 # CBSE [All India]-Set-3-2015

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• Q1

The line AB in the ray diagram represents a lens. State whether the lens is convex or concave.

Marks:1

The lens is concave in nature as it slightly diverges the path of the refracted light.

• Q2

Distinguish between emf and terminal voltage of a cell.

Marks:1

 Emf Terminal voltage It is the maximum potential difference that can be delivered by a cell when no current flows through the circuit. It is the potential difference across the terminals of the load when the circuit is switched on and current flows through it. It is represented by E and remains constant for a cell. It is represented by V and depends on the internal resistance of the cell.

• Q3

Draw a graph to show variation of capacitive-reactance with frequency in an a.c. circuit.

Marks:1

Here, X
C is the capacitive reactance and v
is the frequency in an a.c. circuit.

• Q4

What is the function of a 'Repeater' used in communication system?

Marks:1

A repeater is used to increase the range of the transmission in communication systems with the help of a set of receiver and transmitter.

• Q5

The field lines of a negative point charge are as shown in the figure. Does the kinetic energy of a small negative charge increase or decrease in going from B to A?

Marks:1

From the given field lines, we can say that the electric field present due to the given point charge will be directed towards the centre. Since, we know that a negative charge always experiences a force in the direction opposite to that of the external electric field present, the negative charge will experience the force away from the centre. This will cause its motion to retard while moving from B to A. Hence, its kinetic energy will decrease in going from B to A.

• Q6

The equivalent wavelength of a moving electron has the same value as that of a photon of energy 6 x 10–17 J. Calculate the momentum of the electron.

Marks:2

According to the de Broglie hypothesis, the momentum of an electron is given by:

• Q7

What is ground wave communication? Explain why this mode cannot be used for long distance communication using high frequencies.

Marks:2

Ground waves are the radio waves that travel along the surface of the Earth. In ground wave propagation, a large portion of wave energy is in space near the surface of the Earth. The propagation of the wave is guided along the Earth's surface and follows the curvature of the Earth.
The propagation of high frequency wave is not possible through ground waves for long distance communication because while progressing, ground waves induce current in the ground and bend round the corner of the objects on the Earth due to which the energy of the ground waves of high frequency is almost absorbed by the surface of the Earth after travelling a small distance. Such signals are also absorbed by the obstacles (like mountains, tall buildings and trees) between the transmitter and the receiver. This loss in power of ground waves increases with the increase in frequency. Thus, ground wave communication is not suited for high frequency.

• Q8

A ray of light passes through an equilateral glass prism such that the angle of incidence is equal to the angle of emergence and each of these angles is equal to 3/4 of angle of prism. Find the angle of deviation.

Marks:2

The angle of deviation for a ray of light in a prism is given by:

• Q9

Calculate the speed of light in a medium whose critical angle is 45°. Does critical angle for a given pair of media depend on the wavelength of incident light? Give reason.

Marks:2

• Q10

How does one explain, using de Broglie hypothesis, Bohr's second postulate of quantization of orbital angular momentum?

Marks:2

According to de-Broglie hypothesis, a stationary orbit is the one that contains an integral number of de-Broglie waves associated with the revolving electron.
For an electron revolving in nth circular orbit
Total distance covered by electron = Circumference of the orbit
For the permissible orbit, we have

where, λ is the wavelength.
According to de-Broglie, wavelength of matter waves is given by

where,

h = Planck's constant

m = mass of electron

vn = speed of electron in nth orbit

This is Bohr's second postulate of quantization of orbital angular momentum.