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ISC Class XII Analysis Of Pupil Performance 2017 : Physics

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Analysis of Pupil Performance Research Development and Consultancy Division Council for the Indian School Certificate Examinations New Delhi Year 2017 _________________________________________________________________________________ _ Published by: Research Development and Consultancy Division (RDCD) Council for the Indian School Certificate Examinations Plot No. 35-36, Sector VI Pushp Vihar, Saket New Delhi-110017 Tel: (011) 29564831/33/37 E-mail: council@cisce.org Copyright, Council for the Indian School Certificate Examinations All rights reserved. The copyright to this publication and any part thereof solely vests in the Council for the Indian School Certificate Examinations. This publication and no part thereof may be reproduced, transmitted, distributed or stored in any manner whatsoever, without the prior written approval of the Council for the Indian School Certificate Examinations. FOREWORD This document of the Analysis of Pupils Performance at the ISC Year 12 and ICSE Year 10 Examination is one of its kind. It has grown and evolved over the years to provide feedback to schools in terms of the strengths and weaknesses of the candidates in handling the examinations. We commend the work of Mrs. Shilpi Gupta (Deputy Head) and the Research Development and Consultancy Division (RDCD) of the Council who have painstakingly prepared this analysis. We are grateful to the examiners who have contributed through their comments on the performance of the candidates under examination as well as for their suggestions to teachers and students for the effective transaction of the syllabus. We hope the schools will find this document useful. We invite comments from schools on its utility and quality. Gerry Arathoon Chief Executive & Secretary November 2017 i PREFACE The Council has been involved in the preparation of the ICSE and ISC Analysis of Pupil Performance documents since the year 1994. Over these years, these documents have facilitated the teaching-learning process by providing subject/ paper wise feedback to teachers regarding performance of students at the ICSE and ISC Examinations. With the aim of ensuring wider accessibility to all stakeholders, from the year 2014, the ICSE and the ISC documents have been made available on the Council s website www.cisce.org. The document includes a detailed qualitative analysis of the performance of students in different subjects which comprises of examiners comments on common errors made by candidates, topics found difficult or confusing, marking scheme for each answer and suggestions for teachers/ candidates. In addition to a detailed qualitative analysis, the Analysis of Pupil Performance documents for the Examination Year 2017 have a new component of a detailed quantitative analysis. For each subject dealt with in the document, both at the ICSE and the ISC levels, a detailed statistical analysis has been done, which has been presented in a simple user-friendly manner. It is hoped that this document will not only enable teachers to understand how their students have performed with respect to other students who appeared for the ICSE/ISC Year 2017 Examinations, how they have performed within the Region or State, their performance as compared to other Regions or States, etc., it will also help develop a better understanding of the assessment/ evaluation process. This will help them in guiding their students more effectively and comprehensively so that students prepare for the ICSE/ ISC Examinations, with a better understanding of what is required from them. The Analysis of Pupil Performance document for ICSE for the Examination Year 2017 covers the following subjects: English (English Language, Literature in English), Hindi, History, Civics and Geography (History & Civics, Geography), Mathematics, Science (Physics, Chemistry, Biology), Commercial Studies, Economics, Computer Applications, Economics Applications, Commercial Applications. Subjects covered in the ISC Analysis of Pupil Performance document for the Year 2017 include English (English Language and Literature in English), Hindi, Elective English, Physics (Theory and Practical), Chemistry (Theory and Practical), Biology (Theory and Practical), Mathematics, Computer Science, History, Political Science, Geography, Sociology, Psychology, Economics, Commerce, Accounts and Business Studies. I would like to acknowledge the contribution of all the ICSE and the ISC examiners who have been an integral part of this exercise, whose valuable inputs have helped put this document together. I would also like to thank the RDCD team of Dr. Manika Sharma, Dr. M.K. Gandhi, Ms. Mansi Guleria and Mrs. Roshni George, who have done a commendable job in preparing this document. The statistical data pertaining to the ICSE and the ISC Year 2017 Examinations has been provided by the IT section of the Council for which I would like to thank Col. R. Sreejeth (Deputy Secretary - IT), Mr. M.R. Felix, Education Officer (IT) ICSE and Mr. Samir Kumar, Education Officer (IT) - ISC. Shilpi Gupta Deputy Head - RDCD November 2017 ii CONTENTS Page No. FOREWORD i PREFACE ii INTRODUCTION 1 QUANTITATIVE ANALYSIS PHYSICS 3 QUALITATIVE ANALYSIS Theory (Paper-1) 10 Practical (Paper-2) 39 This document aims to provide a comprehensive picture of the performance of candidates in the subject. It comprises of two sections, which provide Quantitative and Qualitative analysis results in terms of performance of candidates in the subject for the ISC Year 2017 Examination. The details of the Quantitative and the Qualitative analysis are given below. Quantitative Analysis This section provides a detailed statistical analysis of the following: Overall Performance of candidates in the subject (Statistics at a Glance) State wise Performance of Candidates Gender wise comparison of Overall Performance Region wise comparison of Performance Comparison of Region wise performance on the basis of Gender Comparison of performance in different Mark Ranges and comparison on the basis of Gender for the top and bottom ranges Comparison of performance in different Grade categories and comparison on the basis of Gender for the top and bottom grades The data has been presented in the form of means, frequencies and bar graphs. Understanding the tables Each of the comparison tables shows N (Number of candidates), Mean Marks obtained, Standard Errors and t-values with the level of significance. For t-test, mean values compared with their standard errors indicate whether an observed difference is likely to be a true difference or whether it has occurred by chance. The t-test has been applied using a confidence level of 95%, which means that if a difference is marked as statistically significant (with * mark, refer to t-value column of the table), the probability of the difference occurring by chance is less than 5%. In other words, we are 95% confident that the difference between the two values is true. t-test has been used to observe significant differences in the performance of boys and girls, gender wise differences within regions (North, East, South and West), gender wise differences within marks ranges (Top and bottom ranges) and gender wise differences within grades awarded (Grade 1 and Grade 9) at the ISC Year 2017 Examination. The analysed data has been depicted in a simple and user-friendly manner. 