Explanation Given data: Refer to Figure 16.75 in the textbook. Formula used: Write an expression to calculate the value of step input. u ( t ) = { 0 t < 0 1 t > 0 Write a general expression to calculate the impedance of a resistor in s -domain. Z R = R (1) Here, R is the value of resistance. Write a general expression to calculate the impedance of an inductor in s -domain. Z L = s L (2) Here, L is the value of inductance. Write a general expression to calculate the impedance of a capacitor in s -domain. Z C = 1 s C (3) Here, C is the value of capacitance. Calculation: The given circuit is drawn as shown in Figure 1. For a DC circuit, at steady state condition when time t = 0 − the capacitor acts like open circuit and the inductor acts like short circuit. The value of current source i s ( t ) calculated as follows: Substitute − 1 for t in i s ( t ) . i s ( t ) = 2 [ 1 − u ( ( − 1 ) ) ] A = 2 [ 1 + ( 0 ) ] A { ∵ u ( t ) = 0 for t < 0 } = 2 A Now, the Figure 1 is reduced as shown in Figure 2. Refer to Figure 2, the current through inductor is same as the value of current source. i L ( 0 − ) = i s ( t ) = 2 A The resistor ( R 2 ) and the capacitor are connected in parallel. For the parallel connection, the voltage is same. Therefore, the voltage across the capacitor is calculated as follows: v C ( 0 − ) = ( 12 Ω ) ( 2 A ) = 24 V The current through inductor and voltage across capacitor is always continuous so that, i ( 0 ) = i L ( 0 − ) = i L ( 0 + ) = 2 A v ( 0 ) = v C ( 0 − ) = v C ( 0 + ) = 24 V For time t > 0 , Substitute 1 for t to find i s ( t ) . i s ( t ) = 2 [ 1 − u ( 1 ) ] A = 2 [ 1 − 1 ] A { ∵ u ( t ) = 1 for t > 0 } = 0 A Since the value of current source is zero, it is open circuited and the Figure 1 is reduced as shown in Figure 3. Substitute 8 Ω for R 1 in equation (1) to find Z R 1 . Z R 1 = 8 Ω Substitute 12 Ω for R 2 in equation (1) to find Z R 2 . Z R 2 = 12 Ω Substitute 2 H for L in equation (2) to find Z L . Z L = s ( 2 H ) = 2 s Substitute 1 18 F for C in equation (3) to find Z C . Z C = 1 s ( 1 18 F ) = 18 s Using element transformation methods with initial conditions convert the Figure 3 into s -domain. Apply Kirchhoff’s voltage law for Figure 4. − v ( 0 ) s + 18 s I ( s ) + 8 I ( s ) + 12 I ( s ) + 2 s I ( s ) − ( 2 s ) i ( 0 ) s = 0 I ( s ) ( 18 s + 8 + 12 + 2 s ) = v ( 0 ) s + ( 2 s ) i ( 0 ) s I ( s ) ( 18 s + 20 + 2 s ) = v ( 0 ) s + ( 2 s ) i ( 0 ) s Substitute 24 for v ( 0 ) , and 2 for i ( 0 ) in above equation

EE 98: Fundamentals of Electrical Circuits - With Connect Access

6th Edition

ISBN: 9781259981807

Author: Alexander

Publisher: MCG

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Question

Chapter 16, Problem 52P

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Find the expression of current i(t) and the voltage v(t) for all t>0.

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Chapter 16, Problem 51P

Chapter 16, Problem 53P

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