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Sample Paper (CBSE)

Series SC/SP Code No. SP-16

CBSE-12-Physics- SP ©Educomp Solutions Ltd. 2015-16

Physics

Time Allowed: 3 hours Maximum : 70

General Instructions:

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

(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 questions of five marks each.

(iv) 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 weightage. You have to attempt only one of the choices in such

questions.

(v) You may use the following values of physical constants wherever necessary:

c = 3 × 108 m/s

h = 6.63 × 10-34 Js

e = 1.6 × 10-19 C

μo = 4 × 10-7 T mA-1

= 9 × 109 N m2 C-2

me = 9.1 × 10-31 kg


Mass of Neutrons = 1.675 × 10-27 kg

Mass of proton = 1.673 × 10-27 kg

Avogadro’s number = 6.023 × 1023 per gram mole

Boltzmann constant = 1.38 × 10-23 JK-1

SECTION A

1 Figure shows a point charge +Q, located at a distance R/2 from the

centre of a spherical metal shell. Draw the electric field lines for the

given system.

1

2 Give an example of a material each for which temperature coefficient of

resistivity is (i) positive, (ii) negative.

1

3 A device ‘X’ is connected to an a.c. source V = Vo sin ωt. The variation of

voltage, current and power in one complete cycle is shown in the

following figure.

(i) Which curve shows power consumption over a full cycle?

(ii) Identify the device ‘X’

1


4 An object is placed in front of convex lens made of glass. How does the

image distance vary if the refractive index of the medium is increased in

such a way that still it remains less than the glass?

1

5 Name the network within an institution, made by connecting all or some

of their computers.

1

SECTION B

6 Two cells of E.M.F. 10 V and 2 V and internal resistances 10 Ω and 5 Ω

respectively, are connected in parallel as shown. Find the effective

voltage across R.

2

7 Figure shows a system of two polarizing sheets in the path of initially

unpolarized light. The polarizing direction of first sheet is parallel to x-

axis and that of second sheet is 60˚ clockwise from x-axis. Calculate

what fraction of intensity of light emerges from the system.

2


OR

State Huygen’s principle. Using it, construct a ray diagram for a plane

wave front getting incident on a denser medium.

8 A monochromatic light source of power emits photons per

second. This light ejects photoelectrons from a metal surface. The

stopping potential for this set up is . Calculate the work function of

the metal.

2

9 The following table shows some measurements of the decay rate of a

radionuclide sample. Find the disintegration constant.

2

10 Distinguish between any two types of propagation of Electromagnetic

waves with respect to (i) frequency range over which they are applicable

(ii) communication systems in which they are used.

2

SECTION C

11 Given a uniformly charged plane/ sheet of surface charge density

.

(i) Find the electric field intensity at a point A, away from the

sheet on the left side.

3


(ii) Given a straight line with three points X, Y and Z placed away

from the charged sheet on the right side. At which of these points, the

field due to the sheet remain the same as that of point A and why?

12 The potential difference across a resistor ‘r’ carrying current ‘I’ is Ir.

(i) Now if the potential difference across ‘r’ is measured using a

voltmeter of resistance ‘ ’, show that the reading of voltmeter is less

than the true value.

(ii) Find the percentage error in measuring the potential difference by a

voltmeter.

(iii) At what value of , does the voltmeter measures the true potential

difference?

OR

You are given two sets of potentiometer circuit to measure the emf of

a cell.

Set A: consists of a potentiometer wire of a material of resistivity ,

area of cross-section and length .

Set B: consists of a potentiometer of two composite wires of equal

lengths each, of resistivity , and area of cross-section ,

respectively.

(i) Find the relation between resistivity of the two wires with respect to

their area of cross section, if the current flowing in the two sets is same.

(ii) Compare the balancing length obtained in the two sets.

3


13 (i) Name the machine which uses crossed electric and magnetic fields to

accelerate the ions to high energies. With the help of a diagram, explain

the resonance condition.

(ii) What will happen to the motion of charged particle if the frequency

of the alternating voltage is doubled?

3

14 The magnetic field through a single loop of wire, in radius and

resistance, changes with time as shown in the figure. The

magnetic field is perpendicular to the plane of the loop. Plot induced

current as a function of time.

3

15 Identify the type of waves which are produced by the following way and

write one application for each:

(i) Radioactive decay of the nucleus,

(ii) Rapid acceleration and decelerations of electrons in aerials,

(iii) Bombarding a metal target by high energy electrons.

