Basic Concepts of Electricity. n Force on positive charge is in direction of electric field, negative is opposite

Transcription

1 Basic Cocepts of Electricity oltage E Curret I Ohm s Law Resistace R E = I R 1 Electric Fields A electric field applies a force to a charge Force o positive charge is i directio of electric field, egative is opposite Charges move if they are mobile A electric field is produced by charges (positive ad egative charges) Electric fields ca be produced by time varyig magetic fields (geerator, atea radiatio) 2 1

2 Capacitor (electric field costat betwee parallel plates) oltage Differece oltage differece is the differece i potetial eergy i a electric field E = /d As you move closer to a positive charge the voltage icreases Metal Dielectric Metal Q d 0 -Q 3 Curret A electric curret is produced by the flow of electric charges Curret = rate of charge movemet = amout of charge crossig a surface per uit time I coductors, curret flow is due to electros Covetioal curret is defied by the directio positive charges will flow Directio of electro flow is opposite to directio of covetioal curret 4 2

3 Resistace I materials electros accelerate i a electric field Electros lose eergy whe they hit atoms - lost eergy appears as heat ad light The result is that electros drift with costat velocity (superimposed o radom thermal motio) Resistace is the ratio oltage/curret R = /I 5 oltage, Curret, ad Resistace Flow = curret Pressure = voltage 6 3

4 Material Coductivity Coductors - egligible resistace Isulators - extremely large resistace Semicoductors - some resistace Resistors - are devices desiged to have costat resistace across a rage of voltages 7 Resistor Combiatio Series resistace R1 R2 R1+R2 R1 Parallel resistace R2 R = R1 R2 = R1 R2 R1+R2 1/R=1/R1 + 1/R2 8 4

5 oltage Divider I R1 2 = R1 R1 + R2 + R2 + _ 2 Solutio: Goal: Fid 2 give Fid 2 i terms of I Curret through R2 i terms of I oltage across R1 Fid voltage across R1 ad R2 usig two differet methods 9 Potetiometer (ariable Resistor) B Adjustable cotact termial Resistace material + R X A A X B X = * Distace AX/Distace AB (liear potetiometer) A trimpot is a small variable resistor mouted o a prited circuit board that ca adjusted by a small screwdriver to make semi-permaet adjustmets to a circuit 10 5

6 Iput Trasducers These are devices that produce electric sigals i accordace with chages i some physical effect e.g. covert temperature, light level to a voltage level or resistace e.g. microphoes, strai gauge, photodetectors, io-selective membraes, thermistors Sometimes the defiitio of trasducer is that of a device that coverts o-electrical eergy to electrical eergy 11 Output Trasducers Devices which covert a electrical quatity ito some other physical quatity or effect e.g. relay, loudspeaker, soleoid 12 6

7 Light Depedet Resistors (LDRs) Devices whose resistace chages (usually decreases) with light strikig it (also called photocells, photocoductors) Light strikig a semicoductig material ca provide sufficiet eergy to cause electros to break away from atoms. Free electros ad holes ca be created which causes resistace to be reduced 13 LDRs Typical materials used are Cadmium Sulphide (CdS), Cadmium Seleide (CdSe), Lead Sulphide With o illumiatio, resistace ca be greater tha 1 MΩ (dark resistace). Resistace varies iversely proportioal to light itesity. Reduces dow to s ohms 100ms/10ms respose time 14 7

8 1 MΩ CdS LDR Top view Resistace 100kΩ 10kΩ 1kΩ 100Ω lm/m 2 Illumiatio 15 LDRs have a low eergy gap Operate over a wide wavelegths (some, ito ifrared) Idium atimoide is good for IR. Whe cooled is very sesitive, used for thermal scaig of earth s surface 16 8

9 Capacitors A compoet costructed from two coductors separated by a isulatig material (dielectric) that stores electric charge (+Q, -Q) As a cosequece there is a voltage differece across the capacitor, Capacitace = C = Q/ The dielectric material operates to reduce the electric field betwee the coductors ad so allow more charge to be stored for a give voltage 17 Capacitors Bucket aalogy Metal Dielectric Metal Q Q Q Q C = Q/ (Q = C) A small bucket (capacitor, C) holds less charge (Q) for give level (voltage ) tha a large bucket 18 9

10 Chargig a Capacitor I 0 t The bucket aalogy ca be used to describe capacitor chargig 0 t Whe curret flows i at a costat rate the voltage icreases liearly ad vice versa for curret flowig out 19 Semicoductors Silico is used as a example (other semicoductors iclude Germaium, Gallium Arseide, Gallium phosphide, idium arseide, idium phosphide) Pure silico (itrisic semicoductor) Four valace electros Crystallie structure Reasoably high resistace Silico atom alece electros 20 10

11 Electros ad holes Due to thermal eergy some electros i the valace shell become free Create: Oe free electro + Oe hole i the valece bad that ca be filled by electros from the valace bad i a adjacet silico atom Curret i silico ca flow due to both movemet of electros ad holes 21 -type silico Add door impurities (e.g. Phosphorus, arseic, idium) with 5 electros i the valace bad As oly four electros ca bod with eighbourig silico atoms oe free electro is left Icreases cocetratio of free electros Reduces cocetratio of holes (due to icreased chace of recombiatio) Resistace reduced Free electro + Door io (+) 22 11

12 p-type silico p-type silico is created by addig acceptor impurities which have three valace electros (e.g. boro) This leaves a uboud valace electro i a adjacet silico atom creatig a hole Icreases cocetratio of holes Reduces cocetratio of Hole created free electros P-type silico has lower resistace tha pure silico - Acceptor io (-) 23 Diodes If a piece of -type silico ad p-type silico are joied directly together a diode (di - electrode) device is created P N Aode Cathode 24 12

13 Macro-behaviour A diode is a device that allows curret flow easily i oe directio easily ad allows hardly ay curret flow i the opposite directio 25 Forward bias Curret flows easily if the P regio is positive with respect to the N regio I I=I 0 e b (Strictly I=I 0 (e b -1)) P I + - N 26 13

14 Reverse bias Curret hardly flows if the P regio is egative with respect to the N regio I=-I 0 I P -I N Diode ad resistor circuit I D R R Currets ad voltages determied by: (work backwards to fid D ) 1. D related to I by diode equatio 2. Curret i resistor ad diode equal 3. R = IR 4. voltage across diode ad voltage resistor add up to voltage source Short cut rule of thumb, D is approx volts ad R For LEDs D is about , depedig o colour Forward biased diode 28 14

15 Diode ad resistor circuit Assume o reverse-bias curret flows (ideal case) Therefore o voltage occurs across the resistor + Therefore the full supply voltage appears across the diode Reverse biased diode 29 LEDs Light emittig diode Whe a electro moves dow from the coductio bad to the valece bad it loses eergy I silico ad germaium the eergymometum relatioships mea that this eergy is lost heat I gallium arseide it produces a photo 30 15

16 LEDs The light itesity is proportioal to curret Pure gallium arseide produces ifrared light GaAsP produces red or yellow light GaP produces red or gree 31 Circuit desig usig LEDs LEDs behave just like ormal diodes except that the forward bias voltages are greater (typically ) A typical forward bias curret of ma is used

17 Example I R 680Ω I = = ma D 33 17