AHLCON PUBLIC SCHOOL CLASS XII PHYSICS … PUBLIC SCHOOL ASSIGNMENT - 2 CLASS ± XII PHYSICS (SESSION: 2017-18) CHAPTER : CURRENT ELECTRICITY 1. Two wire one of copper and other of

AHLCON PUBLIC SCHOOL

ASSIGNMENT - I

CLASS – XII PHYSICS (SESSION: 2017-18)

UNIT I : ELECTROSTATICS

1. Two point charges of unknown magnitude and sign are placed at a distance ‘d’ apart. The electric field

intensity is zero at a point, not between the charges but on the line joining them. Write two essential

conditions for this to happen.

2. There is a certain net flux through a Gaussian sphere of radius ‘r’ enclosing an isolated charged

particle. Suppose the enclosing Gaussian surface is changed to

a) A larger Gaussian sphere.

b) A Gaussian cube with edge length equal to ‘r’ c) A Gaussian cube with edge length equal to ‘2r’

In each case, is the net flux through the new Gaussian surface larger than, smaller than or equal to ?

3. In which orientation, a dipole placed in a uniform electric field is in (i) stable (ii) unstable equilibrium?

4. A spherical rubber balloon caries a charge that is uniformly distributed over its surface. As the balloon

is blown up and increases in size, how does the total electric flux coming out of the surface change?

Give reason.

5. Electric lines of force due to two point charges q1 and q2 placed at points A and B respectively are

shown in the fig. Write the nature of charge on them.

6. In which of the following two cases, more work will be done in increasing the separation between the

plates of a charged capacitor and why?

i) The charging battery remains connected to the capacitor.

ii) The battery is removed after charging the capacitor.

7. How much work is done in deflecting an electric dipole of dipole moment p in a uniform electric field E

through 180o from the equilibrium position?

8. Two protons A and B are placed between two parallel plates having a potential difference V as shown in

the figure.

Will these protons experience equal or unequal force?

9. Why does the electric field inside a dielectric decrease when it is placed in an external electric field?

10. Can the electric potential be zero at a point where the electric field is not zero?

NUMERICALS

1. Two small identical electrical dipoles AB and CD each of dipole moment ‘p’ are kept at an angle of

120o as shown in the figure. What is the resultant dipole moment of this combination? If this system is

subjected to electric field

E along +X direction, what will be the magnitude and direction of the

torque acting on this? (P, PE/2).

2. A hollow cylindrical box of length 1m and area of cross section 25cm2 is placed in a three dimensional

co-ordinate system as shown in the fig. The electric field in the region is given by ^

50 ixE

where E is

in N/C and x is in metre. Find

i) Net flux through the cylinder.

ii) Charge enclosed by the cylinder. CNm /10125 23 CQ15101107

3. A 10 F capacitor is charged by a 30V d.c. supply and then connected across an uncharged 50 F

capacitor. Calculate (i) the final potential difference across the combination (ii) the initial and final

energies. How will you account for the difference in energy?

(5V, JJ33 1075.0,105.4 )

4. A test charge ‘q’ is moved without acceleration from A to C along the path from A to B and then from B

to C in electric field E as shown in the fig.

i) Calculate the potential difference between A and C.

ii) At which point (of the two) is the electric potential more and why? (-4E, C)

5. Calculate the equivalent capacitance of the combination between A and B. Also calculate the total

charge that flows in the circuit when 100V battery is connected between the points A and B. (20 F ,

C3102 )


ASSIGNMENT - 2


CHAPTER : CURRENT ELECTRICITY

1. Two wire one of copper and other of maganin have same length and resistance. Find which wire is

thicker.

2. Two resistance wire made of same material but with different length and diameter are connected in

series. Show how does the drift velocity of free e depends upon their length and diameter.

3. If the two graph A and B are drawn between V and I find,

i) Which graph show high temperature if these are drawn for resistance at different temperature.

ii) Find which shows series and which one shows parallel combination. If these two are drawn for

series and parallel combination of two resistance.

4. Find the potential difference between point A and B?

(Ans. 13 V)

5. State the condition in which terminal voltage across a secondary cell is equal to its e.m.f.

6. Give the colour code of 5 and 10 resistance wire with 10% tolerance.

7. What is the effect on power drawn by a lamp after some time of switching on it.

8. Two coils have a combined resistance of 16 when connected in series and 3 when connected in

parallel. Find the resistance of each coil.

9. What happens to the reading of galvanometer if the galvanometer and cell are interchanged at the

balance point of the bridge?

10. Two identical cells when joined in series or in parallel supply same current through an external

resistance of 1 . Find internal resistance of each cell. (Ans. 1 )

11. Explain how electron mobility changes for a good conductor when

i) The temperature of the conductor is decreased at constant potential differences and

ii) Applied potential difference is doubled at constant temperature.

