THIRD TERMINAL EXAMINATION 2011-‘12
042- PHYSICS
(Theory) Max.
Marks: 70
CLASS XII Time : 3 hours.
General Instructions:
(i) All questions are compulsory.
(ii)
There are 30 questions in total.
Questions 1 to 8 carry one mark each,
questions 9 to 18 carry two
marks each, questions 19 to 27 carry three marks
each and questions 28 to 30 carry five marks
each.
(iiic) There is no overall choice.
However, an internal choice has been provided in one question of two
marks, one question of three marks,
and all three questions of five marks. You have to attend only
one of the given choices in such
questions.
(iv)Use of calculators is not permitted.
1. Name the characteristics of em waves that (i) increases (ii) remains constant in the
electromagneticspectrum as one moves from radio wave region towards uv region. 1
2. Calculate the ratio of energies of photons produced
due to transition of electron of hydrogen atom from its,
(i) second permitted energy level to the first level, and (ii) highest
permitted level to the second permitted level. 1
3. The figure given below shows the variation of an
alternating emf with time. What is the average value of
the emf for the shaded part of the
graph? 1
4. Zener diodes have higher dopant densities as compared
to ordinary p-n junction diodes. How does it affect
the (i) width of the depletion layer
and (ii) junction field? 1
5. What is the
phase difference between voltage across inductor and capacitor in an LCR
circuit? 1
6. A photodiode
is operated in the reverse biased condition when used as a photodetector.
Why? 1
7. Four nuclei of an element fuse together to form a heavier nuc leus. If the process is accompanied by
release of energy, which of the two-the parent or the daughter
nucleus- would have higher binding energy per nucleon? 1
8. A radioactive isotope D decays according to the
sequence D 
D1 
D2.
If the mass number
and atomic number of D2 are 176 and 71
respectively, what is (i) the mass number (ii) atomic number of D? 1
9. Name and define the magnetic elements of earth at a
place with the help of relevant diagrams. 2
10. An inductor L of reactance XL is connected
in series with a bulb B to an ac source as shown in figure1.
Briefly explain how does the brightness of the bulb change when (i) number of
turns of the inductor is
increased and ( ii) a capacitor of reactance XC = XL is
included in series with the same circuit?
OR
Fig.2 shows
a light bulb B and iron cored inductor connected to a dc battery through a
switch (S). (i) What
will one observe when the switch S
is closed? (ii) How will the glow of the bulb change when the battery is
replaced by an ac source of rms
voltage equal to the voltage of the dc battery? Justify your answer in each
case. 2
11. How is a N-type semiconductor formed? Name the major
charge carrier in it. Draw the energy band
diagram of a P-type semiconductor. 2
12. Write the Rydberg formula for spectral transitions in a hydrogen atom and specify the symbols used. Using it,
find the shortest
wavelength in the Balmer series. 2
13. The following figure shows the input waveforms (A,B) and the output waveform (Y) of a logic gate. Identify the
gate and write its truth table. 2
14.
What do you mean
by the term LOS communication? Name the type of waves used for this
communication.
Which of the two-height of the transmitting antenna or that of the receiving antenna- can affect the range over which
this mode of communication remains effective? 2
15.
What do the terms
‘depletion region’ and ‘barrier potential’ mean for a p-n junction? 2
16. A nucleus makes a transition from one permitted energy level to another of lower energy. Name the region of e m
spectrum to which the emitted photon belongs. What is the order
of this energy in electron volt? 2
17. Draw a neat labeled diagram of the arrangement used for Rutherford’s alpha particle scattering experiment.
Define impact parameter. 2
18.
Show
diagrammatically two different arrangements used for winding the primary and
secondary coils in a
transformer. Mention two reasons for energy losses in an actual transformer and
their remedy. 2
19.
A pure inductor
is connected across an ac source. Show mathematically that current in it lags
the applied
emf by a phase angle of π/2. What is inductive reactance? Draw a graph showing
the variation of inductive
reactance with the frequency of the ac source.
3
20.
Define the terms
decay constant and half life of a radio active sample. Write their SI units.
Derive the
relation connecting the two.
