CBSE Previous Year Solved Papers Class 12 Physics Delhi 2010

CBSE Previous Year Solved  Papers  Class 12 Physics Delhi 2010

Time allowed : 3 hours                                                                                           Maximum Marks: 70

General Instructions:

1.  All questions are compulsory. There are 26 questions in all.
2.  This question paper has five sections : Section A, Section B, Section C, Section D and Section E.
3. 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.
4.  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.
5. You may use the following values of physical constants wherever necessary:

SET I

Note s Except for the following questions, all the remaining questions have been asked in Previous Set.

Question.1.In which orientation, a dipole placed in a uniform electric
field is in

1.  stable,
2. unstable equilibrium?

Answer :

1.  The dipole is in stable equilibrium when electric dipole is in the direction of electric field.
2. The dipole is in unstable equilibrium when electric dipole is in the opposite direction of electric field.

Question.2.Which part of electromagnetic spectrum has largest penetrating power?
Answer: Gamma rays

Question.3. A plot of magnetic flux (∅) versus current (I) is shown in the figure for two inductors A and B. Which of the two has larger value of self inductance.

Answer: Line A

Question.4.Figure shows three point charges, +2q, -q and +3q. Two charges +2q and —q are enclosed within a surface ‘S’. What is the electric flux due to this configuration through the surface ‘S’?

Answer : Total charge within a surface S = +2q + (-q) = +q

Question.5. A glass lens of refractive index 1.45 disappears when immersed in a liquid. What is the value of refractive index of the liquid?
Answer : The value of refractive index of the liquid is 1.45.

Question.6. What is the ratio of radii of the orbits corresponding to first excited state and ground state in a hydrogen atom?

Question.7. A wire of resistance’ 8R is bent in the form of a circle. What is the effective resistance between the ends of a diameter AB?
Answer : Resistance of each semi-circular part of circle is 4R.

Question.8. State the conditions for the phenomenon of total internal reflection to occur.
Answer : (i) Light should travel from denser to rarer medium (ii) Angle of incidence in denser medium should be greater than the critical angle for the pair of media in contact.

Question.9. Explain the function of a repeater in a communication system.
Answer : A repeater, picks up the signal from the transmitter, amplifies and retransmits it to the receiver sometimes with a change in carrier frequency. Repeaters are used to extend the range of a communication system.

Question.10.(i) Write two characteristics of a material used for making permanent magnets.
(ii) Why is core of an electromagnet made of ferromagnetic materials?
OR
Draw magnetic field lines when a (i) diamagnetic, (ii) paramagnetic substance is placed in an external magnetic field. WTiich magnetic property distinguishes this behaviour of the field lines due to the two substances?
Answer:
(i) Two characteristics of a material used for making permanent magnets are:
(a) High Coercivity
(b) High Retentivity
(ii) Core of an electromagnet made of ferromagnetic materials because of high permeability and low retentivity.
OR
(i) Behaviour of magnetic field lines when a diamagnetic substance is placed in-an external magnetic field.

Question.11. Draw the circuit diagram of an illuminated photodiode in reverse bias. How is photodiode used to measure light intensity?
Answer : Circuit diagram of an illuminated photodiode :

Explanation: The magnitude of the photocurrent depends on the intensity of incident light (photocurrent is proportional to incident light intensity). Thus photodiode can be used to measure light intensity.

Question.12. An electric lamp having coil of negligible inductance connected in series with a capacitor and an AC source is glowing with certain brightness. How does the brightness of the lamp change on reducing the (i) capacitance, and (ii) the frequency? Justify your answer.

The current flows in the circuit and the lamp glows.
(i) On reducing the capacitance C, X_c. increases. Therefore, the brightness of the bulb decreases.
(ii)On reducing the frequency u,X_c increases. Therefore, the brightness of the bulb decreases.

Question.13. Arrange the following electromagnetic radiations in ascending order of their frequencies :

1.  Microwave
2. Radiowave
3. X-rays
4. Gamma rays

Write two uses of any one of these.
Answer : Radiowave <Microwaves < X-rays < Gamma rays Uses of microwaves are :

1.  Microwaves are used in radar systems for aircraft navigations.
2. Microwaves are used in microwave ovens for cooking purposes.

Question.14. The radii of curvature of the faces of a double convex lens are 10 cm and 15 cm. If focal length of the lens is 12 cm, find the refractive index of the material of the lens.