1 Given below is an example showing the comparison tables used in this section and the manner in which they should be interpreted. Comparison on the basis of Gender Gender Girls Boys *Significant at 0.05 level N 2,538 1,051 Mean 66.1 60.1 SE 0.29 0.42 The t-value 11.91* table shows comparison between the performances of boys and girls in a particular subject. The t-value of 11.91 is significant at 0.05 level (mentioned below the table) with a mean of girls as 66.1 and that of boys as 60.1. It means that there is significant difference between the performance of boys and girls in the subject. The probability of this difference occurring by chance is less than 5%. The mean value of girls is higher The results have also been depicted pictographically. In this case, the girls performed significantly better than the than that of boys. It can be interpreted that girls are performing significantly better than boys. boys. This is depicted by the girl with a medal. Qualitative Analysis The purpose of the qualitative analysis is to provide insights into how candidates have performed in individual questions set in the question paper. This section is based on inputs provided by examiners from examination centres across the country. It comprises of question wise feedback on the performance of candidates in the form of Comments of Examiners on the common errors made by candidates along with Suggestions for Teachers to rectify/ reduce these errors. The Marking Scheme for each question has also been provided to help teachers understand the criteria used for marking. Topics in the question paper that were generally found to be difficult or confusing by candidates, have also been listed down, along with general suggestions for candidates on how to prepare for the examination/ perform better in the examination. 2 STATISTICS AT A GLANCE Total Number of Candidates: 37,561 Mean Marks: Highest Marks: 100 65.4 Lowest Marks: 02 3 PERFORMANCE (STATE-WISE & FOREIGN) West Bengal 68.0 Uttarakhand 66.1 Uttar Pradesh 61.8 Tripura 56.1 Tamil Nadu 73.8 Telangana 60.5 Sikkim 61.1 Rajasthan 66.7 Punjab 60.4 Odisha 57.5 Madhya Pradesh 60.2 Manipur 47.7 Meghalaya 75.7 Maharashtra 75.0 Kerala 71.6 Karnataka 74.8 Jharkhand 64.4 Himachal Pradesh 65.7 Haryana 73.4 Gujarat 68.1 Goa 58.0 Delhi 74.0 Chandigarh 67.4 Chattisgarh 55.1 Bihar 67.4 Assam 87.4 Andhra Pradesh 56.1 Foreign 78.2 The States of Assam, Meghalaya and Maharashtra secured highest mean marks. Mean marks secured by candidates studying in schools abroad were 78.2. 4 GENDER-WISE COMPARISON BOYS GIRLS Mean Marks: 65.6 Mean Marks: 65.3 Candidates: 14,773 Candidates: 22,788 Number of Number of Comparison on the basis of Gender Gender Girls Boys N Mean SE t-value 14,773 22,788 65.6 65.3 0.16 0.13 1.79 5 REGION-WISE COMPARISON East North Mean Marks: 66.3 Mean Marks: 63.4 Number of Candidates: 11,333 Number of Candidates: 20,897 Highest Marks: 100 Lowest Marks: 04 Highest Marks: 100 Lowest Marks: 02 REGION Mean Marks: 70.8 Mean Marks: 71.8 Number of Candidates: 3,345 Number of Candidates: 1,841 Highest Marks: 100 Lowest Marks: 12 South Mean Marks: 80.7 Number of Candidates: 145 Highest Marks: 99 Lowest Marks: 41 Foreign 6 Highest Marks: 100 Lowest Marks: 24 West Mean Marks obtained by Boys and Girls-Region wise 63.1 63.6 North 66.8 71.5 65.9 East 72.5 70.2 South 80.5 80.9 71.3 West Foreign Comparison on the basis of Gender within Region Region North (N) East (E) South (S) West (W) Foreign (F) Gender Girls Boys Girls Boys Girls Boys Girls Boys Girls Boys N Mean SE 8,000 12,897 4,484 6,849 1,498 1,847 732 1,109 59 86 63.1 63.6 66.8 65.9 71.5 70.2 72.5 71.3 80.5 80.9 0.21 0.17 0.28 0.23 0.45 0.43 0.70 0.59 1.99 1.65 *Significant at 0.05 level The performance of girls was significantly better than that of boys in the eastern and southern region. In other regions no significant difference was observed. 7 t-value -1.63 2.48* 2.17* 1.28 -0.15 MARK RANGES : COMPARISON GENDER-WISE Comparison on the basis of gender in top and bottom mark ranges Marks Range Top Range (81-100) Bottom Range (0-20) Gender Girls Boys Girls Boys N Mean SE 4,030 6,335 28 53 89.3 89.5 18.6 17.2 0.08 0.07 0.57 0.48 Boys Girls t-value -1.32 1.93 All Candidates 89.5 81 - 100 89.3 89.4 70.8 61 - 80 70.6 70.7 48.5 41 - 60 48.8 48.6 32.6 21 - 40 32.8 32.7 17.2 0 - 20 18.6 17.7 8 GRADES AWARDED : COMPARISON GENDER-WISE Comparison on the basis of gender in Grade 1 and Grade 9 Grades Gender Girls Boys Girls Boys Grade 1 Grade 9 N Mean SE 1,897 3,094 419 822 94.1 94.1 25.8 25.5 2.16 1.69 1.26 0.90 Boys In Grade 1 and Grade 9 no Girls t-value -0.01 0.17 All Candidates 94.1 94.1 94.1 1 significant difference was observed between the 84.5 84.6 84.5 2 average performance of girls and boys. 74.5 74.6 74.6 3 64.5 64.6 64.6 4 57.0 57.1 57.0 5 52.1 51.9 52.0 6 47.0 47.0 47.0 7 42.2 42.2 42.2 8 9 9 25.5 25.8 25.6 THEORY (PAPER-1) Part I (20 marks) Answer all questions. Question 1 A. Choose the correct alternative (a), (b), (c) or (d) for each of the questions given below: (i) The electrostatic potential energy of two point charges, 1 C each, placed 1 meter apart in air is: (a) (b) (c) (d) (ii) (iii) (iv) 9 103J 9 109J 9 10-3J 9 10-3eV A wire of resistance R is cut into n equal parts. These parts are then connected in parallel with each other. The equivalent resistance of the combination is: (a) nR (b) R/n (c) n/R2 (d) R/n2 Magnetic susceptibility of platinum is 0 0001. Its relative permeability is: (a) 1 0000 (b) 0 9999 (c) 1 0001 (d) 0 When a light wave travels from air to glass: (a) (b) (c) (d) its wavelength decreases. its wavelength increases. there is no change in wavelength. its frequency decreases. 10 [5] A radioactive substance decays to 1/16th of its initial mass in 40 days. The half life of the substance, in days, is: (v) (a) (b) (c) (d) B. 20 10 5 2 5 Answer all questions given below briefly and to the point: (i) Maximum torque acting on an electric dipole of moment 3 10-29 Cm in a uniform electric field E is 6 10-25 Nm. Find E. (ii) What is meant by drift speed of free electrons? (iii) On which conservation principle is Kirchoff s Second Law of electrical networks based? (iv) Calculate magnetic flux density of the magnetic field at the centre of a circular coil of 50 turns, having radius of 0 5m and carrying a current of 5 A. (v) An a.c. generator generates an emf where = 314 Sin(50 t) volt. Calculate the frequency of the emf . (vi) With what type of source of light are cylindrical wave fronts associated? (vii) How is fringe width of an interference pattern in Young s double slit experiment affected if the two slits are brought closer to each other? (viii) In a regular prism, what is the relation between angle of incidence and angle of emergence when it is in the minimum deviation position? (ix) A converging lens of focal length 40 cm is kept in contact with a diverging lens of focal length 30 cm. Find the focal length of the combination. (x) How can the spherical aberration produced by a lens be minimised? (xi) Calculate the momentum of a photon of energy 6 10-19J. (xii) According to Bohr, Angular momentum of an orbiting electron is quantised . What is meant by this statement? (xiii) Why nuclear fusion reaction is also called thermo-nuclear reaction? (xiv) What is the minimum energy which a gamma ray photon must possess in order to produce electron-positron pair? (xv) Show the variation of voltage with time, for a digital signal. 