# Bihar Board 12th Physics Objective Answers Chapter 15 Communication Systems

Bihar Board 12th Physics Objective Questions and Answers

## Bihar Board 12th Physics Objective Answers Chapter 15 Communication Systems

Question 1.
Who invented world wide web?
(a) J.C.R. Licklider
(b) Tim Berners-Lee
(c) Alexander Graham Bell
(d) Samuel F.B. Morse
(b) Tim Berners-Lee

Question 2.
Essential elements of a communication system are
(c) transmitter and communication channel
(d) transmitter, communication channel and receiver
(d) transmitter, communication channel and receiver

Question 3.
Which of the following is an example of point to point communication mode?
(b) Television
(c) Telephony
(d) All of these
(c) Telephony

Question 4.
Which among the following is not a basic terminology used in electronic communication systems?
(a) Transducer
(b) Transmitter
(c) Telegraph
(d) Attenuation
(c) Telegraph

Question 5.
Which of the following is not a transducer?
(a) Loudspeaker
(b) Amplifier
(c) Microphone
(d) All of these
(b) Amplifier

Question 6.
The loss of strength of a signal while propagating through a medium is known as
(a) reception
(b) absorption
(c) transmission
(d) attenuation
(d) attenuation

Question 7.
The process of increasing the strenth of a signal using an electronic circuit is called
(a) amplification
(b) modulation
(c) demodulation
(d) transmission
(a) amplification

Question 8.
Modem is a device which performs
(a) modulation
(b) demodulation
(c) rectification
(d) modulation and demodulation
(d) modulation and demodulation

Question 9.
Modulation is the process of superposing
(a) low frequency audio signal on high frequency radio waves
(b) low frequency radio signals on low frequency audio . wave
(c) high frequency radio signal on low frequency audio signal
(d) high frequency audio signal on low frequency radio waves
(a) low frequency audio signal on high frequency radio waves

Question 10.
The device which is a combination of a receiver and a transmitter is
(a) Amplifier
(b) Repeater
(c) Transducer
(d) Modulator
(b) Repeater

Question 11.
Large bandwidth for higher data rate is achieved by using
(a) high frequency carrier wave
(b) high frequency audio wave
(c) low frequency carrier wave
(d) low frequency audio wave
(a) high frequency carrier wave

Question 12.
In a video signal for transmission of picture, what value of bandwidth used in communication system?
(a) 2.4 MHz
(b) 4.2 MHz
(c) 24 MHz
(d) 42 MHz
(b) 4.2 MHz

Question 13.
Which of the following is an example of broadcast mode of communication?
(b) Television
(c) Mobile
(d) Both (a) & (b)
(d) Both (a) & (b)

Question 14.
Which of the following devices is full duplex?
(a) Mobile phone
(b) Walky-talky
(c) Loud speaker
(a) Mobile phone

Question 15.
(a) it is less noisy
(b) reproduction is of much better quality
(c) it is more noisy
(d) both (a) and (b)
(d) both (a) and (b)

Question 16.
The frequency band used in the downlink of satellite communication is
(a) 9.5 to 2.5 GHz
(b) 896 to 901 MHz
(c) 3.7 to 4.2 GHz
(d) 840 to 935 MHz
(c) 3.7 to 4.2 GHz

Question 17.
The radio waves of frequency 30 MHz to 300 MHz belong to
(a) high frequency band
(b) very high frequency band
(c) ultra high frequency band
(d) super high frequency band
(b) very high frequency band

Question 18.
Ground waves have wavelength
(a) less than that of sky waves
(b) greater than that of sky waves
(c) less than that of space waves
(d) equal to that of space waves
(b) greater than that of sky waves

Question 19.
The mode of propagation used by short wave broadcast services is
(a) space wave
(b) sky wave
(c) ground wave
(d) both (a) and (c)
(b) sky wave

Question 20.
The skip zone in radio wave transmission is that range where
(a) there is no reception of either ground wave or sky wave
(b) the reception of ground-wave is maximum but that of sky wave is minimum
(c) the reception of ground wave is minimum, but that of sky wave is maximum
(d) the reception of both ground and sky wave is maximum
(a) there is no reception of either ground wave or sky wave

Question 21.
The waves that are bent down by the ionosphere are
(a) ground waves
(b) surface waves
(c) space waves
(d) sky waves
(d) sky waves

Question 22.
The maximum line-of-sight distance dM between two antennas having heights hT and hR above the earth is

(d)

Question 23.
Frequencies in the UHF range normally propagate by means of
(a) ground waves
(b) sky waves
(c) surface waves
(d) space waves
(d) space waves