Question.15.An electron is accelerated through a potential difference of 100 volts. What is the de-Broglie wavelength associated with it? To which part of the electromagnetic spectrum does this value of wavelength correspond?

Question.16. A heavy nucleus X of mass number 240 and binding energy per nucleon 7.6 MeV is split into two fragments Y and Z of mass numbers 110 and 130. The binding energy of nucleons in Y and Z is 8.5 MeV per nucleon. Calculate the energy Q released per fission in MeV.

Question.17. (a) The bluish colour predominates in clear shy.
(b) Violet colour is seen at the bottom of the spectrum when white light is dispersed by a prism.
State reasons to explain these observations. 
Answer .
(a) As per Rayleighs law (scattering oc 1/λ⁴), lights of shorter wavelengths scattered more by an atmospheric particles. This results in a dominance of bluish colour in the scattered light.
(b) In the visible spectrum, violet light having its shortest wavelength, has the highest refractive index. Hence it is deviated the most.

Question.18. Plot a graph showing the variation of stopping potential
with the frequency of incident radiation for two different photosensitive materials having work functions W₁ and W₂  (W₁>W₂). On what factors does the (i) slope and

Question.19. A parallel plate capacitor is charged by a battery. After sometime the battery is disconnected and a dielectric slab with its thickness equal to the plate separation is inserted between the plates. How will (i) the capacitance of the capacitor, (ii) potential difference between the plates and
(iii) the energy stored in the capacitor be affected?
Justify your answer in each case.
Answer : As battery in dissconnected, charge will remain constant.

Question.20. Write the principle of working of a potentiometer. Describe briefly, with the help of a circuit diagram, how a potentiometer is used to determine the internal resistance of a given cell.
Answer : Working principle :When constant current flows through a wire of uniform cross-section, then potential difference across the wire is directly proportional to the length v00l

Question.21. Write the expression for the magnetic moment due to a planar square loop of side 7’ carrying a steady current I in a vector form.
In the given figure this loop is placed in a horizontal plane near a long straight conductor carrying a steady current Ii at a distance l as shown.
Give reasons to explain that the loop will experience a net force but no torque. Write the expression for this force acting on the loop.

Question.22.(a) Depict the equipotential surfaces for a system of two identical positive point charges placed a distance ‘d apart.
(b) Deduce the expression for the potential energy of a system of two point charges q₁and q₂ brought from
infinity to the points respectively in the presence of external electric field
Answer : (a) Equipotential surfaces of two identical positive point charges placed at a distance ‘d ’ apart.

Question.23. (i) Define‘activity7 of a radioactive material and write its S.I.unit.
(ii)Plot a graph showing variation of activity of a given radioactive sample with time.
(iii)The sequence of stepwise decay of a radioactive nucleus is

If the atomic number and mass number of D2 are 71 and 176 respectively, what are their corresponding values for D?
Answer : (i) The total decay rate (of a sample) at the given instant, i. e., the number of radio nuclides disintegrating per unit time is called the activity of that sample. The SI unit for activity is becquerel (Bq).

.^.. Atomic number of D = 74
Mass number of D = 180

Question.25. A long straight wire of a circular cross-section of radius V carries a steady current T. The current is uniformly distributed across the cross-section. Apply Amperes circuital law to calculate the magnetic field at a point V in the region for (i) r< a (ii) r> a.
OR
State the underlying principle of working of a moving coil galvanometer. Write two reasons why a galvanometer cannot be used as such to measure current in a given circuit. Name any two factors on which the current sensitivity of a galvanometer depends.



Two reasons:
(i) Galvanometer is a very sensitive device, it gives a full-scale deflection for a current of the order of pA.
(ii) For measuring currents, the galvanometer has to be connected in series, and it has a finite resistance, this will change the value of the current in the circuit.
Two factors : The current sensitivity of a moving coil galvanometer can be increased by
(i) increasing the number of turns , (ii) increasing cross-section area of the loop

Question.26. What is space wave propagation? Give two examples of communication system which use space wave mode.
A TV tower is 80 m tall. Calculate the maximum distance upto which the signal transmitted from the lower can be received.
Answer : When waves travel in space in a straight line from the transmitting antenna to the receiving antenna, this mode of propagation is called the space wave propagation. Examples: Television broadcast, satellite communication. Here

Question.27. In a meter bridge, the null point iHound at a distance of 40 cm from A. If a resistance of 12 Q is connected in parallel with S, the null point occurs at 50.0 cm front A. Determine the values of R and S.

