OUTCOME OF THE STUDY AS FEEDBACK FOR IMPROVEMENT OF PERFORMANCE OF STUDENTS IN PHYSICS - XII
(Based on the 1992 Board's Examination)
3.1 GENERAL POINTS FOR IMPROVEMENT OF PERFORMANCE 1. The teachers should tell the students about the possible distribution of marks over the different value points (or different requirements) specially for a long answer question. For example distribution of 5 marks in a descriptive type question may be distributed over different aspects such as diagram, labelling, description, derivation, formula, properties, limitation, precautions, application etc. whatever may be, depending on the question.
2. If the answer is supported by a diagram, then the following points should be taken into account:
i) Placement of the diagram ii) Proper labelling iii) Appropriate reference of diagram in the answer. iv) Specific requirements in the diagram depending on the type of question e.g.
- path of rays or positions of object/image for questions on optics. - direction of electric current in electricity questions. - location of charge in electrostatic questions. - direction of vectors, if used in answers. - use of graphs depicting the requirement.
3. Regarding numerical questions, the students should be conversant with and so should be told the-following points during the class room teaching :
i) Possible distribution of marks over value points. ii) Conversion of units of different -physical quantities into same system. iii) Use of correct formula with standard notations, stating their usual meaning. iv) Use of diagram, if needed for solving numerical. v) Correct substitution of values in the formula and calculation of result in proper form. vi) Expression of result in proper unit.
4. While defining a physical quantity it should be known that it can be defined 'qualitatively' as well as 'quantitatively'. Student should not confuse between actual definition and its mathematical expression. For some physical quantities (e.g. magnetic moment), unit of the quantity should correspond to the definition stated.
5. All answers should be "brief and to the. point". The expected depth, of the answers should be to the extent given in the prescribed text books. Irrelevant or unnecessary long answers involve wastage of time. Length of the answer is usually related with the marks or the time requirement.
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The number of words mentioned in the question papers for different types of answers are only, suggestive. It may be difficult to write the answers within this specified number of words sometimes. Irrespective of the word limit an answer must contain the desired matter. This should be made clear to students in the class.
6. If a student is unable to understand or ascertain about the requirement of a question, it is advisable that the other version of the question (Hindi/English) may be read. A student has to identify the 'action verb' or the key word in the question which specifies the task.
7. It is advisable that students should be fully conversant with the 'Design' of the question paper and the different parameters involved in it (e.g. length of answers, objectives, difficulty level, unit wise marks etc.).
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3.2 TOPIC-WISE SPECIFIC OBSERVATIONS FOR IMPROVING PERFORMANCE
3.2.1 TOPIC : ELECTROSTATIC AND MAGNETISM
Relevant Questions General Observations on Answers Specific Remarks,
To serve as feed-back for improvement if any
1. Draw lines of force to i. The concept of terms like electric field, coulomb, i. Appropriate reinforcement is required.
represent a uniform magnetic capacitances, potential, electric and magnetic dipoles,
field. dipole moments, are not clear to several students.
2. Define electric dipole moment. ii. The basic property of electric/magnetic lines of forces ii. The concept can be cleared by
3. Define electric field at a points, relating to the intensity of field is not clear to the students. drawing clear and detailed diagrams
It appears that they do have some vague idea about the on the blackboard.
4. Why two electric lines of force relation between the two, but exactness of this relation is
do not interest ? missing.
5. Define coulomb. Calculate the iii. The concept of relativistic mass and its application is not iii. This is a difficult concept and so
charge carried by 12.5 x 10*18 clear to many students. needs more elaboration.
electrons. iv. In numerical questions most of the mistakes are due to iv. These require more practice.
6. Calculate the Coulomb force either calculation or lack of understanding of the relevant
between a proton and an concepts.
electron separated by 0.8 x
10-*15m. V. In some cases students failed to understand the depth of v. Appropriate information about the
answers required. The marks allocated provides guidance length of answer on the basis of marks
7. Define coulomb. How many about the length of the answer indirectly. allotted may be given to the students.
electron will have a total vi. In diagrams related to electrostatic questions, the location vi. Necessary guidance is to be given to
electric charge of 1 coulomb. of charges are not properly shown in several cases. the students while teaching.-
8. Calculate the Coulomb force The reason behind the mathematical steps are not given
between two protons in most of the answer scripts.
separated by a distance of
1.6 x 10-*15m. Reasoning, explanation, sequence, diagrams with
9. How does the speed of an explanation, it necessary, should be considered as integral
electrically charged particle part of the descriptive-long answer type questions.
affect its (i) mass and (ii) vii Certain students could not differentiate between capacitor vii. More attention may be given at the
charge. and capacitance. lime of teaching.
