EE100Su08 Lecture 1 June 23rd 2008 Outline Electrical quantities Charge Current Voltage Power The ideal basic circuit element Sign conventions Circuit element I V characteristics Construction of a circuit model Kirchhoff s Current Law Kirchhoff s Voltage Law EE100 Summer 2008 Slide 1 Bharathwaj Muthuswamy Electric Charge Electrical effects are due to separation of charge electric force voltage charges in motion electric flow current Macroscopically most matter is electrically neutral most of the time Exceptions clouds in a thunderstorm people on carpets in dry weather plates of a charged capacitor etc Microscopically matter is full of electric charges Electric charge exists in discrete quantities integral multiples of the electronic charge 1 6 x 10 19 Coulomb EE100 Summer 2008 Slide 2 Bharathwaj Muthuswamy Electric Current Definition rate of positive charge flow Symbol i Units Coulombs per second Amperes A Note Current has polarity i dq dt where q charge Coulombs t time in seconds Andr Marie Amp re s 1775 1836 EE100 Summer 2008 Slide 3 Bharathwaj Muthuswamy Electric Current Examples 1 105 positively charged particles each with charge 1 6 10 19 C flow to the right x direction every nanosecond 5 Q 10 1 6 10 I t 10 9 2 19 1 6 10 5 A 105 electrons flow to the right x direction every nanosecond Q 105 1 6 10 19 5 I 1 6 10 A 9 t 10 EE100 Summer 2008 Slide 4 Bharathwaj Muthuswamy Electric Potential Voltage Definition energy per unit charge Symbol v Units Joules Coulomb Volts V v dw dq Alessandro Volta 1745 1827 where w energy in Joules q charge in Coulombs Note Potential is always referenced to some point a Subscript convention vab means the potential at a minus the potential at b vab va vb b EE100 Summer 2008 Slide 5 Bharathwaj Muthuswamy 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 Watt As a positive charge q moves through a James 1736 1819 drop in voltage v it loses energy energy change qv rate is proportional to charges sec EE100 Summer 2008 Slide 6 Bharathwaj Muthuswamy 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 EE100 Summer 2008 Slide 7 Bharathwaj Muthuswamy EE100 Summer 2008 Slide 8 Bharathwaj Muthuswamy A Note about Reference Directions A problem like Find the current or Find the voltage is always accompanied by a definition of the direction i v In this case if the current turns out to be 1 mA flowing to the left we would say i 1 mA In order to perform circuit analysis to determine the voltages and currents in an electric circuit you need to specify reference directions There is no need to guess the reference direction so that the answers come out positive EE100 Summer 2008 Slide 9 Bharathwaj Muthuswamy Sign Convention Example Suppose you have an unlabelled battery and you measure its voltage with a digital voltmeter DVM It will tell you the magnitude and sign of the voltage With this circuit you are measuring vab a 1 401 DVM b The DVM indicates 1 401 so va is lower than vb by 1 401 V Which is the positive battery terminal Note that we have used the ground symbol for the reference node on the DVM Often it is labeled C for common EE100 Summer 2008 Slide 10 Bharathwaj Muthuswamy Another Example Find vab vca vcb a V c vcd 1 V b v bd d Note that the labeling convention has nothing to do with whether or not v is positive or negative EE100 Summer 2008 Slide 11 Bharathwaj Muthuswamy
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