EE100Su08 Lecture #1 (June 23rd 2008)Electric ChargeElectric CurrentElectric Current ExamplesElectric Potential (Voltage)Electric PowerThe Ideal Basic Circuit ElementSlide 8A Note about Reference DirectionsSign Convention ExampleAnother ExampleSlide 1EE100 Summer 2008 Bharathwaj MuthuswamyEE100Su08 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 LawSlide 2EE100 Summer 2008 Bharathwaj MuthuswamyElectric 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 CoulombSlide 3EE100 Summer 2008 Bharathwaj MuthuswamyElectric CurrentDefinition: rate of positive charge flowSymbol: iUnits: Coulombs per second ≡ Amperes (A)Note: Current has polarity.i = dq/dt where q = charge (Coulombs)t = time (in seconds)1775-1836André-Marie Ampère'sSlide 4EE100 Summer 2008 Bharathwaj MuthuswamyElectric Current Examples1. 105 positively charged particles (each with charge 1.6×10-19 C) flow to the right (+x direction) every nanosecond2. 105 electrons flow to the right (+x direction) every nanosecondQIt=5 195910 1.6 101.6 1010QIt---� �= =+ = �A5 195910 1.6 101.6 1010QI At---� �= =- =- �Slide 5EE100 Summer 2008 Bharathwaj MuthuswamyElectric Potential (Voltage)•Definition: energy per unit charge•Symbol: v•Units: Joules/Coulomb ≡ Volts (V)v = dw/dqwhere w = energy (in Joules), q = charge (in Coulombs)Note: Potential is always referenced to some point.Subscript convention:vab means the potential at a minus the potential at b.abvab ≡ va - vbAlessandro Volta (1745–1827)Slide 6EE100 Summer 2008 Bharathwaj MuthuswamyElectric 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 drop in voltage v, it loses energyenergy change = qvrate is proportional to # charges/secJames Watt1736 - 1819Slide 7EE100 Summer 2008 Bharathwaj MuthuswamyThe Ideal Basic Circuit ElementAttributes:•Two terminals (points of connection)•Mathematically described in terms of current and/or voltage•Cannot be subdivided into other elements+v_i• Polarity reference for voltage can be indicated by plus and minus signs• Reference direction for the current is indicated by an arrowSlide 8EE100 Summer 2008 Bharathwaj MuthuswamySlide 9EE100 Summer 2008 Bharathwaj Muthuswamy- v +A Note about Reference Directions•A problem like “Find the current” or “Find the voltage” is always accompanied by a definition of the direction: •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.iSlide 10EE100 Summer 2008 Bharathwaj MuthuswamySuppose 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. The DVM indicates 1.401, so va is lower than vb by 1.401 V. Which is the positive battery terminal?1.401DVMabNote that we have used the “ground” symbol ( ) for the reference node on the DVM. Often it is labeled “C” for “common.”Sign Convention ExampleSlide 11EE100 Summer 2008 Bharathwaj MuthuswamyFind vab, vca, vcbNote that the labeling convention has nothing to do with whether or not v is positive or negative.++ V1 V++vb dvc dabdcAnother
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