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Understanding Basic Circuit Concepts in EEE 202 Course

Learn electrical quantities, circuit elements, and laws like Ohm’s Law and Kirchhoff’s Laws in this introductory lecture on circuits. Understand sign conventions, passive and active elements, and how to apply these concepts in circuit analysis.

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Understanding Basic Circuit Concepts in EEE 202 Course

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  1. Lecture 1. Getting Started 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

  2. 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

  3. 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

  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 = IV 1 Watt = 1 Volt·Amp = 1 Joule/sec

  5. Water Analogy

  6. I(t) 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

  7. Circuit Element(s) + – V(t) 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. What is electrical potential?

  8. I Circuit Element – + 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

  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. i(t) + The Rest of the Circuit R – Ohm’s Law v(t) = i(t) R - or - V = I R p(t) = i2(t) R = v2(t)/R [+ (absorbing)] v(t)

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

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

  15. Resistors in Series Two or more elements are in series if the current that flows through one must also flow through the other. In series R1 R2 I1 = I2 Not in series R1 R2 I1 ≠ I2

  16. R1 R2 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 R2 Parallel Not Parallel

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

  19. Class Examples • Drill Problems 1, 2, 4

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