Electro Magnetic Waves

SHORT ANSWER

Q1. A Variable frequency A.C. source is connected to capacitor. How will the displacement

current change with decrease in frequency?

Ans. Reactance Xc=1/Cω = 1/2πʋC. On decreasing frequency Xc will increase decreasing the

conduction current. Here id=ic , so displacement current will also decrease.

Q2. Draw a sketch of plane em wave propagating along Z-direction.

Ans.

Q3. Name the constituent radiation of e.m. spectrum which

(1) Is used for crystal structure

(2) Produces intense heating effect

Ans. (1) X-Rays

(2)Infra-Red rays

Q4. A plane e.m. wave of frequency 25 MHz travels in free space X-direction. At an extent, the

electric vector is E= 6.3V/m j^. Calculate B at this point

Ans. B=E/C =2.1*10-8 waber/m2.

Short answer Questions (2 Marks)

Group 5

Chapter: Semi-Conductor Electronics and Communications System

TWO MARKS QUESTIONS

1. What is threshold frequency? Does it depend on the nature of emitter?

Ans.: The minimum value of the frequency of incident radiation below which the photoelectric emission stops altogether is called threshold frequency. Yes threshold frequency depends on the nature of photoelectric emitter.

.2. In photoelectric effect, why should the photoelectric current increase as the intensity of

monochromatic radiation incident on a photosensitive surface is increased? Explain.

(ANS). The photoelectric current increases proportionally with the increase in intensity of

incident radiation. Larger the intensity of incident radiation, larger is the number of

incident photons and hence larger is the number of electrons ejected from the

photosensitive surface.

3.What is the effect on the velocity of the photoelectrons if the wavelength of incident light is

decreased?

Ans:

4. The plot between the maximum kinetic energy of photoelectrons versus frequency of incident

radiations is shown below for two surfaces A and B. Ans. the followings:-

a. Which of the metal has greater work

function?

b. For which of the metals will stopping

potential be more for the same frequency of

incident radiations?

Ans. a. Metal B has greater work function.

b. Metal A has more stopping potential.

5. An electron and a proton are accelerated through the same potential. Which one of

these two has (i) greater value of de Broglie wavelength associated with it and (ii) less

momentum justify your answer.

6. An electron and a proton have the same de Broglie wavelength associated with them. How is

their K. E. related with each other?

Ans.: de Broglie wavelength, 𝜆 =ℎ

√2𝑚𝐸

Given 𝜆𝑒 = 𝜆𝑝 ⇒ [ℎ

√2𝑚𝐸]e = [

ℎ

√2𝑚𝐸]p

⇒ 𝐸𝑒

𝐸𝑝=

𝑚𝑝

𝑚𝑒≈ 1840

Hence K.E. of electron = 1840 × (K.E. of proton)

7 . A proton and a deuteron are accelerated through the same accelerating potential. Which one of the two has

(i) greaterc value of de-Broglie wavelength associated with it, and

Q.1 Draw the circuit diagram showing how a pn junction diode is (i) forward biased and (ii) reverse

biased. How is the width of depletion layer affected in two cases?

(ii) less momentum? Give reasons to justify your answer. (ANS)

8. Write two basic features of photon picture of electromagnetic radiation on which Einstein’s photoelectric equation is based. (ans) Any two of following

9. A proton and an alpha particle are accelerated through the same potential. Which one of the two has (i) greater value of de-Broglie wavelength associated with it and (ii) less kinetic energy? Give reasons to justify your answer. (ANS)

10.

11

For a photosensitive surface threshold wavelength is λ 0. Does photoemission occur if the wavelength of radiation is (a) more than λ 0 (b) less than λ 0. Justify.

Ans: for emission EW0

photoemission occur if the wavelength of radiation is less than λ 0 12

The given graph shows the variation of photoelectric current with applied voltage for two different materials and for two different intensities of the incident radiation. Identify the pair of curves corresponding to different materials but same intensity of incident radiation.

Ans:1,3 and 2,4 because different materials have different stopping potential but same intensity of incident radiation gives same maximum current..