10. Calculate the Coulomb force viii. The basic differences between para, ferro and diamagnetic viii. This should be explained on the basis
between an alpha particle and materials is not clear to many students. of physical properties/electron theory.
proton separated by 5.12 x
10-*15m. ix. While dealing with the questions on Gauss's theorem the ix. These should be taught separately
students failed to differentiate between line integral, closed before teaching Gauss's theorem,
line/surface integral.
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Relevant Questions General observations on Answers Specific Remarks,
To serve as feed-back for improvement it any
11. Explain the differences Concept of electric flux is not understood. The application These need more practice in the
between diamagnetic, para- of Gauss's theorem is not clear to many. class room.
magnetic and ferromagnetic x The tangent law is not properly conceived, hence creates x. Demonstration of setting deflection
substances. difficulty in understanding principle of tangent galvanometer magnetometer and tangent
12. Describe the construction and and its setting in magnetic meridian. galvanometer can help in better
working of a tangent xi. While dealing with the combination of capacitors in series understanding.
galvanometer. and parallel, the variation of charge and potential in the xi. The basic concept of potential drop
13. Define capacitance. Derive an circuit is not understood by many. Often direction of current across different capacitors or sum
expression for the capacitance in capacitor and total current as sum of the currents through of charges must be explained.
of a cylindrical capacitor/ different capacitors are wrongly mentioned.
parallel plate capacitor.
14. State Gauss's law in
electrostatics. Apply this to
show that for a spherical shell,
the electric field inside the shell
vanishes, whereas outside it,
the field is as it all the charge
had been concentrated at the
centre.
15. Show that in a uniform electric
field, a dipole experiences only
a torque but no net force.
16. Three capacitors of
capacitances X 1 X 2 and X 3
are connected (i) in series
and (ii) in parallel, Derive
expressions for the equivalent
capacitance X for each of
these combinations.
17. Show that the energy density
in a parallel plate capacitor is
1/2 E? E 2.
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3.2.2 TOPIC : CURRENT ELECTRICTY
THERMAL, CHEMICAL AND MAGNETIC EFFECTS OF CURRENT
Relevant Questions: General Observations on Answers : Specific Remarks,
To serve as feed-back for improvement if any
1. Define resistivity of a material. i. Most of the students could not distinguish between the
Does it depend on terms:-
temperature? 1 . Resistance and Resistivity i. Since, these terms appear similar,
2. When a battery of e.m.f. E and 2. Conductance and Conductivity more attention may be paid while
internal resistance r is teaching.
connected to a resistance R, 3. faraday and farad
a current I flows through it. 4. Shunt and high resistance
Derive a relation between E,
I, r and R. Illustrative examples
3. Define conductivty of a a) Most of the students were aware of the formula
material. Give its SI unit. R = PI but could -not define p.
4. Apply Kirchoff's laws to obtain A
the condition of balance in a b) Temperature dependence of resistivity was not This concept should be explained
Wheatstone's bridge. understood properly. In many cases it was mentioned on the basis of randomness of
that the resistivity decreases with the increase of atoms and electrons.
5. Explain, under what conditions temperature.
Ohm's law is not obeyed in a c) Generally the conductivity is defined as the reciprocal Idea of conductivity should be
conductor. of resistivity. explained in terms of charge carries
6. Three resistances P, Q and R d) The term faraday was either not defined at all or, and relaxation lime.
are connected in parallel. incorrectly defined by most of the students.
Derive an expression for their
equlivalent resistance X. Some students even used the term farad for faraday.
7. State and explain Faraday's e) Few students have written the use of high resistance The term shunt is used as low
laws of electrolysis. Define shunt in series with galvanometer to convert it into a resistance always in parallel; should
electrochemical equivalent./ voltmeter. be made clear to students.
Define faraday. ii. Generally the students could not derive/state/apply the ii. Proper explanation of Peltier effect
8. An electron travels in a circular following laws completely in terms of raising/lowering of
path of radius 20 cm in a potential by the flow of current in
magnetic field of 2 x 10 3 T. 1. Ohm's law the direction of e.m.f. and in
Calculate the speed of the 2. Faraday's laws of electrolysis opposite direction be given.
electron. What is the potential 3. Law of resistance in parallel
difference through which the 4. Seebeck effect
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Relevant Questions General Observations or Answers: Specific Remarks,
To serve as feed-back for improvement if any
electron must be accelerated 5. Peltier effect
to acquire this speed? 6. Thermoelectric series
9. State and explain Peltier effect. Illustrative examples
Define Peltier coefficient.