11 [15] Comments of Examiners A. (i) Some candidates selected option (d), which was similar to the correct option (c), except for the unit. (ii) A few candidates chose option (b), instead of correct option (d). (iii) Some candidates selected option (b) in place of correct option (c). It was due to confusion in the relation between and r . (iv) Many candidates selected incorrect option in this question, in place of correct option (a). (v) Some candidates selected (d) = 2.5 as an option, in place of correct option (b) = 10 days. B. (i) Some candidates did not express the electric field with unit. Some gave incorrect unit of E. A few candidates could not recall the correct formula, while a few made incorrect calculation of E. (ii) Many candidates could not write the definition correctly. Key words like constant or average velocity or on application of electric field were missing. (iii) Some candidates got confused between Kirchhoff s 1st law and II law and hence, wrote conservation of charge in place of conservation of energy. (iv) Some candidates used incorrect formula of magnetic flux density B. They did not write unit of B. They got confused between magnetic flux and wrote magnetic flux density, B. Hence, expressed incorrect unit of B. (v) Some candidates did not know the correct formula of instantaneous emf e=e o sin ( t) Some of them did not know the relation between and f. (vi) Many candidates answered this question incorrectly. (vii) Some candidates wrote that there is no change in fringe-width whereas some answered it correctly. Some answered, Intensity of bright fringes increases. 12 Suggestions for teachers Train students to use given data in a numerical in SI unit, otherwise convert the final answer in the SI unit, if required. The concept of resistors in series and parallel must be developed in students with proper explanation, followed by numerical problems. After explaining the meaning of magnetic susceptibility and relative permeability, ask them to learn formulae by heart. While teaching refraction of light, explain to them the effect on the speed of light on changing the medium. The concept of half-life should be clarified to students, with the help of numerical problems. Emphasize that proper unit must be given to a physical quantity. Advise students to learn definitions, laws, principles in Physics, by heart and practise writing them. While teaching Kirchhoff s laws of electrical networks, explain the difference between the two laws. In magnetism, explain magnetic flux and magnetic flux density clearly. While teaching wave optics, explain the meaning and importance of the term wave front, types of wave fronts and types of sources of light which produce these wave fronts. Factors affecting fringe-width must be discussed specially after deriving the expression. Train students to read the questions heedfully and write answer in brief and to the point. (viii) Some candidates drew the diagram of a prism, showing various angles. They used the formula i = Familiarise the students with the A+ m Cartesian sign convention and train (ix) Some candidates did not take focal length of them to solve a few numerical concave lens as negative and hence, got incorrect problems. answers. Explain spherical and chromatic A few of them did not write the unit of F. aberrations thoroughly. (x) A few candidates got confused between spherical While teaching pair production, give aberration and chromatic aberration. Hence, they students an idea of energy of gamma wrote about achromatic doublet instead of writing rays which can produce electron the way of minimising spherical aberration produced positron pair with numerical by a lens. problems. (xi) Some candidates used the incorrect formula to Teach the types of signals which are calculate the momentum of a photon. Several used in the field of electronics. Draw candidates did not write the unit of p, and a few labelled V-t graphs for them. wrote incorrect unit of p. (xii) A few candidates wrote mnr = h/2 but did not write what n stands for. Some stated that angular momentum is an integral multiple of h/2, in place of h/2 . (xiii) Many candidates got this question incorrect because they wrote Heat energy is produced/released in this reaction , instead of writing heat energy is required to bring about nuclear fusion . (xiv) A few candidates wrote 1.02 eV, in place of 1.02 MeV. Some wrote a statement, instead of giving the value i.e. 1.02 MeV. (xv) A number of candidates did not know the correct V-t graph for a digital signal. They drew a sine curve. Some did not label the axes. MARKING SCHEME Question 1 A. (i) (c) OR 9 10-3J (ii) (d) OR R/n2 (iii) (c) OR 1 0001 (iv) (a) OR Its wavelength decreases. (v) (b) OR 10 days B. (i) (ii) E = = = or answer expressed with any alternate correct unit. It is the mean distance travelled by a free electron per unit time (second) when an external electric field is applied. Or constant/average speed/velocity on application of potential difference/electric field or voltage or opposite to current or towards +ve terminal. 13 (iii) Energy (iv) B = 4 2 = 10-7 = 10 2 50 5 -4 0 5 = 3 14 10-4 T or answer expressed with any alternate correct unit. (v) = 50 or 2 f = 50 f = 25 Hz (vi) Line source/ linear (vii) Fringe width increases (viii) They are equal, i.e. i = e (ix) 1 1 1 3 4 1 = + = = 40 30 120 120 = 120 = 1 2 (x) By using plano-convex / concave lenses OR With the help of stops. (A diagram showing a lens and a stop is also acceptable) (xi) (xii) P = = = 2 10-27 kg m s-1 Alternate method is also . or answer expressed with any alternate correct unit. It means angular momentum is an integral multiple of OR OR (xiii) l = n = where n is an integer. OR mvr = This is because a lot of heat energy is required to bring about nuclear fusion. OR A very high temperature is required to bring about nuclear fusion. (xiv) 1 02 MeV OR 1.632 x 10-13J. 14 V (xv) 0 0 2 4 6 8 10 t PART II (50 Marks) Answer ten questions in this part, choosing four questions from Section A, three questions from Section B and three questions from Section C. SECTION A Answer any four questions. Question 2 (a) (b) Show that electric potential at a point P, at a distance r from a fixed point charge , is given by: [4] Intensity of electric field at a perpendicular distance of 0 5 m from an infinitely long line charge having linear charge density ( ) is 3 6 103 Vm-1. Find the value of . [1] V= . 