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# ELECTRICITY PAST PAPERS QUESTIONS AND ANSWERS

NAME__________________________________    INDEX NUMBER________________________

CANDIDATE’S SIGNATURE______________

DATE__________________________________

448/1

ELECTRICITY

PAPER I

JULY/ AUGUST 2011

2 ½ HOURS

MWALA DISTRICT JOINT EXAMINATION

Kenya Certificate of Secondary Education.

ELECTRICITY

PAPER 1

JULY/ AUGUST 2011

INSTRUCTIONS TO CANDIDATES

1.You should have the following for this paper.:-

* Mathematical tables.

2.  Answer all questions in Section A and any four questions in section B.

This paper consists of 5 printed pages.

Turn over

SECTION A: 52 MARKS

1.         (a) List down the six sources of electricity                                         (3 marks)

(b) State the source of electricity used to produce electricity in the following devices.

(i) Watch cells.                                                                        (1mk)

(ii) Solar panels.                                                                      (1mk)

(iii) Mobile phone cells                                                            (1mk)

(iv) Bicycle dynamo                                                                (1mk)

2.         For each of the following, identify the fault and state a possible remedy.

(a) Electric shock on touching metal casing of an electric cooker.                                    (2mks)

(b) When a fluorescent lamp is switched on it only glows at the two ends of the tube.   (2mks)

3.         (a) A leclanch cell has an emf of 1.5v on open circuit. When a current of 0.5A passes in the

circuit consisting of a torch bulb connected across the terminals of the cell, the p.d of

the cell drops to 0.9v. Calculate the internal resistance of the cell.                      (2mks)

(b)  Four cells each of 2v and internal resistance of 0.4W are to be connected to supply a load

of 0.4W. By aid of suitable diagram show how they would be connected to supply                                    maximum power.                                                                                            (2 marks)

(c) Name the instrument used to measure the specific gravity of electrolyte of a lead-acid                          cell.                                                                                                                        (1mk)

4.         State the differences between

(a) Single pole switch and double  pole switch.                                                         (2mks)

(b) Double wound transformer and autotransformer.                                                 (2mks)

(c) Earthing lead and earth continuity conductor.                                                      (2mks)

5.         (a) Name the tool used to mark lines on metal surfaces.                                            (1mk)

(b) State any two uses of a steel rule.                                                                         (2mks)

(c) List down any three methods of joining metals.                                                   (3 marks)

6.  A coil of resistance 20W and inductance of 0.08H is connected to a supply

of 240v, 50Hz. Calculate:-

(a) The current in the circuit.                                                                                       (2mks)

(b) The value of a capacitor to be connected in series with the coil so that the

current shall be 12A.                                                                                     (3mks)

7.         Calculate the resistance of 2km of copper wire, given that the diameter of

the wire is 0.006 metres and the resistivity of copper in 0.017mW/m                       (2mks)

8.         Define the following electrical terms/devices

(a) Electronics.                                                                                                            (2mks)

(b) Armature reaction                                                                                                  (2mks)

(c) Thermal runaway                                                                                                   (2mks)

(d) Capacitive time constant.                                                                                      (2mks)

9.         (a) State any two advantages of circuit breakers over fuses.                                      (2mks)

(b) Define the following terms used in connection with fuses.

(i) Current rating         (2mks)

(ii)  Fusing current       (2mks)

2.

10.       Sketch in good proportion the front elevation end elevation and plan in 1st angle projection.          (3marks)

15    15

SECTION B: 48 MARKS

11.       (a) Why is a starter necessary before a motor is started.                                             ( 1 ½ mks)

(b) How can the direction of rotation of the flowing motors be reversed:-

(i) Capacitor start, induction run single phase motor                          (1 ½ mks)

(ii) D.C series motor.                                                                                       ( 1 ½ mks)

(iii) Three phase induction motor                                                                    ( 1 ½ mks)

(c)  Describe the operation of a centrifugal switch and explain why it is used in

a single phase motor                                                                                        (1 ½ mks)

(d)  State any one advantage of wound rotor over squirrel cage rotor.                      ( 1 ½ mks)

(e) What would be the effect of increasing the output current of an alternator on

the frequency.                                                                                                            ( ½ mks)

(f) An alternator has 4 poles and the frequency of the output is 50HZ.

Calculate the speed of the rotor.                                                                     ( 1 ½ mks)

3.

