JEE Questions for Physics Communication System Quiz 3

Two electrons are moving in a opposite direction with speeds 0.8c and 0.4c, where c is the speed of light in vacuum. Then, the relative speed is about

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0.4 c
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0.8 c
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0.9 c
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1.2 c

For a body moving with relativistic speed, if the velocity is doubled, then

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its linear momentum is doubled
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its linear momentum will be less than double
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its linear momentum will be more than double
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its linear momentum remains unchanged

In process of amplitude modulation of signal to be transmitted.
Signal to be modulated is given by m(t) = A_m sin ω_mt , carrier wave is given by c(t) = A_c sin ω_ct , modulated signal c_m (t) is given by

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2)
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None of the obove

Which one of the following is incorrect statement in the transmission of electromagnetic waves?

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Ground wave propagation is for high frequency transmission
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Sky wave propagation is facilitated by ionospheric layers
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Space wave is of high frequency and is suitable for line of sight communication
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Space wave is used for satellite communication
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Very high frequency waves cannot be reflected by the ionospheric layers

What is the range of the characteristic impedance of a coaxial cable?

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Between 150 Ω to 600 Ω
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Between 50 Ω to 70 Ω
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Between 0 Ω to 50 Ω
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Between 100 Ω to 150 Ω

Which one of the following statements is not correct?

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Radio waves in the frequency range 30 MHz to 60 MHz are called sky waves
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2)
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Within the skip distance neither the ground waves nor the sky waves are received
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The principle of fibre optical communication is total internal reflection
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Fibre optical communications is free from electrical disturbances

The maximum electron density in the ionosphere in the morning is 10¹⁰ m^-3 . At noon time, it increases to 2×10¹⁰ m^-3 . Find the ratio of critical frequency at noon and the critical frequency in the morning.

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2.00
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2.82
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4.00
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1.414

In the communication system, AM is used for broadcasting because

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its use avoid receiver complexity
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it is more noise immune than other modulation system
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it requires less transmitting power
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no other modulation system can give the necessary bandwidth for faithful transmission

A radio station has two channels. One is AM at 1020 kHz and the other FM at 89.5 MHz. For good results you will use

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longer antenna for the AM channel and shorter for the FM
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shorter antenna for the AM channel and longer for the FM
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same length antenna will work for both
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information given is not enough to say which one to use for which

Advantage of optical fibre is

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high bandwidth and EM interference
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tow bandwidth and EM interference
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high bandwidth, low transmission capacity and no EM interference
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high bandwidth, high data transmission capacity and no EM interference

Which of the following statements is not correct?

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Ground wave propagation can be sustained at frequencies 500 kHz to 1500 kHz
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Satellite communication is useful for the frequencies above 30 MHz
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Sky wave propagation is useful in the range of 30 to 40 MHz
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Sky wave propagation takes place through tropospheric space
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The phenomenon involved in sky wave propagation is total internal reflection

Electromagnetic waves with frequencies greater than the critical frequency of ionosphere cannot be used for communication using sky wave propagation, because

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the refractive index of the ionosphere becomes very high for f > fc
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the refractive index of the ionosphere becomes very low for f > fc
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the refractive index of the ionosphere becomes very high for f < fc
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None of the above

The transmission of high frequencies in a coaxial cable is determined by

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(LC)2
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the impedance L alone
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the dielectric and skin effect

A double star system consists of two stars A and B which have time period T_A and T_B . Radius R_A and R_B and mass M_A and M_B . Choose the correct option.