CBSE Class-12th
1. PHYSICS
General Instructions :
All questions are compulsory. There are 26 questions in all.
This question paper has five sections : Section A, Section B, Section C, Section D and Section E.
Section A contains five questions of one mark each, Section B contains five questions
of two marks each, Section C contains twelve questions of three marks each, Section
D contains one value based question of four marks and Section E contains three
questions of five marks each.
There is no overall choice. However, an internal choice has been provided in one
question of two marks, one question of three marks and all the three questions of
five marks weightage. You have to attempt only one of the choices in such questions.
You may use the following values of physical constants wherever necessary :
c = 3x108 m/s
h = 6.63x10-34 Js
e = 1.6x10-19 C
πo= 4x10-7 T m A-1
εo= 8.854 10-12 C2N-1M-2
1/4πεo= 9x109 N m2 C-2
Mass of electron = 9.1 x 10-311kg
Mass of neutron = 1.675 x 10-27 kg
Mass of proton = 1.673x10-27 kg
Avogadro’s number = 6.023 x 1023 per gram mole
Boltzmann constant = 1.38 x 10-23JK-1
SECTION A
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.(1)
2. Do electromagnetic waves carry energy and momentum ?(1)
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.(1)
4. Name the phenomenon which shows the quantum nature of electromagnetic radiation.(1)
5. Predict the polarity of the capacitor in the situation described below : (1)
SECTION B
6. Draw the intensity pattern for single slit diffraction and double slit interference. Hence,
state two differences between interference and diffraction patterns.(2)
OR
Unpolarised light is passed through a polaroid P1
. When this polarized beam passes through
another polaroid P2 and if the pass axis of P2 makes angle 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 varies from 0 to 2 .
7. Identify the electromagnetic waves whose wavelengths vary as (2)
(a) 10-12 m <λ 10-8 m
(b) 10-3 m < λ 10-1 m
Write one use for each.
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.(2)
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.(2)
10. Write two properties of a material suitable for making (a) a permanent magnet, and (b)
an electromagnet.(2)
SECTION C
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 4 are connected to the terminals of the source. The emf of the source is 12 V having an internal resistance of 2 . Calculate the
voltmeter and ammeter readings.(3)
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 ? (3)
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.()
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.(3)
15. (a) Monochromatic light of wavelength 589 nm is incident from air on a water surface. If for water is 1·33, find the wavelength, frequency and speed of the refracted light.(3)
(b) 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.
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.(3)
OR
Define self-inductance of a coil. Obtain the expression for the energy stored in an inductor L onnected across a source of emf.
17. (a) Write the principle of working of a metre bridge.
(b) In a metre bridge, the balance point is found at a distance l1 with resistances R and S as shown in the figure. An unknown resistance X is now connected in parallel to the resistance S and the balance point is found at a distance l2. Obtain a formula for X in terms of l1, l2 and S.(3)
18. Draw a block diagram of a generalized communication system. Write the functions of each of the following : (3)
(a) Transmitter
(b) Channel
(c) Receiver
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.(3)
20. (a) Draw a ray diagram depicting the formation of the image by an astronomical
telescope in normal adjustment. (3)
(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.
Lenses Power (D) Aperture (cm)
L1 3 8
L2 6 1
L3 10 1
21. (a) State Biot – Savart law and express this law in the vector form.(3)
(b) Two identical circular coils, P and Q each of radius R, carrying currents 1 A and
√3 A 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.
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.(3)
SECTION D
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.(4)
(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 ?
SECTION E
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.(5)
(a) Use Gauss’s theorem to find the electric field due to a uniformly charged infinitely large plane thin sheet with surface charge density .
(b) An infinitely large thin plane sheet has a uniform surface charge density +. 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.(5)
(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. (5)
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 B
(b) A horizontal conducting rod 10 m long extending from east to west is falling with a speed
5·0 ms1at right angles to the horizontal component of the Earth’s magnetic field, 0·3 x 10-4Wb m-2 Find the instantaneous value of the emf induced in the rod.(5)
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. (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 thefocal lengths of the lenses.(5)
(b) A ray of light passing from air through an equilateral glass prism undergoes minimum deviation when the angle of incidence is of the angle of prism. Calculate the speed of light in the prism.
