## CBSE Previous Year Solved  Papers  Class 12 Physics Outside Delhi 2011

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

Question.1.Define electric dipole moment. Write its S.I. unit.
Answer : Electric dipole moment : Dipole moment is a measure of strength of electric dipole. It is vector quantity whose magnitude is equal to product of magnitude of charge and the distance between them.
p = q x d
SI unit of dipole moment is coulomb-meter (C-m)

Question.2.Where on the surface of Earth is the angle of dip 90°?
Answer : Magnetic dip is the angle made by a compass needle with the horizontal point on earths surface. Positive value of inclination indicates that the field is pointing downwards, into the earth, at the point of measurement. The angle of dip is 90° at the poles.

Question.3. A hollow metal sphere of radius 3 cm is charged such that the potential on its surface is 10 V. What is potential at the centre of the sphere?
Answer : Potential inside the charge sphere is constant and equal to potential on the surface of the conductor so, therefore, potential at the centre of sphere is 10V.

Answer : Production of Radiowaves : These are the electromagnetic waves of frequeficy ranging from 500 kHz to about 1000 MHz. Radiowaves are made by various types of transmitter, depending on the wavelength. They are also given off by starts, sparks and lighting. These waves are used in the field of radio communication.

Question.5. Write any two characteristic properties of nuclear force.
Answer : Characteristic properties of nuclear forces are :

1.  Nuclear forces are strongest forces in nature : Magnitude of nuclear forces is 100 times that of electrostatic force and 1038 times the gravitational force.
2. Nuclear forces are charge independent : Nuclear forces between a pair of protons, a pair of neutrons or a pair of neutron and proton act with same strength.

Question.6.Two bar magnets are quickly moved towards a metallic loop connected across a capacitor ‘C’ as shown in .the figure. Predict the polarity of the capacitor.

Question.7. What happens to the width of depletion layer of a p-n junction when it is

1. forward biased,
2. reverse biased?

1. Reverse biased : Potential drop across the junction increases and diffusion of holes and electrons across the junction ‘decreases. It makes the width of the depletion layer larger.
2. Forward biased : Potential drop across the junction decreases and diffusion of holes and electrons across the junction increases. It makes the width of the depletion layer smaller. .

Question.8. Define the term ‘stopping potential’ in relation to photoelectric effect.
Answer : Stopping potential is the minimum negative (retarding) potential of anode for which photocurrent stops or becomes zero. It is denoted by Vs. The value of stopping potential is different for different metals but it is independent of the intensity of incident light.

Question.9. A thin straight infinitely long conducting wire having charge density X. is enclosed by a cylindrical surface of radius r and length 4 its axis coinciding with the length of the wire. Find the expression for the electric flux through the surface of the cylinder.

Question.10

Question.11. Write the expression for Lorentz magnetic force on a particle of charge ‘q’ moving with velocity v in a magnetic field B. Show that no work is done by this force on the charged * particle.

Question.12. What are eddy currents? Write any two applications of eddy currents.
Answer : When a bulk piece of conductor is subjected to
changing magnetic flux, the induced current, developed in it is called eddy current.
Applications of eddy currents :

1. Magnetic brakes in trains.
2.  Electromagnetic damping.
3.  Induction furnaces.
4. Electric power meter.

Question.13. What is sky wave communication? Why is this mode of propagation restricted to the frequencies only upto few MHz?
Answer : Ionosphere reflection of radiowaves, back towards the earth, is known as sky wave communication. It is the mode of Wave propagation in.which the radiowaves emitted from the transmitter antenna reach the receiving antenna after reflection by the ionosphere.
The electromagnetic waves of frequencies greater than  40 MHz penetrate the ionosphere and escape. The ionosphere
layers act as a reflector for a certain range of frequencies (3-30 MHz).

Question.14. In the given circuit, assuming point A to be at zero potential, use KirchhofFs rules to determine the potential at point B.

