FAKULTEIT INGENIEURSWESE FACULTY OF ENGINEERING. Volpunte: Full marks: Instruksies / Instructions

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1 FAKULTEIT INGENIEURSWESE FACULTY OF ENGINEERING Elektrotegniek 143 Electrotechniques 143 Tydsduur: Duration Eksaminatore: Prof H C Reader Prof J B de Swardt Mnr AD le Roux 1.5 h 1 Beantwoord al die vrae. Answer all the questions. Skryf met n pen. Volpunte: Full marks: Moderator: Dr J P Holtzhausen Instruksies / Instructions Eerste Toets September 007 First Test September 007 Write your answers in pen. 3 Sakrekenaars soos voorgeskryf vir die eerste twee jaar van die BIng mag gebruik word. Calculators prescribed for the first two years of the BEng may be used. 60 Vraag 1 / Question 1 [0 punte] (a) Dui aan watter stroombane in Figuur 1(a) is geldig en watter nie. Moet niks bereken nie sê net of dit geldig is of nie. / Show which of the circuits in Figure 1(a) is valid and which not. Do not calculate anything only state if it is valid or not. (4) Figure 1(a) 5V 10V.. 1A 3A 10 (i) (ii) 10V. 15V. 3A 1A 10 (iii) (iv ) Bladsy 1 van 4 Bladsye Page 1 of 4 Pages

2 (b) Bepaal en teken die Thévenin ekwivalente baan tussen punte a en b vir die stroombaan in Figuur 1(b) / Determine and draw the Thévenin equivalent circuit between points a and b for the circuit in Figure 1(b). (10) Figure 1(b) A 0V. a (c) Gebruik brontransformasies om die spanning v ab in Figuur 1(c) te bepaal. / Use source transformations to determine the voltage v ab in Figure 1(c): (6) b Figure 1(c) 40V A a v ab b Vraag / Question [0 punte] (a) Gebruik die knooppuntspanning metode om die spannings v1 en v te bereken / Use the nodevoltage method to calculate the voltages v1 and v. (8) (b) Gebruik die maasstroom metode om strome i1, i en i3 te bereken / Use the meshcurrent method to calculate currents i1, i and i3. (8) (c) Bereken v vanuit die maasstrome. Lewer kommentaar op die antwoord. / Calculate v from the mesh currents. Comment on the answer. () (d) Hoeveel drywing word met die 7 V bron geassosieer? Word hierdie drywing gelewer of geabsorbeer? / How much power is associated with the 7 V source? Is this power delivered or absorbed? () 4 v1 1 i v A i1 3 i3 7V Bladsy van 4 Bladsye Page of 4 Pages

3 Vraag 3 / Question 3 [0 punte] (a) Vereenvoudig die stroombaan tot n enkele ekwivalente induktor tussen terminale a en b. / Simplify the circuit into a single equivalent inductor between terminals a and b. (3) 1H H a 1H 1H b 50mH (b) Vereenvoudig die stroombaan tot n enkele ekwivalente kapasitor tussen terminale a en b. / Simplify the circuit to a single equivalent capacitor between terminals a and b. (5) a F F 1, 5 F 1, 5 F 6F b 3F (c) Bepaal en skets v(t) in die volgende stroombaan vir i(t) soos aangetoon. / Determine and plot v(t) in the following circuit for i(t) as shown. (6) i( t) [ A] i() t H vt () 4 0 t t t 6t t [ s] (d) Vir die stroombaan in (c), bepaal die energie gestoor in die induktor by t=0s en t=4s. / For the circuit in (c), determine the energy stored in the inductor at t=0s and t=4s. (3) (e) Vir die stroombaan in (c), bepaal die drywing wat die induktor absorbeer by t=4s. / For the circuit in (c), determine the power absorbed by the inductor at t=4s. (3) Bladsy 3 van 4 Bladsye Page 3 of 4 Pages

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5 Department of Electrical and Electronic Engineering Electrotechniques 143 Test 1: Comments & Memo Vraag 1 Ek het die gevoel gekry dat mense of presies weet wat om te doen of glad nie 'n idee gehad het nie. In 1(b) kan daar 'n redelike standaard proses gebruik word om die antwoord te kry, maar heelwat mense het die probleem lukraak aangepak. By vraag 1(c) het baie mense die 16A bron verkeerdelik omgeskakel na 'n Thevenin ekwivalente baan en nie besef presies hoe dit dan in die stroombaan moet lyk nie. Question The most common error was to do with signs in node voltage, mesh current and power absorbed/delivered. See memo. Generally sign errors in a and b also had direction misunderstandings and this led to a loss of 3 or 4 marks. There was no need in mesh current to compute i 1 as it must be the same as the current source. Question 3 (a)&(b) Most of you cashed in on (a) and (b), however some where confused regarding capacitors/inductors capacitors in parallel add and inductors in series add. A few didn t see that the 3,5 F and the 3 F capacitors are not in series because the b connection is between them. (c) This went good in general, most of you new that v = Ldt dt and that the voltage waveform can end up being discontinuous however some ended up integrating the current waveform. Others got the correct equations, but didn t know how to draw straight lines or didn t realize that the answer is an equation not a constant value of voltage. (d) Most of you used w 1 = Li giving 3 easy marks others went for w pdt = which is not so ease to get right because of the integration and the fact that the inductor has stored energy at 0 t = 0, we don t know how p() t looks for t < 0 only that pdt = 4. Some had no idea that there is a difference between energy ( w ) and power ( p ). (e) p( 4) v( 4) i( 4) = easy marks, but some of you didn t realize that the passive sign convention gives power absorbed.

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