Question.28. Describe briefly, with the help of a labelled diagram, the basic elements of an A.C. generator. State its underlying principle. Show diagrammatically how an alternating emf is generated by a loop of wire rotating in a magnetic field. Write the expression for the instantaneous value of the emf induced in the rotating loop.
OR ,
A series LCR circuit is connected to an ac source having voltage v = vm sin ϖt. Derive the expression for the instantaneous current I and its phase relationship to the applied voltage.
Obtain the condition for resonance to occur. Define ‘power factor’. Sate the conditions under which it is (i) maximum and (ii) minimum.
Answer: It consists of a coil mounted on a rotor shaft. The axis of rotation of the coil is perpendicular to the direction of the magnetic field. The coil (called armature) is mechanically rotated in the uniform magnetic field by some external means.
The ends of the coil are connected to an external circuit by means of slip rings and brushes.

Underlying Principle : As the coil rotates in a magnetic field B, the effective area of the loop (the face perpendicular to the field) which is AcosO, where 0 is the angle between area (A) and magnetic field (B) changes continuously. Hence, magnetic flux linked with the coil keeps on changing with time and an induced emf is produced.
The instantaneous value of the emf is e = NBA® sin cor

Question.29. State Huygens principle. Show, with the help of a, suitable diagram, how this principle is used to obtain the diffraction pattern by a single slit.
Draw a plot of intensity distribution and explain clearly why the secondary maxima become weaker with increasing order (n) of the secondary maxima.
OR
Draw a ray diagram to show the working of a compound microscope. Deduce an expression for the total magnification . when the final image is formed at the near point.
In a compound microscope, an object is placed at a distance of 1.5 cm from the objective of focal length 1.25 cm. If the eye-piece has a focal length of 5 cm and the final image is formed at the near point, estimate the magnifying power of the microscope.
Answer : Huygens principle : Each point of wavefront is the source of a secondary disturbance and the wavelets emanating from these points spread out in all directions with the speed of the wave. The common tangent/forward envelope, to all these secondary wavelets gives the new wavefront at later time. Application to diffraction pattern: All the points of incoming wavefront (parallel to the plane of slit) are in phase with plane of slit. However the contributions of the secondary wavelets from different points, at any point, on the screen. Total contribution, at any point, may add up to give a maxima or minima dependent on the phase differences.


The central point is a maxima as the contribution of all secondary wavelet pairs are in phase here. Consider next a point on the screen where an angle θ = 3λ/2a. Divide the slit into three equal parts. Here the first two-thirds of the slit can be divided into two halves which have a λ/2 path difference. The contributions of these two halves cancel. Only the remaining one-third of the slit contributes to the intensity at a point between the two minima. Hence, this will be much weaker than the central maximum (where the entire slit contributes in phase). We can similarly show that there are maxima at θ = (n + 1/2) λ/ a with n = 2, 3, etc. These become weaker with increasing n, since only one- fifth, one-seventh, etc. of the slit contributes in these cases.

Question.30.(a) Explain the formation of depletion layer and potential barrier in a p-n junction.
Output
(b) In the figure given  below the input waveform is converted into the output waveform by a device ‘X’. Name the device and draw its circuit diagram.

(a) With the help of the circuit diagram explain the working principle of a transistor amplifier as an oscillator.
(b) Distinguish between a conductor, a semiconductor and an insulator on the basis of energy band diagrams.
Answer : (a) Depletion region : Due to the concentration gradient across p-side and n-sides, holes diffuse from p-side to n-side (p→n) and electrons diffuse from n-side to p-side (n→p).As the electrons diffuse from n→p,a layer of positive charge (or positive space-charge region) is developed on n-side of the junction. Similarly as the holes diffuse, a layer of negative charge (or negative space-charge region) is developed on the p-side of the junction. The space-charge region on either side of the junction together is known as depletion region.
Barrier potential : The loss of electrons from the n-region and the gain of electron by the p-region cause a difference of potential across the junction of the two regions. The polarity of this potential is such as to oppose further flow of carriers.
>

SET II

Note: Except for the following questions, all the remaining questions have been asked in Previous Set.

