Introduction; Circuit Elements; Ohm's Law; KCLIntroductionBasic Electrical QuantitiesCurrent, IVoltage, VDefault Sign ConventionElectrical Analogies (Physical)Active vs. Passive ElementsIndependent SourcesResistorsOhm’s LawOpen CircuitShort CircuitSeriesParallelKirchhoff’s LawsKCL (Kirchhoff’s Current Law)Class ExamplesLect1 EEE 202 1Introduction; Circuit Elements; Ohm's Law; KCLDr. HolbertJanuary 14, 2008Lect1 EEE 202 2Introduction•Syllabus: textbook, homework, grading, schedule, reading assignments, laboratory•Lectures: let’s avoid “Death by PowerPoint”Lect1 EEE 202 3Basic Electrical Quantities•Basic quantities: current, voltage and power–Current: time rate of change of electric chargeI = dq/dt1 Amp = 1 Coulomb/sec–Voltage: electromotive force or potential, V1 Volt = 1 Joule/Coulomb = 1 N·m/coulomb–Power: P = I V1 Watt = 1 Volt·Amp = 1 Joule/secLect1 EEE 202 4Current, 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 timeI(t)Lect1 EEE 202 5Voltage, VVoltage 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 otherCircuit Element(s)+ –V(t)Lect1 EEE 202 6Default 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 powerCircuit Element+–ILect1 EEE 202 7Electrical Analogies (Physical)Lect1 EEE 202 8Active 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)Lect1 EEE 202 9Independent SourcesAn 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+–VoltageSourceCurrentSourceLect1 EEE 202 10Resistors•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)Lect1 EEE 202 11Ohm’s Lawv(t) = i(t) R - or - V = I Rp(t) = i2(t) R = v2(t)/R [+ (absorbing)]v(t)The Rest of the CircuitRi(t)+–Lect1 EEE 202 12Open Circuit•What if R = ?•i(t) = v(t)/R = 0v(t)The Rest of the Circuiti(t)=0+–i(t)=0Lect1 EEE 202 13Short Circuit•What if R = 0 ?•v(t) = R i(t) = 0The Rest of the Circuitv(t)=0i(t)+–Lect1 EEE 202 14SeriesTwo elements are in series if the current that flows through one must also flow through the other.R1 R2SeriesR1 R2Not SeriesLect1 EEE 202 15ParallelTwo elements are in parallel if they are connected between (share) the same two (distinct) end nodes.Parallel Not ParallelR1R2R1R2Lect1 EEE 202 16Kirchhoff’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 zeroLect1 EEE 202 17KCL (Kirchhoff’s Current Law)The sum of currents entering the node is zero:Analogy: mass flow at pipe junctioni1(t)i2(t) i4(t)i5(t)i3(t)njjti10)(Lect1 EEE 202 18Class 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
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