Lecture 1. Electrical Transport 1.1 Introduction * Objectives * Requirements & Grading Policy * Other information 1.2 Basic Circuit Concepts * Electrical l quantities 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, Spring 2008 Prerequisite EEE 101 Pre- or co-requisites: MAT 274 or MAT 275, PHY 131, 132. Instructor: Dr. NJ Tao (njtao@asu.edu) Where: Schwada Classroom & 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 2
1.2. Basic Circuit Concepts * Electrical quantities current, voltage & power, sign conventions * Circuit elements Passive, active and sources * Basic laws Ohm s law and Kirchhoff s laws 3
Basic quantities: Electrical Quantities Current (I): time rate of change of electric charge I = dq/dt Unit: 1A Amp = 1C Coulomb/sec Voltage (V): electromotive force or 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 Electrical Hydraulic Base quantity Flow variable Potential variable Charge (q) Current (I) Voltage (V) Mass (m) Fluid flow (G) Pressure (p) 5
Current, I The sign of the current indicates the direction of flow Current due to positive & negative charge carried; the moving direction of positive charge is conventionally defined as direct of current. What are charge carries in copper 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, V Voltage is the difference in electrical potentials between, e.g., g, two points in a circuit; it is the energy required to move 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 Convention Passive sign convention : current should enter the positive voltage terminal 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 source (voltage or current) may 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 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 - V = IR p(t) = i 2 (t) R = v 2 (t)/r [+ (absorbing)] i(t) The Rest of the R + v(t) Circuit 12
Open Circuit What if R=? i(t)=0 The + Rest of v(t) the Circuit i(t) () = v(t)/r () = 0 13
Short Circuit What if R=0? i(t) The + Rest of v(t)=0 the Circuit 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. R 1 R 2 I 1 = I 2 In series Not in series R 1 R 2 I I 1 2 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. R 1 R 1 R 2 R 2 Parallel Not Parallel 16
Kirchhoff s Laws Kirchhoff s Current Law (KCL) sum of all currents entering a node is zero sum of currents entering node is equal to sum of currents leaving node Conservation of charge Kirchhoff s Voltage Law (KVL) sum of voltages around any loop in a circuit it is zero Conservation of energy 17
KCL (Kirchhoff s Current Law) i 2 (t) i 1 (t) i 5 (t) i 4 (t) i 3 3( (t) The sum of currents entering the node is zero: n i ( t) = 0 j j= 1 Analogy: mass flow at pipe junction 18
Class Examples Drill Problems 1, 2, 4 19