10. An electron after being a) Most of the students could not state all the conditions The conditions under which Ohm's
accelerated through a potential under which Ohm's law is not obeyed. It seems that law is not obeyed should also be
difference of 10*4V enters a the students are well aware of the condition under explained while teaching Ohm's
uniform magnetic field of 0.04 which Ohm's law is obeyed and not the contrary. law.
T perpendicular to Rs direction b) Many students stated V-I relationship as not unique Explanation in terms of effect of
of motion. Calculate the or non-linear only. temperature, drift velocity of
radius of curvature of its electrons and how V/I does not
trajectory. Second law of Faraday's electrolysis was not stated remain constant may be given.
by many students.
11. An electron moving with a d) Some students could not state the laws in Care should be taken on explaining
speed 10 B ms-1 enters a mathematical form. the laws stated in mathematical
magnetic field of 5 x 10--3T in a form.
direction perpendicular to the e) Most of the students, instead of deriving an
field. Calculate (i) radius of the expression for parallel resistances, derived a condition
path and (ii) frequency of of balance for Wheatstone bridge viz. P/Q = R/S, by
revolution of the electron. mistaking the given symbols for resistors as P, Q, R
12. Explain Seebeck effect. as resistances of Wheatstone bridge.
Practice of using symbols other
Discuss the thermoelectric f) Some students of their own used R 1, R 2 and R 3
series. instead of P, Q and R. than standard symbols be provided
in deriving mathematical relation
13. A chamber is maintained at a g) Many students could not apply both the Kirchoff's
between physical quantities.
uniform magnetic field of 5 x laws satisfactorily to derive the condition of balance
10-3 T. An electron with a in Wheatstone bridge.
speed of 5 x 10-7 MS7-1 enters h) Suitable explanation for Peltier effect was not given
the chamber in a direction by majority of students.
normal to the field. Calculate
(i) radius of the path and (H) i) Thermoelectric series was either not written at all or
frequency of revolution of the wrongly written although the students have a basic
electron. idea of the thermoelectric series as reflected in answer
sheets.
14. Explain how you will convert a
galvanometer into a voltmeter iii. Students could not properly define the following, terms iii. Proper guidance may be given.
of a given range. 1. Electro-chemical equivalent.
2. Peltier coefficient
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Relevant Questions General Observations on Answers : Specific Remarks,
To Serve as feed-back for improvement if any
15. Explain how you will convert a iv. a) Students could not either state or derive or give iv. Students should practice to answer
galvanometer into an ammeter symbols related to formulas of the following questions based on experiments
of a given range. 1. Wheatstone bridge also giving a brief introduction
16. Explain how you will compare before giving the mathematical
the e.m.f. of two cells by a 2. Internal resistance derivation.
potentiometer. 3. Laws of resistance in parallel
4. Conversion of galvanometer into ammeter or
voltmeter.
5. E 1 = I 1
E 2 I 2
b) In many cases the required formula was stated Essential steps in derivations should
directly without giving necessary steps in derivation. be identified and reasons be stated.
c) Reasoning of steps, wherever required, were also
not stated.
v. The following shortcomings were found in the V. Stress should be laid on indicating
diagrams : the direction of current in electrical
1. Absence of properly labelled diagrams. circuit diagram along with labelling.
2. Direction of current not indicated.
3. Incomplete diagram in many cases.
Illustrative example :
In Wheatstone bridge the external battery was not
connected and no indication of current was given.
vi. The following deficiencies were found in most of the cases vi. More practice of understanding and
while attempting the numerical problems: doing numericals with conversion
1 . Use of incorrect formula into proper units and complete
or units of physical quantities calculation be given in the class.
2. Conversion into proper unit.
3. Substitution of values.
4. Calculation and final answer.
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Relevant Questions General Observations on Answers Specific Remarks,
To serve as feed-back for improvement if any
Illustrative example : vii. Reinforcement by teacher is
Substitution of value of potential difference (V) for velocity desirable whenever situation
(v) and vice-versa. comes.
vii. Units of the following physical quantities were not correctly
stated by many students
1 . Conductivity
2. Resistivity
Illustrative example
The unit of conductivity was mostly stated as mho, instead
of mho/m.
