Class - XII Physicsephy.in/wp-content/uploads/2016/04/CBSE_XII_PHYSICS_QP_2016_A… · U79/17, DLF City Phase – III, Gurgaon – 122002 | Tel : +91 124 3926025, 96509 12288 Physics

CBSE QUESTION PAPER 2016 Time: 3 Hrs MM: 70

1 U79/17, DLF City Phase – III, Gurgaon – 122002 | Tel : +91 124 3926025, 96509 12288

web : www.equalsign.in | www.onlinepadho.com | email : [email protected]

Physics Class - XII

General Instructions:

1. All Questions are compulsory. 2. Section A contains five questions of one mark each. 3. Section B contains five questions of two marks each. 4. Sections C contains twelve questions of three marks each. 5. Section D contains one value based question of four marks. 6. Section E contains three questions of five marks each.

SECTION – A

1. Two wires one of copper and other of

manganin have same resistance and equal

length. Which wire is thicker and why?

2. The power factor of an a.c. circuit is 0.5. What

is the phase difference different voltage and

current in the circuit?

3. In which situation is there a displacement

current but not conduction current?

4. Name the logic gates market P and Q in the

given logic circuit.

5. Define the ‘term relaxation time’ in a

conductor.

SECTION – B

6. What is electrostatic shielding? How is this

property used in actual practice? Is the

potential in the cavity of a charged conductor

zero?

7. State two properties of electromagnetic waves.

How can we show that em waves carry

momentum?

8. Write briefly the underlying principle used in

Davison-Germer experiment to verify wave

nature of electrons experimentally. What is the

de-Broglie wavelength of an electron with

kinetic energy (K.E) 120 eV?

9. Write the function of a (i) transducer and (ii)

repeater in a communication system.

10. In the ground state of hydrogen atom, its Bohr

radius is given as 115.3 10 m. The atom is

excited such that the radius becomes 1121.2 10 m. Find (i) the value of the principal

quantum number and (ii) the total energy of

the atom in this excited state.

OR

The energy levels of a hypothetical atom are

shown below. Which of the shown transitions

will result in the emission of photon of

wavelength 275 nm?

SECTION – C

11. Derive an expression for the electric field

intensity at a point on the equatorial line of an

electric dipole of dipole moment P and length 2a. What is the direction of this field?

12. (a) A parallel plate capacitor 1C having

charge Q is connected, to an identical

uncharged capacitor 2C in series. What would

be the charge accumulated on the capacitor

2C ?

(b) Three identical capacitors each of

capacitance 3μF are connected, in turn, in

series and in parallel combination to the

common source of V volt. Find out the ratio of

the energies stored in two configurations.

13. (a) Define the term ‘intensity of radiation’ in

photon picture.



Physics Class – XII

(b) Plot a graph showing the variation of photo

current vs collector potential for three

different intensities 1 2 3I >I >I , two of which

1 2I and I have the same frequency υ and the

third has frequency 1υ υ.

(c) Explain the nature of the curves on the

basis of Einstein’s equation.

14. (i) ,Explain briefly the process of emission of

light by a Light Emitting Diode (LED).

(ii) Which semiconductors are preferred to

make LEDs and why?

(iii) Give two advantages of using LEDs over

conventional incandescent lamps.

15. A capacitor of unknown capacitance, a resistor

of 100 and an inductor of shelf inductance

2L 4 π henry are connected in series to an

ac source of 200V and 50 Hz. Calculate the

value of the capacitance and impedance of the

circuit when the current is in phase with the

voltage. Calculate the power dissipated in the

circuit.

16. (i) For a glass prism μ 3 the angle of

minimum deviation is equal to the angle of the

prism. Calculate the angle of the prism.

(ii) Draw ray diagram when incident ray falls

normally on one of the two equal sides of a

right angled isosceles prism having refractive

index μ 3 .

17. Plot a graph showing the variation of binding

energy per nucleon as a function of mass

number. Which property of nuclear force

explains the approximate constancy of binding

energy in the range 30 < A < 170 ? How does

one explain the release of energy in both the

processes of nuclear fission and fusion from

the graph?

