cbse sample paper class xii physics

8/10/2019 CBSE Sample Paper Class XII Physics

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Physics (Code-A) Sample Question Paper for Class XII

Time : 3 Hours Max. Marks : 70

General Instructions :

(i) All questions are compulsory.

(ii) There is no overall choice. However an internal choice has been provided in one question of 2 marks,

one question of 3 marksand all three questions of 5 markseach. You have to attempt only one of the

choices in such questions.

(iii) Questions 1 to 8 are very short answer type and carry 1 mark each.

(iv) Questions 9 to 16 are short answer type and carry 2 marks each.

(v) Questions 17 to 25 are short answer type and carry 3 marks each.

(vi) Question 26 is a value based question and carry 4 marks

(vii) Questions 27 to 29 are long answer type and carry 5 marks each.

(viii) Use of calculator is not permitted.

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

c= 3 108ms1

h= 6.626 1034Js

e= 1.602 1019C

0= 4 107T mA1

9 2 2

0

19 10 Nm C

4

Mass of neutron m

n= 1.675 1027kg

Boltzmanns constant k = 1.381 1023JK1

Avogadros number NA

= 6.022 1023/mol1

SAMPLE QUESTION PAPERPhysics (Theory) - Class XII (Code-A)

SECTION-A

Very Short Answer Type Questions : [8 1 = 8]

1. A wire of resistivity is stretched to three times its length. What will be its new resistivity? [1]

2. What oscillates in electromagnetic waves? Are these waves transverse or longitudinal? [1]

3. State the essential condition for diffraction of light to occur. [1]

4. The electric current flowing in a wire in direction BtoAis decreasing. What is the direction of induced current

in the metallic loop kept above wire as seen from top? [1]

A B

5. What are the two possible values of the magnifying power of a simple microscope made of convex lens of

focal length 5.0 cm? [1]

6. If the maximum kinetic energy of a photoelectron is 3 eV, what is its stopping potential? [1]


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Sample Question Paper for Class XII Physics (Code-A)

7. The frequency of the alternating current applied to a series circuit containing resistance, inductance and

capacitance is doubled. What happens to R,XLandX

C? [1]

8. An electric dipole of dipole moment 20 106

cm is enclosed in a closed surface. What is net flux comingout of the surface? [1]

SECTION-B

Short Answer Type Questions : [8 2 = 16]

9. Calculate the capacitance of the unknown capacitor C if the equivalent capacitance between P and Q is

40 F. [2]

Q

10 F

80 F

C

P

10. Using Gauss law, derive an expression for electric field intensity due to a uniformly charged spherical shell at

a point : [2]

(a) Outside and

(b) Inside the shell

11. Define self inductance and give its SI unit. Derive an expression for self inductance of a long air cored solenoid

of length l, radius rand turns per unit length n. [2]

12. State Amperes circuital law. Use it to derive an expression for the magnetic field along the axis of a current

carrying toroidal solenoid of Nnumber of turns having radius r. [2]

13. What do you mean by power of a lens? What is its unit? What is the expression for power of two thin lenses

in contact with each other? [2]

14. Draw a neat and labelled diagram of image formed by a compound microscope. Write an expression for its

magnifying power. [2]

15. On the basis of energy band diagrams, distinguish between [2]

(i) A metal

(ii) An insulator

(iii) A semiconductor.

16. A coil has an inductance of 1 H. [2]

(a) At what frequency will it have a reactance of 3142?

(b) What should be the capacitance of a capacitor which has the same reactance at that frequency?

OR

A 15.0 F capacitor is connected to 110 V, 50 Hz source. Find the capacitive reactance and the rms current.


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Physics (Code-A) Sample Question Paper for Class XII

SECTION-C

Short Answer Type Questions : [9 3 = 27]

17. Using Biot-Savarts law, derive an expression for the magnetic field intensity at the centre of a current carrying

circular coil. Also find the magnitude of field due to a semicircular coil of radius r and carrying current I. [3]

18. Define capacitance. Derive an expression for the capacitance of a parallel plate capacitor. How will the

capacitance of the capacitor be affected if a conducting slab of some thickness is partially filled in the space

between two plates. [3]

19. Define the term electron mobility. Explain how electron mobility changes for a good conductor when : [3]

(a) The temperature of the conductor is decreased, keeping potential difference constant.

(b) Applied potential difference is doubled keeping temperature constant.

20. In certain cases when an object and screen are separated by a distance d, two positionsxof the converging

lens relative to the object will give an image on the screen. Show that these two values are

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1 12

d fx

d

.

