Electricity Paper 1 Questions and Answers - KCSE 2021 Past Papers

Share via Whatsapp

« Previous Topic
Drawing and Design Paper 1 Questions and Answers - KCSE 2021 Past Papers

Next Topic »
Electricity Paper 2 Questions and Answers - KCSE 2021 Past Papers

Download PDF for future reference Get on Whatsapp for 50/-

SECTION A (48 marks)

Answer all questions in this section in the spaces provided.

1. List two power authorities that are involved in electrical energy in Kenya. (2 marks)
2. State two IEEE regulations regarding ring final circuit. (2 marks)
1. State Ohm's law. (1 mark)
2. Figure 1 shows three lamps A, B and C having resistances of 14002, 9602 and 570 respectively connected to a 240 V supply by a cable of resistance 2Ω.
  
  Determine the total:
  1. Circuit resistance (3 marks)
  2. Circuit current (2 marks)
1. State two ways of magnetising a ferromagnetic material. (2 marks)
2. List two applications of permanent magnets. (1 mark)
1. In a domestic house, the lights in the bedroom go off while there is still power in the main switch. Outline two possible causes of this anomaly. (2 marks)
2. Digital meters are used in electrical installations. List two advantages and one disadvantage of these meters over analogue meters.
  1. Advantages (2 marks)
  2. Disadvantage (1 mark)
1. Explain the meaning of the term first aid. (2 marks)
2. A businessman sets up an electrical enterprise which performs very well. Outline three qualities that the businessman may have as an entrepreneur. (3 marks)
1. Figure 2 shows a toroid-type transformer.
  
  Name the parts labelled A, B and C. (3 marks)
2. A 500 V d.c. motor takes an armature current of 50A. If the armature resistance is 0.50 determine the back emf. (3 marks)
Sketch the correct electrical installation symbols for each of the following:
1. Intermediate switch (1 mark)
2. Switched socket outlet (1 mark)
3. Twin fluorescent lamp (1 mark)
Figure 3 shows a capacitive circuit.

Determine the total:
1. Capacitance (3 marks)
2. Charge (2 marks)
Figure 4 shows a transistor in a common-base configuration. The voltage drop across the 2KQ resistor is 3V and a = 0.95

Find the:
1. emitter current (2 marks)
2. base current (2 marks)
1. Outline two important items contained in a new installation certificate before Kenya Power and Lighting Company supplies power. (2 marks)
2. Sketch schematic symbols for each of the following: (4 marks)
  1. Zener diode
  2. Variable inductor
  3. Electrolytic capacitor
  4. Variable resistor

SECTION B (52 marks)

Answer any four questions from this section in the spaces provided.

1. Convert:
  1. 8410 to binary
  2. 1111011₂ decimal
2. Outline two advantages and two disadvantages of a cartridge fuse.
  1. Advantages
  2. Disadvantages
3. Figure 5 shows a transistor circuit.
  1. Determine the value of I_c at saturation. (2 marks)
  2. Sketch the d.c. load line on the axes below. (3 marks)
1. State four factors to consider when choosing a wiring system. (2 marks)
2. Figure 6 shows an R-L circuit.
  
  Determine the:
  1. Reactance (2 marks)
  2. Impedance (2 marks)
  3. Current (2 marks)
3. Draw and label the construction of a moving coil instrument. (5 marks)
1. Outline two applications of an auto transformer. (2 marks)
2. An ideal single phase auto transformer has voltage ratio 320 V: 250V and supplies a load of 30 KVA at 250 V.
  Determine the current in the:
  1. Primary coil (2 marks)
  2. Secondary coil (2 marks)
  3. Common part of the winding (1 mark)
3. Figure 7 shows a diagram of a d.c. machine.
  1. Name the parts labelled A to D. (4 marks)
  2. Explain the function of part C. (2 marks)
1. Define the following terms in relation to a.c. resistive circuits:
  1. Instantaneous power (2 marks)
  2. Average power (1 mark)
2. An a.c. source is supplying power to a pure resistive load of 82. The peak value of current is 3A and that of voltage is 24 V.
  Calculate the:
  1. rms values of voltage and current (2 marks)
  2. power dissipated in the resistor (3 marks)
3. Figure 8 shows a call circuit.
  1. Name the parts labelled A to D (3 marks)
  2. Explain the operation of the circuit. (2 marks)
Figure 9 shows three view of an object drawn in first angle projection

Draw an isometric view of the object with point X the lowest (13 marks)