18. Draw a circuit diagram of a common emitter

amplifier using n-p-n transistor. Derive an

expression for the current gain acβ

19. (i) Name the phenomenon on which the

working of an optical fibre is based.

(ii) What are the necessary conditions for this

phenomenon to occur?

(iii) Draw a labelled diagram of an optical fibre

and show how light propagates through the

optical fibre using this phenomenon.

20. Mention any three applications of the internet.

Explain one of these in detail.

21. Two harmonic waves of monochromatic light

1 2y cosωt and cos ωta y a

are superimposed on each other. Show that

maximum intensity in interference pattern is

four times the intensity due to each slit. Hence

write the conditions for constructive and

destructive interference in terms of the phase

angle .

22. (i) Mention two properties of soft iron due to

which it is preferred for making an

electromagnet.

(ii) State Gauss’s law in magnetism. How is it

different from Gauss’s law in electrostatics and

why?

OR

Derive an expression for the axial magnetic

field of a finite solenoid of length 2l and radius

r carrying current I. Under what condition does

the field become equivalent to that produced

by a bar magnet?

SECTION – D

23. During a thunderstorm the ‘live’ wire of the

transmission line fell down on the ground. A

group of boys passing through noticed it and

some of them wanted to place the wire by the

side. As they were approaching the wire and

trying to lift it. Hari noticed it and immediately

pushed them away to prevent them from

touching the wire. Two of them got hurt in the

process. Hari took them to a doctor to get

medical acid.

Based on the above paragraph, answer the

following:

(a) Write two values which Hari displayed

during the incident.

(b) Why is it that a bird can sit over a

suspended ‘live’ wire without any harm

whereas touching it on the ground can give

a fatal shock?

(c) The electric power from a power plant is

set up to a very high voltage before

transmitting it to distant consumers. Write

the reason for it.




24. (i) Find the magnitude and direction of current

in 1 resistor in the given circuit.

(ii) Two students X and Y perform an

experiment on potentiometer separately using

the circuit diagram shown below.

Keeping other things unchanged (a) X increase

the value of resistance R, (b) Y decrease the

value of resistance S in the set up. How will

these changes affect the position of null point

in each case and why?

OR

(a) Use Kirchoff’s rules, calculate the current in

the arm AC of the given circuit.

(b) On what principle does the meter bridge

work? Why are the metal strips used in the

bridge?

25. (i) Express Biot-Savart law in the vector form.

(ii) Use it to obtain the expression for the magnetic field at an axial point, distance d from

the centre of a circular coil of radius R carrying

current I.

(iii) Also, find the ratio of the magnitudes of the

magnetic field of this coil at the centre and at

an axial point for which R 3x

OR

(a) Consider a beam of charged particles

moving with varying speeds. Show how

crossed electric and magnetic fields can be

used to select charged particles of a particular

velocity?

(b) Name another device/machine which uses

crossed electric and magnetic fields. What does

this machine do and what are the functions of

magnetic and electric fields in this machine?

Where do these field exist in this machine?

Write about their natures.

26. When a parallel beam of monochromatic

source of light of wavelength λ is incident on a

single slit of width a, show how the diffraction

pattern is formed at the screen by the

interference of the wavelets from the slit.

Show that, besides the central at θ 0= ,

secondary maxima are observed at

λa

1θ n

2

and the minima at θ nλ a.

Why do secondary maxima get weaker in

intensity with increasing n? Explain.

OR

(i) Draw a ray diagram showing the geometry

of formation of image of a point object situated

on the principal axis and on the convex side of

a spherical surface of radius of curvature R.

Taking the rays as incident from a rarer

medium of refractive index 1n to a denser

medium of refractive index 2n , derive the

relation.

2 1 2 1n n n n

v u R

, where symbols have their

usual meaning.

(ii) Explain briefly how the focal length of a

convex lens changes with increase in

wavelength of incident light.

(iii) What happens to the focal length of convex

lens when it is immersed in water? Refractive

index of the material of lens is greater than that

of water.

CBSE QUESTION PAPER 2016 Time: 3 Hrs MM: 70



Physics Class - XII

SOLUTION

SECTION – A

1. Manganin is an alloy of copper with manganese

and nickel so, manganin have resistivity

greater than copper and the manganin thus to

be thicker to have the same resistance.

