# 12th Physics Paper Solutions Set 1 : CBSE Delhi Previous Year 2015

General Instructions:

(i) There are 26 questions in all. All questions are compulsory.

(ii) This question paper has five sections : Section A, Section B, Section C, Section D and Section E.

(iii) 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 question

Q1 :

Define capacitor reactance. Write its S.I units.

Capacitor reactance is the resistance offered by a capacitor when it is connected to an electric circuit. It is given by
XC=1Ï‰C XC=1Ï‰C
where
Ï‰ = Angular frequency of the source
C = Capacitance of the capacitor
The SI unit of capacitor reactance is ohm (Î©).

Q2 :

What is the electric flux through a cube of side 1 cm which encloses an electric dipole?

As per the Gauss's law of electrostatics, electric flux through a closed surface is given by
Ï•E=âˆ®Eâƒ— .ds-â†’=Q0 Ï•E=âˆ®Eâƒ— .dsâ†’=Q0    .....(i)
where
E = Electrostatic field
Q = Total charge enclosed by the surface
00 = Absolute electric permittivity of free space
In the given case, cube encloses an electric dipole. Therefore, the total charge enclosed by the cube is zero, i.e. Q = 0.
Therefore, from (i), we have
Ï•E=âˆ®Eâƒ— .ds-â†’=00=0 Ï•E=âˆ®Eâƒ— .

Q3 :

A concave lens of refractive index 1.5 is immersed in a medium of refractive index 1.65. What is the nature of the lens?

A concave lens made up of certain material behaves as a diverging lens when it is placed in a medium of refractive index less than the refractive index of the material of the lens, and behaves as a converging lens when it is placed in a medium of refractive index greater than the refractive index of the material of the lens, then it behaves as a diverging lens.
In the given case, concave lens is immersed in the medium having refractive index greater then the refractive index of the material of the lens. Therefore, it will behave as a diverging lens.

Q4 :

How are side bands produced?

Q5 :

Graph showing the variation of current versus voltage for a material Ga As is shown in the figure. Identify the region of
(i) negative resistance
(ii) where Ohm's law is obeyed. Q6 :

A proton and an Î±-particle have the same de-Broglie wavelength. Determine the ratio of (i) their accelerating potentials (ii) their speeds.

Q7 :

Show that the radius of the orbit in hydrogen atom varies as n2, where n is the principal quantum number of the atom.

Q8 :

Distinguish between 'intrinsic' and 'extrinsic' semiconductors.

Q9 :

Use the mirror equation to show that an object placed between f and 2f of a concave mirror produces a real image beyond 2f.

OR

Find an expression for intensity of transmitted light when a polaroid sheet is rotated between two crossed polaroids. In which position of the polaroid sheet will the transmitted intensity be maximum?

Q10 :

Use Kirchhoff's rules to obtain conditions for the balance condition in a Wheatstone bridge.

Q11 :

Name the parts of the electromagnetic spectrum which is
(b) used to treat muscular strain.
(c) used as a diagnostic tool in medicine.
Write in brief, how these waves can be produced.

Q12 :

(i) A giant refracting telescope has an objective lens of focal length 15 m. If an eye piece of focal length 1.0 cm is used, what is the angular magnification of the telescope ?
(ii) 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 3.48 Ã— 106 m and the radius of lunar orbit is 3.8 Ã— 108 m.

Q13 :

Write Einstein's photoelectric equation and mention which important features in photoelectric effect can be explained with the help of this equation.
The maximum kinetic energy of the photoelectrons gets doubled when the wavelength of light incident on the surface changes from Î»1 to Î»2. Derive the expressions for the threshold wavelength Î»0 and work function for the metal surface.

Q14 :

In the study of Geiger-Marsdon experiment on scattering of Î± particles by a thin foil of gold, draw the trajectory of Î±-particles in the coulomb field of target nucleus. Explain briefly how one gets the information on the size of the nucleus from this study.
From the relation R = R0 A1/3, where R0 is constant and A is the mass number of the nucleus, show that nuclear matter density is independent of A.

OR

Distinguish between nuclear fission and fusion. Show how in both these processes energy is released.
Calculate the energy release in MeV in the deuterium-tritium fusion reaction :
H12 + H13 -â†’--

Q15 :

Draw a block diagram of a detector for AM signal and show, using necessary processes and the waveforms, how the original message signal is detected from the input AM wave.

