Q20 of 26 Page 1

(a) Distinguish between point to point and broadcast modes of communication. Give an example of each.

(b) Explain the basic concept of mobile telephoning.


The distinguish between point to point and broadcast modes of communication are as follows:



(b) The telephonic service of phones that can be move around freely and not fixed at a single point is known as the mobile telephony. The terrestrial cellular network of the base stations connects the mobile phones. The satellite phones are connected by the orbiting satellite not the terrestrial network.


The satellite phones and the mobile phones are interconnected to the public switched telephone network. This network allows all the phones in the world to be dialled.


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18

(i) Define ‘activity’ of a radioactive substance.

(ii) Two different radioactive elements with half-lives T1 and T2 have N1 and N2 undecayed atoms respectively present at a given instant. Derive an expression for the ratio of their activities at this instant in terms of N1 and N2


 19

Plot a graph showing the variation of photoelectric current with intensity of light. The work function for the following metals is given: Na: 2.75 eV and Mo: 4.175 eV.

Which of these will not give photoelectron emission from a radiation of wavelength 3300 Å from a laser beam? What happens if the source of laser beam is brought closer?


OR


Define the term “cut off frequency” in photoelectric emission. The threshold frequency of a metal is f. When the light of frequency 2f is incident on the metal plate, the maximum velocity of photo-electrons is v1. When the frequency of the incident radiation is increased to 5f, the maximum velocity of photo-electrons is v2. Find the ratio v1: v2.


 21

Two identical coils P and Q each of radius R are lying in perpendicular planes such that they have a common centre. Find the magnitude and direction of the magnetic field at the common centre of the two coils, if they carry currents equal to I and 3 I respectively.


 22

(i) Define self-inductance. Write its SI units.

(ii) A long solenoid with 15 turns per cm has a small loop of area 2.0 cm2 placed inside the solenoid normal to its axis. If the current carried by the solenoid changes steadily from 2.0 A to 4.0 A in 0.1 s, what is the induced emf in the loop while the current is changing?