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Q1 Which of the following frequencies will be suitable for beyond the horizon communication using sky waves? (a) 10 kHz (b) 10 MHz (c) 1 GHz (d) 1000 GHz Ans: (b) 10 MHz
For beyond-the-horizon communication, it is necessary for the signal waves to travel a large distance. 10 KHz signals cannot be radiated efficiently because of the antenna size. The high energy signal waves (1GHz − 1000 GHz) penetrate the ionosphere. 10 MHz frequencies get reflected easily from the ionosphere. Hence, signal waves of such frequencies are suitable for beyond-the-horizon communication.
Q2 Frequencies in the UHF range normally propagate by means of (a) Ground waves. (b) Sky waves. (c) Surface waves. (d) Space waves. Ans: (d) Space waves
Owing to its high frequency, an ultra high frequency (UHF) wave can neither travel along the trajectory of the ground nor can it get reflected by the ionosphere. The signals having UHF are propagated through line-of-sight communication, which is nothing but space wave propagation.
Q3 Digital signals (i) Do not provide a continuous set of values, (ii) Represent values as discrete steps, (iii) Can utilize binary system, and (iv) Can utilize decimal as well as binary systems. Which of the above statements are true? (a) (i) and (ii) only (b) (ii) and (iii) only (c) (i), (ii) and (iii) but not (iv) (d) All of (i), (ii), (iii) and (iv). Ans: (c) A digital signal uses the binary (0 and 1) system for transferring message signals.
Such a system cannot utilise the decimal system (which corresponds to analogue signals). Digital signals represent discontinuous values.
Q4 Is it necessary for a transmitting antenna to be at the same height as that of the receiving antenna for line-of-sight communication? A TV transmitting antenna is 81m tall. How much service area can it cover if the receiving antenna is at the ground level? Ans: Line-of-sight communication means that there is no physical obstruction between the transmitter and the receiver.
In such communications it is not necessary for the transmitting and receiving antennas to be at the same height.
Height of the given antenna, h = 81 m
Radius of earth, R = 6.4 × 10 6 m
For range, d = 2Rh,
The service area of the antenna is given by the relation: A = πd 2 = π (2Rh)
= 3.14 × 2 × 6.4 × 10 6 × 81 = 3255.55 × 10 6 m 2 = 3255.55~3256 km 2
Q5 A carrier wave of peak voltage 12 V is used to transmit a message signal. What should be the peak voltage of the modulating signal in order to have a modulation index of 75%? Ans: Amplitude of the carrier wave, A c = 12 V
Modulation index, m = 75% = 0.75
Amplitude of the modulating wave = A m
Using the relation for modulation index: m = Am/Ac
Therefore, Am = m Ac
= 0.75 x 12 = 9V
Q6 A modulating signal is a square wave, as shown in Fig. 15.14. The carrier wave is given by (i) Sketch the amplitude modulated waveform (ii) What is the modulation index? Ans: It can be observed from the given modulating signal that the amplitude of the modulating signal, A m = 1 V
It is given that the carrier wave c (t) = 2 sin (8πt)
Therefore, Amplitude of the carrier wave, A c = 2 V
Time period of the modulating signal T m = 1 s
The angular frequency of the modulating signal is calculated as: ωm = 2π/Tm
= 2π rad s-1 .......(i)
The angular frequency of the carrier signal is calculated as: ωc = 8π rad s-1 ....(ii)
From equations (i) and (ii), we get:
= ωc = 4ωm
The amplitude modulated waveform of the modulating signal is shown in the following figure.
( ii ) Modulation index, m = Am/Ac = 1/2 = 0.5
Q7 For an amplitude modulated wave, the maximum amplitude is found to be 10 V while the minimum amplitude is found to be 2 V. Determine the modulation index μ. What would be the value of μ if the minimum amplitude is zero volt? Ans: Maximum amplitude, A max = 10 V
Minimum amplitude, A min = 2 V
Modulation index μ, is given by the relation:
Q8 Due to economic reasons, only the upper sideband of an AM wave is transmitted, but at the receiving station, there is a facility for generating the carrier. Show that if a device is available which can multiply two signals, then it is possible to recover the modulating signal at the receiver station. Ans: Let ω c and ω s be the respective frequencies of the carrier and signal waves.
Signal received at the receiving station, V = V 1 cos (ω c + ω s )t
Instantaneous voltage of the carrier wave, V in = V c cos ω c t
At the receiving station, the low-pass filter allows only high frequency signals to pass through it. It obstructs the low frequency signal ω s .
Thus, at the receiving station, one can record the modulating signal, V1Vs/2 cos ωst which is the signal frequency.
