CBSE Class 12 Marking Scheme 2021 for Physics

Class –XII

PHYSICS (Theory)

SQP Marking Scheme 2020-21

Sr. VALUE POINTS Marks
No.
1 Magnetic dipole moment 1

2 Any one use of micro waves 1

OR

1:1

3 zero 1

4 Remains same 1

OR

7.707A, 50Hz ½+½

5 h/2 1

6 4eV 1

7 Antinutrino 1

OR

Electron

8 Decreases 1

OR

25Hz

9 Dynamic resistance =change in voltage/change in current=1ohm 1

10 Photodiode 1

11 a) Both A and R are true and R is the correct explanation of A 1

12 b)Both A and R are true and R is the correct explanation of A 1

13 c)A is true but R is false 1

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14 b) Both A and R are true but R is NOT the correct explanation of A 1

15 1.c) Copper 4x1 =
4
2.a) car

3.c) zero

4.a) -q

5 b)1.9105 Nm2/C leaving the surface

(any 4 parts to be attempted)

16 1. b) Its critical angle with reference to air is too small 4x1=4

2. a) 2.42

3. c) high refractive index
4. d) increase

5. d) less than first

(any 4 parts to be attempted)

17 Explanation by showing magnetic field directions in all three regions 1
Concluding left of region 1 1

18 Plot of Intensity distribution of diffraction with proper labeling 2

OR

n/d=2/a
n=2d/a ,where d is separation between slit and a width of slit

19 Derivation including both terms electrostatic energy in system and in 1+1
external field

OR

Derivation of relation E=-dV/dr 1+1
Diagram of equipotential surfaces

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20 Circuit diagram showing biasing of LED in F.B ½
Action of LED 1
For emission in visible range least band energy required is1.8eV ½

21 Calculation of magnetic flux =BA cos ,where =300 =√3/2^10-11Wb 1
Calculation of induced emf E=A cosdB/dt=0.5V 1

22 Path difference=3/2 1
Putting value we will get=3cm 1

23 Well labeled energy band diagram of n-type semiconductor 1

n-type semiconductor ½

electrons-majority charge carriers ½

24 Definition of each term ½+½
Diagram showing relation 1

OR

Bv/BH=tan 1
Putting values,=300 1

25 Two characteristics- virtual and enlarged image and same side of 1
object.
As u and v both negative, we get 1/v=1/u-1/f 1
Interpret y=mx+c ,plot of the graph

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26 ½
for
each
plot

Induced current and power, sketch is same as shown above. 1
In case of circular coil, rate of change of area of the loop during its
passage out of field is not constant, hence induced current varies
accordingly.

½

t

27 Circuit diagram 1

1

1
Maximum current drawn will be at R=0

OR
1
Circuit diagram
½
Applying correct formula
½
And caluation of p.d=11.5V

Series resistor limits the current drawn from source 1

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28 De-Broglie reasoned out that nature was symmetrical and two basic 1
physical entities –mass and radiation must be symmetrical.If radiation
shows shows dual aspect than matter should do so.

De-Broglie equation- 1
=h/P
For photon –
P=h/C
Therefore,h/P=C/=
As =h/√2mk

So,alpha particle will be having shortest de-Broglie wavelength 1
compared to deutrons.

OR

Main implications-
1. kinetic energy of emitted electrons depends upon frequency,but not
on intensisty of radiation 1
2.there exist a frequency of radiation below which no photoemission
takes place, how high intensity of radiation may be. 1
Explanation wave nature of radiation fails to explain photoelectric
effect 1

29 Derivation of frequency of radiation emitted when a hydrogen atom de 2
excites from level n to level (n-1).

 = me4 (2n-1) / (4)3 (h/2)3 n2 (n-1)2 1
Comparing for large values of n, with classical freuency  = v / 2r

30 One difference between nuclear fission and nuclear fusion 1

Calculating Q=((m) Fe-2(m) Al)C2 =26.90MeV 1
Justification not possible 1

31 (a) Statement of Gauss law 1

Proof of outward flux due to a point charge Q ,in vacuum within 2
gaussian surface, is independent of its size and shape

(b) Net electric field towards left=/ left 1

Net electric field towards right=/ right 1

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OR

Definitionof ideal dipole +example ½+
½

Derivation of torque 2

Putting values in correct formula and solving,value of charge and 1
potential energy
Q=810-3 C 1
U=-8J

32 (a) Derivation of instantaneous current i=iosin (ωt +/2) 1

Reactance XC=1/C 1

Phasor diagram showing v and i relation in pure C 1

(b) Explanation that adding R it will behave RC series ac circuit 1+1
Calcuation of current and phase angie

OR

(a)Principle of ac generator 1
(b)Well labelled diagram 1
Brief working and emf expression
2
(c) reason
1
33 (a) Definition of wavefront 1

(b) Ray diagram showing shapes of wavefront

1

1

(c) Proof of Snell’s law 2

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OR

(a) choice of objective 1

(b) ray diagram of reflecting type telescope 2+1
Formula of magnifying power

(c) stating two advantages 2

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