matter consists, measured in coulombs (C) 1 C of charge requires electrons Law of conservation of charge: charge cannot be created or

Transcription

1 Basc Concepts Oerew SI Prefxes Defntons: Current, Voltage, Power, & Energy Passe sgn conenton Crcut elements Ideal s Portland State Unersty ECE 221 Basc Concepts Ver Crcut Analyss: Introducton An electrc crcut s an nterconnecton of electrcal elements Charge s an electrcal property of the atomc partcles whch matter conssts, measured n coulombs (C) 1 C of charge requres electrons Law of conseraton of charge: charge cannot be created or destroyed, only transferred. Portland State Unersty ECE 221 Basc Concepts Ver SI Prefxes These are used throughout the currculum and by engneers Multpler Prefx Symbol Example tera T TB 10 9 gga G GB 10 6 mega M MHz 10 3 klo k kω 10 0 V 10 3 mll m mh 10 6 mcro µ µa 10 9 nano n ns pco p pf femto f atto a Portland State Unersty ECE 221 Basc Concepts Ver Flashlght Crcut Swtch Batteres Each Lght Bulb 10 Ω A flashlght crcut has 4 crcut elements We wll use symbols for crcut elements Facltates analyss Portland State Unersty ECE 221 Basc Concepts Ver

2 Electrc Current Electrc Current s the rate of change of charge, measured n amperes (A) 1 A = 1 C/s Two man types Drect Current (DC): Current remans constant Alternatng Current (AC): Current ares snusodally wth tme where = dq dt = current n amperes q = charge n coulombs t = tme n seconds Portland State Unersty ECE 221 Basc Concepts Ver Propagaton Rate 10 Ω Aerage drft elocty s ery small (mm/sec) Propagaton s ery fast (close to speed of lght) Effects appear to be nstantaneous Smlar to a hose full of water Portland State Unersty ECE 221 Basc Concepts Ver Notes on Current e 10 Ω Current n crcuts physcally realzed by moement of electrons Drecton of current must be specfed by an arrow By conenton, current drecton defned as flow of poste charge Note poste charge s not flowng physcally Electrons hae a negate charge They moe n the opposte drecton of current Portland State Unersty ECE 221 Basc Concepts Ver Key Assumptons For the duraton of ths class, we wll make the followng assumptons. Propagaton s nstantaneous (lumpedparameter system) Net charge stored by all crcut elements s 0 (zero) Nothng can store electrons No magnetc couplng between wres or crcut elements Portland State Unersty ECE 221 Basc Concepts Ver

3 Voltage Voltage s the energy absorbed or expended as a unt charge moes through a crcut element Analogous to pressure n a hydraulc system Sometmes called potental dfference Can be created by a separaton of charge Is a measure of the potental between two ponts Voltage pushes charge n one drecton We use polarty ( and on batteres) to ndcate whch drecton the charge s beng pushed = dw dq where = oltage n olts w = energy n Joules q = charge n coulombs Portland State Unersty ECE 221 Basc Concepts Ver Example 1: Voltage Concept Pressure Reseror Sponge Current Voltage Pressure Electrc Current Water Current Sponge Porosty Resstance Portland State Unersty ECE 221 Basc Concepts Ver Voltage Concept 3V 10 Ω The oltage sources push current through the crcut The current s the rate of flow of charge (.e. electrons) The lght bulb (resstor) ressts the flow of current Portland State Unersty ECE 221 Basc Concepts Ver Example 2: Voltage Concept mg Voltage Force due to graty (mg) Electrc Current Marble elocty Vscosty Resstance Portland State Unersty ECE 221 Basc Concepts Ver

4 Power Power: tme rate of expendng or absorbng energy Denoted by p By conenton Crcut elements that absorb power hae a poste alue of p Crcut elements that produce power hae a negate alue of p where p = dw dt p = ± p = power n watts (W = J/s) w = energy n joules (J) t = tme n seconds (s) = oltage n olts (V) = current n amperes (A) Portland State Unersty ECE 221 Basc Concepts Ver Passe Sgn Conenton Passe Sgn Conenton (PSC): Current enters the poste termnal of an element Equalent: Current leaes the negate termnal Most twotermnal crcut elements (e.g. batteres, lght bulbs, resstors, swtches) are characterzed by a sngle equaton that relates oltage to current: = ±f() or = ±g() Called the defnng equaton The PSC determnes the sgn of the relatonshp IfPSCssatsfed: = f() or = g() If PSC s not satsfed: = f() or = g() Ths s also true of power IfPSCssatsfed:p = If PSC s not satsfed: p = Portland State Unersty ECE 221 Basc Concepts Ver Energy Law of Conseraton of Energy: the net power absorbed by a crcut s equal to 0 In other words The total energy produced n a crcut s equal to the total energy absorbed Eery Watt absorbed by an element must be produced by some other element(s) Energy: capacty to do work, measured n joules (J) w = t t0 p(t)dt = t t0 ±(t)(t)dt If current and oltage are constant (DC), w = t t0 p dt = p(t t0) Portland State Unersty ECE 221 Basc Concepts Ver Example 3: Passe Sgn Conenton Suppose the crcut element shown below s characterzed by = ±f() and = ±g(). Determne whether the PSC s satsfed and wrte the equatons for the oltage, current, and power for each of the dagrams below. Portland State Unersty ECE 221 Basc Concepts Ver

5 Example 4: Passe Sgn Conenton Fnd the power (absorbed) for each element. 2 A 2 A 2 A 2 A 4 V 4 V 4 V 4 V Portland State Unersty ECE 221 Basc Concepts Ver Passe Sgn Conenton Remarks Falure to comply wth the PSC wll result n a wrong equaton n the early stages of crcut analyss All of the results that follow wll be wrong Ths translates to many lost ponts on exams One of the key deas s that the defnng equatons depend on the oltage polarty and current drecton Example: p = ± You must examne how the polarty of and the drecton of s labeled on the crcut dagram to determne the sgn Portland State Unersty ECE 221 Basc Concepts Ver Portland State Unersty ECE 221 Basc Concepts Ver In most cases, we wll be concerned wth oltage drops The frst expresson s the most common There s a 5 V drop from b to a There s a 5 V rse from a to b There s a 5 V rse from b to a The followng statements are true and equalent There s a 5 V drop from a to b 1kΩ a b 5 V Voltage Drops & Rses Defned Portland State Unersty ECE 221 Basc Concepts Ver Voltage Current Power (V) (A) (W) a b c 2 6 d e f b b e e b c e f c f f d a c a a d d Fnd the total power absorbed n the crcut. Example 5: Passe Sgn Conenton

6 Ideal Voltage s Vs Vs ci1 Independent (Battery) Independent Dedependent Ideal Voltage : produces Vs olts regardless of the current absorbed or produced by the dece The oltage produced may depend on some other crcut arable (current or oltage) The sgn of Vs can be negate 12 V = 12 V 12 V = 12 V Portland State Unersty ECE 221 Basc Concepts Ver Notes on Ideal s Ideal sources are models used to smplfy analyss These deces do not exst physcally How much power can an deal source produce? How much power can a battery produce? Ideal models sere as a good approxmaton of physcal deces, but only oer a lmted operatng range We wll use and dscuss more accurate models later n the term Portland State Unersty ECE 221 Basc Concepts Ver Ideal Current s Is cv1 Independent Dedependent Ideal Current : produces Is amps regardless of the current n the dece The current produced may depend on some other crcut arable (current or oltage) Note the sgn of Is can be negate 2 A = 2 A 2 A = 2 A Portland State Unersty ECE 221 Basc Concepts Ver