MARKING SCHEME

1(a)	KENGEN KENTRACO REA K.P.L.C Any 2 x 1=2	(2marks)
(b)	Every twin socket outlet counts as single socket outlet. The number of non-fused spurs must not exceed the total number of points on the ring. The face rating of fused spur box must not exceed 13 A and the current rating of all points supplied by the fused spur must not exceed 13 A. Non-fused spurs must supply not more than one single or one double socket outlet or one stationary appliance. For domestic premises of area less than 100m2 a ring circuit may serve as an unlimited number of points. Any 2 x 1 = 2	(2marks)
2(a)	Current flowing through a conductor is directly proportional to voltage across it provided temperature and other physical conditions are kept in constant. 1x 1 = 1	1
(b)	1 = 1 + 1 + 1 RT R1 R2 R3 1 = 1 + 1 + 1 (1) RT 400 960 570 RT = 570 Ω (1) Effective resistance = 570 + 2 = 572 Ω (1) I = V RT = 240 (1) 572 = 0.4 A (1)	3 2
3(a)	Stroking Induction Use of direct current Any 2 x 1=2	2
(b)	In magnetic compass For data storage (magnetic tapes) To hold objects Provide magnetic field for small motors, dynamo, measuring instrument Any 2 x 1	2
4(a)	Blown fuse Short circuit Open circuit Overload Any 2 x 1	2
(b)	Advantages Robust Easy to read No error of parallax Any 2x1 Disadvantages Expensive Requires regular battery charging Any 1x1	2 1
5(a)	Application of skilled principles in the event of a sudden illness or accident by use of readily available facilities until the casualty is taken to the hospital. Any 2 x 1	2
(b)	Hard working Innovative Good interpersonal skills Focused Any 3 x 1	3
6(a)	laminate core primary winding secondary winding (3 x 1)	3
(b)	Eb = Es - Ia Ra (1) Eb = 500-(50x0.5) (1) Eb = 500 − 25 = 475 V (1)	3
7(a)	Intermediate switch	1
(b)	Switched socket outlet	1
(c)	Twin flourescent lamp	1
8 (a)	CT = C1 + C2 + C3 1 = 70+ 50+34 1 = 154 μF 1	3
(b)	Q = CV = 154 x 10⁻⁶ F x 500V 1 = 7.7 x 10⁻²C ½ = 77mc ½	1 1
9(a)	Ic = 3v = 1.5mA 2kΩ α = IC IE = IC = 1.579 mA IE α	1 1
(b)	1_B = 1_E − 1_C= (1.579 − 1.5) mA = 79 μA	1 1
10(a)	Physical inspection results (1) Installation tests results (1) 2 x 1	2
(b)		1 1 1 1
11(a)	84÷2 = 42 rem 0 42+2 = 21 rem 0 21+2 = 10 rem 1 10:2 = 5 rem 1 (1) 5+2 = 2 rem 1 2÷2 = 1 rem 0 ∴ 84₁₀ = 1010100 (1) 1111011₂ = 26 + 25 + 24 + 23 + 02 + 21 + 20 (1) = 64 + 32 + 16 + 8 + 2 + 1 (1) = 123	2 2
(b)	Cartridge fuse Advantages rating is accurately known fuse element is less prone to deteriorate Disadvantages the element cannot be replaced its unsuitable for use where extremely high values of fault current may occur	2 2
(c)	When V_CE = 0 V_CC = V_CE + I_CR_C ⇒ Vcc = IcRc ⇒ Vcc = Ic Rc = 12 = Ic 12 1.0 μA = Ic
12(a)	cost durability type of fapply type of binding (permanent/temporary) (Any 4 x 1⁄2)	2
(b)	XL= 2 πfL ½ = 2π (50 Hz) (10 mH) ½ = 2π x 50 x 10 x 10^-3 ½ = 3.14 Ω ½ Z = √R² + XL² ½ = √42 +3.142 ½ = √16 +9.85 ½ = 5.086 Ω ½ Current, I = V 1 Z = 240 5.086 = 47.18A 1	2 2 2
(c)	Moving coil instrument	5
13(a)	In three phase transmission for stepping-up and stepping-down voltages Interconnect systems that are operating at almost constant voltage To boost voltages on long single and three-phase distribution lines To provide multi-voltage source (Any 2 x 1)	2
(b)	Primary current: Ip = KVA ½ VP Ip = 30 X 1000 ½ 320 = 93.75A (1) Secondary current Is = KVA ½ Vs = 30 x 1000 ½ 250 = 120 A (1) Current in common part of the winding = (120 − 93.75) ½ = 26.25A ½	2 2 1
(c)	- Shaft - Armature - Commutator - Brush (Any 4 x 1) Commutator To produce direct current from an a.c. generator, a commutator is used. It converts generated alternating current into direct current for external use.	4 2
14(a)	Instantaneous power Is the rate of energy conversion at a given instant of the sine wave or cycle. (2 x 1) Average power Is the rate of energy conversion over a duration of time (1 x 1)	2 1
(b)	Irms = 0.707 x 3 A = 2.121 A (1) Vrms 0.707 x 24 V = 16.97 V (1) Power dissipated in the resistor P = VI (1) = 16.97 X 2.121 (1) = 35.98W (1)	2 3
(c)	- Bell - Push button - Source of supply - Relay (Any 3 x 1) Operation When the push button is pressed, the coil is energized and the armature carrying the switch is attracted (1) and closes the circuit, hence the bell rings. (1)	3 2
15	Faces - 9 = (10 x 1) = 10 Line work = 1 Point x directly palced = 1 Neatness = 1 13 marks