CBSE Class 12
2. PHYSICS
SET-2 2017
SECTION – A
1. A point charge Q is placed at point ‘O’ as shown in the figure. Is the potential at point A, i.e. VA , greater, smaller or equal to potential, VB , at point B, when Q is
(i) positive, and (ii) negative charge ? (1)
O. A. B.
2. Write the expression for speed of electromagnetic waves in a medium of electrical permittivity ε and magnetic permeability μ. (1)
3. Does the magnifying power of a microscope depend on the colour of the light used ? Justify your answer. (1)
4. Draw logic symbol of an OR gate and write its truth table. (1)
5. A photosensitive surface emits photoelectrons when red light falls on it. Will the surface emit photoelectrons when blue light is incident on it ? Give reason. (1)
SECTION B
6. Find the intensity at a point on a screen in Young’s double slit experiment where the
interfering waves of equal intensity have a path difference of (i) λ/4, and (ii) λ/3. (2)
7. Write two points of difference between intrinsic and extrinsic semiconductors. (2)
8. Distinguish between broadcast mode and point-to-point mode of communication and give one example for each. (2)
9. A light bulb and a solenoid are connected in series across an ac source of voltage. Explain, how the glow of the light bulb will be affected when an iron rod is inserted in the solenoid.(2)
10. Use the mirror equation to show that an object placed between f and 2f of a concave
mirror forms an image beyond 2f. (2)
OR
(a) State the condition under which a large magnification can be achieved in an
astronomical telescope.
(b) Give two reasons to explain why a reflecting telescope is preferred over a refracting telescope.
SECTION C
11. (a) Define the term ‘modulation index,’ used in communication system. Why is its value kept less than or equal to one?
(b) A message signal of frequency 10 kHz and peak voltage of 10 V is used to modulate a carrier frequency 1 MHz and peak voltage 10 V. Determine the (i) modulation index, and (ii) side bands produced. (3)
12. Using Bohr’s postulates, derive the expression for the orbital period of the electron moving in the nth orbit of hydrogen atom. (3)
13. A charge Q is distributed uniformly over a metallic sphere of radius R. Obtain the
expressions for the electric field (E) and electric potential (V) at a point 0 < x < R. Show on a plot the variation of E and V with x for 0 < x < 2R. (3)
14. In the given circuit, with steady current, calculate the potential difference across the
capacitor and the charge stored in it. (3)
15. A long charged cylinder of linear charge density + λ1 is surrounded by a hollow coaxial conducting cylinder of linear charge density -λ2 . Use Gauss’s law to obtain expressions for
the electric field at a point (i) in the space between the cylinders, and (ii) outside the larger cylinder.
16. Using Biot-Savart law, deduce the expression for the magnetic field at a point (x) on the axis of a circular current carrying loop of radius R. How is the direction of the magnetic field determined at this point ? (3)
The figure shows three infinitely long straight parallel current carrying conductors. Find the
(i) magnitude and direction of the net magnetic field at point A lying on conductor 1, (ii) magnetic force on conductor 2.
17. (a) State the law of radioactive decay. Write the SI unit of ‘activity’.
(b) There are radioactive nuclei in a given radioactive sample. If the half-life of the sample is 20 s, how many nuclei will decay in 10 s ? (3)
18. (a) How are electromagnetic waves produced ? Explain.
(b) A plane electromagnetic wave is travelling through a medium along the +ve z-direction. Depict the electromagnetic wave showing the directions of the oscillating electric and magnetic fields. (3)
19. A source of ac voltage v = v0 sin ωt, is connected across a pure inductor of inductance L.
Derive the expressions for the instantaneous current in the circuit. Show that average power dissipated in the circuit is zero. (3)
20. (a) Draw a plot showing the variation of photoelectric current with collector potential for different frequencies but same intensity of incident radiation.
(b) Use Einstein’s photoelectric equation to explain the observations from this graph.
(c) What change will you observe if intensity of incident radiation is changed but the frequency remains the same? (3)
21. (a) State the condition under which a charged particle moving with velocity v goes undeflected in a magnetic field B.
(b) An electron, after being accelerated through a potential difference of 10 4 V, enters a uniform magnetic field of 0·04 T, perpendicular to its direction of motion. Calculate the radius of curvature of its trajectory. (3)
22. A monochromatic light of wavelength λ is incident normally on a narrow slit of width ‘a’ to produce a diffraction pattern on the screen placed at a distance D from the slit. With the help of a relevant diagram, deduce the conditions for obtaining maxima and minima on the screen. Use these conditions to show that angular width of central maximum is twice the angular width of secondary maximum. (3)
SECTION D
23. Sunil and his parents were travelling to their village in their car. On the way his mother noticed some grey coloured panels installed on the roof of a low building. She enquired from Sunil what those panels were and Sunil told his mother that those were solar panels.