3


16 Consider a two slit interference arrangement (shown in figure) such

that the distance of the screen from the slits is half the distance between

the slits. Obtain the value of in terms of such that the first minima

on the screen fall at a distance from the centre O.

3

17 A compound microscope consists of an objective of focal length and

eye piece of focal length separated by . (a) At what distance

from the objective should an object be placed so that the final image is

formed at least distance of distinct vision? (b) Calculate the angular

magnification in this case.

3

18 Compare the photoelectric effect on the basis of photon theory and wave

theory of light and hence explain why the wave theory failed to explain

it.

3

19 Derive the expression for the magnetic field at the site of a point nucleus

in a Hydrogen atom due to the circular motion of the electron. Assume

that the atom is in its ground state and give the answer in terms of

fundamental constants.

3

20 The graph of potential barrier versus width of depletion region for an

unbiased diode is shown in A. In comparison to A, graphs B and C are

obtained after biasing the diode in different ways. Identify the type of

biasing in B & C and justify your answer.

3


21 Explain the following:

(i) In the active state of the transistor, the emitter base junction acts as

a low resistance while base collector region acts as high resistance.

(ii) Output characteristics are controlled by the input characteristics in

common emitter transistor amplifier.

(iii) LEDs are made of compound semiconductor and not by elemental

semiconductors.

3

22 (i) Write the factors that prevent a baseband signal of low frequency to

be transmitted over long distances.

(ii) What is to be done to overcome these factors? Draw a block diagram

to obtain the desired signal.

3

SECTION D

23 Ria recently read about earth’s magnetic field and its causes. She

became so much fascinated by the topic that she further studied it in

detail. She collected information as follows:

a) The magnitude of magnetic field at the Earth's surface ranges

from gauss.

b) The Earth’s magnetic field varies with time. There are short term

and long term variations.

c) One the scale of million years, the Earth’s magnetic field reverses

its direction, and much more.

She made a power point presentation on the same and shared all this

information with her classmates.

(i) Suggest another activity related to the same topic, which will help a

4


student to internalize the same values gained by Ria.

(ii) Draw a labeled diagram showing the three magnetic elements of

earth.

SECTION E

24 (a) Two isolated metal spheres A and B have radii and

respectively, and same charge . Find which of the two spheres have

greater : (i) Capacitance and (ii) energy density just outside the surface

of the spheres.

(b) (i) Show that the equipotential surfaces are closed together in the

regions of strong field and far apart in the regions of weak field. Draw

equipotential surfaces for an electric dipole.

(ii) Concentric equipotential surfaces due to a charged body placed at

the centre are shown. Identify the polarity of the charge and draw the

electric field lines due to it.

OR

(a) Compare the individual dipole moment and the specimen dipole

moment for H2O molecule and O2 molecule when placed in

(i) Absence of external electric field.

(ii) Presence of external electric field. Justify your answer.

(b) Given two parallel conducting plates of area A and charge densities

and . A dielectric slab of constant and a conducting slab of

thickness each are inserted in between them as shown.

5


(i) Find the potential difference between the plates.

(ii) Plot versus graph, taking at positive plate and at

negative plate.

25 (a) With the help of a diagram, explain the principle and working of a

device which produces current that reverses its direction after regular

intervals of time.

(b) If a charged capacitor is short circuited through an inductor , the

charge and current in the circuit oscillate simple harmonically.

(i) In what form the capacitor and the inductor stores energy?

(ii) Write two reasons due to which the oscillations become damped.

OR

(a) Figure shows the variation of resistance and reactance versus

angular frequency. Identify the curve which corresponds to inductive

reactance and resistance.

5


(b) Show that series LCR circuit at resonance behaves as a purely

resistive circuit. Compare the phase relation between current and

voltage in series LCR circuit for (i) (ii) using phasor

diagrams.

(c) What is an acceptor circuit and where it is used?

26 (a) Draw a ray diagram to show the formation of the real image of a

point object due to a convex spherical refracting surface, when a ray of

light is travelling from a rarer medium of refractive index to a denser

medium of refractive index . Hence derive the relation between object

distance, image distance and radius of curvature of the spherical

surface.

(b) An object is placed in front of right angled prism ABC in two

positions as shown. The prism is made of crown glass with critical angle

of . Trace the path of the two rays from P and Q.

