Subject: Physics
Class XII
Time Allowed: 3 hours
Max. Marks: 70
General Instructions:
(1) There are 35 questions in all. All questions are compulsory
(2) This question paper has five sections: Section A, Section B, Section C, Section D and Section E. All the sections are compulsory.
(3) Section A contains eighteen MCQ of 1 mark each, Section B contains seven questions of two marks each, Section C contains five questions of three marks each, section D contains three long questions of five marks each and Section E contains two case study-based questions of 4 marks each.
(4) There is no overall choice. However, an internal choice has been provided in section B, C, D and E. You have to attempt only one of the choices in such questions.
(5) Use of calculators is not allowed.
SECTION A
1. If an object contains n1 protons and n2 electrons the net charge on the object is
i) (n1 + n2)e
ii) (n1 – n2)e
iii) (n2 – n1)e
iv) Zero
2. When the separation between two identical charges is increased the electric potential energy of the charges
i) increases
ii) decreases
iii) remains same
iv) may increase or may decrease
3. The current – voltage graph for a given metallic wire at two different temperatures T1 and T2 as shown in figure, then
i) Temperature T2 is greater than T1
ii) Temperature T1 is greater than T2
iii) Both T1 and T2 are equal
iv) Resistance of metallic wire is independent of temperature
Explanation: Slope of the graph is inverse proportional resistance and resistance increases with the temperature.
4. An electron travelling west to east enters a chamber having a uniform magnetic field in north to south, then the direction of deflection is
i) Upwards
ii) Downwards
iii) North
iv)South
Explanation: Flemings left hand rule
5. Two rings, each having equal radius R, are placed mutually perpendicular to each other and each have common centre at the origin of coordinate system. If the current i is flowing through each ring then the magnitude of the magnetic field at the common centre is
i) 𝜇0𝑖/√2𝑅
ii) 𝜇0𝑖/2𝑅
iii)𝜇0𝑖/𝑅
d) 𝜇0𝑖/4𝑅
Explanation: he magnetic field mutually perpendicular to each other.
6. Needles N1, N2 and N3 are made of a ferromagnetic, paramagnetic and diamagnetic substance respectively. A magnet is brought close to them will
i) Attract N1 and N2 strongly but repel N3
ii) Attract N1 strongly, but repel N2 and N3 weakly
iii) Attract N1 strongly, N2 weakly and repel N3 weakly
iv) Attract all these of them.
7. An iron cored coil is connected in series with an electric bulb with an AC source as shown in figure. When iron piece is introduced into the coil, the brightness of the bulb will
i) increase
ii) decrease
iii) remains unaffected
iv) fluctuate
Explanation: When it is inserted there will be induction current effect and bulb glows with less intensity.
8. Which of the following rays has minimum frequency?
i) U V rays
ii) X-rays
iii) Microwaves
iv) Infrared rays
9. A Current carrying wire is placed below a coil in its plane, with current flowing as shown. If the current increases.
i)No current will be induced in the coil.
ii) An anti-clock wise current will be induced in the coil.
iii) A clockwise current will be induced in the coil.
iv) The current induced in the coil will be first anti clockwise and then clockwise.
10. The ratio of maximum to minimum intensity in the interference pattern is 25:1 . Then the ratio of amplitudes of the interfering beam is
i) 3:2
ii) 5:1
iii) 2:3
iv) 5:3
11. A metal of work function 4eV is exposed to a radiation of wavelength 1400Å then the maximum energy of photoelectrons
i) 2.43eV
ii) 4.86eV
iii) 5.24eV
iv) 6.24eV
12. The energy of electron in the ground state of Hydrogen atom is -13.6eV. The kinetic energy of the electron is the 4th orbit is
i) 3.4eV
ii) 0.85eV
iii) 1.7eV
iv) 0.425eV
13. Which of the following is not correct about nuclear forces.
i) They are short range attractive forces
ii) They are independent of charge
iii) They change to repulsive at very close distance
iv) They obey inverse square law
14. In an LCR series circuit, the voltage across R, L and C at resonance are 40V, 50V and 50V respectively. The applied voltage is
i) 50V
ii) 40V
iii) 140V
iv) 90V
15. A slab of copper of thickness y is inserted in between the plates of parallel plate capacitor. The separation between the plates is d. If y =d/4, then the ratio of capacitance of the capacitor after and before inserting the slab is
i) 4:3
ii) 1:2
iii) 1:1
iv) 3:2
16. Two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
a) Both A and R are true and R is the correct explanation of A
b) Both A and R are true and R is NOT the correct explanation of A
c) A is true but R is false
d) A is false and R is also false
Assertion: When a diode is forward biased it acts like a conductor.
Reason: Width of depletion layer increases in forward bias.