12. Open circuit voltage of a cell is 6V and short circuit current is 6Amp. Find how much current same cell

will supply when is connected to a load resistance of 5 (Ans.1amp)

13. Give the condition when meter bridge is most sensitive and explain why we cannot find very high or

very small resistance using meter bridge.

2A

14. In the circuit when key K1 is closed and K2 is open current drawn from the battery of negligible

internal resistance is 3A. When both the keys are closed galvanometer does not show any deflection.

Find the value of resistance X and Y.

(Ans. X = 4 , y = 8 )

15. In the circuit diagram shown here what should be the value of R so that there is no current in the branch

containing 6V battery?

(Ans.

12 )

16. A 10m long wire of uniform cross – section and 20 resistance is used in a potentiometer. The wire is

connected in series with a battery of 5V along with an external resistance of 480 . In an unknown emf

E is balanced at 6m length of the wire, calculate

i) The potential gradient of the potentiometer wire

ii) The value of unknown emf E. (Ans. .02V/m, 0.12V)

17. Find the current through 3 , 5 and 9 resistance in the circuit shown.

( Ans. 23/37A, 6/37 A, 12/37 A)

18. A potentiometer wire of length 1m and resistance 5 is supplied by a cell of emf 6V through a series

resistance of 5 . Then find,

i) Potential gradient along the potentiometer wire.

ii) Unknow EMF of the cell which is balanced at 75cm length of this wire.

iii) Maximum EMF which can be measured (Ans. 3V/m, 2.25V, 3V)

19. A silver wire has a resistance 2.1 at 27.5oC and a resistance of 2.7 at 100

oC. Determine the

temperature coefficient of the resistivity of silver. (0.0038/oC)

20. Two resistor of 12 and 6 when connected in series with a battery of negligible internal resistance;

total power consumed in P. What will be the total power consumed when, these two resistance are

connected in parallel across the same supply. (Ans. 9P/2)


ASSIGNMENT - 3


CHAPTER - MAGNETIC EFFECT OF CURRENT & MAGNETISM

1. A straight conductor AB lies along the x axis from 2

ax

to

2

ax

and carries a current I. What is the

magnetic field due to this conductor at a point ax ?

2. A charge ‘q’ moving along the x axis with a velocity

v is subjected to a uniform magnetic field B acting

along the z- axis as it crosses the origin O.

i) Trace its trajectory

ii) Does the charge gain kinetic energy as it enters the magnetic field.

3. A charged particle moving in a uniform magnetic field penetrates a layer of lead and thereby loses one- half of its

kinetic energy. How does the radius of curvature of its path change?

4. Two long wires carrying currents I1and I2 are arranged as shown in fig. Find the force exerted at O2 because of

the wire along the x – axis.

5. A charged particle moves in a uniform magnetic field at right angles to the direction of field. Which of the

following quantities will change? Speed, velocity, momentum, kinetic energy.

6. A beam of electrons projected along +x axis experiences a force along the +y axis is due to a magnetic field.

What is the direction of magnetic field?

7. The coils in certain galvanometers, have a fixed core made of a non – magnetic metallic material. Why does the

oscillating coil comes to reset so quickly in such a core?

8. In fig. the straight wire AB is fixed while the loop PQRS is free to move. In which direction does the loop begin

to move?

9. Two wires of equal lengths are bent in the form of two loops. One of the loop is square shaped where as the other

loop is circular. These are suspended in a uniform magnetic field and the same current is passed through them.

Which loop will experience greater torque? Give reason.’

10. How will the angle of dip vary when one goes from a place, where the acceleration due to gravity is maximum, to

a place where it is minimum on the surface of earth?

11. A long wire is bent as shown in figure. What will be the magnitude and direction of the field at the centre O of

the circular portion, if a current I is passed through the wire? Assume that the various portions of the wire do not

touch at point P.

[Ans: ]1

12

0upward

r

IB

12. A copper wire having a resistance of 0.01 per metre is used to wind a 400 turn solenoid of radius 1.0cm

and length 20cm. Find the e.m.f of a battery which when connected across the solenoid would produce

a magnetic field of 210 T near the centre of the solenoid.

[Ans: IV] 13. A galvanometer with a scale divided into 100 equal divisions has a current sensitivity of 10 dimensions per mA

and a voltage sensitivity of 2 divisions per mV. What adoptions are required to read.

i) 5A for full scale and

ii) ii) 1 division per volt? [Ans: i) 499

5sR ii) R=9995 ]

14. A galvanometer needs 50mV for a full scale deflections of 50 divisions. Find its voltage sensitivity.

What must be its resistance, if its current sensitivity is 1 division / A ?

[Ans: i) 1310 divVVs , ii) 1000gR ]

15.