13.Monochromatic light of frequency 6.0 × 1014 Hz is produced by a laser. The power emitted is

2.0 × 10–3 W. Estimate the number of photons emitted per second on an average by the source.

(ANS)

1

3

2 4

I

V

=5X1015 PHOTONS PER SECOND

14. The frequency of incident light on a metal surface is doubled. How will this affect

the value of kinetic energy emitted photoelectrons?

Ans.

Kinetic energy will become slightly more than double. 15

In the nth orbit of hydrogen atom find the ratio of radius of electron orbit and de Broglie wavelength associated with it. Ans: rn = nh/2π mvn

λ = h/mvn 16

An electron and a proton have the same kinetic energy . Which one of the two has the larger

wavelength and why?

Ans. According to de Broglie equation in terms of kinetic energy as electron has smaller

mass than a proton hence an electron has the larger wavelength because

17

Two lines A and B shown in the graph represent the de Broglie wavelength (λ ) as a function of 1/√ V (V is the accelerating potential) for two particles having the same charge. Which of the two represents the particle of smaller mass?

A.1

Ans: Slope of graph = h/√2mq 1/√m

18. A particle of mass M at rest decays into two particles of masses m1 and m2 having non

zero velocities. What is the ratio of the de Broglie wavelength of the two particles?

Ans: By conservation of linear momentum,

19. An electromagnetic wave of wavelength λ is incident on a photosensitive surface of

negligible work function. The photoelectrons emitted from this surface have de Broglie

wavelength as λ1, prove that:

Ans.

λ

1/ √V

A

B

Forward Biased pn junction diode Reverse Biased junction diode

For forward biased : Width of depletion layer decreases

For reverse biased : Width of depletion layer increases

Q.2 Name the device ‘D’ which is used as a voltage regulator in the given circuit and give its symbol.

A. Zener Diode

Symbol of Zener Diode

20. Write the expression for the de Broglie wavelength associated with a charged particle having

charge

‘q’ and mass ‘m’, when it is accelerated by a potential V. (ans)

Q.3 The a.c current gain of a transistor is 120. What is the change in collector current in the transistor

whose base current changes by 100µA.

A.3 Given β = 120

ΔIb = 100µA = 10-6 x 100 = 10-4A

ΔIc = ?

As we know that

β = ΔIc

ΔIb

ΔIc = β ΔIb = 120 x 10-4 = 12 x 10-3A

ΔIc = 12 mA

Q.4 Can we take one slab of p-type semiconductor and physically join it to another n-type

semiconductor to get p-n junction.

A.4 No. any slab, howsoever flat, will have roughness much larger than the interatomic crystal spacing

and hence continuous contact at atomic level is not possible. The junction will behave as discontinuity

for the following charge.

Q.5 Suppose a pure si crystal has 5 x 10-8 atomsm-3. It is doped by 1 ppm concentration of pentavalent.

As calculate the number of electrons and holes. Given ni = 1.5 x 1016m-3.

Q.6 The current in the forward bias is known to be more than the current in the reverse bias. What is

the reason than to operate the photo diode in reverse bias.

A. Refer to NCERT Example 14.6 page 487

Q.7 For a CE transistor amplifier, the audio signal voltage across the collector resistance of 2 KΩ is 2V.

Suppose the current amplification factor of the transistor is 100, what should be the value of RB in

series with VBB supply of 2V if the dc base current has to be 10 times the signal current? Also calculate

the d.c. drop across the collector resistance.

Q.8 In half-wave rectification, what is the output frequency if the input frequency is 50Hz. What is the

output frequency of a full wave rectifier for the same input frequency.

A. (i) for half wave foutput = 50 Hz

(ii) for full wave fout = 2 fin

fout = 100 Hz

Q.9 Draw the output waveform at X, using the given inputs A and B for the logic circuit shown below.

Also identify the logic operation performed by this circuit.