15 Comments of Examiners (a) Many candidates could not draw the correct diagrams required for this derivation. Majority of candidates did not use limits for integration. Many of them used incorrect limits. Some could not perform integration. A few missed out negative sign in work done dw. (b) Some candidates did not know the correct formula for intensity due to a long line charge. Some either wrote incorrect unit of or didn t write unit of . Suggestions for teachers Explain the role of integral calculus in Physics specially the meaning of definite integral i.e. the meaning of limits. Advise students to study and practise diagrams, along with learning of derivations. Prepare a list of formulae in each chapter of Physics and ask students to learn these formulae by heart and practise. A few numerical problems of different types, based on each formula must be solved in class, for clear understanding. MARKING SCHEME Question 2 (a) dx O Q P r B +q0 x 1 0 F=4 0 2 dw = - F dx = W = V=W/q 0 = So, (b) 1 V= 0 2 / 4 0 Alternate methods are also acceptable. i.e. E= A 16 3 6 103 = 9 109 = 1 10-7 C m-1 Correct substitution with or without formula and correct answer with unit. Question 3 (a) Three capacitors C 1 = 3 F, C 2 = 6 F and C 3 = 10 F are connected to a 50 V battery as shown in the Figure 1 below: C1 C2 A 3 F 6 F [3] B C3 10 F 50V Figure 1 Calculate: (i) The equivalent capacitance of the circuit between points A and B. (ii) (b) The charge on C 1 . Two resistors R 1 = 60 and R 2 = 90 are connected in parallel. If electric power consumed by the resistor R 1 is 15 W, calculate the power consumed by the resistor R 2 . [2] Comments of Examiners (a) (i) After using correct formula for equivalent capacitance of a series combination i.e 1/C 4 =1/2, a few candidates forgot to find C 4 . Some of them did not write the unit of C. (ii) A few candidates answered this part incorrectly as they used incorrect value of C, i.e. C 1 in place of C 4. (b) Some candidates found out current in R 1 and used the same current for R 2 . They got confused whether current is same or potential difference is same in parallel combination. Some candidates used the incorrect formula for the power consumed by the resistor R 2 . 17 Suggestions for teachers Give practice in solving a few numerical problems on capacitors connected in a circuit in series and in clearly parallel combination, explaining the status of charge on each capacitor and potential difference across each capacitor. Make use of equivalent circuits. Explain how to calculate the power developed in a resister with different type of numerical problems. MARKING SCHEME Question 3 (a) (i) C 4 = 1+ 2 1 (ii) (b) 2 3 6 3+6 = 2 Correct substitution with or without formula and correct answer. C = C4 + C3 = 2 + 10 = 12 Q 4 = C 4 V = 2 50 = 100 R V2 = P 1 R 1 = 15 60 = 900 (volt)2 2 P 2 = = 2 900 90 [Unit (volt)2 not necessary.] = 10W Alternate correct methods are also acceptable. Question 4 (a) Figure 2 below shows two resistors R 1 and R 2 connected to a battery having an emf of 40V and negligible internal resistance. A voltmeter having a resistance of 300 is used to measure potential difference across R 1 . Find the reading of the voltmeter. [3] 40V R2 R1 880 200 v Figure 2 (b) A moving coil galvanometer has a coil of resistance 59 . It shows a full-scale deflection for a current of 50 mA. How will you convert it to an ammeter having a range of 0 to 3A? 18 [2] Comments of Examiners (a) Many candidates were unable to calculate equivalent resistance of the circuit correctly and hence got incorrect value of current. Some got the correct value of current but used incorrect value of resistance, hence, got incorrect answers. (b) Many candidates used the formula of conversion of galvanometer to voltmeter, rather than galvanometer to ammeter. A few didn t state that the calculated resistance should be connected in parallel with the galvanometer. Suggestions for teachers Explain to students, the correct concept and treatment of resistances in series and parallel combination. More practice should be given in solving circuit problems in the class. Encourage the habit of drawing equivalent circuits. Encourage students to solve as many numerical problems as possible. Train them to express answers with units, direction, etc. MARKING SCHEME Question 4 (a) Equivalent resistance R 3 of R 1 and V is: R 3 = + = 120 Then, + R = 120 + 880 = 1000 I = = = 0 04 A V = I R3 = 0 04 120 = 4 8 V (b) S = or = ( ) = S = 1 in parallel or shunt of 1 or 1 shunt shown in diagram. 19 Question 5 (a) In a meter bridge circuit, resistance in the left hand gap is 2 and an unknown resistance X is in the right hand gap as shown in Figure 3 below. The null point is found to be 40 cm from the left end of the wire. What resistance should be connected to X so that the new null point is 50 cm from the left end of the wire? 2 [3] X G 0 cm 100 cm 40 cm ( ) Figure 3 (b) The horizontal component of earth s magnetic field at a place is component. Determine the angle of dip at that place. times the vertical [2] Comments of Examiners (a) Some candidates did not understand this numerical on meter bridge. It involved three steps: Calculations of X, resistance to be connected to X and hence the required resistance. Many found out X and left it as answer. A few candidates did not know that metre bridge works on the principle of Wheatstone bridge. (b) Some candidates did not know the relation between B H , B V and . Hence, they obtained incorrect results. A few did not write the unit of angle of dip . 20 Suggestions for teachers Explain meter bridge based numerical problems in the laboratory when students perform experiments. It would give them better understanding of Wheatstone bridge principle. A few numerical problems must be solved on Wheatstone bridge as well as Meter bridge. Train the students to read questions carefully and write the data. Then, they must recall the correct formula on which it is based. Finally, answer must be given with proper unit. MARKING SCHEME Question 5 (a) = x = 3 When balancing length becomes 50 cm: i.e. y = 2 Now, = = + = = = In parallel with X. (b) BH = BV = tan = = 60o Question 6 (a) Using Ampere s circuital law, obtain an expression for the magnetic flux density B at a point X at a perpendicular distance r from a long current carrying conductor. (Statement of the law is not required). [3] (b) PQ is a long straight conductor carrying a current of 3A as shown in Figure 4 below. An electron moves with a velocity of 2 107 ms-1 parallel to it. Find the force acting on the electron. [2] P electron 3A 0 6 m Q Figure 4 21 Comments of Examiners (a) A few candidates applied Biot Savarts law, in place of Ampere Circuital law. Many could not draw correct diagrams required for this derivation, specially dl and B. They did not use the correct symbol of integration. Some did not use the vector notation with B and dl. Some did not solve B.dl (b) A number of candidates did not understand the numerical problems. Some could not get the correct formula to calculate force on the moving electron. Others could not substitute the given values correctly and hence got incorrect answers. Suggestions for teachers Explain Ampere circuital law comprehensively Ask students to practise diagrams, along with derivations. Train students to solve numerical problems interrelated with different topics. MARKING SCHEME Question 6 (a) . C . = As = 90, cos 90 = 1 = = = (b) B. 2 r = = or = 2 4 = 10 7 2 3 0 6 = 1 10-6 T F = Be = 1 10-6 1 6 10-19 2 107 = 3 2 10-18 N Alternate correct methods are acceptable. 22 Question 7 (a) (i) AB and CD are two parallel conductors kept l m apart and connected by a resistance R of 6 , as shown in Figure 5 below. They are placed in a magnetic field B = 3 102 T which is perpendicular to the plane of the conductors and directed into the paper. A wire MN is placed over AB and CD and then made to slide with a velocity 2 ms-1. (Neglect the resistance of AB, CD, and MN.) A M B R Figure 5 C (ii) [3] N 1m D Calculate the induced current flowing through the resistor R. In an ideal transformer, an output of 66 kV is required when an input voltage of 220 V is available. If the primary has 300 turns, how many turns should the secondary have? (b) In a series LCR circuit, obtain an expression for the resonant frequency. [2] Comments of Examiners (a)(i) Some candidates calculated emf and not the induced current, as was required. A few candidates did not write the unit of current. (ii) Some candidates used incorrect formula. A few candidates did not convert output given in kV to volt. (b) A number of candidates obtained the relation for instead of frequency f . Some of them could not derive this simple expression, possibly they had no idea of resonant frequency. 