Question 24.
A ground reciver in line-of-sight communication cannot receive direct waves due to
(a) its low frequency
(b) curvature of earth
(c) its high intensity
(d) smaller antenna
(b) curvature of earth

Question 25.
Which of the following modes is used for line of sight communication as well as satellite communication?
(a) Ground wave
(b) Sky wave
(c) Space wave
(d) All of these
(c) Space wave

Question 26.
Through which mode of propagation, the radio waves can be sent from one place to another
(a) ground wave propagation
(b) sky wave propagation
(c) space wave propagation
(d) all of them
(d) all of them

Question 27.
Which of the following frequency will be suitable for beyond the horizon communication using sky waves?
(a) 10 kHz
(b) 10 MHz
(c) 1 GHz
(d) 1000 GHz
(b) 10 MHz

Question 28.
What should be the length of the dipole antenna for a carrier wave of frequency 3 x 108 Hz?
(a) 1 m
(b) 0.5 m
(c) 2 m
(d) 2.5 m
(b) 0.5 m

Question 29.
A radio can tune to any station in 7.5 MHz to 12 MHz band. The corresponding wavelength band is
(a) 25 m-40m
(b) 10 m-30m
(c) 20-40 m
(d) 25 m – 35 m
(a) 25 m-40m

Question 30.
In frequency modulation
(a) the amplitude of modulated wave varies as frequency of carrier wave
(b) the frequency of modulated wave varies as amplitude of modulating wave
(c) the amplitude of modulated wave varies as amplitude of carrier wave
(d) the frequency of modulated wave varies as frequency of modulating wave
(d) the frequency of modulated wave varies as frequency of modulating wave

Question 31.
Audio signal cannot be transmitted because
(a) the signal has more noise.
(b) the signal cannot be amplified for distance communication.
(c) the transmitting antenna length is very small to design.
(d) the transmitting antenna length is very large and impracticable.
(d) the transmitting antenna length is very large and impracticable.

Question 32.
The modulation in which pulse duration varies in accordance with the modulating signal is called
(a) PAM
(b) PPM
(c) PWM
(d) PCM
(c) PWM

Question 33.
A 300 W carrier is modulated to a depth 75%. The total power in the modulated wave is
(a) 200 W
(b) 284 W
(c) 320 W
(d) 384 W
(d) 384 W

Question 34.
A signal wave of frequency 12 kHz is modulated with a carrier wave of frequency 2.51 MHz. The upper and lower side band frequencies are respectively
(a) 2512 kHz and 2508 kHz
(b) 2522 kHz and 2488 kHz
(c) 2502 kHz and 2498 kHz
(d) 2522 kHz and 2498 kHz
(d) 2522 kHz and 2498 kHz

Question 35.
The maximum amplitude of an amplitude modulated wave is found to be 15 V while its minimum amplitude is found to be 3 V. The modulation index is
$$(a) \frac{3}{2} (b) \frac{2}{3} (c) \frac{1}{2} (d) \frac{1}{3}$$
$$(b) \frac{2}{3}$$

Question 36.
The LC product of a tuned amplifier circuit require to generate a carrrier wave of 1 MHz for amplitude modulation is
(a) 1.5 x 10-14 s
(b) 1.2 x 10-12 s
(c) 3.2 x 10-12 s
(d) 2.5 x 10-14 s
(d) 2.5 x 10-14 s

Question 37.
The essential condition for demodulation is

(b)

Question 38.
Figure shows a communication system. What is the output power when input signal is 1.01 m W?

(a) 90 mW
(b) 101 mW
(c) 112 m W
(d) 120 mW
(b) 101 mW

Question 39.
A 100 m long antenna is mounted on a 500 m tall building. The complex can become a transmission tower for waves with X
(a) ~ 400 m
(b)~25m
(c)~150m
(d)~ 2400 m
(a) ~ 400 m

Question 40.
A male voice after modulation-transmission sounds like that of a female to the receiver. The problem is due to
(a) poor selection of modulation index (selected 0<w<l)
(b) poor bandwidth selection of amplifiers
(c) poor selection of carrier frequency
(d) loss of energy in transmission
(b) poor bandwidth selection of amplifiers

Question 41.
A basic communication system consists of
(A) transmitter
(B) information source
(C) user of information
(D) channel

Choose the correct sequence in which these are arranged in a basic communication system.
(a) ABCDE
(c) BDACE

