NCERT Exemplar for Class 12 Physics Chapter 15

SimplyAcad has provided the NCERT Exemplar for Class 12 Physics below to help students learn all the chapters in a detailed manner. The exemplar will allow students to gain deep insights of all the sections and prepares you for the upcoming examination. Physics is an interesting subject which requires attention to minor details, hence, completing exemplars will be an effective way to increase your marks. The given exemplar contains a total of 18 questions. Students can access the NCERT exemplar for class 12 Physics Chapter 15 Communication Systems by scrolling below. Along with this, there are several NCERT exemplar for class 12 science of all the chapters provided on this platform.

Access the NCERT Exemplar Class 12 Physics Chapter 15 Communication Systems

Q.1 What do you mean by remote sensing? Give its applications.
Ans. Remote Sensing: Remote sensing is a technique by which we can obtain information about a particular geographical area or object, from a distance without being in physical contact.
Applications of remote sensing: The applications of remote sensing are given as follows:-
1. By the help of remote sensing we can find out the information of cultivation of different crops in a particular area.
2. It is used in study and making of different kinds of maps also knowm as cartography.
3. It is used in oceanography i.e. for knowing the climatic condition, temperature and also to identify the potential fishing zones.
4. Remote sensing is useful in finding out the extent of pollution in any particular region and its sources too.
5. Remote sensing is used in predicting the temperature and weather and other climatic condition of a given area in advance.
6. It is also used during the period of natural calamities to assess the damage caused.
7. Remote sensing is used for spying in the field of defence and during war to locate the enemies.
8. It is used in the field of forestry. Remote sensing can give information about extent of forest cover, density of forest and types of plants. It is also helpful in detection and prevention of forest hazards like fire, excessive falling of of trees, etc.

Q.2 What is amplitude modulation? What is modulation index? Derive the expression for modulation index.
Ans. The variation of the amplitude of a carrier wave in accordance with the instantaneous value of the modulating(information) signal is called as amplitude modulation. The concept of generation of amplitude modulated signal is shown in the figure below,

Here the instantaneous value of the modulating signal is given by equation,

em=Emsinm t——(I) ;where, m is the angular frequency of the modulating signal.

Similarly the instantaneous value of the carrier signal is given by,

ec=Ecsinct——–(II) ;where, c is the angular frequency of the carrier signal.

Modulation Index: The ratio of amplitude of modulating signal to the amplitude of carrier is called as modulation index. It is also called as degree of modulation or depth of modulation. It is denoted by ma.

Here we have to find the expression for the ratio Em/Ec.

From fig(b), it follows that:-

Em= (Emax – Emin)/2 ———-(III)

Similarly,

Ec= Emax – E

Subsituting the value of Em from equation (III), we get:-

Ec= Emax – (Emax – Emin)/2

Ec= (Emax – Emin)/2 ————(IV)

Dividing equation (III) by (IV),

ma = Em/Ec = (Emax – Emin)/(Emax + Emin)

Q.3 Explain how communication is possible with the help of sky wave propagation?
Ans. In this type of communication the radiowaves transmitted and directed towards the sky are reflected back to earth at a distant location by the help of layer of atmosphere, also called as ionosphere.
The signal transmitted in this case must belong to the frequency range between 1710 kHz to 54 MHz. These waves are also called as short waves and are used for the transmission of radio signals. Ionosphere is the uppermost layer of the earth’s atmosphere and is located at the altitude of 65 km to 400 km. It contains ionized layers of electrons and ions, which are responsible for reflecting the transmitted signal back to earth. The signals which are above 54 Mhz escapes ionosphere and are not reflected back. Hence we can say that only a range of frequencies can be transmitted back towards earth.

Q.4 What are the processes involved in converting analog signal into digital signal? Explain pulse amplitude modulation (P.A.M).

Ans. The steps involved in converting analog signal into digital signal is shown below with the help of block schematic below, Pulse Amplitude Modulation: In pulse amplitude modulation the pulses of the carrier pulse train is varied in accordance with the instantaneous value of the modulating signal.

Here from the figure below we can say that in pulse amplitude modulation the amplitude of pulses increseases or decreases in accordance with the modulating signal. In case of the pulse amplitude modulation the modulating signal is mixed with the modulating information signal to get the train of pulse amplitude signals. The pulse amplitude signal may be of single polarity or double polarity. If the train of pulses exists only in one direction than it is double polarity type P.A.M and if the pulses are present in both direction then it is called as double polarity P.A.M.