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3.2.3 TOPIC : ALTERNATING CURRENT AND ELECTROMAGNETIC INDUCTION
Relevant Questions: General Observations on Answers Specific Remarks,
To serve as -feed-back for improvement it any
1. When are the voltage and
current in LCR series AC circuit i. For questions related to LCR circuits in all the sets, the i. Numerical problems proved to be a
in phase? part of the questions based on testing knowledge were difficult exercise for many
attempted properly by majority of students. However, candidates. More emphasis should
2. What is the power consumed understanding aspects of these questions were found to be laid for doing numerical
in (i) purely inductive and (ii) be difficult for them. problems in the class.
purely capacitative AC circuits? ii. Although the question on induced e.m.f. basically relates ii. A suitable demonstration can be
3. Express Quality factor (Q) in with testing of knowledge, yet on the whole, the responses shown during the class teaching to
terms of L, C and R. were poor as the questions in set 1, 2 and 3 were for explain the phenomenon of
4. What is the physical testing higher abilities, electromagnetic induction.
significance of Quality factor in Question on eddy current in set 4 invited good response
a LCR AC circuit? though the diagram was not drawn by the candidates.
5. How can you obtain wattless iii. For the question on LCR circuits the response in set 1 is iii. More emphasis to be given to
current in an AC circuit? very good because the question is straight, simple and its practical aspect of all concepts and
6. Name one device through answer is well explained in the text-books while in other answering indirect questions based
which power consumed in an sets the question asked were indirect involving Quality on known concepts.
AC circuit is zero. factor (Q) and so few students gave proper explanation.
7. What is the phase relationship iv. For the question on AC circuits, the knowledge based iv. While teaching, more emphasis
between current and voltage in questions in set 1 and set 4 are well performed while the should be laid on understanding
(i) an inductor and (ii) a understanding based questions in set 2 and set 3 are of rather than simple knowledge.
capacitor in an AC circuit. poor response.
8. Derive an expression for the
induced e.m.f. produced by
changing the area of a
rectangular coil placed
perpendicular to a uniform
magnetic field.
9. Derive an expression for the
work done in maintaining a
steady current i o in an inductor
10. Show that the induced e.m.f.
produced by changing the area
of a rectangular coil placed
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Relevant Questions General Observations on Answers Specific Remarks,
To serve as feed-back for improvement If any
perpendicular to a uniform
magnetic field is B I v.
11. What are eddy currents? Give
their one use.
12. Derive an expresion for the
impedance of a coil in an AC
circuit. A current of 1.1 A flows
through a coil when connected
to a 110 V DC. When 110 V
AC of 50 HZ is applied to the
same coil, only 0.5 A current
flows.
Calculate the (i) resistance, (i)
impedance and (iii) inductance
of the coil.
13. Derive an expression for the
impedance of LCR series AC
circuit. A 4.0 ohm resistor, 3 mH
inductor and 2 fF capacitor are
connected in series to a 110 V,
5000 Hz AC source. Calculate
the value of the current in- the
circuit.
14. Discuss the phenomenon of
resistance in a LCR series AC
circuit. A capacitor, a 15 ohm
resistor and 80 mH-I inductor are
placed in series with a So Hz
AC source. Calculate the
capacity of the capacitor if the
current is observed in phase
with the voltage.
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3.2.4 TOPIC : ELECTROMAGNETIC WAVES
Relevant Questions: General Observations on Answers : Specific Remarks,
To serve as feed-back for improvement if any
1. Give four properties of i. For the questions related with e.m. wave not more than i. More attention to be given for
electromagnetic waves. two correct properties were mentioned while students could leaching of e.m. wave.
2. Give one use of each of the correctly state one of the uses asked for.
following : (i) microwaves, (ii) ii. Regarding ozone layer, its role in nature was well described ii. While teaching, teacher should
infra-red light (ii) ultraviolet while its location was wrongly indicated by many draw the diagram on black-board
radiation and (iv) gamma rays. examinees. to show the location of ozone layer.
3. Where is ozone layer situated iii. Questions on wave forms of AM or FM and on Poynting's iii. Students, who are interested to
in the atmosphere? What role Vector were beyond the comprehension of the students know more, may be encouraged.
does it play for human survival? and so these were not properly attempted.
4. Show diagrams of AM and FM
wave forms.
5. Define Poynting's Vector and
give its SI unit.
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3.2.5 TOPIC : OPTICS AND OPTICAL INSTRUMENTS
Relevant Questions: General Observations on Answers : Specific Remarks,
To serve as feed-back for improvement if any
1. Define elliptically (or circularly) i. Questions of Optics and Optical instruments in all the four i. Teaching of Optics should be made
polarised light. sets are not answered properly. Moreover these questions more interesting so that it is not
2. State the path difference have been attempted at the end of answer book by majoirty neglected.
between two waves for of students. This means these questions form the difficult
destructive (or constructive) or neglected portion of the syllabus.
interference. ii. In few scripts, instead of actual definition, mathematical ii. Special emphasis should be given
3. Define resolving power of a defintion of a physical quantity is given. for showing direction of rays, correct
microscope (or telescope). iii. Some students could not use sign conventions in solving use of sign convention, correct sizes
problems. of objects and images, objective
4. Define lumen. and eye lenses of microscope and
5. What is the speed of light in telescope.
glass having refractive index
1.5?
