Physics

CBSE Class 12

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11.
(a) Calculate the distance of an object of height h from a concave mirror of radius of curvature 20 cm, so as to obtain a real image of magnification 2. Find the location of image also.

(b) Using mirror formula, explain why does a convex mirror always produce a virtual image.

(b) Using mirror formula, explain why does a convex mirror always produce a virtual image.

a)

Given,

Radius of curvature, R = 20 cm

So, focal length, f = R/2 = - 10 cm

Since the image obtained is real, therefore magnification of the image, m = -2

Now, using the formula,

Therefore, the distance of the object is 15 cm in front of the mirror and the position of the image is 30 cm, formed in front of the mirror.

b)

For a convex mirror,

Focal length, f > 0

Position of the object, u < 0

Using mirror formula, we have

That is, the image formed by a convex lens is always behind the mirror and hence is virtual.

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12.
A nucleus with mass number A = 240 and BE/A = 7.6 MeV breaks into two fragments, each of A = 120 with BE/A = 8.5 MeV. Calculate the released energy.

OR

Calculate the energy in the fusion reaction:

OR

Calculate the energy in the fusion reaction:

The B.E. of the nucleus of mass number 240, B_{1} = 7.6 x 240 = 1824 MeV

The B.E of each product nucleus, B_{2} = 8.5 x 120 - 1020 MeV

Then, the energy released as the nucleus breaks is given by,

E = 2B_{2} - B_{1} = 2 x 1020 - 1824 = 216 MeV

OR

Given:

B.E of

B.E of

Energy in the fusion reaction is given by,

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13.

(a) Explain any two factors that justify the need of modulating a low-frequency signal.

(b) Write two advantages of frequency modulation over amplitude modulation.

Modulating low-frequency signals are required because:

i) Size of the antenna: Antenna is required for both transmission and reception of the signal whose size (at least ) should be comparable to the wavelength of the signal so that time variation of the signal can be properly sensed by the antenna. For a low-frequency signal, the wavelength is large. Therefore, a large antenna of such a huge height is practically impossible to construct and operate.

Hence, the need for high-frequency transmission arises.

ii) Overlapping signals from different transmitters: When an information is transmitted using different transmitters, the signals get overlapped and the information is lost. Hence, high-frequency transmission is required. Each band should be allocated to each frequency range in order to avoid mixing of signals.

iii) Higher energy: High-frequency signals have high energy and therefore, even after loss due to attenuation, signals can be carried over longer distances.

b)

Advantage of frequency modulation over amplitude modulation are:

i) FM is more efficient than AM because the amplitude of an FM wave is constant, irrespective of its modulation index. Thus, the transmitted power is constant. Also, in AM the transmitted power goes waste in the transmitted carrier.

ii) FM reception is quite immune to noise in comparison to AM reception. Noise is a form of amplitude variation in the transmitted signal. Thus, using amplitude limiters in the FM receivers, the noise is eliminated.

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14.
Write three characteristic features in photoelectric effect that cannot be explained on the basis of wave theory of light, but can be explained only using Einstein's equation.

The three characteristics of photoelectric effect, which cannot be explained on the basis of wave theory of light are:

1. For a given metal and frequency of incident radiation, the number of photoelectrons ejected per second is directly proportional to the intensity of the incident light.

2. For a given metal, a certain minimum frequency of the incident radiation below which no emission of photoelectrons take place. The is the threshold frequency.

3. Above the threshold frequency, the maximum kinetic energy of the emitted photoelectron is independent of the intensity of the incident light and is dependent only upon the frequency or wavelength of the incident light.

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The value of Brewster's angle for a transparent medium is different for light of different colours. Give reason.

Brewster discovered a relation between polarising angle, i

The law is given by,

When a light is incident on a transparent surface at the polarising angle, the reflected and the refracted rays are perpendicular to each other.

The lights of different colours have different wavelength hence, there are different values of refractive index for a medium. That is why these lights have different values of brewster's angles.