Question.16. Net capacitance of three identical capacitors in series is 1 μF. What will be their net capacitance if connected in parallel?
Find the ratio of energy stored in the two configurations if they are both connected to the same source.

Question.17. Using the curve for the binding energy per nucleon as a function of mass number A, state clearly how the release in .energy in the processes of nuclear fission and nuclear fusion can be explained.

Answer : The above curve shows that:

1.  When a heavy nucleus breaks into two medium sized
nuclei (in nuclear fission) the BE/nucleon increases resulting, in the release of energy.
2. When two small nuclei combine to form a relatively bigger nucleus in nuclear fusion BE/nucleon increases, resulting in the release of energy.

Question.18. In the meter bridge experiment, balance point was observed atJ with AJ = l.

1.  The values of R and X were doubled and then interchanged. What would be the new position of balance point?
2. If the galvanometer and battery are interchanged at
the balance position, how will the balance point get affected?

1.  By doubling and interchanging R and X, there is no change in the position of balance point because
2.  By interchanging galvanometer and battery, there will be no change in the balance point position.

Question.19. A convex less made up of glass of refractive index 1.5 is dipped, in turn, in (i) a medium of refractive index 1.65, (ii) a medium of refractive index 1.33.
(a) Will it behave as a converging or a diverging lens in the two cases?
(b) How will its focal length change in the two media?
Answer : (i) When convex lens is dipped in medium of refractive index 1.65, its focal length (f1) is given by

Question.20. Draw a plot showing the variation of photoelectric current with collector plate potential for two different frequencies, v1 > v2 of incident radiation having the same intensity. In which case will be stopping potential be higher? Justify your answer.

Stopping potential is more for the curve corresponding to the frequency V2 (...v1 > v2)
This is due to the fact that with increase in the frequency, the kinetic energy of emitted photoelectrons also increases. Therefore we need more negative potential to stop these electrons.

Question.22. Use the mirror equation to show that
(a) an object placed between f and 2 f of a concave mirror produces a real image beyond If.
(b) a convex mirror always produces a virtual image independent of the location of the object.
(c) an object placed between the pole and focus of a concave
mirror produces a virtual and enlarged image.

Therefore, v is always +ve and always less than d, So we can say that convex mirror always produces a virtual image between pole and focus.
(c) Object between pole and F we have 0 < 0 < 1 In this case : v is +ve (virtual image) and /v/>c Therefore, we get a virtual and enlarge image.’

Question.23. Draw a labelled diagram of a full wave rectifier circuit state its working principle. Show the inp ut-output waveforms.

Rectification : Rectification means conversion of ac into dc. A p-n diode acts as a rectifier because an ac changes polarity periodically and a p-n diode conducts only when it is forward biased; it does not conduct when reverse biased.
Working : The ac input voltage across secondary S1 and S2 changes polarity after each half cycle. Suppose during the first cycle of input ac signal, the terrginal S1 is positive relative to centre tap and S2 is negative relative to it Then diode Di is forward biased and D2 is reverse biased. Therefore, diode D1 conducts while D2 does not. Thus, die-current in load resistance RL is in the same direction for both half cycles of input ac signal the output current is a continuous series of unidirectional pulses.
In a full wave rectifier, if input frequency is Hertz, then output frequency will be 2f Hertz because for each cycle of input, two positive half cycles of output are obtained.

Question.24.(a) Using de Broglie’s hypothesis, explain with the help of a suitable diagram, Bohr’s second postulate of quantization of energy levels in a hydrogen atom.
(b) The ground state energy of hydrogen atom is -13.6 eV. What are the kinetic and potential energies of the electron in this state? ‘

Question.25. You are given, a circuit below. Write its truth table. Hence, identify the 1 qgic operation carried out by this circuit. Draw the logic symbol of the gate it corresponds to.