For what value of current in the coil does a small magnetic needle, kept at origin, remains undeflected?

What is the direction of current in the coil?

16. Fig: Shows the variation of intensity of magnetization (M) v ersus the applied magnetic field intensity

(H) for two magnetic materials A and B.

a) Identify the materials A and B.

b) Draw the variation of susceptibility with temperature for B.

17. A short magnet of magnetic moment 0.9J/T is placed with its axis at 45o to a uniform magnetic field. If

it experiences a torque of 0.063 J: (a) calculate the magnitude of magnetic field and (b) what orientation

of the bar magnet corresponds to the stable equilibrium in the magnetic field.

[ Ans: B = 0.099T]

AHLCON PUBLIC SCHOOL, MAYUR VIHAR – I, DELHI – 91.

ASSIGNMENT-5


ELECTROMAGNETIC WAVES.

Q.1 Which part of the electro magnetic spectrum has highest frequency?

Q.2 Why are micro waves used in RADAR?

Q.3 What is the order of magnitude of frequency of vibration of the longest and shortest waves in electro

magnetic spectrum?

Q.4 Arrange the given electromagnetic radiation in the descending order of their frequencies: Infra red, X –

rays, Ultraviolet and Gamma rays.

Q.5 Which part of electromagnetic spectrum has largest penetrating power and highest frequency?

Q.6 Identify the part of the electromagnetic spectrum which is

i) Suitable for radar system used in aircraft navigation.

ii) Adjacent to the low frequency end of the em spectrum.

iii) Produced in nuclear reactions.

iv) Produced by bombarding a metal target by high speed electrons.

Q.7 Write the order of frequency range and one use of each of the following electromagnetic radiations:

a) Microwaves b) Ultraviolet rays c) Gamma rays.

Q.8 Name the constituent radiation of electromagnetic spectrum which:

i) is used in satellite communication

ii) is used for studying crystal structure

iii) is similar to radiations emitted during decay of radio active nuclei

iv) has its wavelength range between 390 nm and 770 nm.

v) produce intense effect

vi) is absorbed from sunlight by ozone layer.

Q.9 Find the wavelength of EM waves of frequency Hz9104 in free space. Give its two application.

Q.10 Draw a sketch of a plane electromagnetic wave propagating along the 3 – direction. Depict clearly the

direction of electric and magnetic field varying sinudoially with Z.

Q.11 Write the relationship between amplitude of electrical and magnetic field in free space.

Q.12 The oscillating magnetic field in plane EMW is given by xtSinBy 300102108 116 Tesla

i) Calculate the wavelength of Em wave

ii) Write down the expression for the oscillating electric field.

Q.13 What is the ratio of speed of rays and radio waves in vaccum?

Q.14 State Ampere’s circuital law modified by Maxwell.

Q.15 The wavelength of electromagnetic radiation is doubled. What will happen to the energy of the photon?

Q.16 A variable frequency A.C source is connected to a capacitor. How will the displacement current change

with decreases in frequency?

Q.17 Give two characteristic of EM waves. Write the expression for velocity of EM wave in terms of

permittivity and permeability of the medium.


ASSIGNMENT - 6


CHAPTER –OPTICS

1. Why are mirrors used in search lights parabolic and not concave spherical?

2. A concave mirror is held in water. What would be the change in the focal length of the mirror?

3. A lens of glass is immersed in water. What will be its effect on the power of the lens?

4. A convex lens made of glass of refractive index L is immersed in medium of refractive index M. How

will the lens behave when L <M?

5. Vehicle moving in foggy weather use yellow colour Headlights. Why?.

6. Does the apparent depth of a tank of water change if viewed obliquely? If so does the apparent depth increase or

decrease?

7. A person can see the objects lying between 25 cm and 10m from his eye. His vision can be corrected by using

lens of power 0.1D. Is the statement true or false?

8. A compound microscope has a magnification of 30. The focal length of the eye piece is 5 cm. Assuming that

final image is formed at the least distance of distinct vision (25)cm. Calculate the magnification produced by the

objective. (5)

9. A telescope consist of two lenses of focal lengths 20cm and 5cm. Obtain magnifying power when final image is

(a) at infinity (b) at 25cm from the eye.

(a) .4 (b) –4.8

11. Why does the intensity of secondary maxima go on decreasing with distance from the central maximum?

12. Why do we fail to observe the diffraction pattern from a wide slit illuminated by a monochromatic light?

13. Yellow light is replaced by X rays in a single slit diffraction experiment with slit width 0.6 mm, how will the

diffraction pattern be affected?

14. The phase difference between two light waves emerging from the slits of young’s experiment is radians. Will

the central fringe be bright or dark?

15. What will happen to the interference pattern in young’s experiment if the source is not exactly on the central line

between the slits?

