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ISC Class XII Board Exam 2017 : Electricity and Electronics

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ISC 12th
Indian School Certificate Examination (ISC), New Delhi

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ELECTRICITY AND ELECTRONICS (Maximum Marks: 100) (Time allowed: Three hours) (Candidates are allowed additional 15 minutes for only reading the paper. They must NOT start writing during this time.) --------------------------------------------------------------------------------------------------------------------Answer all questions from Part I (Compulsory) and five questions from Part II. All working, including rough work, should be done on the same sheet as, and adjacent to the rest of the answer. Mathematical tables and squared paper are provided. The intended marks for questions or parts of questions are given in brackets [ ]. --------------------------------------------------------------------------------------------------------------------PART I (40 Marks) Answer all questions. Question 1 With the help of a neat circuit diagram, explain the working of a full wave rectifier. [4] Question 2 (a) (b) Name the component used for each of the following: (i) Filtration of electrical signal. (ii) Amplification of electrical signal. Name an element for each of the following: (i) Pentavalent atom (ii) Trivalent atom [2] [2] Question 3 Explain briefly how a zener diode works where there is increase in: (a) Load (b) I/P voltage [4] -------------------------------------------------------------------------------------------------------------------This Paper consists of 6 printed pages. 1217-866 Turn over Copyright reserved. Question 4 (a) Write any two properties of insulation used in a cable. [2] (b) State any two reasons why conductors are usually made up of copper, aluminium, or galvanised steel. [2] Question 5 Sketch a graph to show the plate characteristics of a triode. inferences drawn from the graph. Also, state any two [4] Fill in the blanks choosing the appropriate word from those given in brackets. Write the correct answer in your answer booklet. [4] Question 6 (a) A metal plate buried deep inside earth in any electrical installation is called_____. (earthing, doping) (b) A safety device used for preventing an appliance from damage is called _______. (fuse, inductor) (c) If a pure semiconductor is heated, its resistance will go _______. (up, down) (d) A device used in regulated power supply to change the voltage is called________. (transformer, capacitor) Question 7 Explain briefly how an inductor coil can be used to produce a rotating magnetic field for a single phase a.c. motor to self-start. Question 8 What is the use of a microphone? With the help of a neat diagram, explain the working of a moving coil microphone. [4] [4] Question 9 With reference to Cathode Ray Oscilloscope (CRO), state the functions of the following knobs on its panel board: (a) Focus (b) Intensity (c) Volt / division [4] (d) Horizontal centring --------------------------------------------------------------------------------------------------------------------2 1217-866 Question 10 With reference to generators and motors, explain what is meant by armature reaction. Also, mention the two important effects of armature reaction. [4] PART II (60 Marks) Answer any five questions. Question 11 (a) Explain the term critical field resistance of a d.c. shunt generator. [3] (b) Explain briefly the necessity of a starter resistor for a motor. [3] (c) Explain how a d.c. motor converts electrical energy to mechanical energy. [6] Question 12 (a) State any two essential qualities of a switch. [2] (b) [4] =50 IB = 240 IE = 30 mA Figure 1 Calculate the rating of the transistor shown in Figure 1 above. determine the value of IC. (all symbols have usual meaning) (c) Hence, Draw a neat circuit diagram of a single stage R-C coupled amplifier circuit and state the importance of: (i) Biasing circuit (ii) Emitter bypass capacitor. [6] Question 13 (a) (b) Name the three sections of an NPN transistor and explain briefly, how they are formed. [5] Explain briefly how electrical power is distributed from a power-house to the [4] consumers. (c) Draw a graph to show reverse biased P-N junction diode. Also, mention why there [3] is a small current flowing through the diode in the reverse biased condition. --------------------------------------------------------------------------------------------------------------------3 1217-866 Turn over Question 14 (a) (b) (c) Give a reason why the final stages of a practical amplifier circuit is a transformer coupled amplifier. Also, state any two features that must be incorporated to obtain high power amplification. Using the following accessories, sketch a simple electrical layout: Distribution board, ceiling rose, switch, cables, fan. State whether the following statements are true or false: (i) If the emitter-base junction of a transistor is reverse-biased, the collector current stops. (ii) Current amplification factor ( ) is given by the ratio of collector current (IC) to that of emitter current (IE). (iii) In the reverse region of its characteristic, P-N junction diode appears as an ON switch. (iv) All the three regions of a transistor are equally doped. [4] [4] [4] Question 15 (a) State any two properties of a germanium crystal. [2] (b) [3] Fill in the blanks choosing the appropriate word from those given in brackets. Write the correct answer in your answer booklet. (i) In lap winding, the armature conductors are divided in such a way that the number of parallel paths is _______ the number of poles (equal to, greater than, less than). (ii) If the field resistance is greater than the critical resistance of the generator, then it will ______. (fail to generate voltage, generate voltage) (iii) The input resistance of a common emitter is ________ (low, high) (c) Calculate the current in the circuit shown in Figure 2 below. (Assume that the diode is ideal). Also, copy the circuit and mention the polarity for the diode. +6V [3] ^^^^^ +4V 200 Figure 2 (d) A 230V d.c. motor has an armature circuit resistance of 0 6 . If the full load armature current is 30A and no-load armature current is 4A, calculate the change in back emf, from no-load to full load. [4] --------------------------------------------------------------------------------------------------------------------4 1217-866 Question 16 (a) (b) (c) With the help of a neat circuit diagram, explain the working of a voltage doubler. Explain how sound is recorded on magnetic tapes. Why are inductors, capacitors and resistors called passive circuit elements? Also, name any two types of capacitors. [5] [4] [3] Question 17 (a) Fill in the blanks choosing the appropriate word from those given in brackets. Write the correct answer in your answer booklet. (i) The forward resistance of a crystal diode is ________ than its reverse resistance. (much less, much greater). (ii) When a high resistance is connected in series with a galvanometer, it becomes ________ (ammeter, voltmeter). [4] (iii) The maximum efficiency of a full-wave rectifier is _______ (81 2%, 41 2%). (iv) The addition of pentavalent impurity to a semiconductor creates ________ (holes, free electrons). (b) The following readings were obtained from the linear portions of the static characteristics of a vacuum triode: EP (plate voltage) 100V 100V 50V IP (Plate current) 10mA 3mA 5 5mA Eg (Grid voltage) 1 0V 2V 1 0V [4] Calculate: (c) (i) a.c. plate resistance. (ii) Mutual or trans conductance. (iii) Amplification factor. With the help of a neat diagram, explain the working of a vacuum tube voltmeter (VTVM). [4] Question 18 (a) Draw the energy band diagram for the following: (i) N-type semi-conductor. (ii) Insulator. [4] --------------------------------------------------------------------------------------------------------------------5 1217-866 Turn over (b) The I/P a.c. voltage to a full wave rectifier is given by: [4] V = 1000 sin . Calculate: (i) Irms (ii) (c) Idc Draw a neat circuit diagram for a common-collector (CC) connection for a transistor and mark the direction of the current in your circuit diagram. Also, define the current amplification factor ( ). [4] --------------------------------------------------------------------------------------------------------------------6 1217-866

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