**UNIVERSITY EXAMINATIONS: 2010/2011 **

**FIRST YEAR STAGE EXAMINATION FOR THE DEGREE OF BACHELOR **

**OF SCIENCE IN INFORMATION TECHNOLOGY **

**BIT 1103: BASIC ELECTRICITY **

**DATE: AUGUST 2011 TIME: 2 HOURS **

**INSTRUCTIONS: Answer question ONE and any other TWO questions**

**Question One**

a) Define Kirchhoff`s current law and Kirchhoff`s voltage law. (4 Marks )

b) By drawing the two circuits which depict Kirchhoff`s laws, show how effective resistance of each

circuit may be derived. (6 Marks )

c) Determine the standard values of the following resistors; giving minimum and maximum values of

each resistor. (4 Marks )

(i) Brown-Green -Brown-Silver

(ii) Orange-Blue-Silver-Gold

d) Give the correct band-colours of the following standard resistors. (4 Marks )

(i) 3.6K ± 5%

(ii) 180R ± 10%

e) (i) Name the factors which affect the resistance of a conductor.

(iii)Show clearly the relationship between the resistance and each of those factors stated in part (i)

(6 Marks)

f) Two resistors R1, R2 are connected in parallel across the dc source of voltage, E. If an ammeter A is

connected to measure the total current and voltmeter V is connected to measure the p.d. across the

resistor R2, draw a neat labeled circuit diagram of the circuit. (6 Marks )

**Question Two**

a)Two resistors R1 = 560 R ± 10% and R2 = 430 R ± 5% are connected in a circuit to a 12V-dc

source. Determine the minimum and maximum values of currents flowing from the 12V-dc

source if; ( answers expressed in mA corrected to 2 dec. places ) (10 Marks)

(i) R1 and R2 are in series.

(ii) R1 and R2 are in parallel

b)For the circuit shown in fig.Q2, determine an expression for the voltage V3 across R3, in terms of

Fig. Q2

c)If in the circuit of fig. Q2, R1 = 15 Ω, R2 = 30 Ω, R3 = 45 Ω and E = 33 V, calculate;

(i) the value of V3,

(ii) the values of currents I2, I3 through R2, R3 respectively.

(6 Marks)

**Question Three**

a)Define the term resistivity of a resistor and show its unit of measurement. (3 Marks)

b)Three resistors (R1, R2, and R3) made from the same materials, have the dimension as shown in

Table Q3

The current, I1 through resistor R1 is 2.0 A for a given voltage V. For the same voltage, applied

separately across R2 and R3, determine the values of currents I2 and I3. (7 Marks)

c) Fig. Q3 (c) shows a simple electrical circuit.

Fig. Q3 (c)

By applying the Kirchhoff`s laws, determine the values all the currents I1, I2 and I3, flowing

through R1, R2 and R3. (10 Marks)

**Question Four**

a)Sketch neatly the variation of sinusoidal voltage for an ac power supply. (2 Marks)

b)Mark and label the following parameters/quantities on the waveform of part (a) (3 Marks)

(i) – Peak-to-peak value

(ii) – r.m.s. value

(iii) – total period

c)An ac power supply of 220V is connected in series with a load resistance of 100 ohms.

Determine the values of the following quantities; (12 Marks)

(i) maximum current, minimum current, average current, and root mean square (r.m.s.) current.

(ii) the power consumed by the load resistance.

d)Give three advantages of ac power supply system over dc power system. (3 Marks)

**Question Five**

a)Explain why it’s advisable (recommended) to switch off supply to computer systems right from

the supply socket when those systems are off and are not being used. (4 Marks)

b)A capacitor C and a resistor R are connected in series as shown in fig. Q 5 (6 Marks)

(i) When the supply V is switched on, sketch the variation of voltage VC across the capacitor

with time.

(ii) State the condition under which the circuit current will become zero.

(iii) Give an expression for the time-constant of the circuit.

c)If in fig.Q5, C = 100 uF, R = 1k Ω and V = 120V dc, determine; (8 Marks)

(i) the time-constant of the circuit,

(ii) the time at which VC = 75 V

d)Explain why an electrolytic capacitor should never be charged by connecting it directly across

the dc supply without a resistor in series with it. (2 Marks)