Sample paper Physics (2021) Class12 CBSE

 Class 12         Sample paper physics CBSE(2021)           M. M 70                                                                                                                

Section A contain ten very short answer questions and four assertion reasoning MCQs of 1 mark each, Section B contain two case based questions of 4 marks each, Section C contain nine  short answer questions of 2 marks each, Section D - five short answer questions of 3 marks each and Section E contains three long answer questions of 5 marks each.

                                                                                                                                                                                     Section A

Q1 In young’s double slit experiment, the separation of the slit is doubled and the distance between the slits and screen is halved. How will it affect the fringe width?

                                                                                                            2 Define wavefront and state Huygens principle.   

                                                                                                            3 A ray of light , incident on an equivalent glass prism(n=√3) moves parallel to the base of the prism , inside it. What is the angle of incidence for this ray?  

                                                                                                             4 A charge of 500μC is placed at the centre of a circle if radius 10cm.What is the work done to  move a 20μC charge from a point on the circle to a diametrically opposite point?

                                                                                                            5 A wire of resistance R is drawn into a new length which is three times it's original length. Assuming that resistivity and density of the wire is not changed, wire is not changed, what is the resistance of the longer wire?   

                                                                                                                                                                                                                                                                                                                                                               6 An object is placed on the focus of an equiconvex lens, how is the image changed if     i) the radius of curvature of the lens is somehow increased slightly,    ii) the refractive index is somehow increased slightly.

                                                                                                                                                  7 State the reason, why two independent sources of light cannot be considered as coherent sources.

                                                                                                                                                                 8 A metal plate is introduced between the plates of a charged parallel plate capacitor. What is its effect on the capacitance of the capacitor?         

                                                                                                                            9 Draw equipotentinal surfaces due to an electric dipole. Locate the points where the potential due to dipole is zero.  

                                                                                                                                                                 10 If the temperature of a good conductor increases, how does the relaxation time of electrons in the conductor change?   

  Read the given assertion and reason questions and choose the correct one out of the following                                                      A) Both Assertion and reason are true and reason is correct explanation of Assertion.                                                                B) Both Assertion and reason are true but reason is not correct explanation of Assertion.                                                              C) Assertion is true reason is false.                                                                                                  D)Both Assertion and reason are false                                                                                                                                                                              11 Assertion: To compare the emfs of two cells using a potentiometer, the emf of the cell in auxillary circuit must be greater than the emfs of both cells.                                         Reason: The area of cross-section of the wire should be uniform and current through the wire should always be kept constant.                                                         

 12 Assertion: A parallel plate capacitor is connected across a battery through a key. A dielectric slab of dielectric constant k is introduced between the plates. The energy, which is stored becomes K times.                                                                     Reason :The surface density of charge on plate remains constant.                                                  

13Assertion :In optical fibre, the diameter of the core is kept small.                                                                                     Reason: This smaller diameter of core ensures that the fibre should have incident angle more than the critical angle required for total internal reflection.                                                                                                                                

14Assertion : In Young's double slit experiment, the fringe width for dark fringes is equal to that for white fringes.                  Reason: It is performed with a source of white light, only black and white fringes are observed.  

                                                                                                                                Section- B    

   15 Read the source and answer following:-

 The Wheatstone bridge works on the principle of null deflection ,i.e the ratio of their resistance are equal and no current flows through the circuit. And the principle of working of meter bridge is wheat stone bridge and it is used to find the resistance of an unknown conductor or to compare two unknown resistance.

1) when a metal conductor connected to the left gap of a meter bridge is heated, the balancing point shifts :                                    a) right     b) left       c) unchanged.       d) none of the above                                                           2) Wheatstone bridge is a/an :                                                         a) a.c bridge b) d.c bridge c) high voltage bridge d) none of these. 3) It is used to measure the resistance of various types of wires for : a) determining their effective resistance.    b) computing the power dissipation     c) quality control of wire.   d) none of these                                                                                                                                                                4) By using the variation on Wheatstone bridge we can measure quantities such as.                                                                          a) voltage, current and power    b) measure high resistance values c)complex power.   d) capacitance, inductance and impedence.                                                                                                                     

