 5.
Predict the polarity of the capacitor in the situation described below:

Answer :
A is positive and
B is negative
Also accept: A is negative and B is positive.

SECTION B

Question 6.
Draw the intensity pattern for single slit diffraction and double slit interference. Hence, state two differences between interference and diffraction patterns.
OR
Unpolarised light is passed through a polaroid P₁ When this polarised beam passes through another polaroid P₂ and if the pass axis of P₂ makes angle θ with the pass axis of P₁ then write the expression for the polarised beam passing through P₂. Draw a plot showing the variation of intensity when θ varies from 0 to 2π.
Answer :

• Interference pattern
• Diffraction pattern
• Two Diffraction

Interference	Diffraction
All maxima have equal intensity	Maxima have different (/rapidly decreasing) intensity
All fringes have equal width.	Different (/changing) width.
Superposition of two wavefronts	Superposition of wavelets from the same wavefront

(Any two)
OR

• Expression for intensity of polarized beam
• Plot of intensity variation with angle

Intensity is y cos² θ (if I₀ is the intensity of unpolarised light.)
Intensity is I cos² θ (if I is the intensity of polarized light.)
(Award 1/2 mark if the student writes the expression as I₀ cos² θ)

Question 7.
Identify the electromagnetic waves whose wavelengths vary as
(a)  10^-12 m < λ < 10^-8 m
(b)  10^-3 m < λ < 10-¹ m
Write one use for each.
Answer :

• Identification
• Uses

(a) X – rays
Used for medical purposes.
(Also accept UV rays and gamma rays and Any one use of the e.m. wave named)
(b) Microwaves
Used in radar systems
(Also accept short radio waves and Any one use of the e.m. wave named)

Question 8.
Find the condition under which the charged particles moving with different speeds in the presence of electric and magnetic field vectors can be used to select charged particles of a particular speed.
Answer :
Condition

• For directions of \(\vec {E } \), \(\vec { B } \), \(\vec { v } \)
• For magnitudes of \(\vec {E } \), \(\vec { B } \), \(\vec { v } \)

(i) The velocity \(\vec { v } \), of the charged particles, and the \(\vec {E } \) and \(\vec { B } \) vectors, should be mutually perpendicular.

(Note: Award 1 mark only if the student just writes: “The forces, on the charged particle, due to the electric and magnetic fields, must be equal and opposite to each other”)]

Question 9.
A 12-5 eV electron beam is used to excite a gaseous hydrogen atom at room temperature. Determine the wavelengths and the corresponding series of the lines emitted.
Answer :
(i) Writing \({ E }_{ n }\propto \frac { 1 }{ { n }^{ 2 } } \)
(ii) Identifying the level to which the electron is emitted
(iii) Calculating the wavelengths and identifying the series of atleast one of the three possible lines, that can be emitted.

(i) We have \({ E }_{ n }\propto \frac { 1 }{ { n }^{ 2 } } \)
(ii) ∴ The energy levels are – 13.6 eV; – 3.4 eV; – 1.5 eV
∴ The 12.5 eV electron beam can excite the electron up to n = 3 level only.
(iii) Energy values, of the emitted photons, of the three possible lines are:
3 —> 1 : (- 1.5 + 13.6) eV = 12.1 eV
2 —> 1 : (- 3.4 + 13.6) eV = 10.2 eV
3—>2: (-1.5+ 3.4) eV = 1.9 eV
The corresponding wavelengths are: 102 nm, 122 nm and 653 nm

(Award this 1 mark if the student draws the energy level diagram and shows (and names the series) the three lines that can be emitted) / (Award these (1/2+1/2 ) marks if the student calculates the energies of the three photons that can be emitted and names their series also.)

Question 10.
Write two properties of a material suitable for making (a) a permanent magnet, and (b) an electromagnet.
Answer :

• Two properties for making permanent magnet
• Two properties for making an electromagnet

(a) For making permanent magnet:
(i) High retentivity
(ii) High coercitivity
(iii) High permeability
(Any two)
(b) For making electromagnet:
(i)  High permeability
(ii) Low retentivity
(iii) Low coercivity
(Any two)

SECTION C

Question 11.
(a) The potential difference applied across a given resistor is altered so that the heat produced per second increases by a factor of 9. By what factor does the applied potential difference change? ’ (b) In the figure shown, an ammeter A and a resistor of Ω are connected to the terminals of the source. The emf of the source is 12 V having an internal resistance of 2 Ω . Calculate the voltmeter and ammeter readings.