16. If the Coherent Sources are placed (i) infinitely close to each other and (ii) far apart from each other. Whether

interference pattern is observed.

17. In a single slit diffraction experiment the size of the slit is increased three times.

What changes are observed in (i) the intensity (ii) the width of the central maxima.

18. A single slit diffraction set up is immersed completely in water without any other change.

How is the width of the central maximum is affected?

19. Name the various processes used to polarize an unpolarised beam of light.

20. A polarizer and an analyser are so oriented that intensity of light transmitted is maximum. What fraction of

maximum light is transmitted when analyzer is rotated through an angle of 60o.

21. Explain, why on viewing clear blue part of the sky through a Polaroid, the intensity of transmitted light varies as

Polaroid is rotated?

22. Two polaroids are placed at 90o to each other. The intensity of transmitted light is zero. Another Polaroid is

introduced between the two making an angle of 45o with either. What is the intensity of the light transmitted?

23. An equiconvex lens is cut into two halves horizontally and vertically. In each case what is the ratio of the focal

length of each half to the focal length of complete lens? (Ans. 1, 2)

24. A convex air bubble trapped in a tank of water behaves as a converging or a diverging lens. Why?

25. A glass lens of power –5D is completely immersed in water. Does the power of the lens increase or decrease.

Explain?

26. How will the magnifying power of the refracting type astronomical telescope is affected on increasing for its eye

piece?

a) Focal length b) Aperture

27. A parallel light is incident on a lens of refractive index 1 kept in a medium of refractive index 2. Draw the

path of light emerging from convex lens if

a) 1 is greater than 2

b) 1 is equal to 2

c) 1 is less than 2

28. Violet light is incident on a converging lens of focal length f. State with reason how focal length of the lens will

change if the violet light is replaced by red light?

29. A monochromatic light of wavelength 589mm is incident from air on a water surface what are the wavelength,

frequency and speed of a

a) Reflected light b) Refracted light

[Ans. (a) no change , b) 443mm, no change, 2.25x108)]

30. A pond of depth 40 cm is half filled with an oil of refractive index 1.4 and other half is filled with water of

refractive index 1.33. What is the apparent depth of the pond when viewed normally?

31. What is the direction in which a fish under water sees the setting sun. Refractive index of the water is 1.33 (Ans.

4103’ with horizontal)

32. Glass has a refractive index of 1.5. A convex lens made of this glass has a focal length of 20cm in air. If

immersed in water of refractive index 4/3 calculate the changed focal length of the lens? (ans. 80 cm)

33. A beam of light converges to a point P. A lens is placed in the path of the convergent beam 12 cms from P. At

what point the beam converge if the lens is :

a) A convex lens of focal length 20 cm

b) Or concave lens of focal length 16cm

(Ans. 7.5 cm, 48cm)

34. An illuminated object and screen are placed 90 cm apart. What is the focal length and nature of the lens required

to produce a clear image on the screen, twice the size of the object.

(Ans :- a convex lens of f = 20 cm)

35. A screen is placed 90 cm from an object. The image of object on a screen is formed by a convex lens in two

different positions separated by 20 cm. What is the focal length of the lens. (Ans.21.4 cm)

36. Find the ratio of intensities of two points x and y on a screen in a young’s experiment, where wave from S1 and S2

have path difference of (i) 0 and (ii) /4. (Ans. 2:1)

37. In Young’s double slit experiment, what is the intensity at a point on screen where two waves arrive having a

phase difference of (1) 60o (ii) 90

o (iii) 120

o ( Ans.- 3Io , 2Io , Io)

38. Two coherent sources have intensities in the ratio of 81:1, what is the ratio of maximum to minimum

intensity (Ans: 49:9)

39. A slit of width ‘d’ is illuminated by light of wavelength 6500Ao. For what values of d will the (i) First minimum

fall at an angle of diffraction 30o

(ii) First maximum fall at an angle of diffraction 30o

40. The critical angle of incidence of water for total internal reflection is 48o for certain wavelength. What is the

polarizing angle and angle of refraction for light on water at this angle? (Ans.53o22´, 36

o 38´)

41. Two sources of light of wavelengths 2500A and 3500A are used in young’s experiment simultaneously. Find out

at which order of the two wavelengths the fringes coincide?

(Ans. 5:7)


ASSIGNMENT - 7 & 8


CHAPTER – DUAL NATURE OF MATTER AND RADIATION ATOMS & NUCLEI.

Conceptual questions:-

Q.1 What determines the maximum velocity of photo electrons?

Q.2 What is the dimensional formula of mv

h?

Q.3 Why are alkali metals most suitable for photo electric emission?

Q.4 The difference between nth

and thn )1( Bohr’s radius of hydrogen atom is equal to th

n )1( Bohr’s

radius. What is the value of n?