A. For given circuit,

X = Y

X = A+B

A B Y’ Y=Y’ X=Y’

0 0 0 1 0

1 1 1 0 1

1 1 1 0 1

0 1 1 0 1

0 0 0 1 0

1 0 1 0 1

Q. Write the truth table for logic circuit shown below and identify the logic operation performed by

this circuit.

A.

A C=A’ B D=B’ Y=C.D

0 1 0 1 0

1 0 0 1 1

0 1 1 0 1

1 0 1 0 1

It behaves as OR gate

Q. Identify the logic gates worked ‘X’ and ‘Y’ in the following figure. Obtain the output Z of the

combination for all possible inputs A and B

A X- NAND

Y – NOT

A B A.B A.B Z=A.B

0 0 0 1 0

0 1 0 1 0

1 0 0 1 0

1 1 1 0 1

Q.(i) Identify the logic gates marked P and Q in the given logic circuit.

(ii) Write down the output at X for the inputs

A=0 B=0 and A=1, B=1

A. P – NOT Gate Q- OR Gate

A B A X=A+B

0 0 1 1

1 1 0 1

Q Which of the diodes is (i) forward biased and (ii) reverse biased in the following circuit?

A. (a) Reverse biased (b) Reverse biased

(c) forward biased (d) forward biased

Q. In the figure given below the inputs waveform is converted into the output waveform by a device ‘X’.

Name the device and draw its circuit diagram.

Q.15 Show the biasing of a photodiode with the help of a circuit diagram. Draw graphs to show

variations in reverse bias current for different illumination intensities.

A. Ref. to NCERT photodiode; fig. 14.2.3, Page – 487

Q.16 In the given block diagram of a receiver, identify the boxes labelled as X and Y and write their

function.

A. X – IF stage

Y – Amplifier

Q.17 In the given block diagram of a simple modulator for obtaining an AM signal, Identify X and Y

A. X – Square law Device

Y – Band Pass Fitter at wc

Q. 18 What is the skywave communication? Why is mode of propagation restricted to the frequencies

only upto few MHz

A. Refer to NCERT 15.6.2, Page 520

Q.19 Draw a schematic sketch showing the (a) Ground wave (b) Skywave and (c) Space wave mode of

propagation. Write the frequency range for each of these modes of propagation.

Frequency Ranges for

(i) Ground wave propagation (500 KHz to 1500 KHz)

(ii) Sky wave propagation (30 MHz - 40 MHz)

(iii) Space wave propagation (Los communication) (> 40 MHz)

Q.20 A transmitting antenna at the top of a tower has a height 32 m and the height of receiving

anteena is 50m. what is the maximum distance between them for satisfactory communication in

LOS mode? Given Re=6.4 x 106

A.

Q. A carrier wave of peak voltage 12 V is used to transmit a message signal. What should be the peak

voltage of modulating signal in order to have a modulation index of 75%?

A. Modulation Index = 75

100 = 0.75

As we know that ,

Modulation index = Am/ Ac

0.75 = Am/ Ac

Am = 0.75 x Ac

Am = 0.75 x 12

Am = 9 volt

Q. A message signal of frequency 10 KHz and peak voltage of 10 volts is used to modulate a carrier

frequency 1 MHz and peak voltage of 20 volts. Determine (a) modulation index (b) the side bands

produced.

A. (a) Modulation Index = 10/20 = 0.5

(b) The side bands are at (1000 + 10) = 1010 KHz and (1000 – 10) = 990 KHz

Q. which mode of propagation is used by short wave broadcast service having frequency range from

few MHz upto 30 MHz. why is there an upper limit to frequency of waves used in this mode?

A. (a) Sky wave propagation

(b) EM waves of frequencies > 30 MHz penetrates the ionosphere and escape out

Q Mention the function of any two of the following used in communication system.

(a) Transducer

(b) Repeater

(c) Transmitter

(d) Band pass filter

A. (a) It converts one form of energy into another

(b) It is combination of transmitter receiver

(c) It processes the message signal and make it suitable for transmission through a channel

and its reception.

(d) It blocks lower and higher frequencies and allows only a band of frequencies to pass

through