Chapter 15 Communication Systems Physics
The last chapter of the book of NCERT Physics Class 12, tell you about the communication systems and this chapter comprises of ten topics. Communication System tells you about the three basic elements of a communication system, bandwidth of signals and transmission medium, concept of wave propagation, satellite communication, propagation of electromagnetic waves, amplitude modulation. There is an exercise at the end of this chapter for the final check of your progress. With the help of Saralstudy, you can the easy learning and more accurate solutions of NCERT Physics Class 12.
Download pdf of NCERT Solutions for Class Physics Chapter 15 Communication Systems Physics
Exercise 1
Popular Questions of Class 12 Physics
- Q:-
What is the force between two small charged spheres having charges of 2 x 10-7 C and 3 x 10-7 C placed 30 cm apart in air?
- Q:-
An infinite line charge produces a field of 9 × 104 N/C at a distance of 2 cm. Calculate the linear charge density.
- Q:-
A polythene piece rubbed with wool is found to have a negative charge of 3 × 10−7 C.
(a) Estimate the number of electrons transferred (from which to which?)
(b) Is there a transfer of mass from wool to polythene?
- Q:-
A 600 pF capacitor is charged by a 200 V supply. It is then disconnected from the supply and is connected to another uncharged 600 pF capacitor. How much electrostatic energy is lost in the process?
- Q:-
A parallel plate capacitor with air between the plates has a capacitance of 8 pF (1pF = 10-12 F). What will be the capacitance if the distance between the plates is reduced by half, and the space between them is filled with a substance of dielectric constant 6?
- Q:- A circular coil of wire consisting of 100 turns, each of radius 8.0 cm carries a current of 0.40 A. What is the magnitude of the magnetic field B at the centre of the coil?
- Q:-
A regular hexagon of side 10 cm has a charge 5 µC at each of its vertices. Calculate the potential at the centre of the hexagon.
- Q:-
A point charge +10 μC is a distance 5 cm directly above the centre of a square of side 10 cm, as shown in Fig. 1.34. What is the magnitude of the electric flux through the square? (Hint: Think of the square as one face of a cube with edge 10 cm.)
- Q:-
A conducting sphere of radius 10 cm has an unknown charge. If the electric field 20 cm from the centre of the sphere is 1.5 × 103 N/C and points radially inward, what is the net charge on the sphere?
- Q:-
A point charge of 2.0 μC is at the centre of a cubic Gaussian surface 9.0 cm on edge. What is the net electric flux through the surface?
Recently Viewed Questions of Class 12 Physics
- Q:-
A difference of 2.3 eV separates two energy levels in an atom. What is the frequency of radiation emitted when the atom makes a transition from the upper level to the lower level?
- Q:-
When a forward bias is applied to a p-n junction, it
(a) raises the potential barrier.
(b) reduces the majority carrier current to zero.
(c) lowers the potential barrier.
(d) None of the above.
- Q:-
Two charged conducting spheres of radii a and b are connected to each other by a wire. What is the ratio of electric fields at the surfaces of the two spheres? Use the result obtained to explain why charge density on the sharp and pointed ends of a conductor is higher than on its flatter portions.
- Q:-
A system has two charges qA = 2.5 × 10−7 C and qB = −2.5 × 10−7 C located at points A: (0, 0, − 15 cm) and B: (0, 0, + 15 cm), respectively. What are the total charge and electric dipole moment of the system?
- Q:-
The electrostatic force on a small sphere of charge 0.4 μC due to another small sphere of charge − 0.8 μC in air is 0.2 N.
(a) What is the distance between the two spheres?
(b) What is the force on the second sphere due to the first?
- Q:-
For a CE-transistor amplifier, the audio signal voltage across the collected resistance of 2 kΩ is 2 V. Suppose the current amplification factor of the transistor is 100, find the input signal voltage and base current, if the base resistance is 1 kΩ.
- Q:-
A plane electromagnetic wave travels in vacuum along z-direction. What can you say about the directions of its electric and magnetic field vectors? If the frequency of the wave is 30 MHz, what is its wavelength?
- Q:-
A horizontal overhead power line carries a current of 90 A in east to west direction. What is the magnitude and direction of the magnetic field due to the current 1.5 m below the line?
- Q:-
Three capacitors each of capacitance 9 pF are connected in series.
(a) What is the total capacitance of the combination?
(b) What is the potential difference across each capacitor if the combination is connected to a 120 V supply?
- Q:-
The number density of free electrons in a copper conductor estimated in Example 3.1 is 8.5 x 1028 m-3. How long does an electron take to drift from one end of a wire 3.0 m long to its other end? The area of cross-section of the wire is 2.0 x 10-6 m2 and it is carrying a current of 3.0 A.
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