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AHLCON PUBLIC SCHOOL
Assignment I – Unit I : Electrostatics
Class : XII – Physics
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)
ii)
The charging battery remains connected to the capacitor.
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

^
E  50 x i where E is in N/C and x is in metre. Find
i)
ii)
Net flux through the cylinder.
Charge enclosed by the cylinder.   125 10 3 Nm 2 / C Q  1107 1015 C



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, 4.5 103 J ,0.75 103 J )
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 , 2 10 3 C )
AHLCON PUBLIC SCHOOL
Class : XII – PHYSICS
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)
ii)
Which graph show high temperature if these are drawn for resistance at different
temperature.
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?
2A
(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)
ii)
The temperature of the conductor is decreased at constant potential differences and
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 metre bridge is most sensitive and explain why we cannot find very high
or very small resistance using meter bridge.
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)
ii)
The potential gradient of the potentiometer wire
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)
ii)
iii)
Potential gradient along the potentiometer wire.
Unknow EMF of the cell which is balanced at 75cm length of this wire.
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 100oC. 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)
AHLCON PUBLIC SCHOOL
Class: XII – PHYSICS
Assignment 3 – Magnetic Effect of Current & Magnetism
a
a
to x 
and carries a current I. What
2
2
is the magnetic field due to this conductor at a point x  a ?
1. A straight conductor AB lies along the x axis from x 

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)
ii)
Trace its trajectory
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 onehalf 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: B 
0 I 
1
1  upward ]
2r   
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 10 2 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)
[Ans: i) Rs 
ii) 1 division per volt?
5
 ii) R=9995  ]
499
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) Vs  103 divV 1 , ii) Rg  1000 ]
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.
PHYSICS ASSIGNMENT-4
CLASS –XI
ELECTROMAGNETIC INDUCTION & ALTERNATING CURRENT
1. A metallic rod held horizontally along east-west direction, is allowed to fall under gravity. Will there be an
emf induced at its ends? Justify your answer.
2. The electric current flowing in a wire in the direction B to A is decreasing. What is the direction of induced
current in the metallic loop kept above the wire as shown in the given figure?
3. In the given figure, A and B are identical magnets. Magnet A is moved away from the coil with a given
speed. Magnet B is moved towards the coil with the same speed. What is the induced emf in the coil.
4. An iron bar falling through the hollow region of a thick cylindrical shell made of copper experiences a
retarding force. What can you conclude about the nature of iron bar?
5. Two similar co-axial loops carry equal currents in the same direction. If the loops be brought nearer, what
will happen to the currents in them?
6. Figure given below shows an arrangement by which current flows through the bulb (X) connected with coil
B, when a.c. is passed through coil A.
Explain the following observations:
a) Bulb lights up
b) Bulb gets dimmer if the coil ‘B’ is moved upwards.