12.       (a) A moving coil instrument has a sensitivity of 10kW/V. The resistance of its coil is 100W.          Calculate:-

(i) The value of shunt that would enable it to measure a maximum current of 5A.   (4 marks)

(ii) The value of multiplier that would enable it to measure a maximum voltage of 250v.                                                                                                                                                    (4mks)

(b) State any two uses of ohmmeter.                                                                          (2mks)

(c) What is the purpose of a galvanometer in a wheatstone bridge circuit                 (2 mks)

13.  The diagram below shows a single stage common emitter amplifier.

10mA 5mA

(a) Calculate:-

(i) The ohmic value of RL                               (2mks)

(ii) Current gain of the amplifier                     (2mks)

(iii) Base voltage (VBE)                                 (2mks)

(iv) Current drawn from the supply.               (2mks)

(b)        What type of transistor is used.                      (1 mk)

(c) Show the terminals for:-

(i) Input                                                           (1 mk)

(ii) Output                                                       (1mk)

(iii)  VCC                                                        (1mk)

4.

14.       Study the series- parallel circuit shown below and answer the questions that follow:-

16W  7W  5W  20W  10W  26W   14W   12W  80v

Calculate :-

(a) The total resistance                                                                                                (3mks)

(b) Voltage drop across 10W  resistor.                                                                        (3mks)

(c)  The current flowing through 26W resistor.                                                           (3mks)

(d)  Power consumed by 16W resistor.                                                                       (3mks)

15.       (a)  Draw a neat well labelled diagram of a lighting circuit having three lamps

controlled from four different positions.                                                        (8mks)

(b) State the two protective devices that can be used to protect the circuit above.   (2mks)

(c)  List down the two cable sizes recommended for use in lighting final sub

circuits.                                                                                                            (2mks)

5.

448/1

ELECTRICITY

PAPER I

JULY/ AUGUST 2011

MWALA DISTRICT JOINT EXAMINATION

Kenya Certificate of Secondary Education.

ELECTRICITY

PAPER 1

JULY/ AUGUST 2011

MARKING SCHEME

1.         (a)        (i) Chemical action                              ( ½ mk)

(ii) magnetism                                     ( ½ mk)

(iii) Light                                             ( ½ mk)

(iv) Friction                                         ( ½ mk)

(v) Heat                                               ( ½ mk)

(vi) Pressure                                        ( ½ mk)

(b)        (i) Chemical action                              (1mk)

(ii) Light                                              (1mk)

(iii)  Chemical action                           (1mk)

(iv) Magnetism                                    (1mk)

2.         (a) The electric cooker is not earthed.             (1mk)

*The electric cooker should be earthed          (1mk)

(b) Faulty starter                                             (1mk)

* The starter should be replaced.                    (1mk)

3.         (a)        E = 1.5V

I = 0.5A

E = V + Ir

Ir = E – V = 1.5 – 0.9 = 0.6                1mk

\r =  0.6   =      1.2W                          1mk

0.5

(b)

2mks

This paper consists of 6 printed pages.

Turn over

(c)  Hydrometer                                                                                                          (1mk)

4.         (a) Single pole switch breaks only one conductor; while double pole switch breaks two

conductors.                                                                                                     (2mks)

(b) Double wound transformer has two windings; while Autotransformer has only one winding.                                                                                                                                       (2mks)

(c)  Earthing lead is the conductor between the earth-continuity  conductor  and the earth

electrode; while earth continuity conductor is the earth wire which runs from consumer

outlets e.g socket outlets, to the distribution board.                                       (2mks)

5.         (a) Scriber                                                                    (1mk)

(b)        (i) Used as a straight edge.                            (1mk)

(ii) Used as  measuring tool                            (1mk)

(c)         (i) Soldering                                                   (1mk)

(ii) Welding                                                     (1mk)

(iii)  Riveting                                                   (1mk)

6.         (a) XL = 2pFL = 2 x px50 x 0.08 = 25.12W

Z = Ö R2 + XL2   = Ö 202 + 25.122    = Ö 1031

= 32.1W (1mk)

\I = V   =  240     =   7.47A                                                  (1mk)

Z       32.1

(b) For 12A, Z = 240/12 = 20W

Since R = 20W, then XL = XC

\ XC = 25.12W                                                         (1 mk)

25.12  =       1

2pfL

\C  =                 1                   106

25.12 x 2 x p x 50                mF                    (1mk)

= 126.7 mF                                                      (1mk)

7.         Diameter = 0.006m = 6mm

CSA = pD2  =  p  x  62 = 18.8mm2                                         (1mk)

6          6

\R = mW x M

mm2

2.