Q16 :

A cell of emf 'E' and internal resistance 'r' is connected across a variable load resistor R. Draw the plots of the terminal voltage V versus (i) R and (ii) the current I.
It is found that when R = 4 â„¦, the current is 1 A and when R is increased to 9 â„¦, the current reduces to 0.5 A. Find the values of the emf E and internal resistance r.

Q17 :

Two capacitors of unknown capacitances C1 and C2 are connected first in series and then in parallel across a battery of 100 V. If the energy stored in the two combinations is 0.045 J and 0.25 J respectively, determine the value of C1 and C2. Also calculate the charge on each capacitor in parallel combination.

Q18 :

State the principle of working of a galvanometer.
A galvanometer of resistance G is converted into a voltmeter to measure upto V volts by connecting a resistance R1 in series with the coil. If a resistance R2 is connected in series with it, then it can measures upto V/2 volts. Find the resistance, in terms of R1 and R2, required to be connected to convert it into a voltmeter that can read upto 2 V. Also find the resistance G of the galvanometer in terms of R1 and R2.

Q19 :

With what considerations in view, a photodiode is fabricated? State its working with the help of a suitable diagram.
Even though the current in the forward bias is known to be more than in the reverse bias, yet the photodiode works in reverse bias. What is the reason?

Q20 :

Draw a circuit diagram of a transistor amplifier in CE configuration.
Define the terms : (i) Input resistance and (ii) Current amplification factor. How are these determined using typical input and output characteristics?

Q21 :

(a) In a double slit experiment using light of wavelength 600 nm, the angular width of the fringe formed on a distant screen is 0.1Â°. Find the spacing between the two slits.
(b) Light of wavelength 5000 Ã… propagating in air gets partly reflected from the surface of water. How will the wavelengths and frequencies of the reflected and refracted light be affected?

Q22 :

An inductor L of inductance XL is connected in series with a bulb B and an ac source. How would brightness of the bulb change when (i) number of turn in the inductor is reduced, (ii) an iron rod is inserted in the inductor and (iii) a capacitor of reactance XC = XL is inserted in series in the circuit. Justify your answer in each case.

Q23 :

A group of students while coming from the school noticed a box marked "Danger H.T. 2200 V" at a substation in the main street. They did not understand the utility of a such a high voltage, while they argued, the supply was only 220 V. They asked their teacher this question the next day. The teacher thought it to be an important question and therefore explained to the whole class.
(i) What device is used to bring the high voltage down to low voltage of a.c. current and what is the principle of its working ?
(ii) Is it possible to use this device for bringing down the high dc voltage to the low voltage? Explain
(iii) Write the values displayed by the students and the teacher.

Q24 :

(a) State Ampere's circuital law. Use this law to obtain the expression for the magnetic field inside an air cored toroid of average radius 'r', having 'n' turns per unit length and carrying a steady current I.
(b) An observer to the left of a solenoid of N turns each of cross section area 'A' observes that a steady current I in it flows in the clockwise direction. Depict the magnetic field lines due to the solenoid specifying its polarity and show that it acts as a bar magnet of magnetic moment m = NIA. OR

(a) Define mutual inductance and write its S.I. units.
(b) Derive an expression for the mutual inductance of two long co-axial solenoids of same length wound one over the other,
(c) In an experiment, two coils c1 and c2 are placed close to each other. Find out the expression for the emf induced in the coil c1 due to a change in the current through the coil c2.

Q25 :

(a) Using Huygens's construction of secondary wavelets explain how a diffraction pattern is obtained on a screen due to a narrow slit on which a monochromatic beam of light is incident normally.

(b) Show that the angular width of the first diffraction fringe is half that of the central fringe.

(c) Explain why the maxima at Î¸=(n+12) Î»a become weaker and weaker with increasing n.

OR

(a) A point object 'O' is kept in a medium of refractive index n1 in front of a convex spherical surface of radius of curvature R which separates the second medium of refractive index n2 from the first one, as shown in the figure.
Draw the ray diagram showing the image formation and deduce the relationship between the object distance and the image distance in terms of n1, n2 and R. (b) When the image formed above acts as a virtual object for a concave spherical surface separatig the medium n2 from n1 (n2 > n1