Q.5 What is the physical meaning of ‘negative energy of an electron’?

Q.6 For a hydrogen like atom, if electrons move from lower energy level to higher energy levels, then what

will happen to its kinetic energy and potential energy?

Q.7 When 7

3 Li is bombarded with a certain particle, two alpha particles are produced. Identify the

bombarded particle.

Q.8 Why are neutrons considered as ideal particle for nuclear reaction?

Q.9 The graphs between the stopping potential ‘V’ and frequency ‘r’ of the incident radiation on two

different metal plates ‘P’ & ‘Q’ are shown in the fig.

i) Which metal, out of P and Q has the greater value of the work function?

ii) What does the slope of the line depict?

Q.10 Ultraviolet light is incident on two photosensitive materials having work functions W1 and W2

(W1>W2). In which case will the kinetic energy of the emitted electrons be greater? Why?

Numericals:-

Q.1 Two nuclei have mass numbers in the ratio 2 : 5. What is the ratio of their nuclear densities? [Ans: 1:1]

Q.2 The radioactive nuclei X and Y initially contain equal number of atoms. Their half life is 1 hour And 2

hours respectively. Calculate the ratio of their rates of disintegration after two hours. [Ans: 1:1]

Q.3 The half life of U238

92 against - decay is S17105.1 . What is the activity of the sample of U

238

92 having

201025 atoms? [Ans – 1155.sec-1

].

Q.4 The binding energy of deuteron 2

1 H and - particle 4

2 He are 1.25 and 7.2 MeV/nucleon

respectively. Which nucleus is more stable? Calculate binding energy / nucleon of 56

26 Fe .

[Given: 56

26 Fem =55.934939 amu

007825.1protonm amu

amuneutronm 008665.1 ]

Q.5 For photo electric effect in sodium, the figure shows the plot of cut – off voltage vs frequency of

incident radiation. [Ans: Hzfo14105.4 , JWo

20107.29 ]

ν×10

14Hz

Calculate (i) the threshold frequency (ii) The work function for sodium.

Q.6 The energy of an atom of an element X is shown the diagram. A photon of wavelength 620 nm is

emitted. This corresponds to which of the transitions: A, B, C, D or E?

[Ans: D]

Q.7 The electron in a hydrogen atom having energy -0.85 eV makes a transition to a state with energy -3.4

eV. Calculate the wavelength of the emitted photon. [Ans: m7107.4 ]

Q.8 How many disintegrations / Sec. will occur in one gram of 238

92U , if its half – life against alpha decay is

171042.1 sec? [Ans: Sec/1023.1 4 ]

VO

Ionisation


ASSIGNMENT – 9


CHAAPTER– SEMICONDUCTOR ELECTRONICS

Q.1 At what temperature would an intrinsic semi conductor behave like a perfect insulator?

Q.2 How does the conductance of a semi conducting material change with rise of temperature?

Q.3 State the factor, which controls

i) Wavelength of light and ii) intensity of light, emitted by a LED.

Q.4 How is the band gap Eg. of a photo diode related to the maximum wavelength m , that can be detected

by it?

Q.5 What happens when both the emitter and the collector of a transistor are forward biased?

Q.6 Can the emitter – base junction of a P – n – P transistor be used as a rectifier diode? Explain.

Q.7 In a transistor, the base region is lightly doped why?

Q.8 The gain of the CE amplifier is given by LmRgA . Does it means that if we keep on increasing LR , the

gain of the amplifier will also increase indefinitely? Explain.

Q.9 The diagram shown a piece of pure semiconductor S in series with a variable resistor R, and a source of

constant voltage V. Would you increase or decrease the value of R to keep the reading of ammeter (A)

constant, when S is heated? Give reasons.

Q.10 A semi conductor has equal electron and hole concentrations of 38 /102 m . On doping with a certain

impurity, the hole concentration increases to 310 /104 m .

a) What type of semi conductor is obtained on doping?

b) Calculate the new electron concentration of semi conductor. [Ans: 106/m

3]

c) How does the energy gap vary with doping?

Q.11 State two reasons why a CE amplifier is preferred to a CB amplifier.

Q.12 What will be the values of input A and B for the Boolean expression BA . 1. BA

Q.13 If the resistance 1R is increased, how will the readings of ammeter and voltmeter change?

Q.14 A germanium Pn junction is connected to a battery with milliammeter in series. What should be the

minimum voltage of battery so that current may flow in milliammeter?

Q.15 Ge and Si junction diodes are connected in parallel. A resistance R, a 12V battery, a milliammeter (mA)

and key (K) are connected in series with them. When key (K) is closed, a current begins to flow in the

milliammeter. What will be the maximum reading of voltmeter connected across R?