c) If a copper sheet is inserted in the gap between the coils how the brightness of the bulb would change.
7. With reference to alternating currents and voltages, state any one fundamental difference between resistance
and reactance.
8. You are given an air coil, a bulb, an iron rod and a source of electricity. Suggest a method to find whether
the given source is d.c. or a.c. Explain your answer.
9. A radio frequency choke is air cored whereas an audio- frequency choke is iron – cored. Give reason for
the difference.
10. a) Out of the arrangements, given below for winding of primary and secondary coils in a transformer, which
arrangement do you think will have higher efficiency and why?
b) Show that in an ideal transformer, when the voltage is stepped up by a certain factor, the current gets
stepped down by the same factor.
c) State any two causes of energy loss in a transformer.
11. A metallic rod of length L rotated at an angular speed  normal to a uniform magnetic filed B. Derive
expressions for the
i) Emf induced in the rod
ii) Current induced and
iii) Heat dissipation, if the resistance of the rod is R.
12. Over a solenoid of 50cm length and 2cm radius and having 500 turns, is wound on another wire of 50 turns
near the centre. Calculate the
i) Mutual inductance of the two coils.
ii) Induced emf in the second coil when the current in the primary changes from 0 to 5A in 0.02 S.
[Ans i) 78.96 H
ii) 19.74 mV]
13. A 1.50 F capacitor is connected to a 220 V, 50Hz source. Find the capacitive reactance and the
current (rms and peak) in the circuit. If the frequency is doubled, what happens to the capacitive
reactance and the current? [Ans Irms = 1.04A Io = 1.47A]
14. A resistance of 2 ohms, a coil of inductance 0.01 H are connected in series with a capacitor, and
put across a 200 volt, 50Hz supply calculate:
i)
ii)
The capacitance of the capacitor so that the circuit resonates.
The current and voltage across the capacitor at resonance. (take   3 )
[Ans i) 1110 4
ii) 100 A and 303.03V]
15. A circuit draws a power of 550W from a source of 220 V, 50Hz. The power factor of the circuit is
0.8. The current in the circuit lags behind the voltage. Show that a capacitor of about
1
10  2 F will have to be connected to bring its power factor to unity.
42
16. The primary of a transformer has 400 turns while the secondary has 2000 turns. If the power
output from the secondary at 1100V is 12.1 KW, calculate the primary voltage if the resistance of
the primary is 0.2 and that of secondary is 2.0 and the efficiency of the transformer is 90%,
calculate the heat losses in the primary & the secondary coil.
[Ans 746.61 W, 242 W]
AHLCON PUBLIC SCHOOL, MAYUR VIHAR – I, DELHI – 91.
PHYSICS ASSIGNMENT-5
CLASS –XII
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)
ii)
iii)
iv)
Q.7
Q.8
Suitable for radar system used in aircraft navigation.
Adjacent to the low frequency end of the em spectrum.
Produced in nuclear reactions.
Produced by bombarding a metal target by high speed electrons.
Write the order of frequency range and one use of each of the following electromagnetic
radiations:
a) Microwaves
b) Ultraviolet rays
c) Gamma rays.
Name the constituent radiation of electromagnetic spectrum which:
i)
ii)
iii)
iv)
v)
vi)
is used in satellite communication
is used for studying crystal structure
is similar to radiations emitted during decay of radio active nuclei
has its wavelength range between 390 nm and 770 nm.
produce intense effect
is absorbed from sunlight by ozone layer.
Q.9
Find the wavelength of EM waves of frequency 4109 Hz 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 By  8 106 Sin 2 1011t  300x Tesla