(a) What were the values displayed by Sunil and his mother ? State one value for each.
(b) In what way would the use of solar panels prove to be very useful?
(c) Name the semiconductor device used in solar panels. Briefly explain with the help of a diagram, how this device works. (4)
SECTION E
24. (a) (i) State the principle on which a potentiometer works. How can a given potentiometer be made more sensitive ?
(ii) In the graph shown below for two potentiometers, state with reason which of the two potentiometers, A or B, is more sensitive.
(b) Two metallic wires, P1 and P2 of the same material and same length but different crosssectional areas, A1 and A2 are joined together and connected to a source of emf. Find the ratio of the drift velocities of free electrons in the two wires when they are connected (i) in series, and (ii) in parallel.
OR
(a) Define the capacitance of a capacitor. Obtain the expression for the capacitance of a parallel plate capacitor in vacuum in terms of plate area A and separation d between the plates.
(b) A slab of material of dielectric constant K has the same area as the plates of a parallel plate capacitor but has a thickness . Find the ratio of the capacitance with dielectric inside it to its capacitance without the dielectric.
25. (a) State Faraday’s law of electromagnetic induction.
(b) The magnetic field through a circular loop of wire 12 cm in radius and 8·5 resistance, changes with time as shown in the figure. The magnetic field is perpendicular to the plane of the loop. Calculate the induced current in the loop and plot it as a function of time.
(c) Show that Lenz’s law is a consequence of conservation of energy.
OR
(a) Describe, with the help of a suitable diagram, the working principle of a step-up
transformer. Obtain the relation between input and output voltages in terms of the number of turns of primary and secondary windings and the currents in the input and output circuits.
(b) Given the input current 15 A and the input voltage of 100 V for a step-up transformer having 90% efficiency, find the output power and the voltage in the secondary if the output current is 3 A.
26. (a) A point object is placed on the principal axis of a convex spherical surface of radius of curvature R, which separates the two media of refractive indices n1 and n2 (n2 > n1). Draw the ray diagram and deduce the relation between the object distance (u), image distance (v) and the radius of curvature (R) for refraction to take place at the convex spherical surface from rarer to denser medium.
(b) A converging lens has a focal length of 20 cm in air. It is made of a material of refractive index 1·6. If it is immersed in a liquid of refractive index 1·3, find its new focal length.
OR
(a) Draw the ray diagram showing refraction of light through a glass prism and hence obtain the relation between the refractive index μ of the prism, angle of prism and angle of minimum deviation.
(b) Determine the value of the angle of incidence for a ray of light travelling from a medium of refractive index into the medium of refractive index μ2 = 1, so that it just grazes along the surface of separation.
CBSE Class 12 th Physics 3. PHYSICS
SECTION – A
1. A point charge +Q is placed in the vicinity of a conducting surface. Draw the electric field lines between the surface and the charge. (1)
2. Define modulation index. Why is it generally kept less than one? (1)
3. In the figure given, mark the polarity of plates A and B of a capacitor when the magnets are quickly moved towards the coil. (1)
4. The objective lenses of two telescopes have the same apertures but their focal lengths are in the ratio 1: 2. Compare the resolving powers of the two telescopes.
5. Define the conductivity of a conductor. Write its SI unit. (1)
SECTION – B
6. (i) Define refractive index of a medium.
(ii) In the following ray diagram, calculate the speed of light in the liquid of unknown
refractive index. (2)
7. Electrons are emitted from the cathode of a photocell of negligible work function, when photons of wavelength are incident on it. Derive the expression for the de Broglie wavelength of the electrons emitted in terms of the wavelength of the incident light. (2)
OR
Derive the Bohr’s quantisation condition for angular momentum of the orbiting of electron in hydrogen atom, using de Broglie’s hypothesis.
8. (a) Write two characteristic features of nuclear force.