[Ans. 11.7 V]

Q.16 A change of 0.2mA in the base current causes a change of 5 mA in the collector current for a common

emitter amplifier.

i) Find the a.c current gain of the transistor.

ii) If the input resistance is 2K , and its voltage gain is 75, calculate the load resistor used in the

circuit. [Ans. i) 25, ii) 6k ]

Q.17 A zener of power rating 1W is to be used as a voltage regulator. If zener has a break down of 5V and it

has to regulate voltage which fluctuated between 3V and 7V, what should be the value of Rs for safe

operation. [Ans. 10 V ]

Q.18 The circuit shown below has two diodes each with forward resistance 50 with infinite reverse

resistance. If the battery voltage is 6V, find the current through the 100 resistance.

Q.19 Draw the output waveform at X, using the given inputs A, B for the logic circuit shown below. Also

identify the gate.

Q.20 For the given combination of gates, find the values of outputs, y1 and y2 in the table given below.

Identify the gates C1 and C2.

Q.21 A P – n junction, when forward biased has a drop of 0.7V which is assumed to be independent of

current. If a 5V battery is to be used to forward bias it, calculate the value of the resistance which

should be used in series with it, for 1mA current to flow in the circuit.

[ Ans: 3103.4 ]

Q.22 The input resistance of a silicon transistor is 665 . Its base current is changed by 15 A which results

in change of collector current by 2mA. This transistor is used as a common emitter amplifier with a load

resistance of 5K . Calculate

i) Current gain

ii) Trans conductance gm

iii) Voltage gain Av of the amplifier. [Ans: i) 133 ii) 0.2 -1, iii) 1000]

Q.23 You are given a circuit below. Write its truth table. Hence identify the logic operation carried out by

this circuit. Draw the logic symbol of the gate it corresponds to.


ASSIGNMENT – 10


CHAPTER – COMMUNICATION SYSTEMS.

Q.1 Name the types of communication system according to the mode of the transmission.

Q.2 Identify the parts X and Y in the following block diagram of a generalized communication system.

X Transmitter Y Receiver

Q.3 Distinguish between sky wave and space wave propagation. Give a brief description with the help of

suitable diagrams indicating how these waves are propagated.

Q.4 Why are high frequency carrier waves used for transmission?

Q.5 Give two examples of communication system which use space wave mode.

Q.6 What type of modulation is required for television broad cast?

Q.7 What is the purpose of modulating a signal in transmission?

Q.8 Would sky waves be suitable for transmission of TV signals of 60 MHz frequency.

Q.9 Two waves A and B of frequencies 2 MHz and 3 MHz respectively are beamed in the same direction for

communication via sky wave. Which one of these is likely to travel longer distance in the ionosphere

before suffering total internal reflection.

Q.10 Explain the function of a repeater in a communication system.

Q.11 Define modulation index. Why is the amplitude of modulating signal kept less than the amplitude of

carrier wave?

Q.12 State two factors by which the range of TV signal can be increased.

Q.13 Draw a plot of variation of amplitude versus w for an amplitude modulated wave.

Q.14 The diagram given below shows a block diagram of a detector for AM signals.

AM WAVE I II Output modulating signal

(b) (c)

Label the unlabelled boxes and show the wave form corresponding to the positions indicated by arrows

(b) and (c)

NUMERICALS

Q.15 A carrier wave of peak voltage 12V 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: 9V)

Q.16 By what percentage will the transmission range of a TV tower be affected when the height of the tower

is increased by 21%. (Ans: 10%)

Q.17 A transmitting antenna at the top of a tower has a height of 36m and the height of the receiving antenna

is 49m. What is the maximum distance between them, for satisfactory communications in the LOS

mode? (Radius of the earth = 6400m) (Ans: 46.2Km)

Q.18 A T.V tower has a height of 400m at a given place. Calculate its coverage range, if the radius of the

earth is 6400 Km.

[Ans. 1.608 1010

m2]


ASSIGNMENT – 11


CHAPTER - PRINCIPLES OF COMMUNICATION

1 A modern communication system can be represented as:

The three basic elements which constitute a communication system are

a) Transmitter: A transmitter processes the incoming message signal so as to make it suitable for

transmission through a channel and subsequent reception.

b) Receiver: A receiver extracts the desired message signals from the received signals at the channel

output.

c) Channel: It is the medium through which the electrical signals from transmitter pass, to reach the

receiver. It is either space or conductors or optical fibres.

2. Modes of communication: There are two basic modes of communication: point – to – point and broadcast.

Point – to – Point: - Communication takes place over a link between a single transmitter and

a receiver.

Example:- Telephony.

Broadcast:- There are a large no. of receivers corresponding to a single transmitter.

Example: Radio & T.V.

3. Analog (Continuous ) Signal. Analog signals are continuous variations of voltage or current. They are

essentially single – valued functions of time.