2. Here, cos 0.5

π

60 or3

3. Time varying voltage (AC) produces

displacement current which can flow through

the insulator while fixed voltage (DC) produces

conduction current which cannot flow through

the insulator.

4. P not gate

Q OR gate

5. It is the time for which an electron moves freely

between two successive collisions within the

atoms of the conductor. For a metallic

conductor, it is of the order of 1410 sec.

SECTION – B

6. Electrostatic shielding is the phenomenon that

uses faraday cage to block the effects of an

electric field. Such a cage can block the effects

of an external fields on its internal contents.

Application: Electrical line men often wear

suits made of faraday cages so as to avoid

electrocution.

The potential in the cavity of a charged

conductor is not zero.

7. Properties of E. M Waves:

(i) E.M Waves are transverse in nature i.e

electric and magnetic field are

perpendicular to each other and to the

direction of wave propagation (ii) These waves do not require any

material medium for propagation and

travel in free space with a speed 83 10 m/s

By measuring the relation pressure exerted on

any surface by em waves we can show that em

waves carry momentum.

Up

C

Here U is energy of em wave

C is speed of em wave

Radiation pressure =

Δp

force Δt

area area

Radiation pressure = p

area t

8. As diffraction is caused by waves, it is a

property associated with waves and the

diffraction of electron after striking the nikle

crystal establishes the wave nature of electron

this was the principle used in Davison –

Germer experiment to verify wave nature of

electron.

34

31 19

34

25

9

hλ

p

hλ

2mk.E

6.6 10λ

2 9.1 10 120 1.6 10

6.6 10λ

59.11 10

λ 0.111 10 m

λ 1.11A

=

=

9. (i) A Tranducer is an electronic device that

convert energy from one form to another

eg: Microphone, antenna, loudspeaker etc.

(ii) In communication system a repeater is a

device that receives a digital signal on an

electromagnetic or optical transmission

medium and regenerates the signal along the

next leg of the medium. In electromagnetic

media, repeaters over come the attenuation.




10. 2

11

n

nr 5.29 10 m

z

Given, 11

nr 21.2 10 m 2

11 11 n21.2 10 5.29 10

z

Here z = 1

2

2

2

2

21.2n

5.29

n 4

n 2

zE 13.6 ev

n

1E 13.6 ev

4

E 3.4 ev

Or

Energy associated with photon of wavelength

275 nm 34 8

9

19

19

19

hc 6.6 10 3 10E

λ 275 10

E 7.2 10 J

7.2 10E ev

1.6 10

E 4.5 ev

Which is B transition.

SECTION – C

11.

1 2

2 2

net 2 1

net 2 2

net 2 2

2 2

22 2

net 22 2

kqE

r a

k qE

r a

E E E

k q kqE

r a r a

1 1E kq

r a r a

r a r ak q

r a

4arE kq

r a

If dipole is short r > > > a

net 4

net 3

net 3

kq 2a 2rE

r

k p 2E

r

2kpE

r

12. (a) common potential

1 1 2 2

1 2

C V C VV

C C

Here 2V 0

1

1

QV

C

1 1

1 2

C VV

C C

Then 2 2Q C V

2 1

1 22

1 2 1 2

QC C

C C QQ

C C C C

(b) s 1 2 3

1 1 1 1

C C C C




s 5

5

p 1 2 3

2 2

s s p p

2

ss

2pp

1 1 1 1 1 3

C 3 3 3 C 3

C 1μF

C C C C 9μF

1 1E C V and E C V

2 2

1C V

E 121E 9

C V2

13. (a) The intensity of radiation is defined as the

rate of emitted energy from unit surface area

through unit solid angle.

Its S.I unit is 2

W

m sr

(b)

(c) maxk.E h υ o

This is called Einstein’s photoelectric equation

Where ohυo work function of the metal

maxK.E υ

The maxυ K graph is a straight line. Comparing

the above relation with m cy x we not that

slope of maxυ K graph is h, x intercept is oυ

and y-intercept is o .