23 Suggestions for teachers Train students to read questions carefully. They should pause and think over the relevant answer. While explaining working of a transformer, give students all the relations/ratios. Encourage them to practice numerical problems based on these formulae. MARKING SCHEME Question 7 (a) (i) e = Bl = 3 10-2 1 2 = 6 10-2 V I= = (ii) Correct substitution with or without formula 6 10 2 6 = 1 10-2 A Alternate correct methods are acceptable. = 66000 = 220 300 300 (b) = 66000 300 220 = 90000 XL = XC or L = 1/ C 1 2 f L = 2 f 2 = = SECTION B Answer any three questions Question 8 (a) (b) (i) State any one property which is common to all electromagnetic waves. (ii) Arrange the following electromagnetic waves in increasing order of their frequencies (i.e. begin with the lowest frequency): (i) (ii) Visible light, rays, X rays, micro waves, radio waves, infrared radiations and ultraviolet radiations. What is meant by diffraction of light? In Fraunhofer diffraction, what kind of source of light is used and where is it situated? 24 [3] [2] Comments of Examiners (a)(i)While many candidates wrote that all electromagnetic waves travel with the same speed (of light), in vacuum was missing in many answers. A few candidates wrote, they all behave like particles. (ii) Some candidates arranged electromagnetic waves in incorrect / reverse order. (b)(i) Many candidates got confused between refraction of light and diffraction. They did not mention bending of light around edges of obstacles . (ii) Most of the candidates were unable to answer this question. They did not mention where the source was placed. Suggestions for teachers A clear understanding of properties of electromagnetic waves is called for. Arrange them in increasing order of wavelength and tell students that the order will reverse in case of frequencies. Explain with the help of wave theory, how light waves spread around edges of opaque bodies to enter geometrical shadow region. Discuss Fraunhofer diffraction in detail in the class. MARKING SCHEME Question 8 (a) (i) - (ii) All electromagnetic waves can travel through vacuum/ free space. They all have , and mutually perpendicular to each other. Transverse in Nature The do not require a material medium for propagation. They can be reflected. All waves are produced by accelerated charged particles/oscillating charged particles. (any one) Any other correct property is . Correct order is: Radio waves, Micro waves, Infra-red radiations, visible light, ultra violet radiations, Xrays and rays. (b) (i) Spreading or bending of light waves around the edges aperture/obstacle/corner/around a body is called diffraction of light. (ii) Monochromatic source of light and it is situated far away. (i.e. at infinity) 25 of an opaque Question 9 (a) (b) In Young s double slit experiment using monochromatic light of wavelength 600 nm, 5th bright fringe is at a distance of 0 48 mm from the centre of the pattern. If the screen is at a distance of 80 cm from the plane of the two slits, calculate: (i) Distance between the two slits. (ii) Fringe width, i.e. fringe separation. (i) State Brewster s law. (ii) Find Brewster s angle for a transparent liquid having refractive index 1 5. [2] Comments of Examiners (a) (i) Many candidates did not understand this numerical problem i.e. they could not understand what was given in the question. Some of them used incorrect formula. (ii) Some of the candidates did not convert unit nm Some did not write the unit of fringe width. (b) (i) Many candidates wrote, At polarising angle, reflected ray is perpendicular to refracted ray . Some of them could not write the statement of Brewster s law. (ii) Some candidates wrote tan i p = 1.5 or i p Suggestions for teachers Give ample practice on numerical problems based on Young s double slit experiment. Give them correct statements of laws, principle, etc and tell them to revise frequently. MARKING SCHEME Question 9 (a) (i) = 5 = 5 = a= = (ii) = 5 5 5 5 600 10 9 0 8 0 48 10 3 5 480 10 6 0 48 a = 5 10-3m = = 1 5 5 = = 0 096 mm 0 48 5 [3] Alternate correct methods are acceptable. 26 (b) (i) When ordinary (unpolarised) light is incident on a transparent medium at an angle of tan-1 ( ), the reflected light is completely polarised. (ii) P = tan-1 (1 50) = 56 3 o . Question 10 (a) Find critical angle for glass and water pair, given refractive index of glass is 1 62 and [2] that of water is 1 33. (b) Starting with an expression for refraction at a single spherical surface, obtain Lens [3] Maker s Formula. Comments of Examiners (a) Many candidates did not know how to find out g w . Some did not know the relation sin c= g w. A few candidates rounded off the value in the intermediate step and got a different answer. (b) Many candidates could not draw the correct diagram; arrows were found to be missing in may diagrams. Some candidates got confused between this derivation and that of refraction at a single spherical surface. Many used the lens formula directly to obtain lens maker s formula. Suggestions for teachers Teach the concept of relative refractive index 1 2 or g w etc and show its application by solving a few numerical problems. Stress upon the fact that in a numerical problem the intermediate step should not be rounded off. Advise them to do this in the final step only. In Geometrical optics (or Ray optics) emphasise on drawing correct ray diagrams Arrows must be given to straight lines to indicate the path of light. Both diagrams and derivations must be practised till they are perfect. MARKING SCHEME Question 10 (a) Sin c = g w = a w / a g = 1.33/1.62 cosec i c = w g = = 0 8210 = 55 2 o P 27 1.62 1.3 (b) Correct diagram showing object O, intermediate image I , final image I, u, v 1, and v. For first spherical surface: 1 1 = 1 1 For second spherical surface: 1 1 = 1 2 Adding, 1 1 1 1 = ( 1) 1 When u = , = 1 1 2 1 1 1 = ( 1) or ( 2 - 1 )/ 1 1 2 Alternate correct methods are acceptable. 1 2 Question 11 (a) A compound microscope consists of two convex lenses of focal length 2 cm and 5 cm. When an object is kept at a distance of 2 1 cm from the objective, a virtual and magnified image is formed 25 cm from the eye piece. Calculate the magnifying power of the microscope. (b) (i) What is meant by resolving power of a telescope? (ii) State any one method of increasing the resolving power of an astronomical telescope. 28 [3] [2] Comments of Examiners (a) Many candidates did not write the unit of Vo. Some did not know which lens is the objective and which one is the eye piece. Some of them did not know the correct formula for magnifying power of compound microscope. Some used incorrect sign convention and got incorrect answers. (b)(i) Most of the candidates did not mention far off i.e. distant objects , in the definition. A few candidates defined magnifying power instead of resolving power of a telescope. (ii)Most of the candidates wrote increasing the objective but did not mention what was to be increased. Suggestions for teachers Train students to draw rough diagrams before solving numerical problems on compound microscope /telescope because these problems are simply based on lenses in combination. Drill them to apply any one sign convention correctly and illustrate it by solving a few numerical problems in the class. Adequate practise should be given to students for learning/understanding various terms and related aspects in Physics correctly. MARKING SCHEME Question 11 (a) For objective: + = + = = = = v0 = = M = Vo/U o + = + = 20 + M = 120 Alternate correct solutions are acceptable. 