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Introduction; Circuit Elements; Ohm's Law; KCL

Introduction; Circuit Elements; Ohm's Law; KCL. Dr. Holbert January 14, 2008. Introduction. Syllabus: textbook, homework, grading, schedule, reading assignments, laboratory Lectures: let’s avoid “Death by PowerPoint”. Basic Electrical Quantities.

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Introduction; Circuit Elements; Ohm's Law; KCL

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  1. Introduction; Circuit Elements; Ohm's Law; KCL Dr. Holbert January 14, 2008 EEE 202

  2. Introduction • Syllabus: textbook, homework, grading, schedule, reading assignments, laboratory • Lectures: let’s avoid “Death by PowerPoint” EEE 202

  3. Basic Electrical Quantities • Basic quantities: current,voltage and power • Current: time rate of change of electric charge I = dq/dt 1 Amp = 1 Coulomb/sec • Voltage: electromotive force or potential, V1 Volt = 1 Joule/Coulomb = 1 N·m/coulomb • Power: P = IV 1 Watt = 1 Volt·Amp = 1 Joule/sec EEE 202

  4. I(t) Current, I • Normally we talk about the movement of positive charges although we know that, in general, in metallic conductors current results from electron motion (conventionally positive flow) • The sign of the current indicates the direction of flow • Types of current: • direct current (dc): batteries and some special generators • alternating current (ac): household current which varies with time EEE 202

  5. Circuit Element(s) + – V(t) Voltage, V Voltage is the difference in energy level of a unit charge located at each of two points in a circuit, and therefore, represents the energy required to move the unit charge from one point to the other EEE 202

  6. I Circuit Element – + Default Sign Convention • Passive sign convention : current should enter the positive voltage terminal • Consequence for P = I V • Positive (+) Power: element absorbs power • Negative (-) Power: element supplies power EEE 202

  7. Electrical Analogies (Physical) EEE 202

  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) EEE 202

  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 Voltage Source Current Source EEE 202

  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) EEE 202

  11. 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) EEE 202

  12. i(t)=0 + The Rest of the Circuit – i(t)=0 Open Circuit • What if R =  ? • i(t) = v(t)/R = 0 v(t) EEE 202

  13. i(t) + The Rest of the Circuit v(t)=0 – Short Circuit • What if R = 0 ? • v(t) = R i(t) = 0 EEE 202

  14. Series Two elements are in series if the current that flows through one must also flow through the other. Series R1 R2 Not Series R1 R2 EEE 202

  15. R1 R2 Parallel Two elements are in parallel if they are connected between (share) the same two (distinct) end nodes. R1 R2 Parallel Not Parallel EEE 202

  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 • Kirchhoff’s Voltage Law (KVL) • sum of voltages around any loop in a circuit is zero EEE 202

  17. 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 EEE 202

  18. Class Examples • Drill Problems P1-1, P1-2, P1-4 • While working these problems, we shall define the term ‘node’ as a point where two or more circuit elements are connected EEE 202

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