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Transcript
CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Autumn Examinations 2009
Module Title: Electrical Science 2
Module Code:
ELEC6019
School:
Electrical and Electronic Engineering
Programme Title:
Bachelor of Engineering in Electrical Engineering – Stage 1
Programme Code:
EELEC_7_Y1
External Examiner(s):
Internal Examiner(s):
Mr. Gerard Beecher and Dr. Maeve Duffy
Mr. Noel Mulcahy
Instructions:
Answer All Questions
Duration:
2 Hours
Sitting:
Autumn 2009
Requirements for this examination: Use of a Scientifc 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 3
Q1.
a. A circuit, having a resistance of 4Ω, an inductance of 0.5H and a variable capacitance in
series, is connected across a 100V, 50Hz supply and a current is drawn. Calculate:
i)
the value of capacitance to give resonance;
(8 marks)
ii) the voltages across the inductance and the capacitance;
(6 marks)
iii) the Q factor of the circuit.
(2 marks)
b. A similar circuit consists of a 115Ω resistor, a 422mH inductor and a 24µF capacitor
connected in series across a 230 V, supply. Determine the resonant frequency of this
circuit.
(4 marks)
Q2.
a. A circuit contains three branches, a resistance of 23Ω, an inductor of 0.08H and a 36µF
capacitor across a 230 V, 50Hz supply. Determine
i)
the current taken by the circuit;
(6 marks)
ii) the phase angle of the circuit;
(3 marks)
iii) the power factor of the circuit;
(2 marks)
iv) the power dissipated in the resistor;
(4 marks)
b. Draw the phasor diagram to represent the current.
(5 marks)
Q3.
A milli-ammeter requires 100mV across its terminals to give full-scale deflection with a current
of 10mA flowing through the coil.
a.
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.
b.
c.
(8 marks)
How could the milli-ammeter be converted to an ammeter with a full-scale deflection
5.0A and what value of resistance would be required?
(8 marks)
Sketch the circuit in each case above.
(4 marks)
Q4.
a. List the relative advantages and disadvantages of lead-acid and alkaline cells.
(6 marks)
b. A battery of 50 cells connected in series is charged through a fixed resistor from a
constant 120 V supply. At the beginning of the charge, the e.m.f per cell is 1.9 V and the
charging current is 4 A. When charging is almost complete, the e.m.f. per cell has risen to
2.2 V. Each cell has an internal resistance of 0.02 Ω. Calculate the value of the external
resistor and the current at the end of the charge.
(14 marks)
Page 2 of 3
Some Useful Formula
Q=CxV
F=BxIxL
E=BxLxv
Specific Heat Capacity of Water = 4.184 J/g/°C
∆Q = m x c x ∆T
P = I x V, P = I2 x R, P = V2 / R
W = ½ x C x V2
XL = 2 x π x f x L
XC =
1
2xπ x f xC
Z = R 2 + ( X L − X C )2
Series circuits p.f. =
R
Z
Parallel circuits p.f. =
IR
IT
Page 3 of 3