17. Two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
a) Both A and R are true and R is the correct explanation of A
b) Both A and R are true and R is NOT the correct explanation of A
c) A is true but R is false
d) A is false and R is also false
Assertion: In Young’s double slit experiment interference pattern disappears when one of the slits is closed.
Reason: Interference occurs due to superposition of light waves from two coherent sources.
18. Two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
a) Both A and R are true and R is the correct explanation of A
b) Both A and R are true and R is NOT the correct explanation of A
c) A is true but R is false
d) A is false and R is also false
Assertion: Maximum photoelectric current in a photocell depends upon intensity of incident light if the frequency of incident light is above threshold frequency.
Reason: Greater the frequency of incident light, greater will be the maximum velocity of emitted electrons
Section B
19. Name the parts of the electromagnetic spectrum which is
(i) suitable for radar systems used in aircraft navigation.
Ans. microwaves
(ii) used to kill germs in water purifier.
Ans. Ultra violet rays
20. (a) The susceptibility of a magnetic materials is – 4.2×10–6. Name the type of magnetic materials it represents.
Ans. Diamagnetic materials
(b) State any two properties of the above type of materials.
Ans. any two properties of diamagnetic materials
21. A difference of 2.3 eV separates two energy levels in an atom. What is the frequency of radiation emitted when the atom makes transition from the upper level to the lower level?
Ans.
OR
Two nuclei have mass numbers in the ratio 1 : 8. What is the ratio of their nuclear radii?
Ans.
22. A ray of light passes through an equilateral glass prism such that the angle of incidence is equal to angle of emergence and each of these angles is equal to 3/4 of angle of prism. What is the value of angle of deviation?
Ans.
23. Write two characteristic features to distinguish between n-type and p-type semiconductors.
Ans. N-type
1. It is formed by doping pentavalent impurities.
2. The electrons are the majority carriers and holes are minority carriers.
P -type
1. It is formed by doping trivalent impurities.
2. The electrons are the minority carriers and holes are majority carriers.
OR
Draw energy band diagrams of an n-type and p-type semiconductor at temperature T > 0 K.
Ans.
24. How does the fringe width of interference fringes change, when the whole apparatus of Young’s experiment is kept in water (refractive index 4/3)?
Ans. Fringe width equation.
fringe width decreases to 𝟑/𝟒 times.
25. Two concentric metallic spherical shells of radii R and 2R are given charges Q1 and Q2 respectively. The surface charge densities on the outer surfaces of the shells are equal. Determine the ratio Q1 : Q2.
Ans.
Section C
26. Deduce an expression for magnetic dipole moment of an electron revolving around a nucleus in a circular orbit. Indicate the direction of magnetic dipole moment. Use the expression to derive the relation between the magnetic moment of an electron moving in a circle and its related angular momentum.
Ans. Derivation
Perpendicular to the plane of loop directed outwards for anticlockwise current in loop and is directed inwards for clockwise current in loop.
Relation between the magnetic moment of an electron moving in a circle and its related angular momentum.
27. A long solenoid with 15 turns per cm has a small loop of area 2.0 cm2 placed inside normal to the axis of the solenoid. The current carried by the solenoid changes steadily from 2A to 4A in 0.1s, what is the induced emf in the loop while the current is changing?
Ans.
28. A series LCR circuit with R=20 Ω, L =1.5 H and C = 35 µF is connected to a variable frequency 200V ac supply. When the frequency of the supply equals the natural frequency of the circuit, what is the average power transferred to the circuit in one complete cycle?
Ans.
OR
Obtain the resonant frequency of a series LCR circuit with L = 2.0 H, C=32 µF and R = 10 Ω. What is the quality factor (Q) of this circuit?
Ans.
29. (a) If the potential difference used to accelerate electrons is doubled, by what factor does the de-Broglie wavelength associated with the electrons change?
Ans.
(b) Show on a graph the variation of the de Broglie wavelength (λ) associated with an electron, with the square root of accelerating potential (V).
Ans.
(c) An electron and a proton have the same kinetic energy. Which one of the two has the larger de Broglie wavelength and why?
Ans. DeBroglie wavelength is inversely proportional to mass for a given value of kinetic. As an electron has a smaller mass than a proton, an electron has larger de Broglie wavelength than a proton for the same kinetic energy.
OR
(a) Draw a graph showing variation of photocurrent with anode potential for a particular intensity of incident radiation. Mark saturation current and stopping potential.
Ans.
(b) How much would stopping potential for a given photosensitive surface go up if the frequency of the incident radiations were to be increased from 4×1015 Hz to 8 × 1015 Hz?
Ans.
30. A photon emitted during the de-excitation of electron from a state n to the first excited state in a hydrogen atom, irradiates a metallic cathode of work function 2 eV, in a photo cell, with a stopping potential of 0.55 V. Obtain the value of the quantum number of the state n.