29 (b) (i) It is the ability of a telescope to form separate images of two distant objects. (close to each other.) (ii) By increasing the diameter or aperture of the objective. SECTION C Answer any three questions. Question 12 (a) Plot a labelled graph of | | where is stopping potential versus frequency f of the incident radiation. [3] (i) Find the de Broglie wavelength of electrons moving with a speed of 7 106 m s-1. [2] (ii) Describe in brief what is observed when moving electrons are allowed to fall on a thin graphite film and the emergent beam falls on a fluorescent screen. (i) (ii) (b) State how will you use this graph to determine the value of Planck s constant. Comments of Examiners (a)(i) Many candidates did not draw the graph correctly. A few did not label the axes/ interchanged the axes. (ii) Some candidates wrote the slope of the graph as Planck s constant. (b)(i) A few candidates did not know the correct formula to find de Broglie wavelength. Some did not write the unit of . (ii) Most of the candidates could not write the correct answer though many alternate options were considered. 30 Suggestions for teachers Students must be taught how to draw correct and labelled graphs. Ask students to learn all the formulae in Physics, with proper understanding of symbols and to practice numerical problems. Electrons diffraction should be explained to students with the help of diagrams, photographs, etc. MARKING SCHEME Question 12 (a) (i) or | | fo Not passing Through the origin f f (ii) eV s = hf + ( - ) V s = + slope of the line = Or h = slope of the line e (b) (i) (ii) = 6.6 10 34 = 9.1 7 106 10 31 1 0 10 10 m Alternate bright and dark Circular fringes Diffraction is seen/ alternate dark and bright figure /scintillation Or any correct alternative answer. Question 13 (a) (b) Draw energy level diagram for hydrogen atom, showing first four energy levels corresponding to n=1, 2, 3 and 4. Show transitions responsible for: (i) Absorption spectrum of Lyman series. (ii) Emission spectrum of Balmer series. (i) Find maximum frequency of X-rays produced by an X-ray tube operating at a tube potential of 66 kV. (ii) State any one difference between characteristic X-rays and continuous X-rays. 31 [3] [2] Comments of Examiners (a)(i) Many candidates could not draw energy level diagrams correctly, some started with n=0, whereas some showed equal spacing between all energy levels. (ii) Many candidates did not show arrows/transitions correctly. They showed downward transition to all levels. Some showed formation of other series, which were not asked for. A few candidates drew (circular) orbits, instead of energy levels. (b)(i) Some candidates calculated wave length min instead of frequency. Others did not know the correct formula. A few of them did not convert potential of the tube from kV to volt. (ii) Most of the candidates were unable to answer this question correctly. They wrote about hard X rays and soft X rays. Suggestions for teachers Teach them how to draw energy level diagram of H atom, correctly and the method to mark for the absorption spectrum and the emission spectrum. Ask students to read questions carefully and to answer only as per the question. Before substituting in a formula, all quantities must be converted to SI systems. - Differences between continuous X rays and characteristic X rays must be highlighted while teaching X rays. MARKING SCHEME Question 13 n=4 (a) n=3 n=2 Emission Spectrum or Balmer Series (1) n=1 Absorption Spectrum or Lyman Series (1) (b) (i) v max = 1 6 10 19 66 103 6 6 10 34 v max =1 6 1019 Hz Or any alternative correct method to calculate v max. (ii) 1. Characteristic X rays are characteristic of the material of the target whereas continuous X rays are not. 2. Characteristic X rays are fewer in number whereas continuous X rays are infinite in number. 32 3. Continuous X rays have lower intensities whereas characteristic X rays have higher intensities. 4. min of continuous X rays depends on applied voltage whereas of characteristic X-rays does not. (any one) Question 14 (a) Obtain a relation between half life of a radioactive substance and decay constant ( ). [2] (b) Calculate mass defect and binding energy per nucleon of , given Mass of = Mass of = u Mass of = u [3] Comments of Examiners (a) Many candidates wrote t for half-life instead of t 1/2 or T. A few of them did not write: 1 At t=T, N=2 N o Suggestions for teachers Some candidates wrote the final relation without giving the in between steps. (b) Many candidates calculated binding energy but not binding energy per nucleon. Quite a few candidates did not know how to calculate mass defect ( ) and binding energy. Advise students to use standard symbols and notations. Teach them the concept of half-life, mean life and disintegration constant and method to obtain relations between them. Adequate practise should be given to students in solving numerical problems on mass defect, binding energy and binding energy per nucleon. MARKING SCHEME Question 14 (a) = When t = T, T= = . N = (with working) 33 (b) { = + ( ) } = {10 1 007825+10 1 008665} {19 992397} = {10 07825 + 10 08665} {19 992397} = or 2.8635 x kg B.E. = MeV = 0 1725 931 = 160 6 MeV or 2.5696 x J B.E./nucleon = or = 8 03 MeV or 1.2848 x J Question 15 (a) (b) With reference to a semi-conductor diode, what is meant by: (i) Forward bias (ii) Reverse bias (iii) Depletion region [3] Draw a diagram to show how NAND gates can be combined to obtain an OR gate. (Truth table is not required). [2] Comments of Examiners (a)(i) A few candidates were confused between forward bias and reverse bias. Some candidates used incorrect terms like P type diode and N type diode. (ii) Some candidates wrote incorrect statements e.g. P region is connected to positive terminal of the battery in reverse bias while some did not mention the N region. (iii) Many candidates could not write the meaning of depletion region correctly or completely. (b) A few candidates used the incorrect symbol of NAND gate. Some showed one input to the NAND gate. A few of them drew the complete diagram but forgot to join the input terminals. Some candidates gave TRUTH Table, which was not required. 34 Suggestions for teachers Ensure that students understand the terms pertaining to semiconductor diode viz. potential barrier, depletion region, drift current and diffusion current, forward bias and reverse bias etc. Tell them to practice drawing labelled diagrams. Explain to students the Logic Gates and their combinations to obtain all basic gates. MARKING SCHEME Question 15 (a) (i) Forward bias: It means p region is connected to positive terminal and n region is connected to negative terminal of a cell / battery. OR P N OR (ii) Reverse bias: It means p region is connected to negative terminal and n region is connected to positive terminal of a cell/battery OR P N OR (iii) Depletion region is a charge free region between p and n regions of a semi-conductor diode. 