= 0.017 x 2000

18.8                 = 1.8W                                     (1mk)

8.         (a) Electronics

That branch of science and technology which is concerned with the study of the phenomena of    conduction of electricity in a vacuum in a gas and in semi-conductors; and with the application            of devices using those phenomena. (2mks)

(b) Armature reaction:

This is the distortion of the main field by the armature field.                                    (2mks)

(c)  Thermal runaway

This is the overheating of a transistor due to increase of colletor current.     (2mks)

(d) Capacitive time constant.

This is the time taken to charge a capacitor to 63.2% of the charging voltage.                                                                                                                                                          (2mks)

9.         (a)        (i) They are more sensitive than fuses.                                                            (1mk)

(ii) Their current rating cannot be altered.                                                      (1mk)

(b)        (i) The maximum current that a fuse can pass without undue deterioration. (2mks)

(ii) The minimum current that would blow a fuse element                             (2mks)

10.  (1mk)  (1mk)  (1mk)

SECTION B, 48 MARKS

11.       (a) Starter is necessary because during starting there is no back emf for controlling the

armature current                                                                                              ( 1 ½ mk)

(b)        (i) Interchanging the connections of the starting winding                              ( 1 ½ mks)

(ii) Interchanging the connections of either field winding or

armature winding.                                                                              ( 1 ½ mks)

(iii)  Interchanging the connections of any two phase lines of the three

incoming phase lines.                                                                         (1 ½ mks)

3.

(c) The centrifugal switch consists of stationary and rotating part. It has normally

closed contacts. As the rotating part accelerates, the contacts open.

The centrifugal switch is used in single phase motor to disconnect the starting winding

after the rotor gains speed.                                                                                                                                                                                                                             ( 1 ½ mk)

(d) The starting torque can be improved                                                                     ( 1 ½ mk)

(e)  No effect.                                                                                                             ( 1 ½ mk)

(f) f = pn \n = f/p, where n = rev/s

\n  =        50Hz

2prs of poles.

= 25 rev/s OR 1500rev/m                                            ( 1 ½ mk)

12.       (a) FSD I =      1         =          0.0001A

10,000

V of the instrument = 0.0001 x 100 = 0.01V

(i) Shunt R =     Shunt V    =     0.01                                      (2mks)

Shunt I            4.999

= 0.002W (2mks)

(ii) Multiplier R = Multiplier V   =    249.99                           (2mks)

Multiplier I            0.0001

= 2,499,900W (2mks)

(b)        (i) Measuring resistance                                              (1mk)

(ii) Checking continuity                                              (1mk)

(c)  To indicate when the bridge is balanced.                         (2mks)

13.       (a) (i) RL =    12 – 8    =      4       =    800W                           (2mks)

0.005       0.005

(ii) Current gain           =        IC      =    0.005

IB          0.00001

= 500                                                   (2mks)

(iii)  VBE = 10mA x 1100KW = 11V

\VBE = 12V – 11V = 1V                                         (2mks)

(vi)  IT = (5 + 0.01) mA

= 5.01mA        (2mks)

4.

(b)        NPN                                                                            (1 mk)

(c)         (i) A and B                                                                 (1mk)

(ii) D and E                                                                 (1mk)

(iii) C and E                                                                (1mk)

14.       (a) 1st step          7  +    (14 + 2)26       =   7 + 13 = 20W

(14 + 2) + 26

2nd step            16 +   5 x 20      = 16 + 4 = 20W

5 + 20

3rd step             20 x 20 = 10W

20 + 20

Last step          10 + 10 = 20W                                                            (3mks)

(b) Voltage drop across 10W resistor

= 10 x IT         IT =   Vs     =  80  =  4A

RT        20

\V = 10 x 4 = 40V                                                    (3mks)

(c)  Current through 26W resistor

= IT ¸ 4

= 4/4 = 1A                                                                    (3mks)

(d) Power consumed by 16W resistor

= I2R

= 22 x 16

= 64Watts                                           (3mks)

15.  (a)

(8 mks)

5.

(b)        (i)  Circuit breaker                               (1mk)

(ii) Fuse                                               (1mk)

(c)        (i) 1.0mm2                                           (1mk)

(ii) 1.5mm2                                          (1mk)

6.

NAME__________________________________    INDEX NUMBER________________________

CANDIDATES SIGNATURE______________

DATE__________________________________

448/2

ELECTRICITY

PAPER 2

PRACTICALS

JULY/ AUGUST 2011

2 ½ HOURS

MWALA DISTRICT JOINT EXAMINATION

Kenya Certificate of Secondary Education.

