# vce physics unit 3: electronics & photonics base notes

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- 1. VCE PhysicsUnit 3 - Topic 2Electronics &Photonics

2. 1.0 Unit Outline apply the concepts of current, resistance, potential difference (voltage drop),power to the operation of electronic circuits comprising diodes, resistors,thermistors, and photonic transducers including light dependent resistors (LDR),photodiodes and light emitting diodes (LED); V = IR, P = VI calculate the effective resistance of circuits comprising parallel and seriesresistance and unloaded voltage dividers; describe energy transfers and transformations in opto-electronic devices describe the transfer of information in analogue form (not including the technicalaspects of modulation and demodulation) using Light intensity modulation i.e. changing the intensity of the carrier wave toreplicate the amplitude variation of the information signal so that the signalmay propagate more efficiently Demodulation i.e. the separation of the information signal from the carrierwave design, investigate and analyse circuits for particular purposes using technicalspecifications related to potential difference (voltage drop), current, resistance,power, temperature, and illumination for electronic components such as diodes,resistors, thermistors, light dependent resistors (LDR), photodiodes and lightemitting diodes (LED); analyse voltage characteristics of amplifiers including linear voltage gain(VOUT/VIN) and clipping; identify safe and responsible practices when conducting investigations involvingelectrical, electronic and photonic equipment 3. Chapter 1 Topics covered: Electric Charge. Electric Current. Voltage. Electromotive Force. Electrical Energy. Electric Power. 4. 1.0 Electric Charge The fundamental unit of electricalcharge is that carried by the electron(& the proton). This is the smallest discrete chargeknown to exist independently and iscalled the ELEMENTARY CHARGE. Electric Charge (symbol Q) ismeasured in units called COULOMBS(C). The electron carries - 1.6 x 10-19C. The proton carries +1.6 x 10-19C.If 1 electron carries 1.6 x 10-19CThen the number of electrons in 1 Coulomb of Charge= 1 C1.6 x 10-19= 6.25 x 1018electrons 5. 1.1 Flowing Charges When electric charges (in particularelectrons) are made to move or flow,an Electric Current (symbol I) is said toexist. The SIZE of this current depends uponthe NUMBER OF COULOMBS ofcharge passing a given point in a givenTIME.Section of Current Carrying WireMathematically:I = Q/twhere:I = Current in Amperes (A)Q = Charge in Coulombs (C)t = Time in Seconds (s)If 1 Amp of current is flowingpast this point,then 6.25 x 1018electronspass here every second. 6. 1.2 Electric Current Electric CURRENTS usually flow alongwires made from some kind ofCONDUCTING MATERIAL, usually, butnot always, a METAL. Currents can also flow through aLiquid (electrolysis), through aVacuum (old style radio valves), orthrough a Semiconductor (ModernDiodes or Transistors). A Current can only flow around aCOMPLETE CIRCUIT. A break ANYWHERE in the circuitmeans the current stops flowingEVERYWHERE, IMMEDIATLY. The current does not get weaker as itflows around the circuit, BUTREMAINS CONSTANT. It is the ENERGY possessed by theelectrons (obtained from the battery orpower supply) which gets used up asthe electrons move around the circuit. In circuits, currents are measured withAMMETERS, which are connected inseries with the power supply.Typical Electric CircuitConnectingWiresResistor (consumesenergy)BatteryCurrentAMeasuresCurrentFlow 7. 1.3 Conventional Current vsElectron CurrentPositive Terminal Negative TerminalConventional vs Electron CurrentResistorElectron Current:Never shown onCircuit DiagramsConventional Current:Always shown onCircuit DiagramsWell before the discovery ofthe electron, electric currentswere known to exist.It was thought that thesecurrents were made up of astream of positive particles andtheir direction of movementconstituted the direction ofcurrent flow around a circuit.This meant that in a Direct Current(D.C.) circuit, the current would flowout of the POSITIVE terminal of thepower supply and into the NEGATIVEterminal.Currents of this kind are calledConventional Currents, and ALLCURRENTS SHOWN ON ALLCIRCUIT DIAGRAMS EVERYWHEREare shown as Conventional Current,as opposed to the real orELECTRON CURRENT. 8. 1.4 Voltage To make a current flow around acircuit, a DRIVING FORCE is required. This driving force is the DIFFERENCEin VOLTAGE (Voltage Drop orPotential Difference) between thestart and the end of the circuit. The larger the current needed, thelarger the voltage required to drivethat current. VOLTAGE is DEFINED as theENERGY SUPPLIED TO THE CHARGECARRIERS FOR THEM TO DO THEIRJOB ie.TRAVEL ONCE AROUND THECIRCUIT. So, in passing through a Voltage of1 Volt, 1 Coulomb of Charge picksup 1 Joule of Electrical Energy. OR A 12 Volt battery will supply eachCoulomb of Charge passingthrough it with 12 J of Energy.Mathematically;V = W/Qwhere:V = Voltage (Volts)W = Electrical Energy (Joules)Q = Charge (Coulombs)Alessandro Volta 9. 