Under what conditions will no image be formed? [3]

21. What is electron emission? Discuss various forms of electron emission in brief. Establish Einsteins photoelectric

equation? [3]

22. A series LCR circuit is connected to an AC source. Using the phasor diagram, derive the expression for the

impedance of the circuit. Plot a graph to show the variation of current with frequency of source, explaining the

nature of its variation. [3]

23. Mention three modes of propagation used in communication system. Explain with the help of a neat and labelled

diagram how long distance communication can be achieved by ionosphere reflection of radio waves. [3]

24. Obtain the binding energy of the nuclei 5626 Fe in units of MeV from the following data : [3]

1.007825 up

m

1.008665 unm

5626Fe 55.934939 um . Also calculate binding energy/nucleon.

25. With the help of an example, explain how the neutron to proton ratio changes during -decay. Also briefly explain

what are nuclear forces. Give their important properties. [3]

OR

A doubly ionised lithium atom is hydrogen like with atomic number 3. Find

(a) The wavelength of radiation required to excite the electron in Li++from the first to the third Bohrs orbit.

(b) How many spectral lines are observed in the emission spectrum of the above excited system?

26. Mr. Feynmann and his students went to north pole on a picnic. They found beautiful coloured curtain like

structures are hanging from the polar sky. What is the reason behind this? [4]


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Sample Question Paper for Class XII Physics (Code-A)

SECTION-D

Long Answer Type Questions : [3 5 = 15]

27. (a) In Youngs double slit experiment, derive the condition for (i) Constructive interference and (ii) Destructive

interference at a point on the screen. Also derive expression of fringe width.

(b) Estimate the distance for which ray optics is a good approximation for an aperture of 4 mm and wavelength

400 nm. [3 + 2 = 5]

OR

State Huygens Principle. Using the geometrical construction of secondary wavelets, explain the refraction of a

plane wavefront incident at a plane surface. Hence verify the Snells law. Illustrate with the help of a diagram the

action of (i) Convex lens and (ii) concave mirror on a plane wavefront incident on it.

28. (a) Explain briefly with the help of a circuit diagram, how V-Icharacterstics of ap-njunction diode are obtained

in(i) Forward bias

(ii) Reverse bias

Draw the shapes of curves obtained.

(b) A photodiode is fabricated from a semiconductor with a band gap of 2.8 eV. Can it detect a wavelength of

6000 nm? Justify. [3 + 2 = 5]

OR

(a) With the help of a circuit diagram, explain working of a transistor as an amplifier.

(b) Calculate the base current, collector current and collector to emitter voltage for the circuit given

RC= 2 kR

B= 300 k

C

B

VCC= 9 V

E = 50

29. (a) Prove that for a short magnetic dipoleaxial

equatorial2

BB .

(b) The susceptibility of magnesium at 300 K is 1.2 105. At what temperature will the susceptibility increase

to 1.8 105? [3 + 2 = 5]

OR

(a) State Biot-Savarts Law. Use it to obtain the magnetic field at an axial point, distancexfrom the centre of a

circular coil of radius a, carrying a current I.

(b) A galvanometer having 30 divisions has a current sensitivity of 20A/div. It has a resistance of 25. How will

you convert it into an ammeter of range 0 - 1 A?


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Physics (Code-A) Solutions of Sample Question Paper for Class XII

SECTION-A

1. No change, resistivity is material dependent.

2. Electric and magnetic fields. Transverse waves.

3. Diffraction of light occurs when size of obstacle (or aperture) is comparable to the wavelength of light used.

4. By Lenz law it is in clockwise direction.

5. When image is formed at distinct vision,

251 1 6

5

DM

f

When image is formed at infinity,

255

5

DM

f

6. KEmax

= eVSP

3 eV3 volt.

SPV

e

7. R- Remains unchanged

XL- Becomes doubled

XC- Becomes halved

8. Since dipole consists of equal and opposite charge hence net charge and thus net flux is zero.

SECTION-B

9. 80 F and 10 F are in parallel

Therefore, equivalent is 80 + 10 = 90 F

This is in series with C

Hence, equivalent

9040

90

C

C

72 FC

Physics (Theory) - Class XII (Code-A)

SOLUTIONS


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Solutions of Sample Question Paper for Class XII Physics (Code-A)

10. (a) Outside the shell,

enc

0

qE ds

2

0

4 q

E r

R r R>

204

qE

r

(b) Inside the shell, Take Gaussian surface

enc

0

qE ds

as enc 0q

r

R0E

11. The property of an electric circuit by virtue of which it opposes any change of flux or current in it by inducing acurrent in itself is called self induction. It is numerically equal to the flux linked when unit current flows through it.

i

Li

Li

SI unit is henry.

Consider, air cored solenoidI I

BAN

20ni r nl

2 20L n r l

i

12. Amperes circuital law :The magnetic circulation around any closed curve is equal to 0times the electric

current threading the curve.