OR

(a) Calculate the value of θ, for which light incident normally on face AB

5


grazes along the face BC. .

(b) Draw a graph showing the variation of angle of deviation ‘ ’ with

that of angle of incidence ‘ ’ for a monochromatic ray of light passing

through a glass prism of refracting angle ‘ ’. What do you interpret from

the graph? Write a relation showing the dependence of angle of

deviation on angle of incidence and hence derive the expression for

refractive index of the prism.


SECTION A

1

2 (i) Cu (metals, alloys)

(ii) Si (semiconductor)

3 (i) Curve A

(ii) Capacitor

4 We know that:

And

If refractive index of a medium increases, decreases and hence image

distance increases.

5 Local Area Network (LAN)

6 Given:

Answer Key


7 Light emerging from first sheet is:

Light emerging from second sheet is:

OR

According to Huygens’ Principle:

1) Each point on the primary wave front acts as a source of secondary

wavelets, sending out disturbance in all directions in a similar manner

as the original source of light does.

2) The new position of the wave front at any instant is the envelope of the

secondary wavelets at that instance.


Ray diagram using Huygens’s construction:

8 Given:

Photon energy is given as:

Work function is given as:

9 We know,

This implies that the slope of vs is

From the table:


10 Type of

propagation of

electromagnetic

wave

Frequency

range

Communication system in

which they are used

Ground wave 500-1500 KHz Standard AM broadcast

Space wave Above 40

MHz

Television

11 Given:

(i) At A, Electric field intensity,

directed away from the sheet .

(ii) At point Y the field due to sheet remains the same as that of point A.

Because at , the charge sheet acts as a finite sheet and thus the magnitude

remains same towards the middle region of the planar sheet.

12 Given: Without voltmeter, the potential difference across resistor is:

(i ) When voltmeter of resistance is connected in parallel to resistor , the

resultant resistance will be:

Voltmeter reading will be:

Thus,

(ii) Percentage error is given as:


(iii) When ,

OR

(a) For set A:

For set B:

Thus,

(b) Potential gradient of the potentiometer wire for set A is:

Potential drop across the potentiometer wire in set B is:

Here,

Which is equal to

Therefore, balancing length obtained in the two sets is same.


13 (i) Cyclotron

Resonance condition:

An alternating voltage is applied between the dees to accelerate charged

particles by an electric field. When charged particles are introduced between the

dees , a magnetic field that is perpendicular to the plane of the dees exerts a

force on the particles. This force is given by:

This force makes the charged particles to move in a circular trajectory. When

the charged particles reach the dees the polarity is reversed and the particles

are once more accelerated. This process continues and highly energetic beams of

charged particles are obtained.

Magnetic force provides the necessary centripetal force to the charged particles

to move in a circular path. That is


The time spent inside a dee to cover semi circular path is given by:

Therefore time period is given by:

Thus, cyclotron frequency is:

When the angular frequency of the rotating charged particle and the angular

frequency of the alternating voltage applied across the dees of the cyclotron are

matched. Then this condition is called resonance.

(ii) If the frequency of the alternating voltage is doubled, the particle will

accelerate and decelerate alternately. However, the radius of the path will

remain unchanged.

14 Given:

Using the relations:

and

We get,


Thus, induced current Vs time graph is:

15 (i) Type of wave: Gamma rays

Application: treatment of tumors

(i) Type of wave: Radio waves

Application: Radio and television communication system

(i) Type of wave: X-rays

Application: Study of crystals

16 Here:


Minima will occur when:

17 Given:

(a) Using the relations:

And

We get:

(b) Using the relation:

We get:

18 The phenomenon of ejection of electrons from a metal surface, when light of

sufficiently high frequency falls upon it, is known as the photoelectric effect.


Three salient features observed in photoelectric effect:

(i) For photoelectric emission to take place, the frequency of the incident

radiation must be greater than the threshold frequency.

(ii) The maximum kinetic energy of the emitted photoelectrons depends

linearly on the frequency of the incident radiation.

(iii) The number of photoelectrons emitted depends on the intensity of

incident radiation.

Wave nature of radiation cannot explain:

(i) The instantaneous emission of photoelectrons.

(ii) The existence of threshold frequency for a metal surface.

(iii) The fact that kinetic energy of the emitted electrons is independent of

the incident light and depends on its frequency.

Thus, photoelectric effect cannot be explained on the basis of wave nature of

light.