Answer :

• The factor by which the potential difference changes
• Voltmeter reading Ammeter Reading

Question 12.
(a) How is amplitude modulation achieved?
(b)The frequencies of two side bands in an AM wave are 640 kHz and 660 kHz respectively. Find the frequencies of carrier and modulating signal. What is the bandwidth required for amplitude modulation?
Answer :

• Achieving amplitude modulation
• Stating the formulae
• Calculation of v_c and v_m
• Calculation of bandwidth

(a) Amplitude modulation can be achieved by applying the message signal, and the carrier wave, to a non linear (square law device) followed by a band pass filter.
(Alternatively, The student may just draw the block diagram.)

(Alternatively, Amplitude modulation is achieved by superposing a message signal on a carrier wave in a way that causes the amplitude of the carrier wave to change in accordance with the message signal.)

Question 13.
(a) In the following diagram, is the junction diode forward biased or reverse biased?

(b) Draw the circuit diagram of a full wave rectifier and state how it works.
Answer :
(a) The nature of biasing
(b) Diagram of full wave rectifier
Working
(a) Reverse Biased
(b) Diagram of full wave rectifier

Working: The diode D₁ is forward biased during one half cycle and current flows through the resistor, but diode D₂ is reverse biased and no current flows through it. During the other half of the signal, D₁ gets reverse biased and no current passes through it, D₂ gets forward biased and current flows through it. In both half cycles current, through the resistor, flows in the same direction. (Note: If the student just draws these graphs (but does not draw the circuit diagram), award 1/2 mark only.

Question 14.
Using photon picture of light, show how Einstein’s photoelectric equation can be established. Write two features of photoelectric effect which cannot be explained by wave theory.
Answer :

• Photon picture plus Einstein’s photoelectric equation
• Two features

In the photon picture, energy of the light is assumed to be in the form of photons, each carrying an energy hv.
Einstein assumed that photoelectric emission occurs because of a single collision of a photon with a free electron.
The energy of the photon is used to
(i) free the electrons from the metal.
[For this, a minimum energy, called the work function (=W) is needed].
And
(ii) provide kinetic energy to the emitted electrons.

This is Einstein’s photoelectric equation
Two features (which cannot be explained by wave theory):
(i) ‘Instantaneous’ emission of photoelectrons
(ii) Existence of a threshold frequency
(iii) ‘Maximum kinetic energy’ of the emitted photoelectrons, is independent of the intensity of incident light
(Any two)

Question 15.
(a) Monochromatic light of wavelength 589 nm is incident from air on a water surface. If μ for water is 1-33, find the wavelength, frequency and speed of the refracted light.
A double convex lens is made of a glass of refractive index 1.55, with both faces of the same radius of curvature. Find the radius of curvature required, if the focal length is 20 cm.
Answer :

• Calculation of wavelength, frequency and speed
• Lens Maker’s Formula
• Calculation of R

Question 16.
Define mutual inductance between a pair of coils. Derive an expression for the mutual inductance of two long coaxial solenoids of same length wound one over the other.
OR
Define self-inductance of a coil. Obtain the expression for the energy stored in an inductor L connected across a source of emf.
Answer :

• Definition of mutual inductance
• Derivation of mutual inductance for two long solenoids

(i) Mutual inductance is numerically equal to the induced emf in the secondary coil when the current in the primary coil changes by unity.
Alternatively: Mutual inductance is numerically equal to the magnetic flux linked with one coil/ secondary coil when unit current flows through the other coil /primary coil.

OR

• Definition of self inductance
• Expression for energy stored

(i)  Self inductance of a coil, is numerically equal to the emf induced in that coil when the current in it changes at a unit rate.
(Alternatively: The self inductance of a coil equals the flux linked with it when a unit current flows through it.)
(ii) The work done against back /induced emf is stored as magnetic potential energy.
The rate of work done, when a current i is passing through the coil, is

Question 17.
(a) Write the principle of working of a meter bridge.
(b) In a meter bridge, the balance point is found at a distance l₁ with resistances R and S as shown in the figure.
Answer :

• Principle of meter bridge
• Relation between l₁,l₂,S.