Amperian curve

0 enclosedB dl I

Magnetic field due to a toriod

r

Currentin eachturn is I

0 enclosedB dl I

02B r NI

02

NIB r


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13. Power of lens is a measure of its ability to converge or diverge a light beam falling on it. Power of lens is defined

as the tangent of the angle by which the lens converges (or diverges) a beam of light falling on it at a unit distancefrom its optical centre.

tan h

f

when h= 1,1

tanf

Power of a lens1

tanPf

f

h F

Unit of power of lens is dioptre (D) or m1.

When lenses are in contact

eq 1 2P P P

14.

Magnification :

(a) For strained eye0

1e

L DM

f f

(b) For relaxed eye0 e

LDM

f f

15. (i) Metal :A solid is a conductor if its valence band overlaps its conduction band.

VB

CB


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(ii) Insulator :A solid is an insulator if the valence band and the conduction band do not overlap and are separatedby an energy gap between 3 eV and 6 eV.

VB

CB

Eg(~3 to 6 eV)

(iii) Semiconductor :A solid is a semiconductor if the energy gap is much smaller as compared to that in caseof insulators. Band energy gap is in the range (0.1 - 1.0 eV)

VB

CB

Eg(~0.1 to 1.0 eV)

16. (a) Here, L= 1 H,XL= 3142

XL= 2fL

f= 500 Hz

(b)1

0.10 F2C

X CfC

OR

15 FC

1212.3

2CX

fC

rmsrms 0.52 A

C

VI

X

SECTION-C

17. 0 3 ,4

Idl rdB B

r

directed outwards

024

IdldB

r

r

dl

I

0 024 2

Idl I B dB

r r

For half coil 04

IBr


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18. Capacitance of a conductor is defined as the ratio of charge on it to its potential

Numerically,Q

C

V

.

Numerically, capacitance of a conductor is equal to the amount of charge required to raise potential through unity.

0

E

VE

d

+++++++++

E

P

Q Q

0

dV

0AQ

C V d

On insertion of a conducting slab, capacitance of the conductor increases as 0ACd t

, tis thickness of the

conducting slab inserted between the plates.

19. Mobility is defined as the magnitude of drift velocity per unit electric field.

,de d eV eE

VE m

ee

e

m

(i) When temperature of the conductor decreases, e decreases and consequently e decreases.(ii)

e doesnt depend upon potential difference and thus it remains unchanged.

20. For a lens1 1 1

f v u

Since the object is real, u= x

Thus,vx

fv x

Further, as the distance between the object and the screen is d,

v d x ,

Thus,

d x xf

d x x

2 0x dx fd

2 4

2

d d fd x

For the image to be formed,xshould be real,

i.e. 1 4 0f

d

4d f When d< 4f, no image is formed.


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21. The emission of free electron from metal surface is called electron emission. It can take place through any of

following physical processes.

(i) Thermionic Emission :The release of electron from metal as a result of its temperature, i.e. by heating.

(ii) Field emission :It is a kind of electron emission in which a very strong electric field pulls the electron out of

metal surface.

(iii) Photoelectric Emission :It is that kind of electron emission in which light of suitable frequency ejects the

electrons from a metal surface.

Photoelectric effect equation max 0KE h h .

22. Phasor diagram,

VRI0

E0

VC

V L VC

VL

0sinE E t

L R C

0 0,R L LV I R V I X and 0C CV I X

22

0 R L CE V V V

22

0 0 L CE I R X X

Impedance 22

L CZ R X X

0I is max, when L C

12

2fL

fC

( )I0 max

I

r

1

2f

LC


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23. Modes of propagation

(i) Ground wave propagation :Travel along curved earths surface from transmitter to the receiver.

(ii) Sky wave propagation :Radio waves travel skywards and if its frequency is below certain critical frequency(typically 30 MHz). It is returned to the earth by ionosphere.

(iii) Space wave :In space wave propagation, radio waves travel in a straight line from transmitting antenna to therecieving antenna.

Sky wave :The sky wave below critical frequency travels from the transmitting antenna to recieving antennavia ionosphere. The ionosphere consists of layers of air molecules which have become positively charged byremoval of electrons by suns ultraviolet radiations. On striking the earth, the sky wave bounces back to theionosphere where it is again gradually refracted and returned earthwards as if by reflection. This continuesuntil it is completely attenuated.