6. What is the magnifying power
of a telescope whose objective
and eye piece have focal length
180 cm and 3 cm respectively?
7. What are optical fibres? Give
their one use.
8. Explain briefly the formation of
mirage in deserts.
9. Why do stars twinkle?
10. An object is placed at a
distance 5 cm from a convex
lens of focal length 1 0 cm.
Calculate the positon and
magnification of the image.
11. Radii of curvature of a double
convex lens are 15 cm and
30 cm and Rs refractive index
is 1.5. Calculate the focal length
of the lens.
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Relevant Questions General Observations on Answers Specific Remarks,
To serve as feed-back for improvement if any
12. Two lenses, one converging of
power 8 Dioptre and the other
diverging of power 4 Dioptre,
are combined together.
Calculate the focal length and
power of the combination.
13. Deduce the laws of refraction
(or reflection) on the basis of
Huygen's principle.
14. What do you understand by
polarisation of light? How can
the transverse nature of light be
demostrated experimentally.
15. Explain the working of a simple
microscope, and show that Rs
magnification `M' is gven by-
M = 1 - D/f.
16. Draw a labelled diagram of a
compound microscope (or an
astronomical telescope) and
explain Rs working.
17. Give an expression for the
resolving power of an
astronomical telescope.
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3.2.6 TOPIC: ATOMS, MOLECULES AND NUCLEI
Relevant Questions: General Observations on Answers : Specific Remarks,
To serve as feed-back for improvement if any
1. What is equivalent energy of a i. It has been found that generally candidates have failed i. Students should have enough
10 mg mass? to convert units i.e. milligrams to kg., electron volt into practice in converting units from one
2. Give an equation respresenting joule. system into another.
the decay of a free neutron. ii. In the decay of a free neutron, appearance of neutrino ii. Neutrino emmission of different
3. What is the de Broglie requires more explanation to explain conservation of energies is not normally explained.
wavelength of a 2 kg object energy. Teachers should take care of it.
moving with a speed of 1 ms-1? iii. Many students have failed to understand the meaning of iii. Teacher has to play the appropriate
4. Define half-life of a radioactive various symbols in the formula-p = h/y role.
substance. A radioactive iv. Many students could not do the calculation of half-fife by iv. Though calculation by using log is
substance decays to 1/32 of its log method. easy yet it seems that it is not
original activity in 25 days. taught.
Calculate its half-life. V. Elements having mass number (A) 230 is fissionable by v. It should be taught that heavy
5. Give two important neutrons and U 235 only undergoes natural fission. This elements undergo fission and very
characteristics of nuclear concept is not clear to some students. light nuclei undergo fusion at high
forces. temperature.
6. Define decay constant of a vi. Few students could not differentiate between potential vi. Care should be taken to make the
radioactive substance. How is difference 'V' and velocity 'v'. difference clear and standard
it related to half-life? symbol must be known well by
7. Explain the phenomenon of students.
fission. Give one representative
equation.
8. Explain the phenomenon of
fusion. Give one representative
equation.
9. Distinguish between fission and
fusion.
10. Name the four essential
components of a nuclear
reactor.
11. What is photoelectric effect?
Calculate the threshold
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Relevant Questions General Observations on Answers Specific Remarks,
To serve as feed-back for Improvement If any
frequency of photon for
photoelectric emmission from a
metal of work-function 0.1 eV.
12. What is photoelectric effect?
Calculate the kinetic energy of
a photoelectron (in eV) emitted
on shining light of wavelength
5 x 10-6 m on a metal surface
of work function 0.15 eV.
13. Show that the de Broglie
wavelength of electrons
accelerated through a potential
of V volts can be expressed
as- x = h/v2meV.
20
3.2.7 TOPIC: SOLIDS AND SEMICONDUCTOR DEVICES
Relevant Questions: General Observations on Answers : Specific Remarks,
-To serve as feed-back for improvement if any
1. What is depletion region in a i. Students found difficulty in answering the question on i. Teachers have to explain to what
pn-junction? depletion region carrying 1 mark in a single sentence, or extent the answer is expected to
2. What types of charge carriers so. Some of them wanted to explain it in terms of the be written which depends on the
are there in a p-type semi- diffusion of carrier of charges at the junction and resultant marks allotted.
conductor? recombination of mobile charge carriers thus creation of a Mechanism behind formation of
electric field or potential barrier. However, the attempts depletion region should be
3. Explain through a labelled were poor as the mechanism or the concept behind it was explained step by step to the
circuit diagram, the working of not well understood. students. While explaining it, help
a transistor as an amplifier of diagram may be taken.