Since wave is a fundamental analog signal. Sound and picture signals in TV are analog in nature.

4. Digital signals are those which can take only discrete stepwise values.’ Binary coded decimal (BCD) and American Standard Code for Information Interchange (ASCII) are

popular digital code to represent numbers, letters and certain characters.

Binary system that is extensively used in digital electronics employs just two levels of a signal.’0’ corresponds to low level and ‘1’ corresponds to a high level of voltage / current.

5. Transducer:- Any device that converts one form of energy into another can be termed as a transducer.

An electrical transducer may be defined as a device that converts some physical variable (pressure,

displacement, force, temperature etc) into corresponding variations in the electrical signal at its output.

Microphone, loudspeaker and photo detector are examples of transducers.

6. Attenuation:- Power loss or loss of information strength while propagating through a medium is called

attenuation.

Communication system

Message Transmitted Received Message

Information Transmitter Channel Receiver User of

Source Signal Signal Signal Signal information

Noise

Block diagram of a generalized communication system.

7. Amplification. It is the process to boost the signal. Amplification is done at a place between the source

and the destination wherever signal strength becomes weaker than the required strength.

8. Range:- It is the largest distance between a source and a destination upto which the signal is received with

sufficient strength.

9. Repeater:- To increase the range of the transmission of microwaves, number of in between sets of

receivers and transmitters are erected. A repeater, picks up the signal from the transmitter, amplifies and

retransmits it to the receiver sometimes with a change in carrier frequency.

10. Bandwidth:- It is the frequency range over which an equipment operates or the portion of the spectrum

taken by the signal.

11. Bandwidth of some signals

Signal Frequency range Bandwidth required

Speech 300 – 3100 Hz 3100 – 300 = 2800 Hz

Music High frequencies 20 kHz

Produced by musical

Instrument

(audible range = 20Hz – 20kHz)

Video - 4.2 MHz

TV Contains both voice and Picture 6MHz

12. Bandwidth of transmission media.

Service Frequency range Remarks

1 Wire (Most common:

coaxial cable)

750 MHz

(Bandwidth)

Normally operated

below 18GHz.

2

Free space

(radio waves) 540 kHz – 4.2 GHz

i) Standard AM

Broadcast 540 – 1600 kHz

ii) FM 88 – 108 MHz

iii) Television

54 – 72 MHz

76 – 88 MHz

174 – 216 MHz

420 – 890 MHz

VHF (Very high

frequencies) TV

UHF (Ultra high

frequencies) TV

iv) Cellular mobile radio

896 – 901 MHZ

840 – 935 MHZ

Mobile to base station

Base station to mobile

v) Satellite

Communication

5.925 – 6.425 GHz

3.7 – 4.2 GHz

Uplinking

Downlinking.

3 Optical communication using

using fibres

ITHz – 1000 THz

(microwaves – ultra

violet)

One single optical fibre

offers

Bandwidth> 100GHz

13. Space Communication refers to communication process utilizing the physical space around the earth.

Modes of propagation are:

a) Ground wave propagation

b) Sky wave propagation

c) Space wave propagation.

14. Ground Wave Propagation

a) The radio waves which travel through atmosphere following the surface of earth are known as ground

waves or surface waves and their propagation is called ground wave propagation or surface wave

propagation.

b) The attenuation of surface waves increases very rapidly with increase in frequency. Thus ground wave

propagation is not suitable for high frequency waves.

c) The ground wave propagation is suitable for low and medium frequency i.e. upto 20 MHz only.

d) The ground wave propagation is generally used for local band broadcasting and is commonly called

medium wave.’ e) The maximum range of ground or surface wave propagation depends on two factors.

i) the frequency of the radio waves and

ii) power of the transmitter.

15. Sky Wave propagation

a) The sky wave propagation is based on the ionospheric reflection of radio waves.

b) It is suitable for short wave broadcast services for frequency between 3MHz to 30 MHz.

c) EM waves of frequencies higher than 30 MHz penetrate the ionosphere and escape and thus sky wave

propagation is not suitable for higher frequencies.

d) The signals received due to sky wave propagation are subjected to fading in which the strength of

signal varies with time.

16. Space wave propagation.

a) The space waves are the radiowaves of very high frequency (i.e between 30 MHz to 300 MHz or

more)

b) At these frequencies, the antennas are relatively smaller.

c) The space waves can travel through atmosphere from transmitter antenna to receiver antenna either

directly or after reflection from ground in the earth’s troposphere region. That is why the space wave

propagation is also called as tropospherical propagation or line of sight propagation.

d) The Line of Sight distance (LOS) is the distance between transmitting antenna and receiving antenna

at which they can see each other which is also called range of communication. The LOS is limited to

frequencies above 40MHz as at these frequencies antennas are relatively smaller and can be placed at

heights of many wavelengths above the ground.

e) The range of communication of space wave propagation can be increased by increasing the heights of

transmitting and receiving antenna.

f) The space wave communication is utilized in television communication, radar communication and

microwave communication.