35 It is a narrow space (region) between p and n regions which does not contain charge carriers (i.e. electrons and holes). (b) Note: For questions having more than one correct answer/solution, alternate correct answers/solutions, apart from those given in the marking scheme, have also been accepted. 36 Topics found difficult by candidates Concepts in which candidates got confused Capacitors in series and parallel. Derivation of electric potential at a point. Resistors in series and parallel. Numerical problems on electric circuits, voltmeter, meter bridge, etc. Angle of dip. Ampere circuital law: applications. Derivation of lens maker s formula. Resolving power of a telescope. Energy level diagram of H atom. Depletion region Kirchoff s 1st law and II law. Biot Savart s law and Ampere s circuital law. Spherical aberration and chromatic aberration. Lens formula and lens maker s formula. Arranging electro-magnetic waves according to their frequencies. Emission spectrum and absorption spectrum of hydrogen atom. Characteristic X rays and continuous X rays. B.E and Binding Energy per nucleon. Forward bias and reverse bias of a junction diode. Frequency (f) and angular frequency ( ). 37 Suggestions for candidates Study regularly. Practise conversion of one system of unit to other system of unit. Prepare a list of formulae, definitions, laws, derivations, etc from each chapter. Learn the laws, principles, definitions, etc by heart. Focus on key words and terminology. Learn all the formula with meaning of each and every term involved. Learning with proper understanding is more important than just learning by rote. Try to understand various concepts involved in Physics. Refer to different text books, encyclopaedia etc, for reference. Practise derivations and numerical problems regularly. Practise drawing diagrams, ray diagrams, circuit diagrams, etc regularly. Solve past years question papers and sample paper of ISC. During examination read every question carefully and answer to the point. Draw labelled diagrams. In Ray optics, don t forget to put arrows to the rays. While solving numerical, read the question carefully and write the given data. Before substituting in a relevant formula, ensure that all the given quantities are in SI units. Make proper conversions (if required). Be careful with units like mm, cm, nm, A, C and F, electron volt etc. These must be converted to SI units. Write complete answer with unit and direction (if it s a vector quantity). Don t spend too much time on any one question. Write only what is asked for. Write in brief and to the point, rather than beating around the bush. 38 PRACTICAL (PAPER-2) Answer all questions. You should not spend more than one and a half hours on each question. [9] Question 1 This experiment determines the focal length of the given convex lens by no parallax method. You are provided with: (a) An optical bench (b) A lens holder (c) A convex lens (d) Two optical pins Note: If an optical bench is not available, the experiment may be performed on a table top, using a meter scale. (i) (ii) Determine the approximate focal length f of the given convex lens by projecting the image of a distant object formed by the lens on a wall or a screen. Record the value of f in cm correct upto one decimal place. Arrange the object pin O, the image pin I and the lens L on the optical bench or table top as shown in Figure 1 below. Adjust the height of the object pin O and that of the image pin I so that the tips of O and I lie on the principal axis of the lens. I L O 0 cm u I v 100 cm Figure 1 (iii) (iv) (v) Place the object pin O at the 0 cm mark and the lens L at the 70 0 cm mark so that the object distance u = 70.0 cm (i.e. the distance between L and O) Look at the tip of the object pin O through the lens from a distance so that you see an inverted image (say I ) of the object pin. Now, adjust the position of the image pin I in such a way, that there is no parallax between I and I . Ensure that tip to tip parallax is removed. 39 (vi) At no parallax, note the position of the image pin I and measure the image distance v = LI (i.e. the distance between the lens and the image pin) in cm, correct upto one decimal place. (vii) Repeat the experiment for four more values of u, i.e. 60 0 cm, 50 0 cm, 40 0 cm and 30 0 cm. + (viii) For each value of u, calculate x = and y = . 100 10 (ix) Tabulate all five sets of u, v, x and y with their units. (x) Show the image position when the parallax has been removed, in any one of the readings in (ix) above, to the Visiting Examiner. (xi) Plot a graph of y vs x. Draw the line of best fit. Calculate its slope m using, m= and record its value, correct upto three significant figures. (xii) Find F using, F = and record its value with proper unit, correct upto one 10 decimal place. Comments of Examiners Record Many candidates did not express the approximate focal length of the convex lens up to l decimal place with unit. A few candidates did not express the image distance v up to 1 decimal place. Many candidates wrote the position as L of the lens but not write the object distance OL = u. In several cases, candidates showed x = + Suggestions for teachers instead In a number of cases, units of u, v, x and y were not written or the values of x and y were not rounded off properly. Graph Many candidates did not take a uniform scale from the origin. In a few cases, the scale was not written. Many candidates took kink on the graph. In some cases, the scale taken was inconvenient. Many candidates tended to plot the points as blobs; at times, discontinuous lines were drawn, while a few candidates took fake points to get a straight line. + of x= 100 and y=100 instead of y = 10 . 10 40 Explain the theoretical aspects related to each experiment. Help students to make meaningful observations. In optics practicals, explain about the parallax error and how to eliminate it. Tell students about the trend of the experiment (for record mark) such as, u increases, v should decrease. Give practice to students in finding the least count of different instruments and their ranges. Give practice in correct rounding off up to various d.p. and significant figures. Instruct students to read the question paper properly. Tell students to write observations in a tabular form with unit, decimal place and significant figures, asked as per the question. Graph: give enough practice for development of graphical skills (such as labelling, taking a uniform convenient scale, correct plotting, concept of best fit line, finding slope taking other than plotted points separated by more than 50% of the line drawn and correct calculation of slope). Deduction For slope m plotted points were taken in several cases. In some cases, less than half or 50% of the line was taken; many candidates did not express m in three significant figures. Calculation Many candidates calculated f = without decimal place or unit or both. In some cases, F was out 10 of range. MARKING SCHEME Question 1 RECORD (R) A. Approximate focal length of the lens (1dp/unit) - unit should be in c.m. 4 Correct sets of u and v Correct calculation of x and y at least in six values Note: (i) Correct set means v increases as u decreases (ii) Proper unit should be used at least in one place i.e. in u/v/x/y and three values of v should be up to 1dp GRAPH (G) A. B. C. Axes labelled correctly. The scale should be convenient, uniform starting from the origin. [If the scale taken is uniform on both the axes, without the origin marked, it can