(common emitter). Obtain an
expression for the voltage gain. ii. The use of transistor as an amplifier has not been ii. Transistor symbols, biasing and
4. Explain through a labelled attempted properly by most of the students. Those who working needs thorough teaching.
circuit diagram, the working of have given good description have drawn poor and Flow of current in electric circuits
a transistor as an oscillator. unlabelled diagram without proper biasing. Some have should always be indicated.
made good and correct diagram but poorly described the
working. Many have shown wrong CE and BE biasing.
iii. In an amplifier of power amplification, current gain and iii. Phase inversion current and voltage
voltage gain are not differentiated by students. The output gains should be clarified by the
phase inversion is also not understood by many. teacher.
iv. The long answer question on oscillator is very poorly iv. Besides sine waves, knowledge of
attempted. Students lack basic understanding of feed-back other types of waves i.e. saw-tooth
system for producing sustained oscillations and sources must be given in the class. With
of energy. Diagrams drawn are better but description of proper diagrams working of
feed-back and its phase is not clear. oscillator should be explained.
21
3.2.8 TOPIC : THE UNIVERSE
Relevant Questions: General Observations on Answers : Specific Remarks,
-To serve as feed-back for improvement if any
1. What are comets composed of? i. Although the questions on comets, shooting stars and light- i. It seems that the chapter is some
year are simple knowledge type, the responses were very what neglected in the teaching
2. What are shooting stars? poor. process.
3. Define "Light Year". ii. Respones for the questions on steller spectra, mass or ii. Teachers have to make the
4. Explain briefly steller spectra. size or star were very poor. teaching interesting.
Give its one use.
5. Explain briefly how the mass of
a star can be determined.
6. Explain briefly how you will
determine the size of a star.
22
3.3 SOME TYPICAL GOOD ANSWERS Set 2, Q. No. 30 (or part)
Show that in a uniform electric field, a dipole experiences only a torque but no net force. Derive expression for the torque. (5 marks)
Ans. Consider an electric dipole having charges +q and -q at its two ends separated by a distance 1. It is placed in a uniform electric field E such that it makes an arbitrary angle e with E in the plane of the depicted fig 1.
The force experience by +q at A is +qE in the direction of the field and that due to -q at B is -qE , which is in opposite direction.
Hence, the net force on the dipote F Total = +qE - qE = Zero
The two equal, palled and unlike forces constitute a couple and
gives rise to a torque. This tends to set the dipole along the field.
The dipole moment P makes angle e with E such that
p = lq ... (i)
The magnitude of the torque is given by
tau force x perp. dist.
qE x AC = qE x ABsine = qE Isine
or tau = lq E sine ...(ii)
from (i) and (ii) we get tau=p Esine
By right hand rule we get direction of t c as out of the plane of the paper.
Hence, expression for torque tau = p x E
Points to note :
1. A proper diagram has been drawn which depicts all details.
2. The question carries 5 marks and so the answer is written with all details.
Set 2, Q. No. 20
Derive an expression for the work, done in maintaining a steady current i o in an inductor. (2 marks)
23
Ans. If a current i is passed through a coil of inductance L, then the induced emf produced is
c = L x rate of growth of current= L di dt
Then the energy stored in the coil which is equal to the work done by the source is given by -
The total work done so as to increase the current from i - o to steady final value of i = i o is given by
Points to note :-
1 . There is no need to draw diagram as the question is only of 2 marks involving derivation and hence time is saved.
2. It is convenient to derive the expression showing all steps, although it may take more time. It is better not to jump steps for ensuring correctness of answer.
3. Appropriate notation of physical quantities are used.
Set 3, Q.No. 29
Explain briefly how will you convert a galvanometer into a voltmeter of range 0-10 m V.
(2 marks)
Ans. To measure the p.d. across a resistance r in a circuit, a voltmeter is always connected in parallel to it. The resistance of the voltmeter is considerably high so that negligible current flows through R and the current in the main circuit is not much affected. Thus it measures the correct p.d.
To convert a galvanometer of resistance G into a voltmeter, a high resistance R is connected in series with it, the value of which depends on the wanted range of the voltmeter thus converted.
Let 1 g be the maximum safe current that can be passed through G so that R gives full scale deflection for the range V, such that V = 10 mV = 10 x 10-3V = 10-2 volt. Then by Ohm's law (current X resistance = p.d.)