(b) Draw a plot of potential energy of a pair of nucleons as a function of their separation. (2)
9. State the two points to distinguish between sky wave and space wave modes of propagation. (2)
10. The figure shows a plot of terminal voltage ‘V’ versus the current ‘i’ of a given cell. Calculate from the graph (a) emf of the cell and (b) internal resistance of the cell.(2)
11. A parallel plate capacitor of capacitance C is charged to a potential V by a battery.
Without disconnecting the battery, the distance between the plates is tripled and a dielectric medium of k = 10 is introduced between the plates of the capacitor. Explain giving reasons, how will the following be affected: (3)
(i) capacitance of the capacitor (ii) charge on the capacitor, and (iii) energy density of the capacitor.
12. (a) Draw a graph showing the variation of binding energy per nucleon (BE/A) vs mass number A for the nuclei in
20 ≤ A ≤ 170.
(b) A nucleus of mass number 240 and having binding energy/nucleon 7.6 MeV splits into two fragments Y, Z of mass numbers 110 and 130 respectively. If the binding energy/nucleon of Y, Z is equal to 8.5 MeV each, calculate the energy released in the nuclear reaction. (3)
13. (a) In Young’s double slit experiment, the two slits are illuminated by two different lamps having same wavelength of light. Explain with reason, whether interference pattern will be observed on the screen or not.
(b) Light waves from two coherent sources arrive at two points on a screen with path differences of 0 and /2. Find the ratio of intensities at the points. (3)
14. Using Bohr’s postulates, derive the expression for the total energy of the electron revolving in nth orbit of hydrogen atom. Find the wavelength of H line, given the value of Rydberg constant,
R = 1.1× 107 m–1. (3)
15. Name the e.m. waves in the wavelength range 10 nm to 10–3 nm. How are these waves generated ? Write their two uses. (3)
16. (a) Draw the pattern of magnetic field lines for a circular coil carrying current.
(b) Two identical circular loops X and Y of radius R and carrying the same current are kept in perpendicular planes such that they have a common centre at P as shown in the figure. Find the magnitude and direction of the net magnetic field at the point P due to the loops. (3)
17. State the reason, why the photodiode is always operated under reverse bias. Write the working principle of operation of a photodiode. The semiconducting material used to fabricate a photodiode, has an energy gap of 1.2eV. Using calculations, show whether it can detect light of wavelength of 400 nm incident on it. (3)
18. Draw the circuit diagram of a common emitter transistor amplifier. Write the expression for its voltage gain. Explain, how the input and output signals differ in phase by 180°. (3)
OR
Draw the circuit diagram of a full wave rectifier. Explain its working principle. Draw the input and output waveforms.
19. Briefly explain the three factors which justify the need of modulating low frequency signal into high frequencies. (3)
20. Define the term current sensitivity of a galvanometer. In the circuits shown in the figures, the galvanometer shows no deflection in each case. Find the ratio of R1 and R2 . (3)
21. The current through two inductors of self-inductance 12 mH and 30 mH is increasing with time at the same rate. Draw graphs showing the variation of the
(a) emf induced with the rate of change of current in each inductor
(b) energy stored in each inductor with the current flowing through it. Compare the energy stored in the coils, if the power dissipated in the coils is the same. (3)
22. (a) Explain how the intensity of diffraction pattern changes as the order (n) of the diffraction band varies.
(b) Two wavelengths of sodium light 590 nm and 596 nm are used in turn to study the diffraction at a single slit of size 4 mm. The distance between the slit and screen is 2 m.
Calculate the separation between the positions of the first maximum of the diffraction pattern obtained in the two cases. (3)
SECTION – D
23. Mr Kamath, the chief mechanical engineer, in Northern railways went to Tokyo to attend a seminar on fast moving trains. His friend Mr Hiorki explained how Japanese people are concentrating on energy conservation and saving fossil fuels using maglev trains. Mr Kamath travelled from Tokyo to Osaka in maglev train and found that the sound is less,L travel is smooth and understood the Japanese technology in mass transporting systems. Maglev trains work on the principle of Meissner’s effect. (3+2)
(a) Mention two values which Mr Kamath found in Mr Hiorki.
(b) Which values in Mr Kamath do you appreciate ?
(c) What is Meissner’s effect ? Write the value of magnetic permeability for perfect
diamagnetism.
SECTION – E
24. (a) State Gauss’ law. Using this law, obtain the expression for the electric field due to an infinitely long straight conductor of linear charge density λ.