ELECTRICITY

PAPER 2

PRACTICALS

JULY/ AUGUST 2011

INSTRUCTIONS TO CANDIDATES

1. There are five exercises in this paper

2.  Candidates are allowed 30 minutes for each exercise.

3. Each exercise will be awarded a maximum of 20marks

5.  All dimensions are in millimeters.

6. Attempt all the five exercises.

FOR EXAMINERS USE ONLY

 QUESTION 1 2 3 4 5 TOTAL MARKS

This paper consists of 5 printed pages.

Turn over.

EXERCISE 1

(a) Identify the electrical items marked 1 to 7 and fill the table below.

 ITEM’S NUMBER ITEM’S NAME ANY ONE USE OF THE ITEM 1. 2. 3. 4. 5. 6. 7.

(14 marks)

(b) You are provided with diodes and transistors marked A to F. Using the ohmmeter provided, state      whether the components are good or faulty by filling the table below.

 COMPONENT STATE WHETHER GOOD OR FAULTY DIODE A DIODE B DIODE C DIODE D TRANSISTOR D TRANSISTOR E TRANSISTOR F

(6 marks)

2.

EXERCISE 2

The layout of a surface lighting circuit is shown below. Use tools and materials provided to install the circuit.

X

20 marks

3.

EXERCISE 3

You are provided with the following tools and materials

(1) 4pcs Diodes

(2) 4pcs short cables

(3) 1pc soldering board

(4) 1pc soldering iron

(a)  Solder the diodes and the cables on the soldering board to make a bridge rectifier circuit

and inform the examiner.                                                                                (14 marks)

(b)  Use the masking tape to show the input and output terminals.                           (4 marks)

(c) Where necessary, show the polarity of the terminals.                                           (2 marks)

4.

EXERCISE 4

Use materials tools and equipment to make a key holder as shown below

Diameter 3  R = 12   24  80

20 marks

5.

EXERCISE 5

You are provided with an extension socket outlet

(a) Use the ohmmeter provided and state whether there are any faults___________________________

____________________________________________________________________________

____________________________________________________________________________

____________________________________________________________________________

____________________________________________________________________________

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____________________________________________________________________________

____________________________________________________________________________

____________________________________________________________________________

(b)  Incase of faults:-

(i) List them down

____________________________________________________________________________

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(ii)  What is the effect or dangers of faults mentioned in (b) (i) if any

____________________________________________________________________________

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____________________________________________________________________________

(20 marks)

6.

448/2

ELECTRICITY

PAPER 2

PRACTICALS

JULY/ AUGUST 2011

MWALA DISTRICT JOINT EXAMINATION

Kenya Certificate of Secondary Education.

ELECTRICITY

PAPER 2 PRACTICALS

JULY/ AUGUST 2011

MARKING SCHEME

EXERCISE 1

(A)      CORRECT NAME OF ITEM

= 1 MARK X 7 = 7 MARKS

CORRECT USE OF ITEM

= 1 MARK X 7 = 7 MARKS

(B)       CORRECT CONDITION OF COMPONENT

1MARK X 6 = 6 MARKS

TOTAL MARKS FOR THE EXERCISE

= 7 + 7 + 6 = 20 MARKS

EXERCISE 2

AREA TO BE MARKED                                                               MAXIMUM MARKS

Correct connexion of consumer unit                                                   3

Firmness of accessories                                                                       2

Firmness and quality of terminations                                                  3

Removal of insulation and sheath                                                       2

Neatness of clipping                                                                           2

Quality of cable bend                                                                         2

Correct dimensions                                                                             2

Cable allowance                                                                                  1

Correct working of the circuit                                                                        3

TOTAL MARKS                                                                              20

This paper consists of 2 printed pages.

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EXERCISE 3

AREA TO BE MARKED                                                   MAXIMUM MARKS

Correct diodes polarity                                                           8

Correct input terminals                                                           2

Correct output terminals                                                         2

Correct polarity of polarized terminals                                   2

Quality of soldering work                                                       6

TOTAL MARKS                                                      20

EXERCISE 4

AREA TO BE MARKED                                       MAXIMUM MARKS

Dimensions                                                                 4

Cleaning of the metal                                                  4

Drilling of the hole                                                      4

Neatness of the piece                                                  4

TOTAL MARKS                                                      20

EXERCISE 5

AREA TO BE MARKED                           MAXIMUM MARKS

Identification of fault(s)                                 6

Correct fault(s)                                                8

Correct danger                                                            6

TOTAL MARKS                              20

2.

