1.1 Introduction * Objectives * Requirements & Grading Policy * Other information 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 Lecture 1. Getting Started 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 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: 2

* Electrical quantities  current, voltage & power, sign conventions * Circuit elements  Passive, active and sources * Basic laws  Ohm’s law and Kirchhoff’s laws 1.2. Basic Circuit Concepts 3

4 Electrical Quantities Basic quantities: –Current (I): time rate of change of electric charge I = dq/dt Unit: 1 Amp = 1 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 = I V 1 Watt = 1 Volt·Amp = 1 Joule/sec

5 Water Analogy

6 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 –alternating current (ac): household current which varies with time I(t)

7 Voltage, V Voltage is the difference in electrical potentials between, e.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?

8 Default Sign Convention Passive sign convention : current should enter the positive voltage terminal Passive sign convention: P = I V –Positive (+) Power: element absorbs power –Negative (-) Power: element supplies power Circuit Element + – I

9 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)

10 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 +–+–

11 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)

12 Ohm’s Law v(t) = i(t) R- or -V = I R p(t) = i 2 (t) R = v 2 (t)/R[+ (absorbing)] v(t)v(t) The Rest of the Circuit R i(t)i(t) + –

13 Open Circuit What if R=  ? i(t) = v(t)/R = 0 The Rest of the Circuit v(t)v(t) i(t)=0 + –

14 Short Circuit What if R=0? v(t) = R i(t) = 0 The Rest of the Circuit v(t)=0 i(t)i(t) + –

15 Resistors in Series Two or more elements are in series if the current that flows through one must also flow through the other. R1R1 R2R2 In series R1R1 R2R2 Not in series I 1 = I 2 I 1 ≠ I 2

16 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. Parallel Not Parallel R1R1 R2R2 R1R1 R2R2

17 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 is zero –Conservation of energy

18 KCL (Kirchhoff’s Current Law) The sum of currents entering the node is zero: Analogy: mass flow at pipe junction i1(t)i1(t) i2(t)i2(t)i4(t)i4(t) i5(t)i5(t) i3(t)i3(t)

19 Class Examples Drill Problems 1, 2, 4