(b) A wire AB of length L has linear charge density λ = kx, where x is measured from the end A of the wire. This wire is enclosed by a Gaussian hollow surface. Find the expression for the electric flux through this surface. (3+2)
OR
(a) Derive the expression for the electric potential at any point P, at distance r from the centre of an electric dipole, making angle α, with its axis.
(b) Two point charges 4 μC and +1 μC are separated by a distance of 2 m in air. Find the point on the line-joining charges at which the net electric field of the system is zero.
25. (a) Prove that an ideal capacitor in an ac circuit does not dissipate power.
(b) An inductor of 200 mH, capacitor of 400 μf and a resistor of 10 are connected in series to ac source of 50 V of variable frequency. Calculate the (i) angular frequency at which maximum power dissipation occurs in the circuit and the corresponding value of the effective current, and (ii) value of Q-factor in the circuit. (3+2)
OR
(a) A metallic rod of length l is moved perpendicular to its length with velocity ν in a magnetic field acting perpendicular to the plane in which rod moves. Derive the expression for the induced emf.
(b) A wheel with 15 metallic spokes each 60 cm long, is rotated at 360 rev/min in a plane normal to the horizontal component of earth’s magnetic field. The angle of dip at that place is 60°. If the emf induced between rim of the wheel and the axle is 400 mV, calculate the horizontal component of earth’s magnetic field at the place. How will the induced emf change, if the number of spokes is increased?
26. (a) Explain with reason, how the power of a diverging lens changes when (i) it is kept in a medium of refractive index greater than that of the lens. (ii) incident red light is replaced by violet light.
(b) Three lenses L1, L2, L3 each of focal length 30 cm are placed co-axially as shown in the figure. An object is held at 60 cm from the optic centre of lens L1 . The final real image is formed at the focus of L3 . Calculate the separation between (i) (L1 and L2) and (ii) (L2 and L3). (3+2)
OR
(a) Deduce the expression, by drawing a suitable ray diagram, for the refractive index of a triangular glass prism in terms of the angle of minimum deviation (D) and the angle of prism (A). Draw a plot showing the variation of the angle of deviation with the angle of incidence.
(b) Calculate the value of the angle of incidence when a ray of light incident on one face of an equilateral glass prism produces the emergent ray, which just grazes along the adjacent face. Refractive index of the prism is √2.
CBSE Class 12
th Physics
Last Year Paper
Compartment Delhi SET-1 2017 4. PHYSICS
SECTION – A
1. Draw a plot showing variation of electric field with distance from the centre of a solid conducting sphere of radius R, having a charge of +Q on its surface. (1)
2. State one factor which determines the intensity of light in the photon picture of light. (1)
3. An iron-cored solenoid has self-inductance 2.8 H. When the core is removed, the self-inductance become 2 mH. What is the relative permeability of the core used? (1)
4. An object is kept in front of a concave lens. What is the nature of the image formed ?
5. When light travels from a rarer medium to denser medium, the speed of light decreases.
Does the reduction in speed imply a reduction in the energy? (1)
SECTION – B
6. How is electromagnetic wave produced ? Draw a sketch of a plane e.m. wave propagating
along X-axis depicting the directions of the oscillating electric and magnetic fields. (2)
7. A charge q of mass m is moving with a velocity of V, at right angles to a uniform magnetic field B. Deduce the expression for the radius of the circular path it describes.
8. Calculate the shortest wavelength of light emitted in the Paschen series of hydrogen
spectrum. Which part of the electromagnetic spectrum, does it belong ? Given :
Rydberg constant, R = 1.1 × 10 7 m-1 . (2)
9. A small illuminated bulb is at the bottom of a tank, containing a liquid of refractive index upto a height H. Find the expression for the diameter of an opaque disc, floating
symmetrically on the liquid surface in order to cut-off the light from the bulb. (2)
OR
A ray of light is incident on a glass prism of refractive index and refracting angle A. If
it just suffers total internal reflection at the other face, obtain an expression relating the
angle of incidence, angle of prism and critical angle.