I g (G + R) = V
or R =V - G where V = 10 volt. Ig
Points to note :
1. Proper explanation is given, which is written briefly.
2. Essential part of the circuit diagram is drawn.
3. Conversion into proper unit is essential.
4. Some students may be confused to think the question as numerical problem.
24
Set 1, Q.No. 25
Explain the differences between diamagnetic, paramagnetic and ferromagnetic substances.
(3 marks)
Ans. The differences between these magnetic substances can best be expressed in terms of the effect of external magnetic on substances.
i) Diamagnetic substances (e.g. Au, Ag, Cu) when placed in a magetic field, acquire a feable magnetism in opposite direction to the field and show slight repulsion.
Diamagnetic substances have magnetic permeability (p) 1
ii) Paramagnetic substances (e.g. Al, Pt' 02) placed in a magnetic field acquire a feable magnetism in same direction as the field and so show slight attraction.
Here u 1 and the property decreases with rise of temperature.
iii) Ferromagnetic substances (e.g. Fe, Ni, Co) in a magnetic field acquire a strong magnetism in the same direction as the field even if the field is weak and so show strong attraction.
Here p is several thousand. The property decreases with rise of temperature and above certain temperature called Curie point, a ferromagnetic substance changes into paramagnetic.
Points to note:
1. The answer covers the main points of differences. Further detail is beyond the comprehension of the students of +2 stage.
2. It is convenient to state the differences in tabular form as that will reduce the number of words and save times.
Set 1, Q.No. 20
Derive an expression for the induced emf produced by changing the area of a rectangular coil placed perpendicular to a uniform magnetic field.
Let a U shaped conducting wire be placed across a magnetic field B as shown in the fig. 3. Let the conductor PQ moving with a velocity v towards right occupies a position P'Q' after a time dt, when the area of the circuit changes from PQRS to P'Q'RS.
The change in magnetic flux linked with the circuit is given by-
dO = B (change in area)
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= B (rect. PQQ'P') = B (PQ x QQ')
= B (I x v dt)
By Faraday's law, the induced emf is given by the rate of change of flux
or /E/=dO B IvBdt
or /E/ = B I v
The direction of the induced current is given by Lenz's law as shown.
Points to note :
1. Essential diagram with brief explanation is provided.
2. Fundamental law behind the production of induced emf is properly included.
3. Mathematical steps are clearly shown for the derivation.
4. The number of words for a question of 3 marks mentioned in question paper may be taken as a suggestive guideline. In practice, the number of words needed may be more than the prescribed one.
5. Unnecessary description could be avoided by drawing a labelled diagram in order to save time as the question carries 2 marks and involves derivation only.
Set 2, Q.No. 26
What do you understand by polarisation of light 7 How can the transverse nature of light be demonstrated experimentally? (3 marks)
Ans. i) Light being an electromagnetic wave, is transverse in nature, Its em components vibrate in all possible orientations at right angle to its direction of propagation. This constitutes the ordinary light which is unpolarised. If those transverse components of vibration are restricted to a particular plane then it gives rise to the phenomenon of polarisation of light. This can be produced by passing the light through certain bodies called polaroids. Only those vibrations along the crystal axis of a polaroid, are allowed to pass while all others are stopped.
To demonstrate the transverse nature of light, ordinary light is first passed through a polaroid (called Polariser, P) and one gets plane polarised light as depicted in the fig. 4.
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It is possible only if light is a transverse wave. To confirm this, a second polaroid (called Analyer, A) is placed in the path of the beam. If axes of both polaroids, P and A, are parallel, then the beam is transmitted. If now A is gradually rotated, than light intensity gradually decreases until at right angle to P, no light comes out. Hence, it is confirmed that light is transverse in nature.
Points to note :-
1 . The first part of the question concerns with understanding of the term - Polarisation of light and correspondingly a candidate is expected to write, what does the term mean.
2. As per the requirement of the second part of the question, emphasis has been given on description of the experimental demonstration instead of explanation only.
3. Drawing of labelled diagram is expected and it makes the presentation easier.
4. The explanation by taking the analogy of mechanical transverse wave along a string and passing through linear slits is not desirable or essential for the question and so has been avoided to save time.
5. The question carries 3 marks. It may be difficult to restrict the answers within suggested number of words.
Set 1, Q.No. 16
Explain the phenomenon of fission. Give one representative equation. (2 marks)
Ans. The process of splitting of a heavy nucleus into two or more nuclei of nearby comparable masses with liberation of energy is called nuclear fission.
For example, when 92 U 235 is bombarded with slow neutron, a compound nucleus 92 U 236 is formed. This disintegrates into 56 Ba141 and 36 Kr 92 with the release of 3 neutrons and some energy Q.