1.5 E.M.F.Voltage is measured with a VOLTMETER.The term EMF (ELECTROMOTIVE FORCE)describes a particular type of voltage.It is the VOLTAGE of a battery or powersupply when NO CURRENT is being drawn.This is called the Open Circuit Voltage ofthe battery or supplyVVoltmeterCircuit SymbolWith S closed, a current begins toflow and V drops and nowmeasures voltage available todrive the current through theexternal circuitResistorAVSV measures EMFVoltmeters are placed in PARALLEL withthe device whose voltage is beingmeasured.Voltmeters have a very high internalresistance, so they have little or no effectthe operation of the circuit to which they areattached.ResistorAV 10. 10Electronics & Photonics RevisionQuestion Type:Q1: Which one of the following statements (A to D) concerning thevoltage across the resistor in Figure 1 is true?A. The potential at point A is higher than at point B.B. The potential at point A is the same as at point B.C. The potential at point A is lower than at point B.D. The potential at point A varies in sign with time compared to thatat point B.Potential Difference 11. 1.6 Electrical EnergyThe conversion of ElectricalEnergy when a current passesthrough a circuit element (acomputer) is shown below.MathematicallyW = VQ 1,where:W = Electrical energy (Joule)V = Voltage (Volts)Q = Charge (Coulomb)Current and Charge arerelated through:Q = It.substituting for Q, inequation 1 we get:W = VItVoltage= V voltsCharges (Q) enterwith high energyCharges (Q) leavewith low energyQ Coulombs ofElectricity entercomputerQ Coulombs ofElectricity leavecomputerIn time t, W units of energy are transformed to heat and lightElectrical Energy (W) isdefined as the product of theVoltage (V) across, times theCharge (Q), passing througha circuit element (eg. a lightglobe). 12. Electronics & Photonics RevisionQuestion Type:Q2: Determine the electrical energydissipated in the 100 resistor of Figure 1 in1 second. In your answer provide the unit.Electrical EnergyA: Electrical energy W = VQ = VIt= (4.0)(40 x 10-3)(1)= 0.16 Joule 13. 1.7 Electrical Power Electrical Power is DEFINED as theTime Rate of Energy Transfer:P = W/twhere P = Power (Watts, W)W = Electrical Energy (Joule)t = Time (sec) From W = VI t we get:P = VI From Ohms Law (V = IR) [see nextchapter] we get:P = VI = I2R = V2/Rwhere: I = Current (Amps)R = Resistance (Ohms)V = Voltage (Volts)Electrical Power is sold toconsumers in units of Kilowatt-Hours. (kW.h)A 1000 W (1kW) fan heater operatingfor 1 Hour consumes 1kWh ofelectrical power.Since P = W/t or W = P x t, we can say:1 Joule = 1 Watt.secso1000 J = 1kW.secso3,600,000 J = 1 kW.houror3.6 MJ = 1 kW.h 14. 1.8 A.C. Electricity There are two basic types of currentelectricity:(a) D.C. (Direct Current) electricitywhere the current flows in onedirection only.(b) A.C. (Alternating Current) where thecurrent changes direction in aregular and periodic fashion. The Electricity Grid supplies domesticand industrial users with A.C.electricity. A.C. is favoured because:(a) it is cheap and easy to generate(b) it can be transformed; its voltagecan be raised or lowered at will bypassage through a transformer. The only large scale use of highvoltage D.C. electricity is in publictransport, ie. trams and trains.VoltageTimeVP VPtoPTA.C. ELECTRICITY - PROPERTIESVPtoP = Peak to Peak Voltagefor Domestic Supply VPtoP = 678 VT = Periodfor Domestic Supply T = 0.02 secVP = Peak Voltagefor Domestic Supply VP = 339 V 15. 1.9 R.M.S. Voltage and CurrentVt339-3390V2t1.15 x 1050 0Mean V25.8 x 104t0Mean V2240tGRAPHICAL DEVELOPMENT OF THE RMS VOLTAGE FROM AN A.C. VOLTAGEWith an A.C. supply, the average valuesfor both voltage and current = 0,so Vav and Iav cannot be used by thePower Companies to calculate theamount of electric power consumed byits customers.To get around this problem R.M.S. orRoot Mean Square values for ACvoltage and current were developed.RMS values are DEFINED as:The AC Voltage/Current whichdelivers the samevoltage/current to an electricaldevice as a numerically equalD.C. supply would deliver.An AC source operating at 240V RMS delivers the same powerto a device as a DC source of240 V.Yet, AC circuits do consume power,so a method of calculating it had tobe found. 16. 1.10 Peak versus RMS Values In AC supplies, the Peakand RMS values are relatedthrough simple formulae: For Voltage:VRMS = VP/2 For Current:IRMS = IP/2 In Australia DomesticElectricity is supplied at240 V, 50 Hz The Voltage quoted is theRMS value for the ACsupply. Thus the Peak value forvoltage isVP = VRMS x 2= 240 x 1.414= 339 VVoltage (V)Time (s)VP+339 V- 339 VVP to P240 V 17. Chapter 2 Topics covered: Resistance. Ohms Law. Resistors in Series and Parallel. Voltage Dividers Impedance Matching 18. 2.0 Resistance Electrical Resistance is a property ofALL materials, whether they beclassed as conductors, insulators orsomething in between. (ieSemiconductors) The size of the resistance dependsu

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