Family Name ……………………………….. Given Name ………………………………… Student No. …………………………………. Signature ……………………………………
THE UNIVERSITY OF NEW SOUTH WALES School of Electrical Engineering & Telecommunications
MID-SESSION EXAMINATION S2 2016
ELEC1111
Electrical and Telecommunications Engineering
TIME ALLOWED:
TOTAL MARKS:
TOTAL NUMBER OF QUESTIONS:
50 minutes 40
4
THIS EXAM CONTRIBUTES 20%TO THE TOTAL COURSE ASSESSMENT
This paper contains 3 pages.
Candidates must ATTEMPT ALL questions.
Answer all questions in the answer booklet provided.
Marks for each question are indicated beside the question.
This paper may NOT be retained by the candidate.
Print your name, student ID and question number on the front page of each answer book. Authorised examination materials:
Candidates should use their own UNSW-approved electronic calculators.
This is a closed book examination.
Assumptions made in answering the questions should be stated explicitly.
All answers must be written in ink. Except where they are expressly required, pencils may only be used for drawing, sketching or graphical work.
QUESTION 1 [10 marks]
a) For the resistor network in Figure 1a below
i. [4 marks] Write down an expression for the equivalent resistance Rab as seen
from the terminals a and b in terms of individual resistor R1, R2, R3, R4, R5, R6
and R7.
ii. [2 marks] Calculate Rab when R1= R2= R3= R4=60 and R5=R6= R7=20.
b) A practical battery having voltage Vs and internal resistor Rs is connected to the terminals a-b of the resistor network in part (a-ii) as shown in Figure 1b.
i. ii.
[2 marks] Find the current Is suppling by the battery
[2 marks] Calculate the total power dissipated by the resistor network and Rs and the power supplied by the voltage source Vs. Show that power is conserved in this circuit.
R1 a
R1 a
b
Rs=15
+ Vab
R2
–
R4
Is
R7
R3
Figure 1a
Rab
b
R7
+ –
Vs=9V
R3
R6
R4
R2
R6
R5
R5
Figure 1b
QUESTION 2 [10 marks]
For the circuit below in Figure 2
i. [6 marks] Apply nodal analysis to write down the node voltage equations at
nodes Va and Vb.
ii. [2 marks] Solve the voltage equations in part (i) to find the voltages Va and Vb.
iii. [2 marks] Find the currents i1, i2 and i3 based on your results in part (ii)
i3
4
Figure 2
Va i1Vb
3A 2
+-
2
i2
2
2i2
Page 2
QUESTION 3 [10 marks]
For the circuit below in Figure 3, find the current i in the 4 resistor using: (a) [6 marks] The superposition principle.
(b) [4 marks] The Thevenin equivalent of the circuit to the left of terminal pair a-b. a
·
i
20 V 1
+-
3 2
Figure 3
QUESTION 4 [10 marks]
For the circuit below in Figure 4, the switch has been in the same position for a long time before changing position as shown at time t = 0.
3
+- t=0 2A 4 2F
vc
Figure 4
(a) [3 marks] Find the voltage vc and the current i when t<0
(b) [3 marks] Find vc and i when t>0 and in steady state condition, i.e. vc(+) and i(+) (c) [4 marks] Write down expressions for voltage vc(t) and current i(t) when t>0
END OF PAPER
2 A
4
i
·
b
12 V
Page 3
–
+