CBSE DelhiSet22015
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Q1
What is the electric flux through a cube of side 1 cm which encloses an electric dipole?
Marks:1Answer:
According to Gauss’s Law
Where,
E = Electrostatic field
Q = total charge enclosed by the cube
= Absolute electric permittivity of free space
In case of a dipole, net charge is equal to zero; therefore, electric flux through the cube is also zero.

Q2
A concave lens of refractive index 1.5 is immersed in a medium of refractive index 1.65. What is the nature of the lens?
Marks:1Answer:
If a concave lens is immersed in a medium with the refractive index greater than its own refractive index then it will behave as a diverging lens.

Q3
How are side bands produced?
Marks:1Answer:
During the process of modulation, the audio frequency modulating signal wave is superimposed on a high frequency wave called carrier wave. Any form of modulation produces frequencies that are the sum and difference of the carrier and the modulating frequencies. These frequencies are called side bands. Side bands are a band of frequencies higher than or lower than carrier frequency.

Q4
Graph showing the variation of current versus voltage for a material GaAs is shown in the figure. Identify the region
(i) Of negative resistance
(ii) Where Ohm’s law is obeyed
Marks:1Answer:
(i) The slope of the curve is negative in the region DE; hence, DE represents the area of negative resistance.
(ii) According to Ohm’s law, current varies linearly with voltage. Thus, BC represents the region where Ohm’s law is obeyed.

Q5
Define capacitor reactance. Write its S.I. units.
Marks:1Answer:
Capacitor reactance is the resistance offered by a capacitor when connected to an electric circuit. It is denoted by X_{c }and is given by the expression:
whereω = Angular frequency
C = Capacitance of the capacitor

Q6
Show that the radius of the orbit in hydrogen atom varies as n^{2}, where n is the principle quantum number of the atom.
Marks:2Answer:
Centripetal force required by an electron of mass m and velocity v during its orbit is given by
Also, when the charge on the nucleus is Ze, the centripetal force due to force of electrostatic attraction between the electron and nucleus is given by:

Q7
Distinguish between ‘intrinsic’ and ‘extrinsic’ semiconductors.
Marks:2Answer:
Intrinsic Semiconductors
Extrinsic Semiconductors
 These types of semiconductors have no impurity atoms and hence, are pure.
These types of semiconductors are prepared by doping a small amount of impurity atoms.
 The number of free electrons in conduction band is equal to the number of the holes in the valence band.
The number of electrons and holes are never equal. The number of electrons is greater than number of holes in ntype semiconductor whereas number of holes is greater than electrons in ptype semiconductors.

Q8
Use the mirror equation to show that an object placed between f and 2f of a concave mirror produces a real image beyond 2f.
Marks:2Answer:

Q9
Find an expression for intensity of transmitted light when a polaroid sheet is rotated between two crossed polaroids. In which position of the polaroid sheet will the transmitted intensity be maximum?
Marks:2Answer:
Let the two polaroids P_{A} and P_{B} cross each other at an angle of 90˚. A polaroid sheet P_{C} is placed between them such that it makes an angle θ with Polaroid P_{A}.
Then, intensity of polarized light emerging from P_{C }is given by
I = I_{0 }cos^{2}θ
As P_{A} and P_{B} are perpendicular to each other, angle between P_{B} and P_{C} = (90  θ)
Therefore, intensity of light emerging from PB is given by
I = [I_{0 }cos^{2}θ] cos^{2 }(90  θ)
= [I_{0 }cos^{2}θ] sin^{2}θ
So, the intensity of light is maximum when
sin2θ = 1
2θ = 90˚
θ = 45˚
The maximum intensity of light is expressed as

Q10
Use Kirchhoff’s rules to obtain conditions for the balance condition in a whetstone bridge.
Marks:2Answer:
Let us obtain the balance condition for the arrangement given below.