10. Depict the behaviour of magnetic field lines near (i) diamagnetic and (ii) paramagnetic
substances. Justify, giving reasons. (2)
SECTION – C
11. Draw a graph showing the variation of de Broglie wavelength of a particle of charge q
and mass m with the accelerating potential. Proton and deuteron have the same de Broglie
wavelengths. Explain which has more kinetic energy. (3)
12. Explain the term, ‘amplitude modulation’ of a signal. For an amplitude modulated wave,
the maximum amplitude is 10 V and the minimum amplitude is 2 V. Calculate the
modulation index. (3)
13. State the Lorenz’s force and express it in vector form. Which pair of vectors are always
perpendicular to each other? Derive the expression for the force acting on a current carrying
conductor of length L in a uniform magnetic field ‘B’. (3)
14. An optical instrument uses eye-lens of power 16 D and objective lens of power 50 D and has a tube length of 16.25 cm. Name the optical instrument and calculate its magnifying power if it forms the final image at infinity.
15. Explain the two processes involved in the formation of a p-n junction diode. Hence define the term ‘barrier potential’.
16. (a) Write two properties by which electric potential is related to the electric field.
(b) Two point charges q1 and q2, separated by a distance of r12 are kept in an externalelectric field. Derive an expression for potential energy of the system of two charges in field.
OR
State Gauss’s law in electrostatics. Derive an expression for the electric field due to an
infinitely long straight uniformly charged wire. (3)
17. State Lenz’s law. Explain, by giving examples that Lenz’s law is a consequence of conservation of energy. (3)
18. A capacitor of unknown capacitance is connected across a battery of V volt. A charge of 360 μC is stored in it. When the potential across the capacitor is reduced by 120 V, the charge stored in the capacitor becomes 120 μC. Calculate V and the unknown capacitance. What would have been the charge on capacitor if the voltage were increased by 120 V? (3)
19. A plane wavefront propagating from a rarer into a denser medium is incident at an angle of incidence i on a refracting surface. Draw a diagram showing incident wavefront and refracted wavefront. Hence verify Snell’s laws of refraction. (3)
20. Distinguish between sky wave and space wave modes of communication. What is the
main limitation of space wave mode ? Write the expression for the optimum separation between the transmitting and receiving antenna for effective reception of signals in this mode of communication. (3)
21. Using the wave forms of the input A and B, draw the output waveform of the given logic circuit. Identify the logic gate obtained. Write also the truth table. (3)
SECTION – D
23. Ram was a daily wage worker in a factory. He was suffering from Cancer. On hearing this, most of his co-workers, started avoiding him under the impression that it was a contagious disease. When Prof. Srivatsava came to know about this case, he took him to a leading radiologist, who examined him and told that it was at the beginning stage. He advised that it could be easily cured and also certified that it was not a communicable disease. After this, Ram was given proper treatment by the doctor and got cured completely.
(1) What moral values did Prof. Srivatsava display ?
(2) How is mean life of a radioactive element related to its half life ? (4)
(3) A radioactive sample has activity of 10,000 disintegrations per second after 20 hours. After next 10 hours its activity reduces to 5,000 dps. Find out its halflife and initial activity.
SECTION – E
24. In the following circuit, calculate (a) the capacitance of the capacitor, if the power factor of the cir cuit is unity, (b) the Q-factor of this circuit. What is the significance of the Q-factor in a.c. circuit ? Given the angular frequency of the a.c. source to be 100/s. Calculate the average power dissipated in the circuit.
OR (a) Prove that the current flowing through an ideal inductor connected across a.c. source, lags the voltage in phase by 2.
(b) An inductor of self-inductance 100 mH, and a bulb are connected in series with a.c.
source of rms voltage 10 V, 50 Hz. It is found that effective voltage of the circuit leads the
current in phase by . Calculate the inductance of the inductor used and average power dissipated in the circuit, if a current of 1 A flows in the circuit. (5)
sunlight. An incident beam of light of intensity Io is made to fall on a polaroid A. Another polaroid B is so oriented with respect to A that there is no light emerging out of B. A third polaroid C is now introduced mid-way between A and B and is so oriented that its axis bisects the angle between the axes of A and B. Calculate the intensity of light transmitted by A, B andC.
OR
(a) In Young’s double slit experiment, a monochromatic source of light S is kept
equidistant from the slits S1 and S2
. Explain the formation of dark and bright fringes on the screen.
(b) A beam of light consisting of two wavelengths, 650 nm and 520 nm, is used to obtain interference fringes in a Young’s double-slit experiment.
(i) Find the distance of the third bright fringe on the screen from the central maximum for wavelength 650 nm.
(ii) What is the least distance from the central maximum where the bright fringes due to both the wavelengths coincide?
