# 12th Physics Paper Solutions Set 1 : CBSE Delhi Previous Year 2012

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 :

When electrons drift in a metal from lower to higher potential, does it mean that all the free electrons of the metal are moving in the same direction?

No, when electric field applied the electrons will have net drift from lower to higher field but locally electrons may collide with ions and may change its direction of motion.

Q2 :

The horizontal component of the earth”™s magnetic field at a place is B and angle of dip is 60Â°. What is the value of vertical component of earth”™s magnetic field at equator?

On the equator, the values of both angle of dip (Î´) and vertical component of earth”™s magnetic field is zero. So, in this case, Bv = 0.

Q3 :

Show on a graph, the variation of resistivity with temperature for a typical semiconductor.

The following curve shows the variation of resistivity with temperature for a typical semiconductor. This is because, for a semiconductor, resistivity decreases rapidly with increasing temperature.

Q4 :

Why should electrostatic field be zero inside a conductor?

Q5 :

Name of physical quantity which remains same for microwaves of wavelength 1 mm and UV radiations of 1600 Ã… in vacuum.

Q6 :

Under what condition does a biconvex lens of glass having a certain refractive index act as a plane glass sheet when immersed in a liquid?

Q7 :

Predict the directions of induced currents in metal rings 1 and 2 lying in the same plane where current I in the wire is increasing steadily. Q8 :

State de-Broglie hypothesis.

Q9 :

A ray of light, incident on an equilateral prism moves parallel to the base line of the prism inside it. Find the angle of incidence for this ray.

Q10 :

Distinguish between “Analog and Digital signals”™.

OR

Mention the function of any two of the following used

(i) Transducer

(ii) Repeater

(iii) Transmitter

(iv) Bandpass Filter

Q11 :

A cell of emf E and internal resistance r is connected to two external resistance R1 and R2 and a perfect ammeter. The current in the circuit is measured in four different situations:

(i) without any external resistance in the circuit

(ii) with resistance R1 only

(iii) with R1 and R2 in series combination

(iv) with R1 and R2 in parallel combination

The currents measured in the four cases are 0.42 A, 1.05 A, 1.4 A and 4.2 A, but not necessarily in the order. Identify the currents corresponding to the four cases mentioned above.

Q12 :

The susceptibility of a magnetic material is − 2.6 Ã— 10−5. Identify the type of magnetic material and state its two properties.

Q13 :

Two identical circular wires P and Q each of radius R and carrying current “I”™ are kept in perpendicular planes such that they have a common centre as shown in the figure. Find the magnitude and direction of the net magnetic field at the common centre of the two coils. Q14 :

When an ideal capacitor is charged by a dc battery, no current flows. However, when an ac source is used, the current flows continuously. How does one explain this, based on the concept of displacement current?

Q15 :

Draw a plot showing the variation of (i) electric field (E) and (ii) electric potential (V) with distance r due to a point charge

Q16 :

Define self-inductance of a coil. Show that magnetic energy required to build up the current I in a coil of self inductance L is given by Q17 :

The current in the forward bias is known to be more (~mA) than the current in the reverse bias (~Î¼A). What is the reason, then, to operate the photodiode in reverse bias?

Q18 :

A metallic rod of “L”™ length is rotated with angular frequency of “Ï‰”™ with one end hinged at the centre and the other end at the circumference of a circular metallic ring of radius L, about an axis passing through the centre and perpendicular to the plane of the ring. A constant and uniform magnetic field B parallel to the axis is presents everywhere. Deduce the expression for the emf between the centre and the metallic ring.

Q19 :

The figure shows a series LCR circuit with L = 10.0 H, C = 40 Î¼F, R = 60 Î© connected to a variable frequency 240 V source, calculate

(i) the angular frequency of the source which drives the circuit at resonance,

(ii) the current at the resonating frequency,

(iii) the rms potential drop across the inductor at resonance. Q20 :

A rectangular loop of wire of size 4 cm Ã— 10 cm carries a steady current of 2 A. A straight long wire carrying 5 A current is kept near the loop as shown. If the loop and the wire are coplanar, find (i) the torque acting on the loop and

(ii) the magnitude and direction of the force on the loop due to the current carrying wire.

