KENDRIYA VIDYALAYA SANGATHAN, CHENNAI REGION

Class-XII Common Pre-Board Examination

Physics(Theory)-Marking Scheme

1. Potentiometer because it has infinite resistance. ie, it does not draw any current from the cell at null point (1)

2. As r =

qB

mv

=

qB

p

r α p ( as q and B are have same values for both the particles (1/2) Thus re : rp = 1:1 (1/2)

3. Short wave band means higher frequency of transmission. Higher the frequency longer is the distance (1)

4. No influence (1)

5. No (1/2)

Energy depends on amplitude and frequency and not on speed (1/2) 6. (a) Since Q = CV, It doubles (1/2)

(b) Since UE = 1 2 CV2, it quadruples (1/2)

7. Correct diagram (1)

8. (a) Which draw Irms = 10A, costs more to operate as it consumes more power. Since Irms for 12A is

2

12

= 8.5 A. (1)

9. K.E = h Υ – hΥ0 (1)

(i) Correct definition (1/2)

(ii) Correct definition (1/2)

10. Refer NCERT text book Vol – II Page No.322 fig b & c (1 + 1) 11. Definition (1)

Energy stored in the Inductor u = 1 2 LI2 (1)

12. Let x be the capacitance of each capacitors.

In series

c

1

=

x

1

+

x

1

+ +n (terms) (1)

c

1

= n.

x

1

x = nc

In parallel y=ne + nc + +n (terms)

=n2c (1)

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13. Power due to the force p = . V (1/2)

p = F.V. cos900 (1/2)

= 0 (1)

(or)

λ =

h

2

(with proper substitution of values) (1)

λ1

λ2

= 4 V2

V1

= 8 = 2 2

1

(1)

14. r1 = ( V

V )R = (1.5 1.4

1.4 )8.5 (1 1 2)

r1 =0.61 Ὠ ( 1 2 )

r=2 r1 = 2 x 0.61 = 1.22 Ὠ (1/2)

15.

16. Current gain β = 25 (1)

Voltage gain Av = β R

R

RL = 6k Ὠ (1)

17.

Binding energy per nucleon of 1H2 = 1.1 MeV

Total B.E of two 1H2 = 2.2 MeV

Total B.E of two deutron nuclei = 2 x 2.2 = 4.4 MeV (1) B.E per nucleon of 2He4 = 7MeV

Total B.E. of helium 2He4 = 4 x 7 = 28 MeV (1)

Energy released in fission = 28 – 4.4

= 23.6 MeV (1)

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18. For the objective

1

0

= 1

0

- 1

0

(1/2)

0 = 7.2 cm (1/2)

h y

1

= 1

- 1

(1/2)

= 2.27 (1/2)

Separation between objective and the eyepiece = V0 + (ue ) = 9.47 cm M.P =

[ 1 +

] = 88 (1)

19. For correct definition (1)

For correct unit (1/2)

Mass number of D, A = 180

Atomic number of D, Z = 72 (1 1 2 )

20. For Correct Definition (1)

μ =

(1)

μ =

+ (1)

OR

For correct Definition of Space wave propagation (1) Used in TV, RADAR, Microwave communication (1)

The range is maximum when the two antenna have a height h/2 each (1) 21. For correct Principle (1)

For Correct diagram (1)

1

2

= 1

2

(1)

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22. For correct Principle (1)

For Correct diagram (1)

Working (1)

23. The rays will emerge out of the face ac for which i < C where C is the critical angle (1/2)

(1 1 2)

aμg = 1

here i=450 aμg = 2 = 1.41 (1/2)

Thus the condition is aμg < 1.41

only Red Ray will emerge out. (1/2)

24. (a) Magnetic field must oscillate along

ie ( Z – axis) because = ( x ) (1 1 2)

(b) 0

0

= c (speed of light in vacuum ) (1 1 2)

25. Biot – savart’s law (1)

Diagram (1/2)

For correct derivation (1 1 2 )

26. (a) Electromagnetic Induction (1)

(b)No. Because refrigerator is a heat liberating agent (1)

(i) Creating awareness, presence of mind covrage (1) www.ncerthelp.com

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27. Circuit Diagram (1/2)

Phasor diagram (1/2)

For derivation of Impedance (2)

(1)

Resonance in the circuit (1)

(or)

For correct labelled diagram (1)

Principle (1)

Construction (1)

Workings (2)

28. Assumptions (1/2)

Sign conventions (1)

Diagram (1/2)

Correct derivation and lens maker’s formula (3)

(or)

Correct answers of wave front and ray (1/2)

(i) Correct diagram (1/2)

(ii) Correct diagram (1/2)

Principle (1)

Diagram (1)

For proving (1 1 2 )

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29. (a) Correct circuit diagram (2)

Input wave form (1/2)

Output wave form (1/2)

β = ΔI

ΔI, IE = IB +IC

Ic = βIb

IB = I

1+β = 0.1 mA (1)

Ic = 59 x 0.1 = 5.9 mA (1)

(or)

(a) Correct diagram

Metals (1)

Insulators (1)

Semiconductors (1)

(b) Correct answer (1)

No it cannot detect (1)

[hΥ has to be greater than Eg ]

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