# Physics: 2018: CBSE: [All India]: Set - III

To Access the full content, Please Purchase

• Q1

Which mode of propagation is used by short wave broadcast services?

Marks:1

Sky wave propagation: A radiowave directed towards the sky and reflected by the ionosphere towards the desired location on the earth is called a sky wave. Radiowavesof frequencies between2 MHz to 20 MHz can be reflected by the ionosphere. This range is also called as short wave band. This mode of propagation is used by short wave broadcast service.

• Q2

Four nuclei of an element undergo fusion to form a heavier nucleus, with release of energy. Which of the two - the parent or the daughter nucleus - would have higher binding energy per nucleon?

Marks:1

Nuclear fusion: The process in which two lighter nuclei combine to form a single heavier nucleus is called nuclear fusion.

The mass of the heavier nucleus formed is less than the sum of masses of the combining nuclei.

As, mass defect is more in daughter nuclei so binding energy per nucleon is more in daughter nuclei.

• Q3

Name the electromagnetic radiations used for (a) water purification, and (b) eye surgery.

Marks:1

(a) For water purification, ultra violet (uv) rays is used.

(b) For eye surgery, infrared rays is used.

• Q4

Draw graphs showing variation of photoelectric current with applied voltage for two incident radiations of equal frequency and different intensities. Mark the graph for the radiation of higher intensity.

Marks:1

• Q5

A proton and an electron travelling along parallel paths enter a region of uniform magnetic field, acting perpendicular to their paths. Which of them will move in a circular path with higher frequency?

Marks:1

As we know that when the charge particle enters into perpendicular magnetic field it moves along a circular path.

Since, electron has less mass so, electron will move with high frequency.

• Q6

A carrier wave of peak voltage 15 V is used to transmit a message signal. Find the peak voltage of the modulating signal in order to have a modulation of 60%.

Marks:2

• Q7

If light of wavelength 412.5 nm is incident on each of the metals given below, which ones will show photoelectric emission and why?

 Metal Work Function (eV) Na 1.92 K 2.15 Ca 3.20 Mo 4.17

Marks:2

Thus, metals having work-function less than the energy of photons of incident light will show photoelectric effect.

So Na and K will show photo electric effect.

• Q8

A 10V cell of negligible internal resistance is connected in parallel across a battery of emf 200V and internal resistance as shown in the figure. Find the value of current in the circuit.

Marks:2

Applying Kirchoff's loop ABCDA

• Q9

In a potentiometer arrangement for determining the emf of a cell, the balance point of the cell in open circuit is 350 cm. When a resistance of is used in the external circuit of the cell, the balance point shifts to 300 cm. Determine the internal resistance of the cell.

Marks:2

The internal resistance of the cell,

Here,

• Q10

(a). Why are infra-red waves are often called heat waves? Explain.

(b). What do you understand by the statement, "Electromagnetic waves transport momentum"?

Marks:2

(a) Infra-red waves have frequencies lower than those of the visible light. They have the ability to vibrate not only the electrons but the entire atom or molecules of a body. This vibration increases the internal energy and temperature of the body.

(b) As electromagnetic wave contains both electric and magnetic fields, if charges are present they will have sustained motion by electric and magnetic fields of the em wave. The charges thus acquire energy and momentum from the waves. This just illustrates the fact that an electromagnetic wave carries energy and momentum.

If total energy transferred to a surface in time t is u, it can be shown that the magnitude of total momentum delivered to the surface

where, p is the momentum.

Electromagnetic wave transfer momentum but as is very large so,the amount of momentum transferred is extremely small.