Given: the separation between the slits is 4 mm and the distance between the screen and plane of the slits is 1.2 m. (5)
26. (a) Draw a circuit diagram of a meter bridge used to determine the unknown resistance R of a given wire. Hence derive the expression for R in terms of the known resistance S.
(b) What does the term ‘end error’ in a metre bridge circuit mean and how is it corrected ? How will the balancing point be affected, if the positions of the battery and galvanometer are interchanged in a metre bridge experiment ? Give reason for your answer.
OR
(a) State the working principle of a potentiometer with help of the circuit diagram, explain how the internal resistance of a cell is determined.
(b) How are the following affected in the potentiometer circuit when (i) the internal
resistance of the driver cell increases and (ii) the series resistor connected to the driver cell is reduced? Justify your answer. (5)
5. PHYSICS
SECTION A
Question Paper 2016 Central Outside Delhi Set 1
1. In what way is the behaviour of a diamagnetic material different from that of a paramagnetic, when kept in an external magnetic field?
2. The plot of the variation of potential difference across a combination of three identical cells in series, versus current is shown below. What is the emf and internal resistance of each cell?
3. Why does sun appear red at sunrise and sunset?
4. A charge ‘q’ is moved from a point A above a dipole of dipole moment ‘p’ to a point B below the dipole in equatorial plane without acceleration. Find the work done in the process.
5. Name the essential components of a communication system.
SECTION B
6. Calculate the de-Broglie wavelength of the electron orbiting in the n = 2 state of hydrogen
atom.
7. A battery of emf 12V and internal resistance 2 V is connected to a 4 V resistor as shown in
the figure.
(a) Show that a voltmeter when placed across the cell and across the resistor, in turn, gives
the same reading.
(b) To record the voltage and the current in the circuit, why is voltmeter placed in parallel and ammeter in series in the circuit?
8. Define ionization energy.
How would the ionization energy change when electron in hydrogen atom is replaced by a particle of mass 200 times that of the electron but having the same charge
OR
Calculate the shortest wavelength of the spectral lines emitted in Balmer series.
[Given Rydberg constant, R = 107 M-1]
9. Define modulation index. Why is it kept low? What is the role of a bandpass filter?
10. A ray PQ incident normally on the refracting face BA is refracted in the prism BAC made of material of refractive index 1.5. Complete the path of ray through the prism. From which face will the ray emerge? Justify your answer.
11. (i) Derive an expression for drift velocity of free electrons.
(ii) How does drift velocity of electrons in a metallic conductor vary with increase in
temperature ? Explain.
12. (a) Write the basic nuclear process involved in the emission of in a symbolic form, by a radioactive nucleus. (b) In the reactions given below :
(i)11C6→zBy + x + v
(ii) 12C6 + 12C6→20 Ne + c He b
Find the values of x, y, andz and a, b and c.
13. Sketch the graphs showing variation of stopping potential with frequency of incident radiations for two photosensitive materials A and B having threshold frequencies vA > vB.
(i) In which case is the stopping potential more and why ?
(ii) Does the slope of the graph depend on the nature of the material used ? Explain.
14. (i) State law of Malus.
(ii) Draw a graph showing the variation of intensity (I) of polarised light transmitted by an analyser with angle (u) between polariser and analyser.
(iii) What is the value of refractive index of a medium of polarising angle 60°?
15. Define an equipotential surface. Draw equipotential surfaces :
(i) in the case of a single point charge and
(ii) in a constant electric field in Z-direction.
Why the equipotential surfaces about a single charge are not equidistant ?
(iii) Can electric field exist tangential to an equipotential surface ? Give reason.
16. Use Biot-Savart law to derive the expression for the magnetic field on the axis of a current carrying circular loop of radius R.
Draw the magnetic field lines due to a circular wire carrying current I.
17. Define the term wave front. State Huygen’s principle.
Consider a plane wave front incident on a thin convex lens. Draw a proper diagram to show how the incident wave front traverses through the lens and after refraction focusses on the focal point of the lens, giving the shape of the emergent wave front.
OR
Explain the following, giving reasons:
(i) When monochromatic light is incident on a surface separating two media, the reflected and refracted light both have the same frequency as the incident frequency.
(ii) When light travels from a rarer to a denser medium, the speed decreases. Does this decrease in speed imply a reduction in the energy carried by the wave ?