Question.26. A compound microscope uses an objective lens of focal length 4 cm and eyepiece lens of focal length 10 cm. An object is placed at 6 cm from the objective lens. Calculate the magnifying power of the compound microscope. Also calculate the length of the microscope.
OR
A giant refracting telescope at an observatory has an objective lens of focal length 15 m. If an eyepiece lens of focal length 1.0 cm is used, find the angular magnification „ of the telescope.
If this telescope is used to ‘view the moon,’ what is the diameter of the image of the moon formed by the objective lens? The diameter of the moon is 3.42 x 106 m and the radius of the lunar orbit is 3.8 x 108m.

Question.27. Two heating elements of resistances Ri and R2 when operated at a constant supply of voltage, V, consume powers P1 and P2 respectively. Deduce the expressions for the pqwer of their combination when they are, in turn, connected in (i) series and (i) parallel across the same voltage supply.
Answer: Two resistance are Ri and R2 Both consumed the power Pi and P2 respectively.

Question.28. (a) Using Amperes circuital law, obtain the expression for the magnetic field due to a long solenoid at a point inside . the solenoid on its axis.
(b) In what respect is a toroid different from a solenoid? Draw and compare the pattern of the magnetic field lines in the two cases.
(c) How is the magnetic field inside a given solenoid made
strong?
OR
(a) State the principle of the working of a moving coil galvanometer, giving its labelled diagram.
(b) “Increasing the current sensitivity of a galvonometer may not necessarily increase its voltage sensitivity.” Justify this statement.
(c) Outline the necessary steps to convert a galvanometer of resistance RQ into an ammeter of a given range.
Answer: Magnetic field inside a long solenoid is uniform energy where and approximately zero outside it. Fig shows a sectional view of long solenoid, current comming out of the plane of the papers at points marked and current entering the plane of the paper at point marked at any point inside consider a rectangular loop abed as Amperian loop.
(b) Difference: In a toroid, magnetic lines do not exist outside the body. Toroid is closed whereas the solenoid is open on both sides.
Magnetic field is uniform inside a toroid whereas for solenoid, it is different at the two ends and centre
to Ampere circuital law.

(c) Strength of magnetic field :
(1) By inserting the ferromagnetic substance inside the solenoid.
(2) By increasing the current through the solenoid.

Principle : Its working based on the fact that when a current carrying coil is placed in a magnetic field, it experiences a torque.
Working: When current (I) is passed in the coil, torque x acts on the coil, given by

Question.29. State the working of a.c. generator with”the help of a labelled diagram.
The coil of an a.c. generator having N turns, each of area A, is rotated with a constant angular velocity to. Deduce the expression for the alternating e.m.f. generated in the coil. What is the source of energy generation in this device.
OR
(a) Show that in an a.c. circuit containing a pure inductor, the voltage is ahead of current by π/2 in phase.
(b) A horizontal straight wire of length L extending from east to west is hilling with speed v at right angles to the horizontal component of Earth’s magnetic field B.

1.  Write the expression for the instantaneous value of the e.m.f. induced in the wire.
2. What is the direction of the e.m.f.?
3. Which end of the wire is at the higher potential?

Answer : Working : When a coil (armature) rotates inside a uniform magnetic field, magnetic flux linked with the coil changes w.r.t. time. This produces an e.m.f according to Faradays law.
For first half of the rotation the current will be from one end (first ring) to the other end (second ring). For second half of the rotation it is in opposite sense.
To calculate the magnitude of e.m.f. induced,

Question.30. State the importance of coherent sources in the phenomenon of interference.
In Youngs double slit experiment to produce interference pattern, obtain the conditions for constructive and destructive interference. Hence deduce the expression for the fringe width. How does the fringe width get affected, if the entire experimental apparatus ofYoung is immersed in water?
Answer : Two sources of light which continuously emit light waves of same frequency with a zero or constant phase difference between them are called coherent sources. They are necessary to produce sustained interference pattern.
A thin film of oil spread over water shows beautiful colors due to interference of light.
If coherent sources are not taken, the phase difference, between the two interfering waves, will change continuously and a sustained interference pattern will not be obtained.

