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Lecture 2

# Lecture 2 - ECE 201 Lecture 2 Announcements The first...

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1 ECE 201, Lecture 2 The first homework is due 2pm EE 264. Late first homework due 2pm Friday (half credit maximum) 4 lowest scores in homework can be dropped. Office hours: Monday, Wednesday, and Friday 8:30 – 9:30am, EE 252 Announcements

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2 (1) The battery or source (2) The resistor (3) The capacitor (4) The inductor (5) The operational amplifier Simple Circuit Elements
3 Circuit elements and circuits Four passive circuit elements Resistance Capacitance Inductance Mutual inductance or transformers Two independent sources Independent voltage source Independent current source 6 0 . 3 F R, Ohm, C, Farad, F 2 H L, Henry, H + _ 6 V V or v, Volt, V 4 A I or i, Ampere, A 1-100000 0.000001 0.001

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4 Dependent sources Voltage controlled voltage source Current controlled voltage source Voltage controlled current source Current controlled current source – V x , I y , V z and I w are voltages and currents specified elsewhere in the circuit. Each dependent source provides an equation in circuit analysis. Operational amplifier 1 . 3 I w 2 . 6 V z + _ 3 . 2 I y + _ 2 . 5 V x
5 Revisit the Circuit Model Resistor V in (t) 40 k 10 k 0.5 μ F 20mH The ability to transform the circuits is a very important skill!

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6 Circuit Variables Current How to quantify it? Analogy: traffic flow Scenario 1: very few cars, but each car travels at high speed Low traffic Scenario 2: many cars, each car travels at very slow speed slow traffic Scenario 3: many cars, each car travels at high speed high traffic flow Traffic has two direction, forward or backward
7 What is your definition of traffic flow? You sit at a spot and count how many cars passing you in one second: Scenario 1: small # of cars per second Scenario 2: small # of cars per second Scenario 3: large # of cars per second How would you define current? The amount of charges passing the cross- section in a unit time

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8 Complications in Currents There are positive and negative charges Current direction is along the flow direction of positive charges Positive charges moving along one direction = negative charges moving to the opposite direction Currents can be either positive or negative + or – sign, magnitude and reference direction, collectively, define a real current. 1 A - 1 A
9 Relationship between Charge and Current In a time interval t, a total or net charge of q flow by a given point, then Current in terms of charge Charge in terms of current If t 1 =0 and write t for t 2 , then If t 1 is - and q(- ) = 0, then Q (or q) in C and I (or i) in A dt ) t ( dq t q ) t ( i = = - 2 1 t t 1 2 dt ) t ( i ) t ( q ) t ( q - = t dt ) t ( i ) t ( q 0 ( ) (0) ( ) t q t q i t dt = +

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