19 The force of attraction between the nucleus and the electron provides the

centripetal force to the electron. Thus,

From Bohr quantization condition:

Magnetic field at the centre of the atom is:

But

Thus,


20 Graph B represents reverse biasing as the reverse bias increases the potential

barrier.

Graph C represents forward biasing as it decreases the potential barrier.

21 (i) Emitter base junction is forward biased whereas base collector junction is

reverse biased.

(ii) Small change in the current in the base circuit controls the larger current

in the collector circuit as .

(iii) Elemental semiconductor’s band gap is such that the emitted wavelength

lies in IR region. Hence they cannot be used for making LED.

22 (i) Factors that prevent a baseband signal of low frequency to be transmitted

over long distances are:

1. Size of the antenna

2. Effective power radiated by the antenna

3. Mixing up of signals from different transmitters

(ii) Modulation is done to overcome these factors.

Block diagram of amplitude modulation:

23 (i) Values gained by Ria are: Adventurous and helping.

Suspend a bar magnet freely. It will align in geographic north south direction.

(ii)


Here, is magnetic declination, δ is magnetic inclination or dip and BH is

horizontal component of earth’s magnetic field.

24 (a) Given: Radius of sphere A =

Radius of sphere B =

Charge on sphere A =

Charge on sphere B =

(i) Capacitance of sphere A is:

Capacitance of sphere B is:

(ii) Energy density is given by:

(b) (i) We know electric field is:


For same change in potential dV:

where ‘ ’ represents the distance between equipotential surfaces.

Thus, the equipotential surfaces are closed together in the regions of strong field

and far apart in the regions of weak field.

Diagram of equipotential surface due to a dipole:

(ii) Polarity of charge is negative.

Direction of electric field is radially inward as shown:

OR

(a) (i) In the absence of external electric field:


Dipole

moment

O2 molecule H2O molecule

Individual Does not exist Exist

Specimen Does not exist Does not exist

as dipoles are

randomly

oriented

(ii) In the presence of external electric field:

Dipole

moment

O2 molecule H2O molecule

Individual Exist Torque acts on

the molecule to

align them

parallel to

electric field

Specimen Exist Net dipole

moment exist

(b) (i)

The potential difference is given as:

Where the applied electric field and electric potential difference inside the

conductor is zero.

Thus,


(ii)

25 (a) AC generator

Basic elements of an A.C. generator:

(i) Rectangular coil: Also called as armature

(ii) Strong permanent magnets: Magnetic field is perpendicular to the axis

of rotation of coil.

(iii) Slip rings

(iv) Brushes

Principle: It is based on the principle of electromagnetic induction. That is,


when a coil is rotated about an axis perpendicular to the direction of uniform

magnetic field, an induced emf is produced across it.

Working:

(b) (i) The capacitor stores energy in the form of electric field and the inductor

stores energy in the form of magnetic field.

(ii) Oscillations become damped due to:

a) Resistance of the circuit

b) Radiation in the form of EM waves

OR

(a) Curve B corresponds to inductive reactance and curve C corresponds to

resistance.

(b) At resonance,

Therefore, impedance is given as:

Thus, series LCR circuit at resonance behaves as a purely resistive circuit.


For Therefore phasor diagram is:

Here, is phase difference.

For Therefore phasor diagram is:

(c ) Series resonance LCR circuit is called an acceptor circuit.

They are widely used in the tuning mechanism of a radio or a TV.

26 (a)


Here,

For small angles:

In NOC,

In NCI,


By Snell’s law:

For small angles:

Applying Cartesian sign convention:

(b)


OR

(a)

When light is incident normally on face AB, it will refract without any deviation.

For face BC, angle of incidence is theta and angle of refraction is 90˚. Therefore,

from Snell’s law:


(b)

As the angle of incidence increases, the angle of deviation decreases. At a

particular value of the angle of incidence equal to the angle of emergence, the

angle of deviation attains a minimum value. If we further increase the angle of

incidence, the angle of deviation start increasing again.

Any given value of angle of deviation, except for i = e, corresponds to two values

i and hence of e.

Angle of deviation remains the same if angle of incidence and angle of

emergence are interchanged. Physically, this is related to the fact that the path

of ray can be traced back, resulting in the same angle of deviation.

In minimum deviation position:

As,

We have,


Documents

Physicsdips.educomp.com/.../files/sample_papers_2016/class_… · · 2016-11-29question of two marks, one question of three marks and all three questions of five ... There are short