Introduction; Circuit Elements; Ohm's Law; KCL Dr. Holbert January 14, 2008 Lect1 EEE 202 1 Introduction • Syllabus: textbook, homework, grading, schedule, reading assignments, laboratory • Lectures: let’s avoid “Death by PowerPoint” Lect1 EEE 202 2 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, V 1 Volt = 1 Joule/Coulomb = 1 N·m/coulomb – Power: P=IV 1 Watt = 1 Volt·Amp = 1 Joule/sec Lect1 EEE 202 3 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 I(t) • Types of current: – direct current (dc): batteries and some special generators – alternating current (ac): household current which varies with time Lect1 EEE 202 4 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 Circuit Element(s) + Lect1 V(t) EEE 202 – 5 Default Sign Convention • Passive sign convention : current should enter the positive voltage terminal I + Circuit Element – • Consequence for P = I V – Positive (+) Power: element absorbs power – Negative (-) Power: element supplies power Lect1 EEE 202 6 Electrical Analogies (Physical) Base quantity Flow variable Potential variable Lect1 Electric Hydraulic Charge (q) Mass (m) Current (I) Fluid flow (G) Voltage (V) Pressure (p) EEE 202 7 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) Lect1 EEE 202 8 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 Lect1 EEE 202 Current Source 9 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) Lect1 EEE 202 10 Ohm’s Law v(t) = i(t) R - or p(t) = i2(t) R = v2(t)/R V=IR [+ (absorbing)] i(t) The Rest of the Circuit Lect1 + R v(t) – EEE 202 11 Open Circuit • What if R = ? i(t)=0 + The Rest of the Circuit v(t) – i(t)=0 • i(t) = v(t)/R = 0 Lect1 EEE 202 12 Short Circuit • What if R = 0 ? i(t) The Rest of the Circuit + v(t)=0 – • v(t) = R i(t) = 0 Lect1 EEE 202 13 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 Lect1 R2 EEE 202 14 Parallel Two elements are in parallel if they are connected between (share) the same two (distinct) end nodes. R1 R1 Lect1 R2 R2 Parallel Not Parallel EEE 202 15 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 Lect1 EEE 202 16 KCL (Kirchhoff’s Current Law) i1(t) i5(t) i2(t) i4(t) i3(t) The sum of currents entering the node is zero: n i (t ) 0 j 1 j Analogy: mass flow at pipe junction Lect1 EEE 202 17 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 Lect1 EEE 202 18