Question 42.
Identify the mathematical expression for amplitude modulated wave.
$$(a) A_{c} \sin \left[\left\{\omega_{c}+k_{1} v_{m}(t)\right\} t+\phi\right]$$
$$(b) A_{c} \sin \left\{\omega_{c} t+\phi+k_{2} v_{m}(t)\right\}$$
$$(c) \left\{A_{c}+k_{2} v_{m}(t)\right\} \sin \left(\omega_{n} t+\phi\right)$$
$$(d) A_{c} v_{m}(t) \sin \left(\omega_{c} t+\phi\right)$$
$$(c) \left\{A_{c}+k_{2} v_{m}(t)\right\} \sin \left(\omega_{n} t+\phi\right)$$

Hints And Explanations

Question 29.
(a) Wavelength corresponding to 7.5 MHz frequency
$$\lambda_{1}=\frac{c}{v}=\frac{3 \times 10^{8}}{7.5 \times 10^{6}}=40 \mathrm{m}$$
Wavelength corresponding to 12 MHz frequency
$$\lambda_{2}=\frac{3 \times 10^{8}}{12 \times 10^{6}}=25 \mathrm{m}$$

Question 33.
Pc = 300 W, μ, = 75% = 0.75

Question35.
(b) Here, Ac + Am = 15 V …………………. (i)
Ac – Am = 3V ………………………… (ii)
Solving (I) and (ii), we get, Ac =9 V, Am = 6 V
∴ Modulation index, $$\mu=\frac{A_{m}}{A_{c}}=\frac{6}{9}=\frac{2}{3}$$

Question 36.
(d) Frequency of tuned amplifier is $$v=\frac{1}{2 \pi \sqrt{L C}}$$
As per question, u = 1 MHz = 106 Hz

Question 38.

Here, transmission path = 5 km
Loss suffered in transmission path
= – 2 dB km-1 x 5 km = – 10 dB
Total amplifier gain = 10 dB + 20 dB = 30 dB
Overall gain of signal = 30 dB – 10 dB = 20 dB

Question 39.
(a) Length of antenna, ¡ = 100 m,
As l = $$\frac{\lambda}{4}$$ or λ = 4l = 4 x 100 = 400m.

Very Important Questions

Question 1.
The electric Held at a point near an infinite thin sheet of charged conductor is :

(c)

Question 2.
Two capacitors C1 = 2μF and C2 = 4μF are connected in series and a potential difference (p.d.) of 1200 V is applied across it. The Potential difference across 2μF will be :
(a) 400 V
(b) 600 V
(c) 800V
(d) 900V
(c) 800V

Question 3.
In the figure, ifnet force on Q is zero then value of $$\frac{Q}{q}$$ is:

(b)

Question 4.
When a body is charged, its mass
(a) increases
(b) decreases
(c) remains same
(d) may increase or decrease
(d) may increase or decrease

Question 5.
The electric potential due to a small electric dipole at a large distance r from the center of the dipole is proportional to :
$$(a) r (b) \frac{1}{r} (c) \frac{1}{r^{2}} (d) \frac{1}{r^{3}}$$
$$(c) \frac{1}{r^{2}}$$

Question 6.
Minimum number of capacitors of 2pF each required to obtain a capacitance of 5fxF will be
(a) 4
(b) 3
(c) 5
(d) 6
(a) 4

Question 7.
The specific resistance of conductor increases with :
(a) increase of temperature
(b) increase of cross-sectional area
(c) decrease in length
(d) decrease of cross-sectional area
(a) increase of temperature

Question 8.
The Drift velocity (vd) and applied electric field (E) of a conductor are related as :
$$(a) \mathrm{V}_{\mathrm{d}} \propto \sqrt{\mathrm{E}} (b) \mathrm{V}_{\mathrm{d}} \propto \mathrm{E} (c) \mathrm{V}_{\mathrm{d}} \propto \mathrm{E}^{2} (d) v_{d}= Constant$$
$$(b) \mathrm{V}_{\mathrm{d}} \propto \mathrm{E}$$

Question 9.
A charge ‘q’ moves in a region where electric field ‘E’ and magnetic field ‘B’ both exist, then force on it is :

(b)

Question 10.
A bar magnet of magnetic moment M is cut into two parts of equal length. The magnetic moment of either part is :
(a) M
(b) M/2
(c) 2M
(d) Zero
(b) M/2

Question 11.
The dimensional formula for $$\frac{1}{2} \varepsilon_{0} \mathbf{E}^{2}$$ is identical to that of:

(a)

Question 12.
A circular coil of radius R carries a current I. The magnetic field at its center is B. At what distance from the center, on the axis of the coil, the magnetic field will be B/8.
(a) $$\sqrt{2} \mathrm{R}$$
(b) 2R
(c) $$\sqrt{3} \mathrm{R}$$
(d) 3R
(c) $$\sqrt{3} \mathrm{R}$$

