ISC Class XII Exam Question Papers 2012: Electricity and Electronics

 

ELECTRICITY AND ELECTRONICS

(Three hours)

(Maximum Marks: 100) (Candidates are allowed additional 15 minutes for only reading the paper. They must NOT start writing during this time.)

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Answer all questions from Part I (Compulsory( and five questions from Part 1I. 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 part of questions are given in brackets [ ].

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PART I

Answer all questions

Question 1

Describe the working of a fully wave rectifier with the help of a diagram.

 

Question 2

Name the various losses in a D.C. motor. Explain any two.

 

Question 3

Nature the various types of microphones and draw the diagram of any one.

 

Question 4

Why is the earthing pin of a three pin plug made larger than the other pins? Explain the need of earthing.

 

Question 5

Explain the difference between a triode value and a transistor.

 

Question 6

Explain the mechanism of’Hole current flow’ in a semiconductor with the help of a diagram.

 

Question 7

Fill in the blanks choosing the appropriate word from those given in the brackets. Write the correct answer in your answer booklet:

(a)        In an npn transistor_______________ are minority carriers. (free electrons/holes)

(b)       Emitter of a transistor is_____________doped, lightly/heavily)

(c)        The collector-base junction of a transistor is (forward biased/reverse biased)

(d) The voltage gain of a transistor connected in ~ arrangement is the highest, icommon base/common emitter)

 

Question 8

Draw the diagram of an A.C. series motor and label it.

 

Question 9

With reference to an induction motor, write short notes on:

(a) Rotor               (b) Stator

 

Question 10

With the help of a diagram explain how dual switching of lamps can be used on a staircase. Show diagrams for both ‘on’ and ‘off conditions.

PART II

Answer any five questions.

 

Question 11

With the help of a circuit diagram, explain the working of the following self-existed D.C. generators:

(a) Shunt generators                                    (b) Series generators                        (c) Compound generators

 

Question 12

(a) Explain the following triode parameters:

(i) Amplification factor (ip)                         (ii) A.C. plate resistance( ir)

(iii) Trans conductance (gm)

 

Question 13

(a)    With the help of a diagram, explain armature reaction of D.C. generator.

(b)   State the various applications of a V’IXq~.

(c)    Write the merits and demerits of a VTVM.

 

Question 14                                                                       /

(a) Explain the term Ripple factor. How is it different in full-wave rectification and half-wave rectification?

(b) Explain the working of a Zener diode. How is it used as a voltage stabilizer?.~

(c) Define the following terms:

(i) Biasing.        (ii) Barrier potential.

 

Question 15

(a)Describe any three types of switches in an electrical circuit. Explain the working of any one.

(b) Explain two types of power distribution systems.

(c) How can a capacitor be used in various electrical circuits?

 

Question 16

(a)  Give any two reasons for the slow running of a split phase motor.

(b)

• (i) Find the value of/3 if ct = 0.9.
• (ii)The base current in a transistor is 0.01mA and emitter current is 1mA. Calculate the value of a and B ‘.

(c) The collector leakage current in a transistor is 300 g A in CE arrangement. If the transistor is not connected in CB arrangement, what will be the leakage current? Given that/3 =120.

 

Question 17

(a)    Explain single phase three wire distribution system of D.C. electric power.

(b)   Describe how three wire power distribution is made possible in D.C. system.

(c)    With the help of diagrams, explain two phase wire distribution system of electric power.

 

Question 18

(a)    Give the principle and the working of a D.C. motor.

(b)   Differentiate between a wire and a cable.

(c)    Give reasons why the D.C motor should not be started without a load. What is the significance of ‘counter emf  in a d.c. motor?

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