ISC Class 12 Sample Paper Electricity and Electronics

ELECTRICITY AND ELECTRONICS
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Maximum Marks: 80
Time Allowed: Three hours
(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 in Section A, Section B and Section C.
The intended marks for questions or parts of questions are given in brackets [ ].
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SECTION A – 16 MARKS

Question 1

(i) The purpose of synchronising control in a CRO is to: [1]

(a) focus the spot on the screen.

(b) lock the display of the screen.

(c) adjust the amplitude of the display.

(d) control the intensity of the spot.

(ii) For starting a D.C. motor, a starter resistor is required because: [1]

(a) it limits the speed of the motor.

(b) it limits the starting current to a safe value.

(c) it starts the motor.

(d) this motor is not self-starting.

(iii) A single phase induction motor is: [1]

(a) self-starting.

(b) not self-starting.

(c) self-starting with the help of an auxiliary winding.

(d) None of the above.

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(iv) In an NPN transistor ___________ are the minority charge carriers. [1]

(a) free electrons

(b) donor ions

(c) acceptor ions

(d) holes

(v) A semiconductor diode has __________________. [1]

(a) one p-n junction

(b) two p-n junctions

(c) three p-n junctions

(d) four p-n junctions

(vi) The material used for making the stator of an induction motor is ______________ [1]
(copper, silicon steel).

(vii) A multimeter cannot measure _____________. (Capacitance, resistance) [1]

(viii) Inductors are _________ devices. (active, passive). [1]

(ix) An earphone converts __________________. (sound energy to electrical energy, [1]
electrical energy to sound energy)

(x) What happens to the speed of a D.C shunt motor if the load is increased? [1]
(reduces slightly, increases slightly)

(xi) Match the following: [4]

(a) DC generator (1) Voltage regulator

(b) Zener diode (2) Centrifugal switch

(c) Induction motor (3) Transducers

(d) Microphone (4) Split rings

(xii) State whether the following statements are True or False: [2]

(a) The insulation colour used for ‘Live’ wire is yellow.

(b) Minority charge carriers in a p type semiconductor are electrons.

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SECTION B - 32 MARKS

Question 2

(i) Explain why a single phase induction motor is not a ‘self starting’ machine. [2]

(ii) Answer the following questions with reference to the circuit diagram shown in
Figure 1 below.

MAIN WINDING
I IM

IS
•
S

V

C
ROTOR

STARTING WINDING

Figure 1

(a) What is the use of the centrifugal switch ‘S’? [1]

(b) State the function of the capacitor ‘C’ in this circuit. [1]

Question 3 [4]

A diagram of a semiconductor diode is shown below. Label the parts from 1 to 4 by using
technical terms/names.

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Question 4 [4]

(i) Which one of the graphs given below, shows that the semiconductor diode is a
forward biased?

(a)

(b)

(c)

(d)

(ii) Name the three terminals of vacuum triode.

4

Question 5 [4]

Name the triode parameter which can be obtained from the ratio of each of the following:

(i) Change in plate current (∆𝐼𝑝 ) against change in grid voltage (∆𝑉𝑔 ) at constant plate
voltage.

(ii) Change in plate voltage (∆𝑉𝑝 ) against change in plate current (∆𝐼𝑝 ) at constant grid
voltage.

(iii) Change in plate voltage (∆𝑉𝑝 ) against change in grid voltage (∆𝑉𝑔 ) at constant plate
current.

(iv) write the relationship between all the three triode parameters

Question 6 [4]

(i) Draw a neat circuit diagram of a full wave rectifier. Also, sketch the input and
output graphs.

(ii) Draw a neat, labelled diagram of a crystal microphone.

Question 7 [4]

Name the material used in the making of the following:

(i) Filament of an electric bulb.

(ii) Brushes for generators and motors.

(iii) Core of a transformer.

(iv) connecting wires

Question 8 [4]

(i) (a) What is the use of a lamp holder? Mention any two types of lamp holders.

(b) Write a short note on junction box.

OR

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(ii) (a) Briefly explain why ‘earth pin’ is made thicker and longer in a three-pin
plug.

(b) Write a short note on ceiling rose.

Question 9 [4]

(i) Draw a neat circuit diagram of a transistorised voltmeter.

OR

(ii) Draw a neat diagram of a cathode ray tube.

SECTION C - 32 MARKS

Question 10

(i) With the help of a neat diagram, explain how A.C. electrical power is distributed [4]
in a 3 phase 4 wire system.

(ii) Write a short note on any one type of insulation used for copper wires. [2]

(iii) Draw two separate diagrams to show how a dual switch can be used to turn ‘on’ [2]
and ‘off’ a lamp from two different places.

Question 11

(i) Draw a neat, labelled circuit diagram of a transistor (NPN or PNP) as an amplifier. [4]

(ii) Obtain the relationship between α and β (symbols have their usual meaning). [2]

(iii) What is the difference between active and passive circuits elements? Name any [2]
one active circuit elements.

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Question 12

(i) Explain how ‘self excitation’ is achieved in a shunt generator. [4]

(ii) A 441V shunt wound D.C. motor has armature current (𝐼𝑎 ) = 60A. If armature [4]
resistance (𝑅𝑎 ) = 0·8Ω, calculate:

(a) back emf.

(b) power supplied to the armature.

OR

(i) State two effects of armature reaction. [2]

(ii) What is meant by commutation in a D.C. motor? [2]

(iii) Name the two types of generators based on ‘excitations.’ [2]

(iv) State the function of the following parts of a generator: [2]

(a) Poles

(b) Brushes

Question 13 [8]

(i) With the help of a neat circuit diagram, explain the filtering action of a 𝜋 −section
filter.

(ii) Draw a neat circuit diagram of a power amplifier.

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