Announcements. Lecture #2

Transcription

1 Announcements Lectures wll be n 4 LeConte begnnng Frday 8/29 Addtonal dscusson TA Denns Chang (Sectons 101, 105) Offce hours: Mo 2-3 PM; Th 5-6 PM Lab sectons begn Tuesday 9/2 Read Experment #1 onlne Download Pre-Lab #1 and complete t before gong to the lab (140 Cory) Dscusson sectons begn Tuesday 9/2 Lecture 2, Slde 1 Lecture #2 OUTLINE Introducton to crcut analyss Electrcal quanttes Charge Current Voltage Power The deal basc crcut element Sgn conentons Readng Chapter 1 Lecture 2, Slde 2

2 Electrcal System Desgn Process 1. Identfy system performance requrements desgn specfcatons 2. Concee of approach desgn concept 3. Deelop an electrc crcut model (mathematcal model that approxmates the behaor of an actual electrcal system) usng deal crcut components (mathematcal models of actual electrcal components) 4. Buld and test a physcal prototype Lecture 2, Slde 3 Crcut Analyss Crcut analyss s used to predct the behaor of the electrc crcut, and plays a key role n the desgn process. Comparson between desred behaor (desgn specfcatons) and predcted behaor (from crcut analyss) leads to refnements n desgn In order to analyze an electrc crcut, we need to know the behaor of each deal crcut element (n terms of ts oltage and current) and the constrants mposed by nterconnectng the arous elements. Lecture 2, Slde 4

3 Electrc Charge Macroscopcally, most matter s electrcally neutral most of the tme. Exceptons: clouds n a thunderstorm, people on carpets n dry weather, plates of a charged capactor, etc. Mcroscopcally, matter s full of electrc charges. Electrc charge exsts n dscrete quanttes, ntegral multples of the electronc charge 1.6 x coulombs Electrcal effects are due to separaton of charge electrc force (oltage) charges n moton electrc flow (current) Lecture 2, Slde 5 Classfcaton of Materals Solds n whch all electrons are tghtly bound to atoms are nsulators. Solds n whch the outermost atomc electrons are free to moe around are metals. Metals typcally hae ~1 free electron per atom (~ free electrons per cubc cm) Electrons n semconductors are not tghtly bound and can be easly promoted to a free state. nsulators Quartz, SO 2 delectrc materals semconductors S, GaAs metals Al, Cu excellent conductors Lecture 2, Slde 6

4 Electrc Current Defnton: rate of poste charge flow Symbol: Unts: Coulombs per second Amperes (A) = dq/dt where q = charge (n Coulombs), t = tme (n seconds) Note: Current has polarty. Lecture 2, Slde 7 Electrc Current Examples postely charged partcles (each wth charge C) flow to the rght (x drecton) eery nanosecond electrons flow to the rght (x drecton) eery mcrosecond Lecture 2, Slde 8

5 Current Densty Defnton: rate of poste charge flow per unt area Symbol: J Unts: A / cm 2 Example 1: Wre attached to end C1 2 cm 1 cm 10 cm Suppose we force a current of 1 A to flow from C1 to C2: Electron flow s n -x drecton: C / sec = C / electron 1 18 X Semconductor wth free electrons per cm 3 C2 electrons sec Lecture 2, Slde 9 Current Densty Example (cont d) The current densty n the semconductor s Example 2: Typcal dmensons of ntegrated crcut components are n the range of 1 µm. What s the current densty n a wre wth 1 µm² area carryng 5 ma? Lecture 2, Slde 10

6 Electrc Potental (Voltage) Defnton: energy per unt charge Symbol: Unts: Volts (V) = dw/dq where w = energy (n Joules), q = charge (n Coulombs) Note: Potental s always referenced to some pont. a b Subscrpt conenton: ab means the potental at a mnus the potental at b. ab a - b Lecture 2, Slde 11 Electrc Power Defnton: transfer of energy per unt tme Symbol: p Unts: Joules per second Watts (W) p = dw/dt = (dw/dq)(dq/dt) = Concept: As a poste charge q moes through a drop n oltage, t loses energy energy change = q rate s proportonal to # charges/sec Lecture 2, Slde 12

7 The Ideal Basc Crcut Element Polarty reference for oltage can be ndcated by plus and mnus sgns Reference drecton for the current s ndcated by an arrow Attrbutes: Two termnals (ponts of connecton) Mathematcally descrbed n terms of current and/or oltage Cannot be subdded nto other elements Lecture 2, Slde 13 A Note about Reference Drectons A problem lke Fnd the current or Fnd the oltage s always accompaned by a defnton of the drecton: - In ths case, f the current turns out to be 1 ma flowng to the left, we would say = -1 ma. In order to perform crcut analyss to determne the oltages and currents n an electrc crcut, you need to specfy reference drectons. There s no need to guess the reference drecton so that the answers come out poste, howeer. Lecture 2, Slde 14

8 Sgn Conenton Example Suppose you hae an unlabelled battery and you measure ts oltage wth a dgtal oltmeter (DVM). It wll tell you the magntude and sgn of the oltage. a b DVM Wth ths crcut, you are measurng ab. The DVM ndcates 1.401, so a s lower than b by V. Whch s the poste battery termnal? Note that we hae used the ground symbol ( ) for the reference node on the DVM. Often t s labeled C for common. Lecture 2, Slde 15 Another Example Fnd ab, ca, cb a 2 V c 1 V cd b bd d Note that the labelng conenton has nothng to do wth whether or not s poste or negate. Lecture 2, Slde 16

9 Sgn Conenton for Power Passe sgn conenton p = p = - If p > 0, power s beng delered to the box. If p < 0, power s beng extracted from the box. Lecture 2, Slde 17 Summary Current = rate of charge flow Voltage = energy per unt charge created by charge separaton Power = energy per unt tme Ideal Basc Crcut Element 2-termnal component that cannot be sub-dded descrbed mathematcally n terms of ts termnal oltage and current Passe sgn conenton Reference drecton for current through the element s n the drecton of the reference oltage drop across the element Lecture 2, Slde 18