THE UNIVERSITY OF TRINIDAD & TOBAGO

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1 THE UNIVERSITY OF TRINIDAD & TOBAGO FINAL EXAMINATIONS SEPTEMBER/ DECEMBER 2014 Course Code and Title: ELCT1001 Electrical and Electronic Fundamentals for ICT I Programme: Diploma in Computer, Network and Telecommunication Engineering Date: Tuesday 9 th December 2014 Time: 9:00 am 12:00pm Duration: 3 hrs PLEASE READ ALL INSTRUCTIONS CAREFULLY BEFORE YOU BEGIN THIS EXAMINATION Read all instructions carefully before you begin this examination. Materials supplied: 1. Question Paper and Answer booklet Instructions to Candidates 1. This question paper has four (4) sections and fourteen (14) pages. 2. You are required to answer ALL questions in Section I, ANY TWO in Section II, ONE in Section III and Section IV is COMPULSARY. 3. Begin each question on a new page. 4. You must return the question paper along with your answer booklet and other writing paper to the Invigilator at the end of the examination. Key Examination Protocol 1. Students please note that academic dishonesty (or cheating) includes but is not limited to plagiarism, collusion, falsification, replication, taking unauthorised notes or devices into an examination, obtaining an unauthorised copy of the examination paper, communicating or trying to communicate with another candidate during the examination, and being a party to impersonation in relation to an examination. 2. The above mentioned and any other actions which compromise the integrity of the academic evaluation process will be fully investigated and addressed in accordance with UTT s academic regulations. 3. Please be reminded that speaking without the Invigilator s permission is NOT allowed. ELCT1001 Page 1 of 14

2 Section II Answer any TWO Questions Question 1 a) What quantity is always the same for resistors in series with each other? Explain. b) What quantity is always the same for resistors in parallel with each other? Explain. c) Are the voltages across resistors in series generally the same? If not, are there special cases where they could be the same? Explain. d) Are the currents through parallel resistors generally the same? If not, are there special cases where they could be the same? Explain. e) Three resistors that are rated at 5, 2 and 1 are connected in series to a battery. Which gets the most power and why? (Draw a circuit diagram) f) Three resistors that are rated at 5, 2 and 1 are connected in parallel to a battery. Which gets the most power and why? (Draw a circuit diagram) g) What is the equivalent resistance for the resistors in this diagram? Figure 1.0 ELCT1001 Page 7 of 14

3 h) Find the current through and the voltage across the 10 resistor in the circuit shown in figure 2.0. Figure 2.0 (6 Marks) Question 2 1. Figure 3.0 below represents an electric circuit consisting of four resistors and a 12-volt battery. Figure 3.0 a) What is the equivalent resistance of the circuit shown? b) What is the current measured by ammeter A shown in the diagram? (4 Marks) ELCT1001 Page 8 of 14

4 2. A 50.-ohm resistor, an unknown resistor R, a 120-volt source, and an ammeter are connected in a complete circuit. The ammeter reads 0.50 ampere. Figure 4.0 a) Calculate the equivalent resistance of the circuit shown in figure 4.0. b) Determine the resistance of resistor R shown in figure 4.0. (4 Marks) 3. A 3.0-ohm resistor, an unknown resistor, R, and two ammeters, A 1 and A 2, are connected as shown below with a 12-volt source. Ammeter A 2 reads a current of 5.0 amperes. Figure 5.0 a) Determine the equivalent resistance of the circuit shown in figure 5.0. b) Calculate the current measured by ammeter A 1 in figure 5.0. c) Calculate the resistance of the unknown resistor, R in figure 5.0. (6 Marks) 4. The load across a 40-V battery consists of a series combination of three resistances R 1, R 2, and R 3. R 1 is 240 and R 3 is 120. The potential difference across R 1 is 24 V. a) Find the current in the circuit. b) Find the equivalent resistance of the circuit. c) Find the resistance of R 2. (6 Marks) ELCT1001 Page 9 of 14

5 Question 3 a) For the circuit of Figure 6.0 shown below, the total resistance is specified. Find the unknown resistances and the current I. Figure 6.0 (4 marks) b) Determine the current I and its direction for circuit in the Figure 7.0 below. Before solving for I, redraw the circuit with a single voltage source. Figure 7.0 (5 marks) ELCT1001 Page 10 of 14

6 c) For the circuit shown in Figure 8.0. Figure 8.0 i. Determine V 2 ii. Calculate V 3 iii. Determine R 3 iv. Calculate the source current I. \ (5 marks) d) For Figure 9.0, Find R 1, R 2 and R 3 such that that I 2 = 4I 1 and I 3 =3I 2 Figure 9.0 (6 marks) ELCT1001 Page 11 of 14

7 Section III Question 1 Answer only ONE QUESTION from this Section. Suppose a DC generator is powering an electric motor, which we model as a 100 Ω resistor: Figure 10.0 Calculate the amount of current this generator will supply to the motor and the voltage measured across the motor's terminals in Figure 10.0, taking into account all the resistances shown (generator internal resistance r gen, wiring resistances R wire, and the motor's equivalent resistance). Now suppose we connect an identical generator in parallel with the first shown in Figure 11.0, using connecting wire so short that we may safely discount its additional resistance: Figure 11.0 Use the Superposition Theorem to re-calculate the motor current and motor terminal voltage, commenting on how these figures compare with the first calculation (using only one generator). (20 marks) ELCT1001 Page 12 of 14

8 Question 2 A windmill-powered generator and a battery work together to supply DC power to a light bulb. Calculate the amount of current through the light bulb, given the values shown in the schematic diagram in Figure Assume internal resistances of the generator and battery to be negligible: I bulb = Figure 12.0 (20 marks) ELCT1001 Page 13 of 14

9 Section IV This Question is COMPULSARY Question 1 Figure 13.0 Answer the following questions based on Figure 13.0 above: In the circuit above, V1=6 volts. R1= 50Ω, R2= 500Ω, R3= 800Ω, R4= 3000Ω 1. If you place a load resistor (RL) of 3048 ohms between A and B, what would be the voltage across AB? What is the current? (8 marks) 2. Find the Thevenin voltage between A and B. (Load removed.) (2 marks) 3. Find the Thevenin resistance. (Load removed) (3 marks) 4. If you place a load resistor (RL) of 3048 ohms between A and B of the Thevenin Equivalent circuit what would be the voltage across AB? What is the current? Comment on your results. (4 marks) 5. State the condition for maximum power transfer and calculate the output power to the load resistor under the stated condition. (3 marks) END OF EXAMINATION ELCT1001 Page 14 of 14