16 Read the source and answer the following:                            When an object is moved against the electric field it gains some amount of energy which is defined as electric potential energy. For any charge, the electric potential is obtained by dividing the potential energy by the quantity of charge and the surface which is the locus of all points at same potential called equipotential surface.                                                                                                                                                          1) Equipotentials at a great distance from a collection of charge whose total sum is not zero are approximate:                                   a) sphere b) planes c) paraboloids d) ellipsoids                                                             2) work done in carrying an electron from A to B lying equipotential surface on one volt potential is :                               a) 1eV b) 10 eV c) 1 volt d) zero                                                                                                            3) Electric potential is :                                                                     a) scalar b) vector c) both d) none of these                                                      4) The shape of the equipotential for a point charge:                      a) sphere b) rectangular c) circle d)irregular 

Section - C 

17 Two identical charged bodies have 12μC and -18μC charge respectively. These bodies experience a force of 48N at certain separation. The bodies are touched and placed at the same separation again. Find the new force between the bodies.

                                                                                                         18 A giant telescope in an observatory has an objective of focal length 19 m and an eye-piece of focal length 1.0 cm. In normal adjustment the telescope is used to view the moon.What is diameter of the image of the moon formed by objective? The diameter of the moon is 3.5x10^6m and the radius of the lunar orbit around earth is 3.8x10^8m.                                                         

19 A ray of light is incident at an angle of incidence i on one surface of a prism of small angle A and it is found to emerge normally from the opposite surface. If the refractive index if the material of the prism is n, calculate the angle of incidence                                                                 

20 In young’s double slit experiment, the intensity of light at a point on a screen with path difference lambda is I. Calculate the intensity at nearby point where the path difference is lambda/3 Take intensity of two coherent source to be equal.                                                              

21 Draw temperature dependence of resistivity graph for semiconductor & alloys.                                                                   

22 Eight capacitors of capacitance  'C’ each connected in parallel to a source store an energy of 64 units.How much energy will be stored if the capacitors are now joined in series to the same source.     

                                                                                                                                                    23 Calculate the amount of work done in rotating a dipole, of dipole moment 3x 10^-8 Cm, from its position of stable equilibrium to the position of unstable equilibrium,in a uniform field of intensity 10^4N/C.                                                                                                                                      

24 Derive the relation between the refractive index and critical angle for a given pair of optical media. 

Section-D     

  25 State Gauss law in electrostatics. Using this law, derive an expression for the electric field due to a uniformly charged infinite plane sheet.                                                                         

26 A beam of light consisting of two wavelengths 650nm and 520nm, are used to obtain interference fringes in Young’s double slit experiment.    A) Find the distance of the third bright fringe on the screen from the central maximum for wavelength 650nm.           B) what is the least distance from the central maximum where the bright fringes due to both wavelength coincide.                                     

27Find the ratio if the potential differences that must be applied across the parallel and the series combination of two capacitors, C1 and C2 with their capacitances in the ratio 1:2,so that energy stored in two cases becomes same.     

                                                                                                         28 i) Derive the relation delta= (n-1)A                                           ii) Use Huygens’ principle to show that propagation of a plane wavefront from denser to rarer medium . Hence find the ratio of the speeds of wavefronts in the two media.      

                                                                                                                                  29i) converging lens has a focal length of 20cm in air. It is made of a material of refractive index 1.6. It is immersed in a liquid of refractive index 1.3. Calculate its new focal length.                ii)Two thin lenses of power +6D and -2D are in contact. What is the focal length of the combination.                                             

30i) In a meter bridge, two unknown resistance R and S when connected in the two gaps, give a null point at 40cm from one end. What is the ratio of R and S?                                                  ii) Two cells of emf E1 and E2 having internal resistance r1 and r2 resp, are connected in series. Deduce an expression for equivalent emf and internal resistance.

Section E 

 31 In young’s double slit experiment deduce the condition for a) constructive snd b) distructive interference at a point on screen, also give angular position and fringe width.Draw a graph showing a variation of intensity pattern against x.

32 Give principle, construction and working of potentiometer. Explain how it is used i) to compare emfs of two cells & ii) determine the internal resistance of a cell.

33 i)Derive an expression for parallel plate capacitor. ii) Derive an expression for energy stored in capacitor and energy per unit volume. 

   Or 

Derive an expression for convex refracting surface of an object kept in denser medium and hence give lens maker's formula. With proper diagram.

 

( This Sample paper contains questions only from 5 chapters - Electric charges &fields, Electrostatic potential and capacitance, Current Electricity, Ray and wave Optics.)