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ECE 20100 –Fall 2014

Exam #2

October 22, 2014

Section (circle below)

Bermel – 002 Peleato-Inarrea – 004 Qi – 005

Gray – 006 Allen – 007 Lin - 010

Name ____________________________ PUID____________

Instructions

1. DO NOT START UNTIL TOLD TO DO SO.

2. Write your name, section, professor, and student ID# on your Scantron sheet. We may check PUIDs.

3. This is a CLOSED BOOKS and CLOSED NOTES exam.

4. Calculators are NOT allowed (and not necessary).

5. If extra paper is needed, use the back of test pages.

6. Cheating will not be tolerated. Cheating in this exam will result in, at the minimum, an F grade for the

course. In particular, continuing to write after the exam time is up is regarded as cheating. 7. If you cannot solve a question, be sure to look at the other ones, and come back to it if time permits.

8. Exam #2 provides evidence for satisfaction of this ECE 20100 Learning Objective:

ii) An ability to analyze 1st order linear circuits with sources and/or passive elements.

The minimum score needed to satisfy this objective will be posted on Blackboard after the exam has

been graded. Remediation options will be posted in Blackboard if you fail to satisfy any of the course

outcomes.

9. The last page of the exam contains potentially useful formulas.

Honor Pledge: I have neither given nor received unauthorized assistance on this exam

Signature:________________________________

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1. For the Thevenin equivalent circuit seen by the load resistor LR , what is the Thevenin

equivalent voltage THV ?

(1) 0 VTHV (2) 3 VTHV (3) 6 VTHV (4) 9 VTHV

(5) 12 VTHV (6) 15 VTHV (7) 18 VTHV (8) VTHV

(9) None of the above

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2. For the Norton equivalent circuit seen by the inductor 1L , what is the Norton equivalent

resistance NR ?

(1) 1 NR (2) 2 NR (3) 3 NR (4) 4 NR

(5) 5 NR (6) 6 NR (7) 7 NR (8) 0 NR

(9) NR (10) None of the above

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3. What value of resistor LR will result in the maximum power being absorbed by LR ?

(1) 1 (2) 2 (3) 3 (4) 4 (5) 8

(6) 12 (7) 16 (8) 0 (9) (10) None of the above

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4. Find the equivalent capacitance seen between nodes A and B, in terms of capacitance C .

(1) 0.5C (2) 2C (3) 2.5C (4) 4C (5) 4.5C

(6) 6C (7) 6.5C (8) 8C (9) 8.5C (10) None of the above

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5. What is the voltage 4 ( )v t across the 4 H inductor?

(1) 2 V (2) 4 V (3) 8 V (4) 16 V (5) 2 Vt

(6) 4 Vt (7) 8 Vt (8) 16 Vt (9) 0 V (10) None of the above

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6. A 2 µF capacitor is charged to a voltage of 20 V in 50 ms. Assuming the energy stored in the

capacitor was initially 0 J, what is the energy stored in the capacitor at time 50 mst ?

(1) 0 JCW (2) 20 mJCW (3) 40 mJCW (4) 80 mJCW

(5) 100 J CW (6) 200 J CW (7) 300 J CW (8) 400 J CW

(9) None of the above

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7. A 9 mF capacitor has an initial voltage of (0) 5 VCv . If the current through the capacitor is

given by 2( ) 9 mA Ci t t for 0 st , what is (3)Cv ?

(1) 1 V (2) 2 V (3) 3 V (4) 4 V (5) 5 V

(6) 6 V (7) 7 V (8) 8 V (9) 9 V (10) None of these

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Use this circuit for questions 8, 9, and 10. The switch is closed for a long time and then

opened at 1 st . At 0t , the inductor current is known to be (0 ) 0 ALi .

8. For 0 ≤ t < 1 s, what is the time constant, τ?

(1) 0 s (2) 1

s4

(3) 1

s2

(4) 1 s (5) 2 s

(6) 4s (7) 8s (8) 16s (9) s (10) None of the above

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Use this circuit for questions 8, 9, and 10. The switch is closed for a long time and then

opened at 1 st . At 0t , the inductor current is known to be (0 ) 0 ALi .

9. For t ≥ 1 s, what is the time constant, τ?

(1) 0 s (2) 1

s4

(3) 1

s2

(4) 1 s (5) 2 s

(6) 4s (7) 8s (8) 16s (9) s (10) None of the above

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Use this circuit for questions 8, 9, and 10. The switch is closed for a long time and then

opened at 1 st . At 0t , the inductor current is known to be (0 ) 0 ALi .

10. As t → ∞, what is ( )Li ?

(1) ( ) 1 ALi (2) ( ) 2 ALi (3) ( ) 3 ALi (4) ( ) 4 ALi

(5) ( ) 5 ALi (6) ( ) 6 ALi (7) ( ) 8 ALi (8) ( ) 12 ALi

(9) ( ) 0 ALi (10) None of the above

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11. The current through an inductor was kept at ( ) 8 ALi t for all time until 0t . The

inductor was then connected to the circuit as shown below at 0t . Find the current through

the inductor, ( )Li t , for 2 st .

(1) 2 2 4( ) 1 8 At t

Li t e e

(2) 2 2 4( ) 1 8 At t

Li t e e

(3) 2 2 4( ) 1 8 At t

Li t e e

(4) 2 2 4( ) 1 8 At t

Li t e e

(5) 2 2 4( ) 4 4 8 At t

Li t e e

(6) 2 2 4( ) 4 4 8 At t

Li t e e

(7) 2 2 4( ) 4 8 4 At t

Li t e e

(8) 2 2 4( ) 4 8 4 At t

Li t e e

(9) None of the above

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12. A capacitor is charged to (0 ) 4 VCv . At 0t , the switch in the circuit below is closed.

At what time 1t in seconds will the capacitor voltage be 1( ) 3 VCv t ?

(1) 1 5

ln2 2

(2) 1

ln102

(3) 5

2ln2

(4) 2ln10

(5) 5 1

ln2 2

(6) 5

ln 22

(7) 1

10ln2

(8) 10ln 2

(9) None of the above

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Potentially Useful Formulas

00

( )/( ) ( ) ( ) ( )

t tx t x x t x e , where THR C or

TH

L

R

00

2 2

0 1 1 0

( )( )

1( ) ( ) ( )

( , ) ( ) ( )2

LL

t

L L Lt

L L L

di tv t L

dt

i t i t v t dtL

LW t t i t i t

00

2 2

0 1 1 0

( )( )

1( ) ( ) ( )

( , ) ( ) ( )2

CC

t

C C Ct

C C C

dv ti t C

dt

v t v t i t dtC

CW t t v t v t

1ln lnx

x