be considered correct] Origin may be shifted. (ii) Inter change of axes are permitted (iii) Kink is not allowed. (iv) Last plot must cover 50% of either axes. At least four points plotted correctly (i) Note: (i) Correct plot means, plotted points may be 50% of 1 division on both sides from the actual point to be plotted. (ii) Points must be thin and encircled (optional) (Like, x) . (iii) A blob is not a point. Best fit line: (i) Thin and uniform and passes through at least four points (even for blobs) or within five divisions / 1cm. perpendicular distance of both side of the line drawn. (ii) The line must be extended on either side with respect to any four plots. DEDUCTION (D) (i) Correct calculation of slope (m) of the best fit line using two distant points separated by 50% of the line considered, taking at least one unplotted point. & can be read out directly from the graph (ii) Slope m = (Fractional value may be considered) QUALITY Correct calculation of F using candidate s m should lies within the range of 7.5cm. to 12.5cm. (F must be expressed correct upto 1d.p. with proper unit). Unit should be in cm 41 [4+2] Question 2 This experiment determines the potential gradient (K) of a potentiometer wire. You are provided with: (a) A 100 cm long and uniform metallic wire AB attached to a metre scale on a wooden board. It is provided with connecting terminals at its ends. (b) A 4 V d.c. source E. (c) A dry cell . (d) An ammeter A of range 0 - 1 A. (e) A voltmeter V of range 0 - 3 V. (f) A galvanometer G. (g) A plug key K. (h) A jockey J. (A) (i) (ii) Determine and record the least count of the given ammeter and voltmeter. Arrange the circuit as shown in Figure 2(a) below. Make sure that all connections are tight. E K A ( ) C A 0 cm J B V Figure 2(a) (iii) Keep the value of E at about 3 5V to 4V. (iv) Close the key K. Record the ammeter reading I, in your answer booklet. (v) Place the jockey J at a point C on the wire AB such that AC = 20 0 cm. Note and record the reading of the voltmeter. (vi) Repeat the experiment to obtain four more values of l, i.e. AC = 40 0 cm, 60 0 cm, 80 0 cm and 100 0 cm. Each time, note and record the reading of the voltmeter. (vii) For each value of V, calculate K = correct upto three decimal places. (viii) Tabulate all five sets of values of V, l, and K with their units. (ix) Show any one of the readings in (viii) above, to the Visiting Examiner. (x) Find K 0 , the mean of all the five values of K and record its value with unit, in your answer booklet. 42 (B) This part of the experiment determines the emf of the given dry cell . (i) Replace the voltmeter in the Figure2(a) with a dry cell and a central zero galvanometer G and set up a new circuit as shown in Figure 2(b), below: E ( ) A 0 cm CJ l A K B G Figure 2(b) (ii) Close the key K, touch the jockey J near the ends of A and B of the wire AB. The galvanometer needle must show deflection in the opposite directions. (iii) Place the jockey gently at different points on the wire AB till at a certain point C, the galvanometer G shows no deflection. Note the length AC = l. (iv) Now calculate emf of dry cell . = K 0 l where K 0 is the mean value obtained in Question 2(A). (v) Record the value of correct upto two decimal places with its unit, in your answer booklet. Comments of Examiners (i) Many candidates did not write the least count of the ammeter and voltmeter with unit; in some cases, the least count written did not match with the supplied least count of ammeter and voltmeter. The voltmeter readings were not consistent with the least count of voltmeter in many cases; (ii) Proper trend between v and l was not observed. (iii) Average value of k as k o was not calculated or calculated incorrectly. In a few cases, the units k or k o were written as volt per meter (Vm-1) instead of volt per cm (Vcm-1). (iv) Many candidates did not express the value of e.m.f correct up to two decimal places with unit. Suggestions for teachers 43 Give practice in finding the least count of different instruments such as the ammeter, voltmeter, meter scale, etc.) and recording them with proper practical units. Provide students different types and ranges of ammeters and voltmeters and give them enough practice in writing the least count with units and their ranges. Tell students the trend of each experiment mathematically. Give practice to students in connection of the circuit as per the diagram given. The concept of potential gradient must be explained along with its proper unit. MARKING SCHEME Question 2 RECORD (R) (A) L.C. of ammeter and voltmeter with their units (a) (b) Three correct sets of l and V Note: Correct set means: (i) As l increases, V increases (ii) V recorded correctly with or without unit but in multiple of L.C. of V. (at least in 3 values) Any values of l can be taken within 0cm to 100cm (iii) DEDUCTION (D) Correct calculation of potential gradient K in at least three sets with unit.( (V 1, or V 1 ) either in K or K 0 (B) (i) Record of balance length l Correct calculation of = K 0 l (Correct upto 2 dp with proper rounding off, with unit) Note: the value of K 0 must be taken from Q2(A). If mean value K 0 is not calculated, then any recorded value of K can be taken. (ii) 44 Topics found difficult by candidates Concepts in which candidates got confused Suggestions for candidates Removal of parallax error Concept of significant figures. Proper rounding off of any value up to 1 d.p., 3 d.p, or significant figures. Record of readings in consistence with the least count of instruments. In graph: marking of origin, choosing proper uniform and convenient scale, concept of best fit line, finding of slope from best fit line. Concept of significant figures Concept of convenient scale Concept of best fit line Finding slope (taking 2 plotted points separated more than the 50% of the best fit line drawn) Proper choice of scale Least Count of instruments Read the question carefully and follow the instructions, using only the formula given in the question paper for all the calculations. Do not waste time by writing unwanted things like apparatus required, theory, circuit diagram, etc. Understand the theoretical concepts behind the experiment and understand the trend of the two variables in the experiment. Learn the correct use of instruments such as, meter scale, optical bench, Vernier callipers, screw gauge, ammeter, voltmeter and galvanometer; Ensure that all observations are consistent with L.C. of the measuring instrument and recorded in tabular form with unit. Note down the L.C. of the instruments used before starting the experiment. All values calculated should be calculated upto the decimal place or significant figures asked for the in the question. Scale should be uniform and convenient with axes properly labelled. Plots should be small encircled dots, correct to the nearest division of the graph sheet. Line of best fit means the aggregate of all plotted points drawn symmetrically and extended on both sides of the last plotted points. Slope calculation should be from two widely separated, unplotted points lying on the best fit line. The scale of the graph should be such that at least 2/3 of the graph paper is used. 45

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Additional Info : ISC Class XII Analysis Of Pupil Performance 2017 : Physics
Tags : ISC Board, Class 11th, Class 12th, NDA/NA Entrance Examination  

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