14. (i) Light emitting diode is an electronic device

which converts electrical energy into light

energy under forward bias condition.

When a p-n junction is forward biased, the size

of depletion layer decreases. The movement of

majority carrier takes place across the

junction. The electrons moves from n-side to p-

side through the junction and holes moves

from P-side to n-side through and junction. As

a result of it concentration of the minority

carriers increases rapidly on the two sides of

the junction boundary. These excess minority

carriers on either side of the junction boundary

recombine with majority carriers there. In

each recombination of electron and hole the

electron will fall from higher energy state to

grand state. As a result of it, a photon is

released whose energy is nearly equal to the

energy gap Eg. If λ is the wavelength of photon

emitted then.

Eg = hc hc

λλ Eg

(ii) gallium arsenide (GaAS), gallium

phosphide (GaP) and gallium – arsenide

phosphide (Ga AS P)are the material which are

preferred to make LED’s because in case of Ge

and Si the larger percentage of energy released

is mainly transferred into thermal energy of

the vibrating lattice. As a result of it no light is

emitted.

(iii) (a) LED has less power and low

operational voltage

(b) LED is cheap and easy to handle

15. 2

4R 100 , L H

π

2

L

22

L C

V 200 v, ω 2πf 2π 50 100 π rad/sec

4 400X ωL 100π

π π

X 127.38

Z R X X

L

Here, 0°

L CX X

z R 100




C

C

4

4

rms rms

1 1X C

ωC ωX

1 110 F

400 4100π

π

C 0.25 10 F

V 200i = 2A

Z 100

P = V I cos

P 200 2 400w

=

16. (i)

minA δsin

2μ =A

sin2

Here minA δ

sinAμ

Asin

2

sin A3

Asin

2

Here A 60

(ii) here, 1sin c c sinμ 3

Clearly i < C so TIR will note take place

17.

(i) When we move from the heavy nuclei

region to the middle region of the plot we find

that there will be a gain in the overall binding

energy and hence release of energy. This

indicates that energy can be released when a

heavy nucleus A 240 break into two

roughly equal fragments. This process is called

nuclear fission

(ii) similarly, when we move from lighter

nuclei to heavier nuclei, there will be gain in

the overall binding energy and hence release of

energy. This indicates that energy can be

released when two or more lighter nuclei fuse

together to form a heavy nucleus. This process

is called Nuclear fusion.

18. Acc. to Kirchhoff’s law

e b cI I I 1

If CEV is collector voltage

cc CE C L

CE CC C L

V V I R

V V I R 2

a. c current gain: It is defined as ratio of change

in collector current CΔI to the change in base

current bΔI at constant collector voltage. It is

denoted by acβ

cac

b

ΔIβ

ΔI

19. (i) Total internal Reflection

(ii) The necessary condition for this

phenomenon to occur is that angle of incidence

of light must be greater than the critical angle

for the fibre material w.r.t its coating.

(iii)

20. Application of internet:-

(i) E-mail




(ii) E-banking

(iii) Internet surfing

Internet surfing is moving on internet from one

web page/website to another. It is the best way

of searing and viewing information on any

topic of interest.

21. 1 cos ωt 1y a

2 cos ωt 2y a

Acc. to superposition principle the

displacement y of the resultant wave at time t

would be

1 2 cosωt cos ωt 3

cosωt cosωt cos sin ωt sin

cosωt cos sin ωt sin

y y y a a

y a a a

y a a a

Let cos Rsin 4a a

And sin R cos 5a

R sinθcosωt R cos sin ωt

R sin ωt θ 6

y

y

This is the resultant harmonic wave of

amplitude R

2 2

4 5

2 22 2 2

2 2 2 2 2 2 2

2 2 2

2 2

2 2 2

2 2

Resultant

R cos θ sin θ a a cos a sin

R a a cos 2a cos a sin

R 2a 2a cos

R 2a 1 cos

R 4a cos2

I R 4a when 2nπ

For constructive interference

cos max 1 0, 2π, 4π

i.e 2nπ=

For destructive interference

I should be minimum, cos min

π, 3π, 5π

Or

2n 1 π

22. The properties of soft iron due to which it is

preferred for making an electromagnet are:-

(i) high permeability

(ii) low coercivity

(i) Gauss’s law in magnetism:

Acc. to gauss’s law in magnetism the net

magnetic flux B through any closed surface

is always zero.