24. In 5626Fe nucleus, there are 26 protons and 30 neutrons

Mass defect 562626 30 Fep nm m m

= 0.528461 u

Binding energy = (mass defect)c2

0.528461 931.5 MeV 492.26 MeV

BE 492.26 MeV8.76 MeV

Nucleon 56


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25. Consider the -decay of 23892U

238 234 492 90 2U Th He

Neutron-proton ratio before -decay =238 92

1.58792

Neutron-proton ratio after -decay =234 90

1.690

Thus, neutron-proton ratio increases during -decay

Nuclear Force :Force between nucleons is called nuclear forces. It is one of the four fundamental forces in

nature.

Properties :

(i) It is an attractive force

(ii) It is independent of the interacting nucleons

(iii) It is a short range force.

(iv) It is a non-central force.

OR

(a) 2

2

13.6eVn

ZE

n

For Li , 3Z

2

122.4eVnE

n

For transition from n= 1 to n= 3

3 1

hcE E E

114.2

(b) No. of spectral lines = 3

26. During solar flare, a large number of electrons and protons are ejected from the sun, some of them get trapped in

the earths magnetic field and move in helical paths along the field lines. The magnetic field lines come closer to

each other near the magnetic pole. Hence, density of charges increases near the poles. These particles collide

with atoms and molecules of the atmosphere. Excited oxygen atom emit green light and erected nitrogen atoms

emits pink light.


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SECTION-D

27. (a) Consider point Pon the screen, path difference

2 1x S P S P

2

2 22 2

dS P y D

2

2 21 2

dS P y D

Q

P

S

D

y

d

S1

S2

2 22 1 2S P S P yd

2 1 2 12

2

ydS P S P S P S P D

D

ydx

D

For constructive Interference,

x n yd

nD

0,1,....n D

y nd

For destructive interference

2 12

x n

2 1

0,1,....2

n Dy n

d

Hence, fringe widthD

d

(b) Fresnel distance,

2

F

aZ

40 mF

Z

OR

Huygens Principle :Every point on wavefront may be considered as a source that produces secondary wavelets.

These wavelets propagate in the forward direction with a speed equal to speed of wave motion. The surface whichtouches these wavelets at any later instant is the position of new wavefront, called secondary wavefront.

1 2

AD CE

v v

sin AD

iCD

from CAD

sin CE

rCD

from CED

1

2

sinsinvi AD CD

r CD CE v


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As 1 2

2 1

,vc

vv

2

1

sinsin

ir

sini = constant (Snells law)

Convex Lens

Refracted wavefrontIncident wavefront Refractedwavefront

Incidentwavefront

28. (a) V-Icharacterstic

VF(V)

VR (V)

IF (mA)

IR ( A)

MinorityCarriers Forward BiasReverse

Breakdown

Breakdown

MajorityCarriers

V (Knee Voltage)K

80

60

40

20

80 60 40 20

1

2

Forward Bias

Pn

V

Reverse Bias

Pn

V

(b)1242 eV nm

0.207 eV6000 nmg

hcE

Since gE E , the photodiode cannot detect wavelength of 6000 nm.


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OR

(a) Transistor as a common - emitter amplifier.

Circuit Diagram.

ac input ac output

VBB

BE

BIB

C IC

VCE

RL

VCCI

E

Operation :

(i) With no signal input

CE CC C LV V I R

(ii) With signal applied to the emitter base circuit for positive half cycle, increases the forward bias resultingin an increase in collector current. During negative half cycle, the input signal opposes the forward biasof the input circuit, thereby reducing the emitter and consequently the collector current.

dc current gain C

B

I

I

(b)3

9 V30 A

300 10

CC BE CC B

B B

V V VI

R R

(asBE CC V V )

50 30 A 1.5 mAC BI I

3 39 V 1.5 10 A 2 10 6 VCE CC C C V V I R

29. (a) Magnetic field at a point on Axial line : End on position

0

24Sm

Br a

0

24Nm

Br a

2a

r

m +m

S N

0

22 2

4

4P S Nm ra

B B Br a

Since r>> a

03

2

4PM

Br

Magnetic field at a point on equatorial line

0

2 24Nm

Br a

0

2 24S mB r a

SB

PB

NB

S N

P


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0

3/22 2

2

4P N Sam

B B Br a

r>> a

0

34PM

Br

axial equatorial2B B

(b) As mC

T

'

'm

m

T

T

' 200 KT

OR

(a) Biot Savarts law,

dB

the magnetic field at point P

02

sin

4

IdldB

r

r

dl

P

I

net 2 sindB dB

0net 2

2 sin4

IdldB

r

0net 2

24

Idl adB

r r

dB

dB

x

r

a

x

I0

net 3

2

4

IadB dl

r

20

net 3/22 2

IaB

a x

(b)420 A 30 div 6 10 A

div

gI

Shunt required 0.0151

g

GS

I

I

G

S

Ammeter

cbse sample paper class xii physics

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