Download Electrical Engineering Exam - Cork Institute of Technology, Stage 1 and more Exams Electrical Engineering in PDF only on Docsity!
Cork Institute of Technology
Bachelor of Engineering in Electrical Engineering – Stage 1
(NFQ Level 7)
Autumn 2007
Electrical Engineering
(Time: 3 Hours)
Attempt Five Questions. Examiners: Mr. John Hurley Mr. M. Hennessy Prof. E. McQuade
1 (a) A capacitor C is charged to a voltage of V volts. Give expressions for the charge stored in the capacitor and the energy stored in the capacitor. (6 marks) (b) With reference to the circuit shown, calculate the charge and the energy stored in each capacitor. (14 marks) 8 μF
14 μF
5μF
24 μF
90 V
2 (a) State Kirchhoff's Laws and apply them to the solution of the following problem. (5 marks)
(b) Two batteries, A and B, are connected in parallel, and a 10 Ω resistor is connected across the battery terminals. The e.m.f. and the internal resistance of battery A are 40 V and 5 Ω respectively, and the corresponding values for battery B are 36 V and 6 Ω respectively. Find (a) the value and direction of the current in each battery and (b) the terminal voltage.
40 V 36 V
(15 marks)
- (a) Define the term Temperature Coefficient of Resistance. What are the units in which it is measured? (4 marks) (b) A coil of wire is found to have a resistance of 20 Ω at 15 oC When the temperature of the coil is increased to 50 o^ C it is found that the resistance of the coil is 21.2 Ω. Calculate the temperature coefficient of resistance of the coil material. (12 marks) (c) Some materials exhibit a negligible temperature coefficient of resistance. Give two areas of application of such materials. (4 marks)
- (a) Define the term Flux Density as applied to a magnetic circuit and state the units in which flux density is measured. (4 marks) (b) (b)An iron ring, having a mean circumference of 800 mm and a cross-sectional area of 500 mm^2 , is wound with a magnetizing coil of 120 turns. Using the following data, calculate the current required to set up a magnetic flux of 630 μWb in the ring. Flux density B (T) 0.9 1.1 1.2 1. Magnetising Force H (Am-1^ ) 260 450 600 820 (8 marks) (c) The air gap in a magnetic circuit is 1.1 mm long and 2000 mm^2 in cross-section. Calculate: (i) the reluctance of the air gap, and (ii) the m.m.f. required to send a flux 700 μWb across the air gap. (8 marks)
Useful Formulae D.C. Formulae
Resistance and Temperature
R 1 = R 0 ( 1 + α t 1 ) (Ω)
α = Temperature Coefficient of resistance
1
2 1
2 1
t
t R
R
Electrostatics
Capacitors in Series C C C C C N
1 2 3
Capacitors in Parallel C = C 1 + C 2 + C 3 +−− CN (F)
Charge Q = CV (C)
Energy stored 2 2
1 CV (J)
Electromagnetism
Magneto Motive Force (^) NI (A)
Magnetizing Force l
H =^ NI (A/m)
Flux Density A
B = φ (Wb)
Absolute Permeability H
μ = B μo = permeability of free space = 4πx10-7^ H/m μr = relative permeability of magnetic material
A.C. Formulae
Inductive Reactance X L = ω L = 2 π fL = j ω L (Ω)
Series Circuit Impedance (^) Z = R^2 +( XL − XC )^2 (Ω)
Z
Cos θ= R
Power VICos θ (W)
I^2 R (W)
Series Resonance
X L = X C (Ω)
fC
fL
Z = R (Ω)
LC
f (^) o
= 1 (Hz)
Voltage Magnification, Q-Factor R
fL C
L
Q R
=^1 =^2 π
