 CBSE Delhi-Set-2-2016

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• Q1

Write the underlying principle of a moving coil galvanometer.

Marks:1

When a current-carrying coil is placed in a magnetic field, it experiences a torque. This is the underlying principle of a moving coil galvanometer.

• Q2

Why are microwaves considered suitable for radar systems used in aircraft navigation?

Marks:1

Microwaves are considered suitable for radar system used in aircraft navigation because they have a short wavelength range from 10-3 m to 0.3m, which makes them suitable for long distance communication.

Another reason for using microwaves is that they can penetrate through clouds also.

• Q3

Define’ quality factor’ of resonance in series LCR circuit. What is its SI unit?

Marks:1

The Q factor of series resonance circuit is defined as the ratio of the voltage developed across the inductor or capacitor at resonance to the impressed voltage, which is the voltage across R

It is dimensionless hence, it has no units.

• Q4

A point charge +Q is placed at point O as shown in the figure. Is the potential difference VA – VB positive, negative or zero?

Marks:1

• Q5

How does the electric flux due to a point charge enclosed by a spherical Gaussian surface get affected when its radius is increased?

Marks:1

According to Gauss’s law, flux through a closed surface is given by

Where q is the charge enclosed by the Gaussian surface.

Since, on increasing the radius of the Gaussian surface, charge, q remains unchanged, so the flux through the spherical Gaussian surface will not be affected when its radius is increased.

• Q6

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

Marks:2

The binding energy of the nucleus of mass number 240 is, B1 = 7.6 x 240 = 1824 MeV

The binding energy of each product nucleus is, B2 = 8.5 x 120 = 1020 MeV

The energy released as the nucleus breaks is

E = 2B2 – B1 = 2 x 1020 – 1824 = 216 MeV

Alternatively: Gain in binding energy for nucleon is about 0.9 MeV.

Hence, the total gain in binding energy

DE = 240 × 0.9

DE = 216 MeV.

• Q7

Calculate the energy in fusion reaction.

Marks:2

• Q8

Two cells of emfs 1.5 V and 2.0 V having internal resistances 0.2 Ω and 0.3 Ω  respectively are connected in parallel. Calculate the emf and internal resistance of the equivalent cell.

Marks:2

Given,

E1 = 1.5 V

E2 = 2 V

r1 = 0.2 Ω

r2 = 0.3 Ω

Effective EMF of two cells connected in parallel can be calculated as:

• Q9

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

Marks:2

Brewster law : The law states that the tangent of the polarising angle of incidence of a transparent medium is equal to its refractive index.

μ = tan ip

The refractive index of a material depends on the color or wavelength of light. As the polarizing angle depends on refractive index (μ = tan ip), so it also depends on wavelength of light.