17. Coverage of T.V transmission.

Distance wise (Range):

The maximum distance (d) up to which the TV signal can be received, from a TV antenna of height (h)

is

Rhd 2

Where R is radius of earth

If hT and hR be the heights of the transmitting and receiving antennas above the earth, then the maximum

line of sight distance dM between the two antennas is

RTM RhRhd 22

Area covered by a TV tower of height h

RhA 2 (when h<<R)

Population covered = population density area covered.

18. Satellite communication is a mode of communication of signal between transmitter and receiver through

satellite.

19. Factors affecting the choice of transmitting frequency:

A. Size of the antenna or aerial. Antenna should have a size comparable to the wavelength of the signal

(at least 4/ in dimension) so that the time – variation of the signal is properly sensed by the antenna.

Since f/1 , the information contained in low frequency baseband signal has to be translated into

high or radio frequencies before transmission.

B. Effective power radiated by an antenna. Power radiated (length of antenna / 2) i.e, for the same

antenna length, the power radiated by short wavelength or high frequency signals would be large. For

a good transmission, we need high powers and hence, this required use of high frequency

transmission.

C. Mixing up of signals from different transmitters. This can be solved by using communication of high

frequencies and then allotting a band of frequencies to each user.

20. Modulating / Input / Baseband signal. These are electrical voltages or currents which contain the

information to be transmitted or to be retrieved.

21. Modulation is the process by which some characteristic (called parameter) of carrier signal is varied in

accordance with the instantaneous value of the baseband signal.

For sinusoidal continuous carrier waves, the types of modulation are:

i) Amplitude Modulation (AM)

ii) Frequency Modulation (FM)

iii) Phase Modulation (PM)

For pulsed carrier waves, the various modes of modulation are:

i) Pulse Amplitude Modulation (PAM)

ii) Pulse Time Modulation (PTM)

a) Pulse Position Modulation (PPM)

b) Pulse Width Modulation (PWM) or Pulse Duration Modulation (PDM)

iii) Pulse Code Modulation (PCM)

PCM is the preferred modulation scheme for digital communication while others are more suited

to analog system.

22. Need for Modulation

i) To avoid interference.

ii) To design antennas of practicable size.

iii) Variation required in antenna size can be reduced to minimum.

23. Modulated signal. The output signals from modulator are known as modulated signals.

24. AM (Amplitude Modulation). The information signal is used to vary the amplitude of the carrier so that

the profile of amplitude of the carrier wave follows the signal shape of the information signal.

If twAtc cC sin)( ……. Carrier wave

twAtm mm sin)( ……. Message signal

i) Modulation Index, C

m

A

A

= Amplitude of message signal/ Amplitude of carriers wave.

is kept 1 to avoid distortion.

ii) Side bands are mc ww and mc ww . mc ww is called lower side frequency and mc ww

is called upper side frequency.

iii) Message signals are also known as base band or modulating signals.

If the broadcast frequencies (carrier waves) are sufficiently spaced out so that sidebands do not

overlap, different stations can easily operate without interfering with each other.

25. FM (frequency Modulation). The instantaneous value of the angular frequency of carrier signal 1 is

varied such that it is equal to the frequency c of the unmodulated carrier signal plus a time varying

component proportional to modulating signal plus a time varying component proportional to modulating

signal VM(t).

26. PM (Phase Modulation). Here the phase angle of the carrier signal varies in accordance with the

modulating voltage.

A sinusoidal carrier wave

Am modulating signal

Amplitude modulation

Frequency modulation

Phase modulation.

Amplitude modulation index

Ac

Amma

For modulated wave

minmax

minmax

AA

AAma

27. Production of Amplitude Modulated Wave.

Block diagram of a simple modulator for obtaining an AM signal

The modulated signal cannot be transmitted as such. The modulator is followed by a power amplifier to

provide the necessary power and then it is fed to an antenna of proper size for final radiation.

Block diagram of a transmitter

28. Reception of Amplitude Modulated Wave

Block diagram of a receiver

If refers to “intermediate frequency”

29. Detection of Amplitude Modulated Wave. Detection is retrieving the modulating signal from the

modulated carrier wave.

Block diagram of a detector for AM signal

The quantity on y – axis can be current or voltage.

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AHLCON PUBLIC SCHOOL CLASS XII PHYSICS … PUBLIC SCHOOL ASSIGNMENT - 2 CLASS ± XII PHYSICS (SESSION: 2017-18) CHAPTER : CURRENT ELECTRICITY 1. Two wire one of copper and other of