Q.5 What is satellite communication ? Explain its working.
Ans. The communication of signal between transmitter and receiver with the help of satellite is called as satellite communication.
Working: A satellite receives the signals from earth (which is beamed by transmitter), processes the signals and transmits them back to earth. The signal beamed by the satellite is received back on a distant location on earth, from which the original information signal is extracted by the process called demodulation.
The frequency at which satellite receive the signal is called as uplink frequency while the frequency at which satellite returns the signal towards earth is called as downlink frequency. The downlink frequency and uplink frequency are different so that there is no interference between these two signals.

Q.6 What are the merits and demerits of satellite communication?
Ans. The merits of satellite communication are given as follows:
1. It has a wide coverage area as compared to other communication systems.
2. Transmission of data at very high rate is possible through satellite communication.
3. Communication can be done from any part of earth just by using three geostationary satellites.
4. Communication done by the help of a satellite is much cheaper than other available means.
Demerits of satellite communication are given as follows:
1. The cost of launching satellite is very high and needs lot of technology expertise.
2. Every satellite has got its own time frame of working, after which it expires.
3. If there anything goes wrong in the satellite then there is no way we can fix the problem.
4. There is time delay between transmission and reception of signals, since they have to travel long distance(2×36,000=72,000 km).

Q.7 How would the following be affected in the absence of atmosphere around the earth?
a)Surface temperature of earth.
b)Range of radio waves transmission.
Give brief explanation in support of your.

Ans.a) The temperature of earth will decrease since the greenhouse effect will not take place in absence of atmosphere.
b) Range of radiowaves will decrease since reflecting layer(ionosphere) will be absent.

Q.8 A T.V tower has a height of 80m.What is its coverage area?
Ans. The coverage area is given by,

d= ( 2hR )1/2, where h=Height of the tower and R=radius of earth.

d= ( 2 x 80 x 6.4 x 106 )1/2 = 22627.5 m = 22.62 km.

Q.9 What is geostationary satellite ? What are its two characteristics?
Ans. Geostationary satellite: The satellite which appears to be at a fixed position to an observer on earth is called geostationary satellite.

The two characterstics of geostationary satellite are given as follows:

1. It remains at a height of about 36,0000 km above the equator of earth.
2. Its period of revolution should be equal to rotation of earth about its own axis i.e. 24 hours.

Q.10. Give two advantages of optical fibre communication?
Ans. The two advantages of optical fibre communication are given as follows:-
1. It can carry large amount of information signals at once.
2. It is able to carry even the high bandwidth carrier signals.

Q.11 What are the four primary constants of a transmission line?
Ans. The four primary constants of a transmission line are given as follows:-

1. Resistance(R).
2. Inductance(L).
3. Capacitance(C).
4. Conductance(G).

Q.12 Why is short wave band used for long distance radio broadcast?
Ans. Short waves have the frequency range of 2MHz to 20MHz. These waves can be reflected to earth by a layer of atmosphere called ionosphere. Hence we can use these waves for long distance radio transmission.

Q.13 Find the length of the dipole antenna for a carrier wave of wavelength ?
Ans. The length of the dipole antenna will be equal to / 2.

Q.14 What should be the length of a dipole antenna for a carrier wave having frequency 3×108 Hz?
Ans. Length of dipole antenna = / 2—–I.

We are given with the frequency of carrier wave, evaluate wavelength from it by relation,

= c / f = ( 3 x 108 ) / ( 6 x 108 ) = 0.5 m.

Substituting the above value in equation I we get,

Lemgth of dipole antenna=(0.5 / 2) = 0.25 m.

Q.15 What do you mean by bandwidth?
Ans. The total spread of a signal over a range of frequencies is called its bandwidth.

Q.16 How does the power radiated by an antenna vary with wavelength?
Ans. The power radiated by an antenna is related to wavelength by relation,

P ( 1 / 2).

Q.17 Name the type of modulation scheme preferred for digital communication.
Ans. For digital communication we prefer pulse code modulation (P.C.M).

Q.18 What is sampling frequency?
Ans. The number of samples taken per second for a given analog signal is called its sampling frequency.

NCERT Exemplar For Class 12 Science

Students must practise these additional questions for their own benefits, the ncert exemplar are curated by the best subject-matter experts to boost your knowledge on the presented topic. Students can easily access the ncert exemplar for class 12 science by visiting our website SimplyAcad and solve all the questions listed to secure maximum marks.

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