i)
ii)
Q.13
 

Calculate the wavelength of Em wave
Write down the expression for the oscillating electric field.
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.
AHLCON PUBLIC SCHOOL
Class : XII - PHYSICS
Assignment 6 –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.
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?
How will the lens behave when L <M?
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.
the path of light emerging from convex lens if
Draw
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.
37.
(Ans. 2:1)
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) 90o (iii) 120o ( 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´, 36o 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)
AHLCON PUBLIC SCHOOL
Class: XII - PHYSICS
Assignment 7 & 8 – 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
Q.3
Why are alkali metals most suitable for photo electric emission?
Q.4
The difference between nth and (n  1)th Bohr’s radius of hydrogen atom is equal to (n  1) th
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 3 Li 7 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)
ii)
Q.10
h
?
mv
Which metal, out of P and Q has the greater value of the work function?
What does the slope of the line depict?
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
having 2510
Q.4
20
U against  - decay is 1.5  1017 S . What is the activity of the sample of
238
92
238
92
U
atoms? [Ans – 1155.sec ].
-1




The binding energy of deuteron 1 H 2 and  - particle 2 He 4 are 1.25 and 7.2 MeV/nucleon
respectively. Which nucleus is more stable? Calculate binding energy / nucleon of  26 Fe56 .
[Given: m 26 Fe56 =55.934939 amu
m proton  1.007825 amu
mneutron  1.008665amu ]
Q.5
For photo electric effect in sodium, the figure shows the plot of cut – off voltage vs frequency of
incident radiation. [Ans: fo  4.5 1014 Hz , Wo  29.7 10 20 J ]
VO
ν×1014Hz
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
is emitted. This corresponds to which of the transitions: A, B, C, D or E?
[Ans: D]
620 nm
Ionisation
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:   4.7 10 7 m ]
Q.8
How many disintegrations / Sec. will occur in one gram of
decay is 1.42 1017 sec? [Ans: 1.23 10 4 / Sec ]
92
U 238 , if its half – life against alpha
AHLCON PUBLIC SCHOOL
Class : XII - PHYSICS
ASSIGNMENT – 9 – 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 A  g m RL . Does it means that if we keep on increasing
RL , 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 2 108 / m 3 . On doping with a
certain impurity, the hole concentration increases to 4 1010 / m3 .
a) What type of semi conductor is obtained on doping?
b) Calculate the new electron concentration of semi conductor.
c) How does the energy gap vary with doping?
[Ans: 106/m3]
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 A  B . A.B  1
Q.13
If the resistance R1 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)
ii)
Find the a.c current gain of the transistor.
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: 4.3  103  ]
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)
ii)
iii)
Q.23
Current gain 
Trans conductance gm
Voltage gain Av of the amplifier.
[Ans: i) 133 ii) 0.2  -1, iii) 1000]
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.
AHLCON PUBLIC SCHOOL
Class : XII - PHYSICS
ASSIGNMENT – 10 – 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
(b)
Output modulating signal
(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  1010m2]
AHLCON PUBLIC SCHOOL
Class : XII - PHYSICS
PRINCIPLES OF COMMUNICATION
1
A modern communication system can be represented as:
Communication system
Message
Information
Source Signal
Transmitted
Transmitter
Received
Channel
Signal
Signal
Message
Receiver
User of
Signal information
Noise
Block diagram of a generalized communication system.
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.
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
Speech
300 – 3100 Hz
Music
Video
TV
Bandwidth required
3100 – 300 = 2800 Hz
High frequencies
Produced by musical
Instrument
(audible range = 20Hz – 20kHz)
Contains both voice and Picture
20 kHz
4.2 MHz
6MHz
12. Bandwidth of transmission media.
Service
1
Wire (Most common:
coaxial cable)
Free space
(radio waves)
i) Standard AM
Broadcast
2
Frequency range
750 MHz
(Bandwidth)
Remarks
Normally operated
below 18GHz.
540 kHz – 4.2 GHz
540 – 1600 kHz
ii) FM
88 – 108 MHz
iii) Television
54 – 72 MHz
76 – 88 MHz
174 – 216 MHz
420 – 890 MHz
896 – 901 MHZ
VHF (Very high
frequencies) TV
UHF (Ultra high
frequencies) TV
Mobile to base station
iv) Cellular mobile radio
840 – 935 MHZ
v) Satellite
Communication
Optical communication using
3
using fibres
5.925 – 6.425 GHz
3.7 – 4.2 GHz
ITHz – 1000 THz
(microwaves – ultra
violet)
Base station to mobile
Uplinking
Downlinking.
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)
ii)
the frequency of the radio waves and
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
d  2Rh
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
d M  2RhT  2RhR
Area covered by a TV tower of height h
A  2Rh
(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 1 / f , 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)
ii)
iii)
Amplitude Modulation (AM)
Frequency Modulation (FM)
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 c(t )  AC sin wc t ……. Carrier wave
m(t )  Am sin wm t ……. Message signal
Am
AC
= Amplitude of message signal/ Amplitude of carriers wave.
 is kept  1 to avoid distortion.
i) Modulation Index,  
ii)
iii)
Side bands are wc  wm  and wc  wm  . wc  wm  is called lower side frequency and
wc  wm  is called upper side frequency.
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.
Am
Ac
A max  A min
For modulated wave ma 
A max  A min
Amplitude modulation index
ma 
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.