(a) The principle of working of a meter bridge is same as that of a balanced Wheatstone bridge.
(Alternatively:

Question 18.
Draw a block diagram of a generalized communication system. Write the functions following:
(a) Transmitter
(b) Channel
(c) Receiver
Answer :
Diagram of generalized communication system Function of (a) transmitter (b) channel (c) receiver

(a) Transmitter: A transmitter processes the incoming message signal so as to make it suitable for transmission through a channel and subsequent reception.
(b) Channel: It carries the message signal from a transmitter to a receiver.
(c) Receiver: A receiver extracts the desired message signals from the received signals at the channel output.

Question 19.
(a) Write the functions of the three segments of a transistor.
(b) The figure shows the input waveforms A and B for ‘AND’ gate. Draw the output waveform and write the truth table for this logic gate.

Answer :
(a) Function of each of the three segments
(b) Diagram of output wave form Truth table
(a) Emitter: Supplies a large number of majority charge carriers.
Base: Controls the flow of majority carriers from the emitter to the collector.
Collector: It collects the majority carriers from the base/majority of those emitted by the emitter.

Question 20.
(a) Draw a ray diagram depicting the formation of the image by an astronomical telescope in normal adjustment.
(b) You are given the following three lenses. Which two lenses will you use as an eyepiece and as an objective to construct an astronomical telescope? Give reason.

Answer :

• Ray diagram for astronomical telescope in normal adjustment
• Identification of lenses for objective and eyepiece Reason

(a) Ray diagram of astronomical telescope

(Note: Deduct ½ mark if the ‘arrows’ are not marked)
(b) Objective Lens: Lens L₁
Eyepiece Lens: Lens L₂
Reason: The objective should have large aperture and large focal length while the eyepiece should have small aperture and small focal length.

Question 21.
(a) State Biot — Savart law and express this law in the vector form.
(b) Two identical circular coils, P and Q each of radius R, carrying currents 1 A and√3A respectively, are placed concentrically and perpendicular to each other lying in the XY and YZ planes. Find the magnitude and direction of the net magnetic field at the centre of the coils.
Answer :
(a) It states that magnetic field strength, d \(\vec { b } \), due to a current element, Id \(\vec { l }\) ,  at a point., having a position vector r relative to the current element, is found to depend (I) directly on the current element. (ii) inversely on the square of the distance \(\left| r \right| \) , (iii) directly on the sine of angle between the current element and the position vector r.

Question 22.
Two identical parallel plate capacitors A and B are connected to a battery of V volts with the switch S closed. The switch is now opened and the free space between the plates of the capacitors is filled with a dielectric of dielectric constant K. Find the ratio of the total electrostatic energy stored in both capacitors before and after the introduction of the dielectric.

Answer :

• Formula for energy stored
• Energy stored before
• Energy stored after
• Ratio

SECTION D

Question 23.
Asha’s mother read an article in the newspaper about a disaster that took place at Chernobyl. She could not understand much from the article and asked a few questions from Asha regarding the article. Asha tried to answer her mother’s questions based on what she learnt in Class XII Physics.
(a) What was the installation at Chernobyl where the disaster took place ? What, according to you, was the cause of this disaster ?
(b) Explain the process of release of energy in the installation at Chernobyl.
(c) What, according to you, were the values displayed by Asha and her mother?
Answer :

• Name of the installation, the cause of disaster
• Energy release process
• Values shown by Asha and mother

(a) (i) Nuclear Power Plant:/‘Set-up’ for releasing Nuclear Energy/Energy Plant
(Also accept any other such term)
(ii) Leakage in the cooling unit/ Some defect in the set up.
(b) Nuclear Fission/Nuclear Energy  Break up (/ Fission) of Uranium nucleus into fragments
(c) Asha: Helpful, Considerate, Keen to Learn, Modest
Mother: Curious, Sensitive, Eager to Learn, Has no airs
(Any one such value in each case)

SECTION E

Question 24.
(a) Derive an expression for the electric field E due to a dipole of length ‘2a’ at a point distant r from the centre of the dipole on the axial line.
(b) Draw a graph of E versus r for r>> a.
(c) If this dipole were kept in a uniform external electric field E₀, diagrammatically represent – the position of the dipole in stable and unstable equilibrium and write the expressions for the torque acting on the dipole in both the cases.
OR
(a) Use Gauss’s theorem to find the electric field due to a uniformly charged infinitely large plane thin sheet with surface charge density σ.
(b) An infinitely large thin plane sheet has a uniform surface charge density +σ. Obtain the expression for the amount of work done in bringing a point charge q from infinity to a point, distant r, in front of the charged plane sheet.
Answer :

