CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 2 Examinations 2010/11
Module Title:
Electrical Science S2
Module Code:
ELEC6019
School:
Electrical and Electronic Engineering
Programme Title:
Bachelor of Engineering in Electrical Engineering – Year 1
Programme Code:
EELEC_7_Y1
External Examiner(s):
Internal Examiner(s):
Dr Maeve Duffy
Mr Gerard Beecher
Mr Noel Mulcahy
Instructions: Answer All Questions
Duration:
2 Hours
Sitting:
Summer 2011
Requirements for this examination:
Use of a Scientific Calculator is permitted
Note to Candidates: Please check the Programme Title and the Module Title to ensure that you have received the
correct examination paper.
If in doubt please contact an Invigilator.
Page 1 of 4
Q1.An alternating waveform is given by v =125 sin (120 π t - 0.35) volts Find:
a. The amplitude
(2 marks)
b. The peak-to-peak value
(2 marks)
c. The average value of one complete waveform
(2 marks)
d. The rms value of one complete waveform
(3 marks)
e. The periodic time
(4 marks)
f. The frequency
(2 marks)
g. The phase angle relative to v = 125 sin 120 π t
(5 marks)
Q2. A circuit as detailed in the diagram below consists of a resistance of 20Ω, a variable
inductance and a capacitance of 20.26μF in series, is connected across a 140V, 50Hz supply.
Calculate:
(a)
the inductance to give resonance;
(8 marks)
(b)
the current flowing at resonance;
(3 marks)
(c)
the voltages across the inductance and the capacitance at resonance;
(3 marks)
(d)
the phase angle of current at resonance;
(2 marks)
(e)
the Q factor of the circuit at resonance .
(4 marks)
20.26F
20
VR
VL
VC
140V, 50 Hz
Q3. A coil having resistance 38 Ω, and an inductance of 0.04 H is connected in parallel with a
75μF capacitor across a 230 V, 50Hz supply. Determine
(a)
the current in each branch of the circuit (I1 & I2);
(8 marks)
(b)
the total current taken by the circuit; (IT)
(6 marks)
(c)
the phase angle between the supply current and voltage;
(2 marks)
(d)
the power factor of the circuit;
(2 marks)
(e)
the power consumed by the circuit;
(2 marks)
Page 2 of 4
I1
R = 38
L = 0.04H
I2
C = 75F
IT
230 V, 50 Hz
Q4.
a. A battery of 10 cells is to be charged at a constant current of 8A from a generator of
e.m.f. 35 V and internal resistance 0.2Ω. The internal resistance of each cell is 0.08Ω
and its e.m.f. ranges from 1.85V discharged to 2.15V charged. Determine the
maximum and minimum values of series resistor with must be included in the circuit.
(14 marks)
b. A battery when fully charged can provide an output of 22Amps at 12Volts for 1 hour.
For a particular application an output of 60Amps at 48V is required for 1 hour.
Determine the number and configuration of batteries required to provide this output
and then sketch your configuration.
(6 marks)
Q5.
a. A milli-ammeter requires 100mV across its terminals to give full-scale deflection with
a current of 10mA flowing through the coil. Calculate the resistance required in series
with the meter so that the combination of meter plus resistor would give a full-scale
deflection for 50 V.
(6 marks)
b. How could the milli-ammeter above be converted to an ammeter with a full-scale
deflection 5.0A and what value of resistance would be required?
(8 marks)
c. Sketch the circuit in each case above.
(6 marks)
Page 3 of 4
Some Useful Formula
Capacitive Reactance
XC 
1
2. . f .C
[]
Inductive Reactance
X L  2. . f .L
[]
Impedance
Z  R 2   X L  X C 2
[]
Series Circuit:
(a)
VT  VR2  (VL VC )2
(b)
p. f .  Cos 
(a)
IT  I R2  (I L  IC )2
(b)
IT 
(c)
p. f . 
Parallel Circuit:
R
Z
VT
Z
Power
P  V .I .Cos
Series Resonant Frequency
fr 
[V]
(power factor)
[A]
[A]
IR
IT
(power factor)
[W]
1
2. . LC
[Hz]
Page 4 of 4