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Transcript

Lecture 1. Electrical Transport 1.1 Introduction * Objectives * Requirements & Grading Policy * Other information 1.2 Basic Circuit Concepts * El Electrical t i l quantities titi ⇒ current, voltage & power, sign conventions * Circuit elements ⇒ Passive, active and sources * Basic laws ⇒ Ohm’s law and Kirchhoff’s laws 1 EEE 202: Circuits 1 1, Spring 2008 Prerequisite EEE 101 Pre- or co-requisites: MAT 274 or MAT 275, 275 PHY 131 131, 132 132. Instructor: Dr. NJ Tao ([email protected]) Wh Where: S h d Classroom Schwada Cl & Offi Office 150 When: Tu and Th 3:15-4:30 pm Office Hours: Tu and Th 2:00 - 3:00 p.m. or by appointment. Office Location: GWC618 Class Website: http://www.public.asu.edu/ ntao1/Teaching/ECE202/EEE202web.htm http://www.public.asu.edu/~ntao1/Teaching/ECE202/EEE202web.htm 2 1.2. Basic Circuit Concepts * Electrical quantities ⇒ current, current voltage & power, power sign conventions * Circuit elements ⇒ Passive, active and sources * Basic laws ⇒ Ohm’s law and Kirchhoff’s laws 3 Electrical Quantities • Basic quantities: – Current (I): time rate of change of electric charge I = dq/dt Unit: 1 A Amp = 1 C Coulomb/sec l b/ – Voltage g ((V): ) electromotive force or p potential Unit: 1 Volt = 1 Joule/Coulomb = 1 N·m/coulomb – Power (P): rate at which work is done P=IV 1 Watt = 1 Volt·Amp = 1 Joule/sec 4 Water Analogy Base quantity Flow variable Potential variable Electrical Hydraulic Charge (q) Mass (m) Current (I) Fluid flow (G) Voltage (V) Pressure (p) 5 Current,, I • The sign of the current indicates the direction of flow positive & negative g charge g carried;; the • Current due to p moving direction of positive charge is conventionally defined as direct of current. What are charge carries in copper wire wire, Silicon and salt solution? • DC & AC currents: – direct current (dc): batteries and some special generators I(t) – alternating current (ac): household current which varies with time 6 Voltage, g ,V • Voltage is the difference in electrical potentials between, g , two p points in a circuit;; it is the energy gy required q to move e.g., an unit charge from one point to the other. • Voltage with respect to a common point or “ground”. • Positive (high) and negative (low) voltages. Circuit Element(s) + V(t) – What is electrical potential? 7 Default Sign g Convention • Passive sign convention : current should enter the positive iti voltage lt tterminal i l I + Circuit Element – • Passive sign convention: P = I V – Positive (+) Power: element absorbs power – Negative (-) Power: element supplies power 8 Active vs. Passive Elements • Active elements can generate energy – Voltage and current sources – Batteries • Passive elements cannot generate energy – Resistors – Capacitors and Inductors (but CAN store energy) 9 Independent Sources An independent p source ((voltage g or current)) may y be DC (constant) or time-varying (AC), but does not depend on other voltages or currents in the circuit + – 10 Resistors • A resistor is a circuit element that dissipates p electrical energy (usually as heat) • Real-world devices that are modeled by resistors: incandescent light bulbs, heating elements (stoves, heaters, etc.), long wires • Resistance is measured in Ohms (Ω) 11 Ohm’s Law v(t) = i(t) R - or p(t) = i2(t) R = v2(t)/R V=IR [+ (absorbing)] i(t) The Rest of the Circuit + R v(t) – 12 Open p Circuit • What if R R=∞? ∞? i(t)=0 The Rest of the Circuit + v(t) – • i(t) ( ) = v(t)/R () =0 13 Short Circuit • What if R R=0? 0? i(t) The Rest of the Circuit + v(t)=0 – • v(t) ( ) = R i(t) ()=0 14 Resistors in Series Two or more elements are in series if the current that flows through one must also flow through the other. R1 R2 In series I1 = I2 Not in series R1 R2 I1 ≠ I2 15 Resistors in Parallel • Two or more elements are in parallel if they are connected between (share) the same two (distinct) end nodes; • The voltages across these elements are the same. R1 R1 R2 R2 Parallel Not Parallel 16 Kirchhoff’s Kirchhoff s Laws • Kirchhoff Kirchhoff’s s Current Law (KCL) – sum of all currents entering a node is zero – sum of currents entering g node is equal q to sum of currents leaving node – Conservation of charge • Kirchhoff’s Voltage Law (KVL) – sum off voltages lt around d any loop l iin a circuit i it iis zero – Conservation of energy 17 KCL ((Kirchhoff’s Current Law)) i1(t) i5(t) i2(t) i4(t) i3((t)) The sum of currents entering the node is zero: n ∑ i (t ) = 0 j =1 j Analogy: mass flow at pipe junction 18 Class Examples p • Drill Problems 1,, 2,, 4 19