(iii) In the wave picture of light, intensity of light is determined by the square of the
amplitude of the wave. What determines the intensity in the photon picture of light ?
18. For a CE-transistor amplifier, the audio signal voltage across the collector resistance of 2 kV is 2 V. Suppose the current amplification factor of the transistor is 100, find the input
signal voltage and base current, if the base resistance is 1 kV.
19. (i) Identify the part of the electromagnetic spectrum which is :
(a) suitable for radar system used in aircraft navigation,
(b) produced by bombarding a metal target by high speed electrons.
(ii) Why does a galvanometer show a momentary deflection at the time of charging or discharging a capacitor ? Write the necessary expression to explain this observation.
20. (i) Which mode of propagation is used by shortwave broadcast services having frequency range from a few MHz upto 30 MHz ? Explain diagrammatically how long distance communication can be achieved by this mode.
(ii) Why is there an upper limit to frequency of waves used in this mode ?
21. (i) Explain with the help of a diagram the formation of depletion region and barrier potential in a pn junction.
(ii) Draw the circuit diagram of a half wave rectifier and explain its working.
22. (i) When an AC source is connected to an ideal inductor show that the average power supplied by the source over a complete cycle is zero.
(ii) A lamp is connected in series with an inductor and an AC source. What happens to the brightness of the lamp when the key is plugged in and an iron rod is inserted inside the inductor ? Explain
23. Ram is a student of class X in a village school. His uncle gifted him a bicycle with a
dynamo fitted in it. He was very excited to get it. While cycling during night, he could light the bulb and see the objects on the road. He, however, did not know how this device works. He asked this question to his teacher. The teacher considered it an opportunity to explain the
working to the whole class. Answer the following questions:
(a) State the principle and working of a dynamo.
(b) Write two values each displayed by Ram and his school teacher.
SECTION - E
24. (i) Derive the mathematical relation between refractive indices n1 and n2 of two radii and radius of curvature R for refraction at a convex spherical surface. Consider the object to be a point since lying on the principle axis in rarer medium of refractive index n1 and a real image formed in the denser medium of refractive index n2. Hence, derive lens maker’s
formula.
(ii) Light from a point source in air falls on a convex spherical glass surface of refractive index 1.5 and radius of curvature 20 cm. The distance of light source from the glass surface is 100 cm. At what position is the image formed?
OR
(a) Draw a labelled ray diagram to obtain the real image formed by an astronomical
telescope in normal adjustment position. Define its magnifying power.
(b) You are given three lenses of power 0.5 D, 4 D and 10 D to design a telescope.
(i) Which lenses should he used as objective and eyepiece ? Justify your answer.
(ii) Why is the aperture of the objective preferred to be large?
25. (i) Use Gauss’s law to find the electric field due to a uniformly charged infinite plane sheet. What is the direction of field for positive and negative charge densities ?
(ii) Find the ratio of the potential differences that must be applied across the parallel and series combination of two capacitors C1 and C2 with their capacitances in the ratio 1 : 2 so that the energy stored in the two cases becomes the same.
OR
(i) If two similar large plates, each of area A having surface charge densities and
are separated by a distance d in air, find the expressions for (a) field at points between the two plates and on outer side of the plates. Specify the direction of the field in each case.
(b) the potential difference between the plates.
(c) the capacitance of the capacitor so formed. (ii) Two metallic spheres of radii R and 2R are charged so that both of these have same surface charge density s. If they are connected to each other with a conducting wire, in which direction will the charge flow and why?
26. (i) Draw a labelled diagram of a step-down transformer. State the principle of its working.
(ii) Express the turn ratio in terms of voltages.
(iii) Find the ratio of primary and secondary currents in terms of turn ratio in an ideal transformer.
(iv) How much current is drawn by the primary of a transformer connected to 220 V supply when it delivers power to a 110 V-2550 W refrigerator ?
OR
(a) Explain the meaning of the term mutual inductance. Consider two concentric circular coils, one of radius r1 and the other of radius r2(r1 < r2) placed coaxially with centres coinciding with each other. Obtain the expression for the mutual inductance of the arrangement.
(b) A rectangular coil of area A, having number of turns N is rotated at ‘f ‘ revolutions per second in a uniform magnetic field B, the field being perpendicular to the coil. Prove that the maximum emf induced in the coil is 2 NBA.
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