• Derivation of E along the axial line of dipole
• Graph between E vs r
• (i) Diagrams for stable and unstable equilibrium of dipole
  (ii) Torque on the dipole in the two cases

(Note: Award Vi mark only if the student does not draw the diagrams but just writes:
(i) For stable Equilibrium: \(\vec { p } \) is parallel to \(\vec { E } \) .
(ii) For unstable equilibrium: \(\vec { p } \) is antiparallel to \(\vec { E } \)).
Torque = 0 for (i) as well as case (ii).
(Also accept, \(\vec { \tau } \) = \(\vec { p } \) x \(\vec { E } \) / τ = pE sin θ)
OR
(a) Using Gauss’s theorem to find E due to an infinite plane sheet of charge
(b) Expression for the work done to bring charge q from infinity to r

The electric field E points outwards normal to the sheet. The field lines are parallel to the Gaussian surface except for surfaces 1 and 2. Hence the net flux = ∫ E.ds = EA + EA where A is the area of each of the surface 1 and 2.

Question 25.
A device ‘X’ is connected to an ac source V = V₀ sin rat. The variation of voltage, current and power in one cycle is shown in the following graph:

(a) Identify the device ‘X’.
(b) Which of the curves A, B and C represent the voltage, current and the power consumed in the circuit ? Justify your answer.
(c) How does its impedance vary with frequency of the ac source? Show graphically.
(d) Obtain an expression for the current in the circuit and its phase relation with ac voltage.
OR
(a) Draw a labelled diagram of an ac generator. Obtain the expression for the emf induced in the rotating coil of N turns each of cross-sectional area A, in the presence of a magnetic field \(\vec { B } \) .
(b) A horizontal conducting rod 10 m long extending from east to west is falling with a speed 5-0 ms^-1 at right angles to the horizontal component of the Earth’s magnetic field, 0-3 x 10^-4 Wb m^-2. Find the instantaneous value of the emf induced in the rod.
Answer :

• Identification
• Identifying the curves
  Justification
• Variation of Impedance with frequency
  Graph
• Expression for current
  Phase relation

The device X is a capacitor
Curve B → voltage
Curve C → current
Curve A → power


(Note : If the student identifies the device X as an inductor but writes correct answers to parts (c) and (d) (in terms of an inductor), the student be given full marks for (only) these two parts)
OR

• Labelled diagram of ac generator
  Expression for emf
• Formula for emf
  Substitution
  Calculation of emf

Question 26.
(a) Define wavefront. Use Huygens’ principle to verify the laws of refraction.
(b) How is linearly polarised light obtained by the process of scattering of light? Find the Brewster angle for air – glass interface, when the refractive index of glass =1-5.
OR
(a) Draw a ray diagram to show the image formation by a combination of two thin convex lenses in contact. Obtain the expression for the power of this combination in terms of the focal lengths of the lenses.
(b) A ray of light passing from air through an equilateral glass prism undergoes minimum deviation when the angle of incidence is 3/4th of the angle of prism. Calculate the speed of light in the prism.
Answer :

• Definition of wavefront
  Verifying laws of refraction by Huygen’s principle
• Polarisation by scattering
  Calculation of Brewster’s angle

(a) The wavefront is the common locus of all points which are in phase (/surface of constant phase)

Let a plane wavefront be incident on a surface separating two media as shown. Let V₁ and v₂ be the velocities of light in the rarer medium and denser medium respectively. From the diagram

This proves Snell’s law of refraction.
(b) When unpolarised light gets scattered by molecules, the scattered light has only one of its two components in it.
(Also accept the following diagrammatic representation)

OR

• Ray diagram
  Expression for power
• Formula
  Calculation of speed of light
  

Two thin lenses, of focal length ƒ₁ and ƒ₂ are kept in contact. Let O be the position of object and let u be the object distance. The distance of the image (which is at I₁), for the first lens is v₁ This image serves as object for the second lens. Let the final image be at I. We then have

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