EE40 Lecture 2 Venkat Anantharam 1 25 08 Reading Chap 1 EE40 Spring 08 Slide 1 Venkat Anantharam The Ideal Basic Circuit Element i v Polarity reference for voltage can be indicated by plus and minus signs Reference direction for the current is indicated by an arrow Attributes Two terminals points of connection Mathematically described in terms of current and or voltage Cannot be subdivided into other elements EE40 Spring 08 Slide 2 Venkat Anantharam Electric Power Definition transfer of energy per unit time Symbol p Units Joules per second Watts W p dw dt dw dq dq dt vi Concept As a positive charge q moves through a James Watt 1736 1819 drop in voltage v it loses energy energy change qv rate is proportional to charges sec EE40 Spring 08 Slide 3 Venkat Anantharam Passive Sign Convention i p vi v If p 0 power is being delivered to the box The element is a passive element If p 0 power is being generated from the box The element is an active element How can a circuit element absorb power By converting electrical energy into heat resistors in toasters light light bulbs or acoustic energy speakers by storing energy charging a battery EE40 Spring 08 Slide 4 Venkat Anantharam Sign Convention for Power Passive sign convention Active sign convention p vi p vi i v i i v v i v Irrespective of the sign convention If p 0 power is being delivered to the box If p 0 power is being extracted from the box EE40 Spring 08 Slide 5 Venkat Anantharam Electrical Resistance Resistance the ratio of voltage drop and current The circuit element used to model this behavior is the resistor R Circuit symbol Units Volts per Ampere ohms The current flowing in the resistor is proportional to the voltage across the resistor Georg Simon Ohm 1789 1854 v i R Ohm s Law where v voltage V i current A and R resistance EE40 Spring 08 Slide 6 Venkat Anantharam Current vs Voltage I V Characteristic What is the I V characteristic for an ideal resistor i i v v The slope must be positive and the characteristic must go through the origin EE40 Spring 08 Slide 7 Venkat Anantharam Electrical Conductance Conductance is the reciprocal of resistance Symbol G Units siemens S or mhos Example Consider an 8 resistor What is its conductance Werner von Siemens 1816 1892 EE40 Spring 08 Slide 8 Venkat Anantharam Electrical Sources An electrical source is a device that is capable of converting non electric energy to electric energy and vice versa Examples battery chemical electric dynamo generator motor mechanical electric Ex gasoline powered generator Bonneville dam Electrical sources can either deliver or absorb power EE40 Spring 08 Slide 9 Venkat Anantharam Ideal Voltage Source Circuit element that maintains a prescribed voltage across its terminals regardless of the current flowing in those terminals Voltage is known but current is determined by the circuit to which the source is connected The voltage can be either independent or dependent on a voltage or current elsewhere in the circuit and can be constant or time varying Device symbols vs independent EE40 Spring 08 vs vx vs ix voltage controlled current controlled Slide 10 Venkat Anantharam I V Characteristic of Ideal Voltage Source a Vab i i b vs i 0 v Vs 0 1 Plot the I V characteristic for vs 0 For what values of i does the source absorb power For what values of i does the source release power 2 Repeat vs 0 power i 0 absorb power Vs 0 1 i 0for release 3 What is the I V characteristic for an ideal wire EE40 Spring 08 Slide 11 Venkat Anantharam Ideal Current Source Circuit element that maintains a prescribed current through its terminals regardless of the voltage across those terminals Current is known but voltage is determined by the circuit to which the source is connected The current can be either independent or dependent on a voltage or current elsewhere in the circuit and can be constant or time varying Device symbols is independent EE40 Spring 08 is vx is ix voltage controlled current controlled Slide 12 Venkat Anantharam I V Characteristic of Ideal Current Source i i v is v 1 Plot the I V characteristic for is 0 For what values of v does the source absorb power For what values of v does the source release power V 0 absorb power V 0 release power EE40 Spring 08 Slide 13 Venkat Anantharam
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