Q21 :

(a) Using Bohr”™s second postulate of quantization of orbital angular momentum show that the circumference of the electron in the nth orbital state in hydrogen atom is n times the de Broglie wavelength associated with it.

(b) The electron in hydrogen atom is initially in the third excited state. What is the maximum number of spectral lines which can be emitted when it finally moves to the ground state?

Q22 :

In the figure a long uniform potentiometer wire AB is having a constant potential gradient along its length. The null points for the two primary cells of emfs Îµ1 and Îµ2 connected in the manner shown are obtained at a distance of 120 cm and 300 cm from the end A. Find (i) Îµ1/ Îµ2 and (ii) position of null point for the cell Îµ1.

How is the sensitivity of a potentiometer increased? OR

Using Kirchhoff”™s rules determine the value of unknown resistance R in the circuit so that no current flows through 4 Î© resistance. Also find the potential difference between A and D. Q23 :

(i) What characteristic property of nuclear force explains the constancy of binding energy per nucleon (BE/A) in the range of mass number “A”™ lying 30 < A < 170?

(ii) Show that the density of nucleus over a wide range of nuclei is constant-independent of mass number A.

Q24 :

Write any two factors which justify the need for modulating a signal.

Draw a diagram showing an amplitude modulated wave by superposing a modulating signal over a sinusoidal carrier wave.

Q25 :

Write Einstein”™s photoelectric equation. State clearly how this equation is obtained using the photon picture of electromagnetic radiation.

Write the three salient features observed in photoelectric effect which can be explained using this equation.

Q26 :

(a) Why are coherent sources necessary to produce a sustained interference pattern?

(b) In Young”™s double slit experiment using monochromatic light of wavelength Î», the intensity of light at a point on the screen where path difference is Î», is K units. Find out the intensity of light at a point where path difference is Î»/3.

Q27 :

Use Huygens”™s principle to explain the formation of diffraction pattern due to a single slit illuminated by a monochromatic source of light.

When the width of the slit is made double the original width, how would this affect the size and intensity of the central diffraction band?

Q28 :

Explain the principle of a device that can build up high voltages of the order of a few million volts.

Draw a schematic diagram and explain the working of this device.

Is there any restriction on the upper limit of the high voltage set up in this machine? Explain.

OR

(a) Define electric flux. Write its S.I. units.

(b) Using Gauss”™s law, prove that the electric field at a point due to a uniformly charged infinite plane sheet is independent of the distance from it.

(c) How is the field directed if (i) the sheet is positively charged, (ii) negatively charged?

Q29 :

Define magnifying power of a telescope. Write its expression.

A small telescope has an objective lens of focal length 150 cm and an eye piece of focal length 5 cm. If this telescope is used to view a 100 m high tower 3 km away, find the height of the final image when it is formed 25 cm away from the eye piece.

OR

How is the working of a telescope different from that of a microscope?

The focal lengths of the objective and eyepiece of a microscope are 1.25 cm and 5 cm respectively. Find the position of the object relative to the objective in order to obtain an angular magnification of 30 in normal adjustment.

Q30 :

Draw a simple circuit of a CE transistor amplifier. Explain its working. Show that the voltage gain, AV, of the amplifier is given by where Î²acis the current gain, RLis the load resistance and riis the input resistance of the transistor. What is the significance of the negative sign in the expression for the voltage gain?

OR

(a) Draw the circuit diagram of a full wave rectifier using p-n junction diode.

Explain its working and show the output, input waveforms.

(b) Show the output waveforms (Y) for the following inputs A and B of

(i) OR gate (ii) NAND gate 