B

S

B dS 0

Gauss law in electrostatic

E

o

qE dS

εS

Where q is the electric charge enclosed by the closed surface. In an electric dipole were

enclosed by the surface equal and opposite

charges in the dipole add upto zero. Therefore

E would be zero. The fact that B 0 indicates

that the simplest magnetic element is a dipole

or current loop. The isolated magnetic poles

called magnetic monopoles are not known to

exist.

SECTION – D

23. (a) (i) Decisiveness

(ii) Generosity

(b) The bird can sit over a suspended live wire

without any harm because circuit is incomplete

and there is no potential difference.

(c) The electric power form a power plant is set

up to a very high voltage before transmitting it

to distant consumers so as to minimize the

power losses.

24. (a)

Appling kVL in loop A E F C

1 1 2

1 2

1 2

30I 40 40 I I 0

70I 40I 40

70 I 40I 40 1

+kVL in loop AB DC




2 1 2

1 2

1 2

80 20I 40 40 I I

40I 60I 120

40 I 60I 120 2

On solving (1) and (1)

2

1

I 2.61A

I 0.91 A

Here negative sign show that 1I in opp.

direction

Current through 1 2AC I I 1.695 A=

(b) Meter bridge works on the principle of

wheat stone bridge. We use metal strip so that

resistant of wire remain constant and is tends

to 0.

25. According to biot – savart’s law the magnitude

of magnetic field induction dB at a point P due

to current element depend upon the following

factor

(i) dB I (ii) dB dl

(iii) dB sinθ (iv) 2

1dB

r

Combining these factor

2

2

I dlsinθdB

r

k I dlsinθdB

r

Here, oμk

4π

In vector form

o

3

I dl rμdB

4π r

(ii)

o

2

1o

2 2

μ I dl sin90dB

4π r

μ I d ldB dB 1

4π R x

Total magnetic field at P

o

2 2

2 2

o

2 2

o

2 2 2 2

dB sin

μ I dl

4π R

asin and dl 2πR

R

μ IB sin dl

4π R

μ I RB 2πR

4π R R

B

Bx

x

x

x x

2

o

2 2

μ 2πR IB

34π R2

x

If n is the no. of turns

2

o

2 2

μ IRB 2πn

34πR

2x

(iii) 2

oo 3

μ IRB 2πn

4π R




oo

2

o

3R 32 2 2

2

o

3

o

o

R 3

μ IB 2πn

4π R

μ IRB 2πn

4πR 3R

μ IR2πn

4π 8R

μ I= 2πn

4π 8R

B 1

B 8

x

x

26.

In OAC

i γ 1

In IAC

r γ β 2

Also,

1 2

1 2

μ i μ r 3

μ γ μ γ β

Using l

θr

1 2

AM AM AM AMμ μ 4

MO MC MC MI

M is very close to P therefore,

1 2

1 2 1 1

1 2 2 1

2 1 2 2

1 1 1 1μ μ

PO PC PC PI

μ μ μ μ

PO PI PC

PO u, PI V, PC R

μ μ μ μ

u V R

μ μ μ μ

V u R

(ii) We know that

1 2

1 1 1μ 1

f R R

As 1

μλ

As λ increase, μ decreases

1

f decreases focal length (f) increases

(iii) a g

1 2

1 1 1μ 1 1

f R R

ω g

ω 1 2

1 1 1μ 1 2

f R R

Relation (2) shows focal length of lens in water

1

2

a gω

ω g

ω

ω

ω

ω

μ 1f1

f 1 μ 1

31

f 2

3f3 1

2 4

1f 2

1f

8

f4

f

f 4f increases 4 times

Documents

Class - XII Physicsephy.in/wp-content/uploads/2016/04/CBSE_XII_PHYSICS_QP_2016_A… · U79/17, DLF City Phase – III, Gurgaon – 122002 | Tel : +91 124 3926025, 96509 12288 Physics