The representative equation is :
92 U 235 + n 1 92 U 236 66 Ba 141 + 36 Kr 92 + n1 + Q
Points to note :-
Note the briefness of the answer, which has been written to the point.
Set 3, Q.No. 15
Define decay constant (lamda) of a radioactive substance. How is it related to half-life?
(2 marks)
Ans. For a radioactive substance, the number of nuclei decaying per second is proportional to the number of nuclei present at that instant
i.e. dN =-(lamda)N dt
where the constant of proportionality given by X dN/dt is called decay constant. N
Hence, it may be defined as the ratio of the instantaneous rate of disintegration to the number of active atoms present at that instant.
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The half-life (T) of a radioactive element is defined as the time taken for half the number of atoms of the element to disintegrate.
The half-life of a radioactive substance is inversely proportional to its decay constant (i) such that the relation between them is given by T 0.693/(lamda)
Points to note
Note that the physical quantities have been defined fully-qualitatively as well as quantitatively.
Set 1, Q.No. 9
What is depletion region in a pn junction? (1 mark)
Ans. : A small region in the vicinity of the junction of two dissimilar semiconductors (e.g. a pn junction) that has no mobile charge carriers or has a lower than usual number of mobile charge carriers is called depletion region.
OR
A small region near the junction of p and n semiconductors in which the density of mobile charge carriers is insufficient to neutralise the fixed charge density of donors and acceptors.
Points to note :-
1. Any one of the above two ways of answering the question may be written. 2. Since the question carries 1 mark, answer has been written in one sentence. 3. If the allotted marks is more than 1 mark, the answer is expected to include diagram and explanation for the formation of depletion region.
Set 3, Q. No. 31
Explain, through a labelled circuit diagram the working of a transistor as an oscillator.
(5 marks)
Ans. Oscillator is a device which generates electrical oscillations of constant amplitude and frequency. It converts d.c. energy obtained from a high tension battery into a.c. energy in an oscillatory L-C circuit called tank circuit.
Transistor as an oscillator : A tank circuit which is a parallel combination of inductance L and capacitance C is connected in the input or emitter base circuit which is forward biased. A coil L is the feedback coil connected in the output or emitter-collector circuit which is reverse biased.
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When the circuit is closed, the collector alternating current through coil L' induce on emf in coil L producing emitter current which increase the collector current and so on. The magnetic flux linked with coil L' and L increases thereby increasing the forward bias which further increase, the emitter and collector current and so on. Charging of the capacitor continues till the collectors current gets saturated.
At saturation, the current through coil L' is not changing and there is no induced emf linked with coil. The emitter current and hence the collector current decrease. The decreasing current through L' induces an emf in L in the opposite direction which further decreases the emitter current and hence the collector current, Charging of the capacitor continues in the opposite direction till the collector current becomes zero, when the induced emf in the coil L' becomes zero. The process is repeated producing oscillations of constant amplitude and frequency given by :-
f = - 1 2nvLC
where L is the inductance of the coil and C is the capacitance of the capacitor.
If a portion of oscillator output energy of an amplifying device like a transistor is not fed back into the L-C circuits in proper amplitude and phase, the energy given to the circuit is dissipated and the oscillations get damped due to dielectic losses in the capacitor and the resistive losses in, the inductor.
Points to note :-
1. Properly labelled circuit diagram has been drawn.
2. The working of transistor as an oscillator has been written precisely to the point.
3. The answer cannot be restricted to the number of words mentioned in the question paper.
Set 3, Q.No. 22
Explain briefly how the mass of a star can be determined.
(2 marks)
Ans. Most of the stars around us form a binary system. In such a system two stars form a gravitationally bound system and revolve around their common centre of mass (CM).
Let the masses of two stars forming a binary system be M 1 and M 2. Let the distance between be 'a'. The respective distance of the two stars from the CM be a 1 and a 2.
Hence, a 1 + a 2 = a... (1)
M 1 a 1 = M 2 a 2 .. (3)
From Kepler's third law it can be shown that- 2 3 M 1 + M 2 = 4n a where G is the gravitational constant. G T 2
If M 1 and M 2 are measured in terms of solar mass and T in years; then G=4 n2
If CM is knwon, then distances a 1 and a 2 can be determined. Using relations (1), (2) and (3), M 1 and M 2 can be obtained individually.
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Points to note :
1. The appropriate figure helps in explaining the situation.
2. It has been stated that the method is applicable to binary system.
3. The minimum contents that has to be written in the answer is not commensurate with marks assigned.
4. For a question of 2 marks, the steps are explained briefly, avoiding the detailed derivations for M 1 and M 2.
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