# 12th Physics Paper Solutions Set 1 : CBSE Delhi Previous Year 2009

General Instructions:
(i) All questions are compulsory.
(ii) There are 30 questions in total. Question Nos. 1 to 8 are very short answer type questions and carry one mark each.
(iii) Question Nos. 9 to 18 carry two marks each, question 19 to 27 carry three marks each and question 28 to 30 carry five marks each.
(iii) 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 three questions of five marks each. You have to attempt only one of the choice in such questions.
(iv) Use of calculators is not permitted.
Q1 :

What is sky wave propagation?

They type of propagation in which radio waves are transmitted towards the sky and are reflected by the ionosphere towards the desired location on earth is called sky wave propagation.

Q2 :

Write the following radiations in ascending order with respect to their frequencies:

X-rays, microwaves, UV rays and radio waves.

The given radiations can be arranged in ascending order with respect to their frequencies as:

Radio waves < Microwaves < UV rays < X-rays

Q3 :

Magnetic field lines can be entirely confined within the core of a toroid, but not within a straight solenoid. Why?

Magnetic field lines form closed loops around a current-carrying wire. The geometry of a straight solenoid is such that magnetic field lines cannot loop around circular wires without spilling over to the outside of the solenoid. The geometry of a toroid is such that magnetic field lines can loop around electric wires without spilling over to the outside. Hence, magnetic field lines can be entirely confined within the core of a toroid, but not within a straight solenoid.

Q4 :

You are given following three lenses. Which two lenses will you use as an eyepiece and as an objective to construct an astronomical telescope?

 Lens Power (P) Aperture (A) L1 3D 8 cm L2 6D 1 cm L3 10D 1 cm

Q5 :

If the angle between the pass axis of polarizer and the analyser is 45Âº, write the ratio of the intensities of original light and the transmitted light after passing through the analyser.

Q6 :

The figure shows a plot of three curves a, b, c, showing the variation of photocurrent vs collector plate potential for three different intensities I1, I2 and I3having frequencies v1, v2and v3respectively incident of a photosensitive surface.

Point out the two curves for which the incident radiations have same frequency but different intensities.

Q7 :

What type of wavefront will emerge from a (i) point source, and (ii) distance light source?

Q8 :

Two nuclei have mass numbers in the ratio 1: 2. What is the ratio of their nuclear densities?

Q9 :

A cell of emf “E”™ and internal resistance “r”™ is connected across a variable resistor “R”™. Plot a graph showing the variation of terminal potential “V”™ with resistance R. Predict from the graph the condition under which “V”™ becomes equal to “E”™.

Q10 :

(i) Can two equi-potential surfaces intersect each other? Give reasons.

(ii) Two charges −q and +q are located at points A (0, 0, −a) and B (0, 0, +a) respectively. How much work is done in moving a test charge from point P (7, 0, 0) to Q (−3, 0, 0)?

Q11 :

By what percentage will the transmission ranges of TV tower be affected when the height of the tower is increased by 21%?

Q12 :

Derive an expression for drift velocity of free electrons in a conductor in terms of relaxation time.

Q13 :

How does a charge q oscillating at certain frequency produce electromagnetic waves?

Sketch a schematic diagram depicting electric and magnetic fields for an electromagnetic wave propagating along the Z-direction.

Q14 :

A charge ‘q’ moving along the X- axis with a velocity is subjected to a uniform magnetic field B along the Z-axis as it crosses the origin O.

(i) Trace its trajectory.

(ii) Does the charge gain kinetic energy as it enters the magnetic field? Justify your answer.

Q15 :

The following figure shows the input waveforms (A, B) and the output waveform (Y) of a gate. Identify the gate, write its truth table and draw its logic symbol.

Q16 :

State Biot-Savart law.

A current I flows in a conductor placed perpendicular to the plane of the paper. Indicate the direction of the magnetic field due to a small element dat point P situated at distance from the element as shown in the figure.

Q17 :

Why are high frequency carrier waves used for transmission?

OR

What is meant by term “modulation”™? Draw a block diagram of a simple modulator for obtaining an AM signal.