Question 13.
Magnetic dipole moment is a vector quantity directed from:
(a) South to North Pole
(b) North to South Pole
(c) East to West direction
(d) West to East direction
(a) South to North Pole

Question 14.
A wire of magnetic dipole moment M and L is bent into shape of a semicircle of radius r. What will be its new dipole moment?
$$(a) \mathrm{M} (b) \frac{M}{2 \pi} (c) \frac{M}{\pi} (d) \frac{2 M}{\pi}$$
$$(d) \frac{2 M}{\pi}$$

Question 15.
In a closed circuit of resistance 10W, the linked flux varies with time according to relation Φ = 6t2 – 5t +1. At t = 0.25 second, the current (in Ampere) flowing through the circuit is :
(a) 0.4
(b) 0.2
(c) 2.0
(d) 4.0
(b) 0.2

Question 16.
In A.C. circuit, the current and voltage are given by i = 5 cos wt, and V = 200 sin wt respectively. Power loss in the circuit is :
(a) 20 W
(b) 40W
(c) 1000W
(d) zero
(d) zero

Question 17.
The energy of an electromagnetic radiation is 13.2 keV.
This radiation is related to which region of spectrum?
(a) Visible
(b) X-rays
(c) Ultra violet
(d) Infrared
(b) X-rays

Question 18.
The image formed by objective lens of a compound Microscope is :
(a) Virtual and diminished
(b) Real and diminished
(c) Real and large
(d) Virtual and Large
(c) Real and large

Question 19.
A convex lens is dipped in a liquid, whose refractive index is equal to refractive index of material of lens. Then its focal length will:
(a) become zero
(b) become infinite
(c) decrease
(d) increase
(b) become infinite

Question 20.
Critical angle for light moving from medium I to medium II is 6. The speed of light in medium I is v. Then speed in medium II is :

(b)

Question 21.
A magnifying glass is to be used at the fixed object distance of 1 inch. If it is to produce an erect image 5 items magnified, its focal length should be :
(a) 0.2″
(b) 0.8″
(c) 1.5″
(d) 5″
(c) 1.5″

Question 22.
To remove hyper metropia lens used is :
(a) Convex
(b) Concave
(c) Cylindrical
(d) Plano-convex
(a) Convex

Question 23.
Which of the following phenomena taken place when a monochromatic light is incident on a prism?
(a) Dispersion
(b) Deviation
(c) Interference
(d) All of the above
(a) Dispersion

Question 24.
Optical fiber communication is based on which of the following phenomena:
(a) Total Internal reflection
(b) Scattering
(c) Reflection
(d) Interference
(a) Total Internal reflection

Question 25.
The value of maximum amplitude produced due to interference of two waves is given by- y1 = 4 sin wt and y2 = 3 cos wt
(a) 7
(b) 5
(c) 1
(d) 25
(b) 5

Question 26.
The phase difference 0 is related to Path difference Δx by:

(c)

Question 27.
The resolving power of human eye (in minute) is :
(a) $$\frac{1}{60}$$
(b) 1
(c) 10
(d) $$\frac{1}{2}$$
(a) $$\frac{1}{60}$$

Question 28.
A particle of mass m and charged q is accelerated through a potential V. The De-Broglie wavelength of the particle will be :

(c)

Question 29.
The number of Photons of frequency 1014 Hz in radiation of 6.62J will be :
(a) 1010
(b) 1015
(c) 1020
(d) 1025
(c) 1020

Question 30.
The minimum angular momentum of electron in Hydrogen atom will be :
$$(a) \frac{h}{\pi} J s (b) \frac{h}{2 \pi} J s (c) \mathrm{h} \pi \mathrm{Js} (d) 2 \pi h J s$$
$$(b) \frac{h}{2 \pi} J s$$

Question 31.
The atomic number and mass number for a specimen are Z and A respectively. The number of neutrons in the atom will be :
(a) A
(b) Z
(c) A + Z
(d) A-Z
(d) A-Z

Question 32.
The quantities, which remain conserved in a nuclear reaction:
(a) Total Charge
(b) Angular momentum
(c) Linear momentum
(d) All the above
(d) All the above

Question 33.
Meaning of “FAX” is :
(a) Full Access Transmission
(b) Facsimile Telegraphy
(c) Factual Auto Access
(d) Feed Auto Exchange
(b) Facsimile Telegraphy

Question 34.
A semiconductor is cooled from T1K to T2K, then its resistance will:
(a) increase
(b) decrease
(c) remain constant
(d) First decrease then increase