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midterm-sol

# midterm-sol - MAE140 Linear Circuits Fall 09 Midterm...

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MAE140 - Linear Circuits - Fall 09 Midterm, November 10 Instructions (i) This exam is open book. You may use whatever written materials you choose, including your class notes and textbook. You may use a hand calculator with no communication capabilities (ii) You have 70 minutes (iii) Do not forget to write your name , student number , and instructor B + - 10 V 0.5 A 10 40 20 10 A (a) Question 1. A v S i S B D C R 2 R 4 R 3 R 1 i 1 i 2 (b) Question 2. Figure 1: Circuits for questions 1 and 2. 1. Equivalente Circuits Part I [4 points] Use source transformations and association of resistors to find the Thevenin equiva- lent to the circuit in Fig. 1(a) as seen from terminals A and B. Part II [4 points] A classmate was arguing with you that one would “extract” the most power out of this circuit by connecting a very small resistance between terminals A and B. You argued that he/she was wrong and that a 20 resistor would be a better choice. Who is right and why? Hint #1: Don’t complicate: all you need to do is compare the two options! Hint #2: Use the Thevenin equivalent computed in Part I to answer the question. Solution: Part I: A possible series of source transformations is shown here:

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10 V + - + - 10 V 40 20 A B 20 (+ 1 point) 20 V + - 40 40 A B 40 A B 0.5 A 40 (+ 1 point) 0.5 A 20 A B (+ 1 point) + - A B 10 V 20 (+ 1 point) Part II: Let’s investigate the two scenarios. In the case of a small resistance, say R = , using the Thevenin equivalent and voltage division we have that v R = 20 + 10 / 2 V , i R = v R /
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midterm-sol - MAE140 Linear Circuits Fall 09 Midterm...

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