Q18 :

A radioactive nucleus ‘A’ undergoes a series of decays according to the following scheme:

The mass number and atomic number of A are 180 and 72 respectively. What are these numbers for A4?

Q19 :

A thin conducting spherical shell of radius R has charge Q spread uniformly over its surface. Using Gauss”™s law, derive an expression for an electric field at a point outside the shell.

Draw a graph of electric field E(r) with distance r from the centre of the shell for 0 â‰¤ r â‰¤ âˆž.

Q20 :

Three identical capacitors C1, C2 and C3 of capacitance 6 Î¼F each are connected to a 12 V battery as shown.

Find

(i) charge on each capacitor

(ii) equivalent capacitance of the network

(iii) energy stored in the network of capacitors

Q21 :

(a) The energy levels of an atom are as shown below. Which of them will result in the transition of a photon of wavelength 275 nm?

(b) Which transition corresponds to emission of radiation of maximum wavelength?

Q22 :

A proton and an alpha particle are accelerated through the same potential. Which one of the two has (i) greater value of de Broglie wavelength associated with it, and (ii) less kinetic energy? Justify your answers.

Q23 :

In a single slit diffraction experiment, when tiny circular obstacle is placed in path of light from a distance source, a bright spot is seen at the centre of the shadow of the obstacle. Explain why?

State two points of difference between the interference patterns obtained in Young”™s double slit experiment and the diffraction pattern due to a single slit.

Q24 :

(a) Define self inductance. Write its S.I. units.

(b) Derive an expression for self inductance of a long solenoid of length l, cross-sectional area A having N number of turns.

Q25 :

The figure shows experimental set up of a meter bridge. When the two unknown resistances X and Y are inserted, the null point D is obtained 40 cm from the end A. When a resistance of 10 Î©is connected in series with X, the null point shifts by 10 cm. Find the position of the null point when the 10 Î©resistance is instead connected in series with resistance “Y”™. Determine the values of the resistances X and Y.

Q26 :

Derive the expression for force per unit length between two long straight parallel current carrying conductors. Hence define one ampere.

OR

Explain the principle and working of a cyclotron with the help of a schematic diagram. Write the expression for cyclotron frequency.

Q27 :

Three light rays red (R), green (G) and blue (B) are incident on a right angled prism “abc”™ at face “ab”™. The refractive indices of the material of the prism for red, green and blue wavelengths are 1.39, 1.44 and 1.47 respectively. Out of the three which colour ray will emerge out of face “ac”™? Justify your answer. Trace the path of these rays after passing through face “ab”™.

Q28 :

(a) Derive an expression for the average power consumed in a series LCR circuit connected to a.c. source in which the phase difference between the voltage and the current in the circuit is Î¦.

(b) Define the quality factor in an a.c. circuit. Why should the quality factor have high value in receiving circuits? Name the factors on which it depends.

OR

(a) Derive the relationship between the peak and the rms value of current in an a.c. circuit.

(b) Describe briefly, with the help of labelled diagram, working of a step-up transformer.

A step-up transformer converts a low voltage into high voltage. Does it not violate the principle of conservation of energy? Explain.

Q29 :

(i) Draw a circuit diagram to study the input and output characteristics of an n-p-n transistor in its common emitter configuration. Draw the typical input and output characteristics.

(ii) Explain, with the help of a circuit diagram, the working of n-p-n transistor as a common emitter amplifier.

OR

How is a zener diode fabricated so as to make it a special purpose diode? Draw I-V characteristics of zener diode and explain the significance of breakdown voltage.

Explain briefly, with the help of a circuit diagram, how a p-n junction diode works as a half wave rectifier.

Q30 :

Trace the rays of light showing the formation of an image due to a point object placed on the axis of a spherical surface separating the two media of refractive indices n1and n2. Establish the relation between the distances of the object, the image and the radius of curvature from the central point of the spherical surface.

Hence derive the expression of the lens maker”™s formula.

OR

Draw the labelled ray diagram for the formation of image by a compound microscope.

Derive the expression for the total magnification of a compound microscope. Explain why both the objective and the eyepiece of a compound microscope must have short focal lengths.