Question.3. The radius of innermost electron orbit of a hydrogen atom is 5.3 x 10-11m. What is the radius of orbit in the second excited state?

Question.6.Which part of the electromagnetic spectrum is absorbed from sunlight by ozone layer?
Answer: Ultravoilet rays

Question.9. (i) When primary coil is moved towards secondary coil S (as shown in figure below) the galvanometer shows momentary deflection. What can be done to have larger deflection in the galvanometer with the same battery?
(ii) State the related law.

Answer: (i) To get the larger deflection in the galvanometer with the same battery, the coil P should be moved faster.
(ii) Electromagnetic Induction : It is the process in which an emf is induced in a circuit placed in a magnetic field when the magnetic flux linked with the circuit changes.

Question.10.What is the range of frequencies used for TV transmission? What is common between these waves and light wave?
Answer : Range of frequencies used for TV transmission is 54-72 MHz (VHF-Very High Frequencies).
The ionosphere is unable to reflect back these waves to earth. Both waves are electromagnetic waves.

Question.11.A bioconvex lens has a focal length 2/3 times the radius of curvature of either surface. Calculate the refractive index of lens material.

Question.14.

(i)Why does the Sun appear reddish at Sunset or Sunrise?
(ii) For which colour the refractive index of prism material is maximum and minimum?
Answer:

1.  At Sunset or Sunrise, the sun’s rays have to pass through a larger distance in the atmosphere. Most of the blue and other shorter wavelengths are removed by scattering. The least scattered light reaching our eyes, therefore, the sun looks reddish.
2.  Refractive index of prism material is
   (a) maximum of violet colour
   (b) minimum of red colour

Question.20.(i) Why is communication line of sight mode limited to frequencies above 40 MHz?
(ii) A transmitting antenna at the top of a tower has a height 32m and the height of the receiving antenna is 50m. What is the maximum distance between them for satisfactory communication in line of sight mode?
Answer: (i) At frequencies above 40 MHz, communication is essentially limited to line-of-sight paths. At these frequencies, the antennas are relatively smaller and can be placed at height of many wavelengths above the ground. Because of the line- of-sight nature of propagation, direct waves get blocked at some point by the curvature of earth.

SET III

Note: Except for the following questions, all the remaining questions have been asked in Set-I and Previous Set.

Question.4.Which part of electromagnetic spectrum is used in radar systems?
Answer: Microwave

Question.5.Calculate the speed of light in a medium whose critical angle is 30°.

Question.7. Write the expression for Bohr’s radius in hydrogen atom.

Question.11. What is the range of frequencies used in satellite communication? What is common between these waves and light waves?
Answer: Range of frequencies used in satellite communication is
Uplink = 5.925 — 6.425 GHz Downlink = 3.7 – 4.2 GHz
Both space waves and light waves are electromagnetic waves.

Question.12. A coil Q is connected to low voltage bulb B and placed near another coil P as shown in the figure. Give reason to explain the following observations :
(a) The bulb ‘B’ lights
(b) Bulb gets dimmer if the coil Q is moved towards left.

Answer: (i) The bulb ‘B’ lights up due to an emf induced in the coil Q.
(ii) When the coil ,Q is moved towards left, the emf induced in the coil Q becomes less. Hence the bulb gets dimmer.

Question.13. Find the radius of curvature of the convex surface of the plano-convex lens, whose focal length is 0.3m and the refractive index of the material of the lens is 1.5.

Question.14. An electron is accelerated through a potential difference of 64 volts. What is the de-Broglie wavelength associated with it? To which part of the electromagnetic spectrum does this value of wavelength correspond?

This wavelength is associated with X-rays (range : lnm to 10^-3 nm)

Question.15. (i) Out of the blue and red light which is deviated more by a prism? Give reason.
(ii) Give the formula that can be used to determine refractive index of material of a prism in minimum deviation condition.

Question.20. In a meter bridge, the null point is found at a distance of l₁ cm from A. If a resistance of X is connected in parallel with S, the null point occurs at l₂ cm. Obtain a formula for X in terms of
l₁,l₂ h and S.

Question.27. A parallel plate capacitor is charged to a potential difference V by a dc source. The capacitor is then disconnected from the source. If the distance between the plates is doubled, state with reason how the following will change;
(i) electric field between the plates
(ii) capacitance, and
(iii) energy stored in the capacitor