### SET II

Question.1. A hollow metal sphere of radius 10 cm is charged such that
the potential on its surface is 5 V. What is the potential at the centre of the sphere?
Answer : Potential inside the charged sphere is constant and equal to potential on the surface of conductor.
Therefore, potential at the centre of sphere is 5V.

Question.2. How are X-rays produced?
Answer : X-rays are produced when electron strike a metal target. The electrons are liberated from the heated filament and accelerated from the high voltage towards the metal target. X-rays are produced’when electrons collide with the atom and nuclei of metal target.

Question.4. Where on the surface of Earth is the angle of dip zero?
Answer : At magnetic equator angle of dip is zero.

Question.12. State the principle of working of a transformer. Can a transformer be used to step up or step down a d c. voltage? Justify your answer.
Answer : Transformer—Principle : It is a device which converts high voltage A.C. into low voltage A.C. and vice -versa. It is based upon the principle of mutual induction. When alternating current passed through a coil, an induced e.m.f. is set up in the neighbouring coil.

Working : When an alternating current is passed through the primary, the magnetic flux through the iron core changes which does two things. It produces e.m.f. in the primary and an induced e.m.f. is also set up in the secondary. If we assume that the resistance of primary is negligible, the back e.m.f. will be equal to the voltage applied to the primary.

A transformer can not be used to step up or step down a d.c. voltage because d.c. can not produce a changing magnetic flux in the core of the transformer.

Question.14. In the given circuit, assuming point A to be at zero potential, use KirchhofFs rules to determine the potential at point B.

Question.17. What is ground wave communication? On what factors does the maximum range of propagation in this mode depend?
Answer : A radiowave that can travel directly from on point to another following the surface of earth is called a ground wave.
The maximum range of ground wave propagation depends upon the following factors :

1.  The frequency of the transmitted wave
2.  Power of transmitter.

Question.22. A convex lens made up of glass of refractive index 1.5 is dipped, in turn, in (i) a medium of refractive index 1.6, (ii) a medium of refractive index 1.3
(a) Will it behave as a converging or a diverging lens in two cases?
(b) How will its focal length change in the two media?

### SET III

Note : Except for the following questions, all the remaining question have been asked in Previous Set.
Question.1. A hollow metal sphere of radius 6 cm is charged such that the potential on its surface is 12 V. What is the potential at the centre of sphere?
Answer : Potential inside the charged sphere is constant and equal to potential on the surface of conductor. So therefore, potential at the centre of sphere is 12V.

Question.3. How are microwaves produced?
Answer : Microwaves are electromagnetic waves with wavelength ranging from as long as meter to as short as one millimeter, or equivalently with frequencies between 300 . MHz and 300 GHz. Microwaves are produced by vacuum
tubes devices that operate on the ballistic motion of electron controlled by magnetic or electric fields. Some different kinds of microwaves emitters are the cavity magnetron, the klystron, the traveling wave tube (TWT), the gyrotron and all starts.

Question.12. Mention various energy losses in transformer?
Answer : Transformers energy losses tend to worsen with increasing frequency. Magnetic core losses are exaggerated with higher frequencies, eddy currents in the iron core, resistance of windings or copper loss, hysteresis lpss and flux leakage are energy losses in transformers.

Question.14. In the given circuit, assuming point A to be at zero potential, , use Kirchhoff’s rules to determine the potential at point B.

Question.18. A thin straight infinitely long conducting wire having charge density X is enclosed by a cylindrical surface of . radius r and length 4 its axis coinciding with the length of the wire. Find the expression for the electric flux through the surface of the cylinder.
Answer: Electric flux through the cylinderical surface

Question.20. A converging lens has a focal length of 20 cm 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.