 # Physics: 2017: CBSE: [Delhi]: Set – III

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• Q1

A long straight current carrying wire passes normally through the centre of circular loop. If the current through the wire increases, will there be an induced emf in the loop? Justify.

Marks:1

There is no change in the magnetic flux linked with the circular loop so there will be no induced emf in the loop. This happened because magnetic field lines are parallel to the plane of the loop.

The angle between the field line and the area vector is 90o and we know that

• Q2

How is the speed of em-waves in vacuum determined by the electric and magnetic fields?

Marks:1

The ratio of peak value of electric and magnetic fields gives the speed of the electromagnetic waves, c in vacuum which is given as

• Q3

Name the junction diode whose I–V characteristics are drawn below:

Marks:1

The given I-V characteristics is of a solar cell.

• Q4

At a place, the horizontal component of earth’s magnetic field is B and angle of dip is 60˚. What is the value of horizontal component of the earth’s magnetic field at equator?

Marks:1

The horizontal component of the magnetic field is given by

• Q5

Does the charge given to a metallic sphere depend on whether it is hollow or solid? Give reason for your answer.

Marks:1

All the charge given to the spheres whether it is hollow or solid will appear on the outer surface of the spheres, so the charge given to a metallic sphere does not depend on whether it is hollow or solid.

• Q6

Define the distance of closest approach. An -particle of kinetic energy ‘K’ is bombarded on a thin gold foil. The distance of the closest approach is ‘r’. What will be the distance of closest approach for an α-particle of double the kinetic energy?

Marks:2

Distance of closest approach: When an alpha particle moves towards the nucleus, its kinetic energy keeps on decreasing because kinetic energy is being consumed in doing work against the force of repulsion of the nucleus. The distance from the nucleus at which velocity of an alpha particle becomes zero is known as the distance of closest approach.

Distance of closest approach,r0 is given as

• Q7

Write two important limitations of Rutherford nuclear model of the atom.

Marks:2

Following are the two important limitations of Rutherford nuclear model of the atom:

1. According to the classical electromagnetic theory, the revolving electron should radiate energy. An electron pursuing a curved path is accelerated and therefore, it should continuously loose energy while rapidly spiralling into the nucleus.

2. It could not explain the emission line spectrum of the hydrogen atom. As per classical electromagnetic theory, the frequency of EM waves emitted by revolving electron should be equal to the frequency of revolution.Thus, it would emit a continuous spectrum, in contradiction to the observed line spectrum.

• Q8

When are two objects resolved? Explain. How can the resolving power of a compound microscope be increased? Use relevant formula to support your answer.

Marks:2

Two point objects are said be just resolved when the central maximum of the diffraction pattern of one lies on the first secondary minimum of the other.

If two point objects placed close to each other are seen through optical instruments, their images or their diffraction patterns might overlap each other.If the overlapping is small, both the point objects are seen as separate and the instrument is said to resolve the two point objects.But if the overlapping is large, the two point objects are not seen as separate and the optical instrument is not able to resolve the two objects.

The formula for resolving power of compound microscope is given by .

The resolving power of the compound microscope can be increased by

1. Decreasing the wavelength () of the light used.

2. Increasing the refractive index (n) of the medium.

• Q9

i. What is line of sight communication?

ii. Why is it not possible to use sky waves for transmission of TV signals? Upto what distance can a signal be transmitted using an antenna of height ‘h’?

Marks:2

(i)Line of sight communication is a type of propagation that can transmit and receive data only where transmit and receive stations are in view of each other without any sort of an obstacle between them. Few examples of line of sight communication are FM radio, microwave and satellite transmission.

(ii)Sky wave propagation is used for short wave broadcast services. It makes communication easier over great distances using simple equipments. It cannot be used for transmission of TV signals, as waves with frequency more than 40 megahertz cannot be reflected back to the earth by ionosphere. Transmission of TV signal is done with the help of radio waves of frequency ranging between 80 MHz and 200 MHz.

The covering range (d) for an antenna of height 'h' is , where R is radius of the earth.

• Q10

An α -particle and a proton are accelerated through the same potential difference. Find the ratio of their de Broglie wavelengths.

Marks:2

We know that,

Charge on the proton = e

Charge on the alpha particle = 2e

Let mass of proton = m

So, mass of alpha particle = 4m

When a particle is of mass m and charge q is accelerated by a potential V, then its de Broglie wavelength is given by