Ans.
31. (a) A point charge of 2.0 nC is at the centre of a cubic Gaussian surface 9.0 cm on edge. What is the net electric flux through the surface?
Ans.
(b) Two large, thin metal plates are parallel and close to each other. On their inner faces, the plates have surface charge densities of opposite signs and of magnitude 17.0 × 10–22 C/m2. Calculate the electric field strength E:
(i) in the outer region of the first plate
Ans. In the outer region of plate I, there is no charge enclosed by plate I. Thus, the intensity of electric field is zero in the outer region of plate I.
(ii) in the outer region of the second plate
Ans. In the outer region of plate II, there is no charge enclosed by plate II. Thus, the intensity of electric field is zero in the outer region of plate II
(iii) between the plates
Ans.
OR
(a) A regular hexagon of side 10 cm has a charge 5 μC at each of its vertices. Calculate the potential at the centre of the hexagon.
Ans.
(b) Derive an expression for the electric potential at an axial point due to an electric dipole of dipole length 2a.
Ans. Derivation
(c) What is the electric potential at any point on the equatorial line of an electric dipole?
Ans. Zero since angle is 900.
32. (a) Plot a graph showing the variation of resistance of a conducting wire as a function of its radius, keeping the length of the wire and its temperature as constant.
Ans.
(b) Prove that the current density of a metallic conductor is directly proportional to the drift speed of electrons.
Ans. Derivation
(c) The number density of free electrons in a copper conductor is 8.5×1028m3. How long does an electron take to drift from one end of a wire 3 m long, to its another end? The area of cross section of the wire is 2.0 ×10–6 m2 and it is carrying a current of 3.0 A.
Ans.
OR
(a) Use Kirchhoff ’s laws to determine the value of current I1 in the given electrical circuit.
Ans.
(b) Draw a circuit diagram showing balancing of Wheatstone bridge. Use Kirchhoff ’s laws to obtain the balance condition in terms of the resistances of four arms of Wheatstone Bridge.
Ans. Diagram, Derivation.
33. (a) The focal length of an equiconvex lens is equal to the radius of curvature of either face. What is the value of refractive index of the material of the lens?
Ans.
(b) Draw the diagrams to show the behaviour of plane wavefronts as they
(i) pass through a thin prism
Ans.
(ii) pass through a thin convex lens
Ans.
(iii) reflect by a concave mirror.
Ans.
OR (a) Draw the intensity distribution for
(i) the fringes produced in interference
Ans.
(ii) the diffraction bands produced due to single slit.
Ans.
(b) Draw a ray diagram for formation of image of a point object by a thin double convex lens having radii of curvature R1 and R2. Hence, derive lens maker’s formula for a double convex lens.
Section E
34. Case Study:
Read the following paragraph and answer the questions.
When light moves from one medium to another, part of the light gets refracted and the remaining part of it will get reflected back into the same medium. Under certain conditions, the whole of the light can be made to be reflected back into the same medium. This phenomenon is called total internal reflection.
(i) Define critical angle for a pair of media.
Ans. It is the angle of incidence for which the angle of refraction is 900.
(ii) Write the relation between the refractive index and critical angle for a given pair of optical media.
Ans.
(iii) What are the two essential conditions required for total internal reflection to take place?
Ans. * Light ray should move from denser medium to rarer medium
* The angle of incidence should be greater than critical angle
OR
(iii) Write any two advantages of totally reflecting prisms over plane mirrors.
Ans. *When light is reflected from total reflecting prism, whole of the light is totally internally reflected Intensity of reflected light is more in prism than that of the light reflected from plane mirror.
*No multiple reflections take place in total reflecting prism as compared with that of plane mirror.
35. Case Study:
Read the following paragraph and answer the questions.
A P-N junction is an interface or a boundary between two semiconductor material types, namely the p-type and the n-type, inside a semiconductor. In a semiconductor, the P-N junction is created by the method of doping. p–n junctions are elementary “building blocks” of semiconductor electronic devices such as diodes, transistors, solar cells, LEDs, and integrated circuits; they are the active sites where the electronic action of the device takes place.
(i) For the same order of doping, why does n-type semiconductor exhibit larger conductivity than p-type semiconductor?
Ans. It is because the mobility of electrons is greater than that of hole.
(ii) Give one application of P-N junction diodes.
Ans. Used as rectifiers (or any other application)
(iii) Why does a potential barrier set up across the P-N junction?
Ans. The accumulation of negative charges in the p region and positive charges in then region sets up a potential difference across the junction.
OR
(iii) What are the two types of biasing in P-N junction diode? Draw the respective diagrams.
Ans. Forward biasing and reverse biasing, Diagrams