448/2

ELECTRICITY

PAPER 2

PRACTICALS

JULY/ AUGUST 2011

MWALA DISTRICT JOINT EXAMINATION

Kenya Certificate of Secondary Education.

ELECTRICITY

PAPER 2 PRACTICALS

JULY/ AUGUST 2011

CONFIDENTIAL INSTRUCTIONS TO EXAMINERS

N.B THESE INSTRUCTIONS SHOULD REACH THE EXAMINATION CENTRES AT LEAST 2 DAYS BEFORE THE EXAMINATION DAY

EXERCISE 1

Provide the following electrical components and instruments marked as shown below.

1. Ohmmeter

2. Faulty diode, marked                                  A

3. Good diode, marked                                   B

4. Faulty diode, marked                                  C

5.  Good Transistor, marked                           D

6. Faulty transistor, marked                            E

7.  Good Transistor, marked                           F

8. Allan key, marked                                       1

9. Light Emitting diode, marked                    2

10. Tuning capacitor, marked                         3

11. Light dependant resistor, marked                         4

12. Electromagnetic relay, marked                 5

13. Solder sucker, marked                              6

14. Integrated circuit, marked                        7

This paper consists of 2 printer pages.

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EXERCISE 2

Provide the following electrical tools and materials for the Exercise 2

1. Working board

2. Consumer unit

3. 1.5mm2 Twin sheathed cable

4. Electricians tool kit

5. Joint box

6. One way switch

7. Shallow pattress box

8. Straight lamp holder

9.  1 ½” cable clips

10. Tack nails

EXERCISE 3

1. Ohmmeter

2. 4pcs Diodes

3. 1pc printed working board

4. 1pc soldering iron, 30W

5.  Solder wire

7.  Side cutter

8. 1M, 0.75mm2 Twin flexible cord.

EXERCISE 4

Provide the following materials for this exercise

1. Flat bar of 80 x 25 x 3 (1pc per student)

2. Drilling machine

3. 3mm drill bit (1pc per student)

4.  Fabrication tool kit

5.  Bench vice.

EXERCISE 5

Provide the following materials for this exercise

1. Ohmmeter

2. An extension socket outlet with the following connections:-

* Live of plug connected to earth of socket

*Earth of plug connected to live of socket

* Neutral of plug connected to Neutral of socket

2.

448/1

ELECTRICITY

PAPER I

JULY/AUGUST 2011

ELECTRICAL ITEMS REQUIRED FOR ELECTRICITY PRACTICAL PAPER.

ITEMS ARE FOR 10 STUDENTS AT ONE SITTING

ELECTRICAL ITEM                                                         QUANTITY

1. Extension socket outlet                                                       2pcs

2. 1.5mm2 Twin sheathed cable                                             20m

3. Ohmmeters                                                                          6pcs

4. Diodes                                                                                 20pcs

5. 30w soldering irons                                                             2pcs

6. Solder wire                                                                          1 Roll

7. Solder Sucker                                                                      2pcs

8. BC 108 transistors                                                              6pcs

9. Allan keys                                                                           2pcs

10. Light Emitting diodes                                                       2pcs

11. Light dependant resistors                                                 2pcs

12. Tuning capacitors                                                              2pcs

13. Electromagnetic relay                                                       2pcs

14. Intergrated circuits                                                            2pcs

15. Consumer units                                                                 2pcs

16. Joint boxes                                                                        2pcs

17. One way switches                                                             2pcs

18.  Straight lamp holders                                                       2pcs

19.  1 ½” cable clips                                                                4pkts

20. ¾” Tack nails                                                                    1 pkt

21. Side cutters                                                                       2pcs

22.  Pliers                                                                                2pcs

23. Electrician’s knife                                                             2pcs

24.  Screw drivers                                                                   2pcs

25.  Phase testers                                                                     2pcs

26. Shallow pattress box                                                         2pcs

27.  1” woodscrews                                                                1pkt

28. Printed working boards                                                    2pcs

30. 0.75mm2 twin flex                                                                        2m

31.  25 x 3 Flat bar                                                                  2m

32.  Drilling machine                                                               2pcs

33. 3mm drill bits                                                                    10pcs

34. Fabrication tool kit                                                            2pcs

35. AA Alkaline cells                                                              6pcs

This paper consists of 1 printed page.