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16.
How are em waves produced by oscillating charges?

Draw a sketch of linearly polarised em waves propagating in the Z-direction. Indicate the directions of the oscillating electric and magnetic fields.

OR

Write Maxwell's generalisation of Ampere's circuital law. Show that in the process of charging a capacitor, the current produced within the plates of the capacitor is I = , where is the electric flux produced during charging of the capacitor plates.

Draw a sketch of linearly polarised em waves propagating in the Z-direction. Indicate the directions of the oscillating electric and magnetic fields.

OR

Write Maxwell's generalisation of Ampere's circuital law. Show that in the process of charging a capacitor, the current produced within the plates of the capacitor is I = , where is the electric flux produced during charging of the capacitor plates.

A moving charge produces both electric and magnetic fields, and an oscillating charge produces oscillating magnetic and electric fields. These oscillating electric and magnetic fields with respect to space and time produce electromagnetic waves.

The propagation of electromagnetic waves can be shown as:

OR

Maxwell's generalization of ampere's circuital law given by,

Consider that a parallel capacitor C is charging in a circuit.

The magnitude of electric field between the two plates will be,

, is perpendicular to the surface of the plate.

Electric flux through the surface will be,

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17.
Two long, straight, parallel conductors carry steady currents, I_{1} and I_{2} , separated by a distance . If the currents are flowing in the same direction, show how the magnetic field set up in one produces an attractive force on the other? Obtain the expression for this force. Hence, define one ampere.

Magnetic field induction at some point P on wire 2 due to current I_{1} passing through wire 1 is given by,

Magnetic field is produced by wire 1 and current carrying wire 2 lies in magnetic field B_{1}.

The unit length of wire 2 will experience a force, given by

F_{2 }= B_{1} I_{2} x 1 =

According to Fleming's left-hand rule, the force on wire 2 acts in the plane of paper perpendicular to wire 2, directed towards wire 1. Similarly, wire 1 also experiences the same force towards wire 2. Thus, both the conducting wires attract each other with the same force F.

One ampere can be defined as the amount of current flowing through two parallel conductors, which are in the same direction or opposite directions, placed at a distance of one metre in free space, and both the wires attract or repel each other with a force of 2 x 10

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18.
(i) Write the functions of three segments of a transistor.

(ii) Draw a circuit diagram for studying the input and output characteristics of a n-p-n transistor in common emitter configuration. Using the circuit, explain how input, output characteristics are obtained.

(ii) Draw a circuit diagram for studying the input and output characteristics of a n-p-n transistor in common emitter configuration. Using the circuit, explain how input, output characteristics are obtained.

i) The function of three segments of a transistor are:

Emitter: Emits the majority charge carriers

Collector: Collects the majority charge carriers

Base: Base provides the interaction between the collector and the base

ii) An n-p-n transistor is as shown below:

Input characteristics: The variation between the base current and the base-emitter voltage is obtained. Input characteristics is used to find input dynamic resistance of the transistor as it is represented by the slope.

Output characteristics: A graph representing the variation of the collector current and emitter voltage is obtained, keeping base current fixed. The slope fo the output characteristic graph gives us the output dynamic resistance.

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19.

Explain the terms (i) Attenuation and (ii) Demodulation used in Communication System.

(i) Attenuation: Attenuation is the process of loss in the strength of signal while propagating through a medium.

(ii) Demodulation: Demodulation is the process of retrieval of information from the carrier wave at the receiver end. It is the reverse process of modulation.

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20.
Plot a graph showing variation of de Broglie wavelength λ versus , where V is the is accelerating potential for two particles A and B, carrying the same charge but different masses m_{1} and m_{2} (m_{1} > m_{2}). Which one of the two represents a particle of smaller mass and why?

Given,

Charge (q) is the same but the mass of both particles is different (m_{1} > m_{2}).

The De-broglie wavelength given by,

The slope of the graph of λ versus =

The slope of the smaller mass is larger; therefore, plat A in the above graph represents mass m_{2}.

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