# CS计算机代考程序代写 Family Name ……………………………….. Given Name ………………………………… Student No. …………………………………. Signature ……………………………………..

Family Name ……………………………….. Given Name ………………………………… Student No. …………………………………. Signature ……………………………………..

THE UNIVERSITY OF NEW SOUTH WALES

School of Electrical Engineering & Telecommunications

MID-TERM EXAMINATION Term 1, 2019

ELEC1111

Electrical and Telecommunications Engineering

TIME ALLOWED:

TOTAL MARKS:

TOTAL NUMBER OF QUESTIONS:

75 min 100

5

THIS EXAM CONTRIBUTES 25% TO THE TOTAL COURSE ASSESSMENT

Reading Time: 5 minutes.

This paper contains 5 pages.

Candidates must ATTEMPT ALL questions.

Answer each question in a separate answer booklet.

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.

Page 1 of 5

QUESTION 1 [20 marks]

a. (10 marks) For the circuit shown in Figure 1,

i. (6 marks) Apply mesh analysis and write down the mesh equations using the labels provided. Note: Simplify the equations, but DO NOT solve them.

ii. (4 marks) Given the values of mesh currents as 𝑖1 = 5.95 𝐴, 𝑖2 = 4.65 𝐴 and 𝑖3 = 0.95 𝐴, find the power in the 5 A current source.

Figure 1

b. (10 marks) Bipolar transistors can serve as amplifiers, producing both current gain and voltage gain. Such amplifiers can be used to furnish a considerable amount of power to devices that convert energy from one form to another, such as loudspeakers or control motors. For the simplified transistor circuit shown in Figure 2,

i. (6 marks) Use nodal analysis to find the nodal voltage 𝑣1.

ii. (4 marks) Use the result of part (i) to calculate the output current 𝑖0 and output voltage 𝑣0.

Figure 2

Page 2 of 5

QUESTION 2 [15 marks]

For the circuit shown in Figure 3,

a. (6 marks) Calculate the equivalent resistance 𝑅eq as seen from terminals a-b.

b. (3 marks) Find current 𝑖𝑔 using the result of part (a).

c. (6 marks) Find the power dissipated in the 8 Ω resistor.

Figure 3

QUESTION 3 [15 marks]

For the circuit shown in Figure 4, use a succession of source transformations (only source transformations) to find an equivalent circuit consisting of a single voltage source and a single resistor.

Figure 4

Page 3 of 5

QUESTION 4 [20 marks]

The circuit shown in Figure 5 is being used to illuminate a mine. Generator 1 is at the entrance of the mine, with 50 metres of cabling connecting the generator to the lighting system. Generator 2 is at a campsite 75 metres away and is connected to Generator 1 to boost the power that can be supplied to the lighting system. The cables have a resistance of 0.01Ω per metre. The lighting system is made of light bulbs in parallel and can be modelled as a single resistor.

a. (10 marks) Use the superposition principle to find the Thevenin voltage at the lighting system terminals (terminals a-b).

b. (5 marks) Calculate the lighting system resistance that will ensure the maximum transfer of power from the circuit to the lighting system.

c. (5 marks) Find the maximum power that can be delivered to the lighting system.

Figure 5

Page 4 of 5

QUESTION 5 [30 marks]

a. (15 marks) Find the voltage across the capacitors in the circuit of Figure 6 under DC

steady state conditions.

Figure 6

b. (15 marks) The circuit shown in Figure 7 is used to estimate the speed of a horse running a 4 km racetrack. The switch closes when the horse begins and opens when the horse crosses the finish line at 𝑡 = 𝑡1 s.

i. (8 marks) Calculate the voltage 𝑣𝑐(𝑡) in the capacitor for 𝑡 < 𝑡1 s. Assume that the capacitor was initially discharged.
ii. (5 marks) If the capacitor is charged to 85.6 V when the horse crosses the finish line, calculate the time instant t1.
iii. (2 marks) Calculate the speed of the horse in m/s.
Figure 7
END OF PAPER
Page 5 of 5