3
21. Define the term amplitude modulation. We do not choose to transmit an audio signal by just directly converting
it to an e m wave of the same frequency. Give two reasons for the same.
OR
(i)
Draw the block diagram of a communication system.
(ii)
What is meant by ‘detection’ of a modulated carrier wave? Describe briefly the
essential steps for
detection. 3
22.
Answer the
following:
(i) Optical and radio telescopes are built on the ground while X-ray astronomy
is possible only from satellites
orbiting the earth. Why?
(ii) Long distance radio broadcasts use short wave bands. Why?
(iii) The amplitude of the magnetic field part of an em wave in vacuum is B0
= 300 nT. What is the amplitude
of the electric field art of the wave? 3
23.
Draw a graph to
show the variation of the binding energy per nucleon with mass number of
different
nuclei.
Calculate the binding energy per nucleon
of 26Fe56 nucleon. Given: Mass of 26Fe56 = 59.934939 u,
Mass of proton = 1.007825 u ,Mass of
neutron = 1.008665 u. 3
24.
With the help of
a labelled circuit diagram, explain briefly the rectification action of a full
wave rectifier.
Show the input and output waveforms
graphically. How is the output of a full wave rectifier smoothened? 3
25. (i) How can a galvanometer of resistance G be converted in to a voltmeter to read a maximum potential
difference of V volts? (ii) A galvanometer coil has a resistance of 30 ohm and the meter shows full scale deflection
for a current of 2 mA. Calculate the value of resistance required to convert it in to an ammeter of range 0 to
1 ampere.
3
26. The ground state energy of hydrogen atom is -13.6 eV. (i) What is the kinetic energy of an electron in the 2nd
excited state? (ii) If the electron jumps to the ground state from the 2nd excited state, calculate the wavelength of
the spectral line emitted. 3
27. How a Zener diode is differs from ordinary diodes? With the help of a circuit diagram, explain the working of a
voltage regulator using
Zener diode. 3
28.
a) With the help
of a neat and labeled diagram, explain the underlying principle and working of
an ac
generator. Write an expression for the emf
generated in the coil in terms of the speed of rotation.
(b)
An armature coil consists of 20 turns of
wire, each of area 0.09 m2 and total resistance 15
. It
rotates in a magnetic field of 0.5 T at
a constant frequency of 
Calculate the value of
(i) maximum (ii) average induced
emf produced in the coil.
OR
(a) Explain with the help of a
diagram, the principle, construction and working of a step up transformer.
(b) A power transmission line feeds
input power at 2300 V to a step down transformer with its primary
winding having 4000 turns.
What should be the number of turns in the secondary in order to get output
power at 230 V? 5
29. An inductor L, a capacitor C and a resistor R are
connected in series in an a.c.circuit. Define impedence of
the circuit and give its SI unit.
Deduce with the help of suitable phasor diagrams, a mathematical expression
for the impedence of this circuit.
What is meant by resonance of this circuit? Obtain the expression for
resonant frequency.
OR
(a) Draw the diagrams to depict the behavior of magnetic field lines near a
bar of :
(i) Copper (ii) Aluminium (iii) Mercury, cooled to a very low
temperature(4.2 K).
(b) The vertical component of earth’s magnetic field at a given place is 
times its horizontal component. If the
total intensity of earth’s magnetic
field at the place is 0.4 G, find the value of: (i) angle of dip (ii) the
horizontal component of earth’s
magnetic field. 5
30. (i) Plot the transfer
characteristics of a base biased transistor. For using transistor as an amplifier,
which
region of this characteristic
would you use?
(ii) Draw the circuit diagram of a
common emitter amplifier using n-p-n transistor and explain its working.
State the phase relation between
input and out put signals.
(iii) The current gain of a transistor
in CE configuration is 200. Find the voltage gain and power gain if load
resistance of output circuit is
400 kW and input resistance is
200 W.
OR
(i) A student has to study the input and output
characteristics of an n-p-n silicon transistor in common
emitter configuration. What
kind of a circuit arrangement should she use for this purpose?
(ii) Draw the typical shape of input and out put
characteristic likely to be obtained by her.
(iii) What do we understand by the
cut off, active and saturation states of the transistor? In which of these
states does the transistor not
remain when being used as a switch. 5