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ECE201 Exam #1 Review1 Exam #1 Review Dr. Holbert February 15, 2006

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ECE201 Exam #1 Review2 Circuit Analysis Techniques While Obeying Passive Sign Convention Ohm’s Law; KCL; KVL Voltage and Current Division Series/Parallel Impedance combinations

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ECE201 Exam #1 Review3 Sign Convention Passive sign convention : current should enter the positive voltage terminal Consequence for P = I V –Positive (+) Power: element absorbs power –Negative (-) Power: element supplies power Circuit Element + – I

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ECE201 Exam #1 Review4 Ohm’s Law V = I Z The Rest of the Circuit V I + – Z

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ECE201 Exam #1 Review5 KCL (Kirchhoff’s Current Law) The sum of currents entering the node is zero: Analogy: mass flow at pipe junction i 1 (t) i 2 (t)i 4 (t) i 5 (t) i 3 (t)

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ECE201 Exam #1 Review6 KVL (Kirchhoff’s Voltage Law) The sum of voltages around a loop is zero: Analogy: pressure drop thru pipe loop +–+– v1(t)v1(t) + + – – v2(t)v2(t) v3(t)v3(t)

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ECE201 Exam #1 Review7 KVL Polarity A loop is any closed path through a circuit in which no node is encountered more than once Voltage Polarity Convention –A voltage encountered + to - is positive –A voltage encountered - to + is negative

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ECE201 Exam #1 Review8 In General: Voltage Division Consider N impedances in series: Source voltage(s) are divided between the elements in direct proportion to their impedances

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ECE201 Exam #1 Review9 In General: Current Division Consider N impedances in parallel: Special Case (2 impedances in parallel)

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ECE201 Exam #1 Review10 Equivalent Impedance If we wish to replace the two parallel impedances with a single impedance whose voltage-current relationship is the same, the equivalent impedance has a value of: Parallel elements share the same 2 end nodes

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ECE201 Exam #1 Review11 Phasors A phasor is a complex number that represents the magnitude and phase of a sinusoidal voltage or current:

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ECE201 Exam #1 Review12 Complex Numbers x is the real part y is the imaginary part z is the magnitude is the phase angle z x y real axis imaginary axis Polar: z = A = x + jy : Rectangular

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ECE201 Exam #1 Review13 Impedance AC steady-state analysis using phasors allows us to express the relationship between current and voltage using a formula that looks likes Ohm’s law: V = I Z Z is called impedance (units of ohms, ) Impedance is (often) a complex number Impedance depends on frequency, f

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ECE201 Exam #1 Review14 Impedance Summary

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