Potential Difference =Voltage=EMFIn a battery, a series of chemicalreactions occur in whichelectrons are transferred fromone terminal to another. There isa potential difference (voltage)between these poles.The maximum potentialdifference a power source canhave is called the electromotiveforce or (EMF), . The term isntactually a force, simply theamount of energy per charge(J/C or V)
A Basic CircuitAll electric circuits have three main parts1. A source of energy2. A closed path3. A device which uses the energyIf ANY part of the circuit is open the device will not work!
Electricity can be symbolic of FluidsCircuits are very similar to water flowing through a pipeA pump basically works on twoimportant principles concerningits flow• There is a pressure differencewhere the flow begins and ends• A certain amount of flow passeseach second.A circuit basically works on twoimportant principles• There is a "potential differenceaka voltage" from where thecharge begins to where it ends• The amount of charge that flowsper second is called current.
CurrentCurrent is defined as the rate at which chargeflows through a surface.The current is in the same direction as theflow of positive charge (for this course)Note: The “I” standsfor intensity
There are 2 types of CurrentDC = Direct Current - current flows in one directionExample: BatteryAC = Alternating Current- current reverses direction many times per second.This suggests that AC devices turn OFF andON. Example: Wall outlet (progress energy)
Ohm’s Law“The voltage (potential difference, emf) is directlyrelated to the current, when the resistance isconstant”IRIRVRRIVResistancealityproportionofconstantVoltage vs. Current0123456789100 0.2 0.4 0.6 0.8 1Current(Amps)Voltage(V)Voltage(V)Since R= V/I, the resistance is theSLOPE of a V vs. I graph
ResistanceResistance (R) – is defined as the restriction of electronflow. It is due to interactions that occur at the atomicscale. For example, as electron move through aconductor they are attracted to the protons on thenucleus of the conductor itself. This attraction doesn’tThe unit for resistance is theOHM,stop the electrons, justslow them down a bit andcause the system towaste energy.
Electrical POWERWe have already learned that POWER is the rate at which work(energy) is done. Circuits that are a prime example of this asbatteries only last for a certain amount of time AND we getcharged an energy bill each month based on the amount ofenergy we used over the course of a month…aka POWER.
POWERIt is interesting to see how certain electricalvariables can be used to get POWER. Let’stake Voltage and Current for example.
Other useful power formulasThese formulas canalso be used! Theyare simply derivationsof the POWERformula with differentversions of Ohms lawsubstituted in.
Ways to Wire CircuitsThere are 2 basic ways to wire a circuit. Keep inmind that a resistor could be ANYTHING ( bulb,toaster, ceramic material…etc)Series – One after anotherParallel – between a set of junctions andparallel to each other
Schematic SymbolsBefore you begin to understand circuits you need to be able todraw what they look like using a set of standard symbolsunderstood anywhere in the worldFor the battery symbol, theLONG line is considered to bethe POSITIVE terminal and theSHORT line , NEGATIVE.The VOLTMETER and AMMETERare special devices you place INor AROUND the circuit tomeasure the VOLTAGE andCURRENT.
The Voltmeter and AmmeterThe voltmeter and ammeter cannot bejust placed anywhere in the circuit. Theymust be used according to theirDEFINITION.Since a voltmeter measures voltage orPOTENTIAL DIFFERENCE it must beplaced ACROSS the device you wantto measure. That way you can measurethe CHANGE on either side of thedevice.Voltmeter is drawn ACROSS the resistorSince the ammeter measures the current orFLOW it must be placed in such a way as thecharges go THROUGH the device.Current goes THROUGH the ammeter
Simple CircuitWhen you are drawing acircuit it may be a wisething to start by drawingthe battery first, thenfollow along the loop(closed) starting withpositive and drawingwhat you see.
Series CircuitIn in series circuit, theresistors are wired one afteranother. Since they are allpart of the SAME LOOPthey each experience theSAME AMOUNT of current.In figure, however, you seethat they all existBETWEEN the terminals ofthe battery, meaning theySHARE the potential(voltage).321)(321)(VVVVIIIITotalseriesTotalseries
Series Circuit321)(321)(VVVVIIIITotalseriesTotalseriesAs the current goes through the circuit, the charges must USE ENERGY to getthrough the resistor. So each individual resistor will get its own individual potentialvoltage). We call this VOLTAGE DROP.isseriesseriesTTTotalseriesRRRRRRRIRIRIRIIRVVVVV321332211321)()(; Note: They may use theterms “effective” or“equivalent” to meanTOTAL!
ExampleA series circuit is shown to the left.a) What is the total resistance?b) What is the total current?c) What is the current across EACHresistor?d) What is the voltage drop acrosseach resistor?( Apply Ohms lawto each resistor separately)R(series) = 1 + 2 + 3 = 6V=IR 12=I(6) I = 2AThey EACH get 2 amps!V1 2 V V3 =(2)(3)= 6V V2 =(2)(2)= 4VNotice that the individual VOLTAGE DROPS add up to the TOTAL!!
Parallel CircuitIn a parallel circuit, wehave multiple loops. Sothe current splits upamong the loops with theindividual loop currentsadding to the totalcurrentIt is important to understand that parallelcircuits will all have some positionwhere the current splits and comes backtogether. We call these JUNCTIONS.The current going IN to a junction willalways equal the current going OUT of ajunction.Junctions OUTINTotalparallelIIIIII:JunctionsRegarding321)(
Parallel CircuitNotice that the JUNCTIONS both touch thePOSTIVE and NEGATIVE terminals of thebattery. That means you have the SAMEpotential difference down EACH individualbranch of the parallel circuit. This meansthat the individual voltages drops are equal.This junctiontouches thePOSITIVEterminalThis junctiontouches theNEGATIVEterminalViPPParallelTTTotalparallelTotalparallelRRRRRRRVRVRVRVIRVIIIIVVVV111111)(;321332211321)(321)(
Example454.01454.019171511PpPRRR)(8 RIIRVTo the left is an example of a parallel circuit.a) What is the total resistance?b) What is the total current?c) What is the voltage across EACH resistor?d) What is the current drop across each resistor?(Apply Ohms law to each resistor separately)2.203.64 A8 V each!987858975 IIIIRV1.6 A 1.14 A 0.90 ANotice that theindividual currentsADD to the total.
Compound (Complex) CircuitsMany times you will have series and parallel in the SAME circuit.Solve this type of circuitfrom the inside out.WHAT IS THE TOTALRESISTANCE?3.1133.33803.33;50110011sPPRRR
Compound (Complex) Circuits3.1133.33803.33;50110011sPPRRRTTTTTIIRIV)3.113(120Suppose the potential difference (voltage) is equal to 120V. What is the totalcurrent?1.06 AWhat is the VOLTAGE DROP across the 80 resistor?8080808080)80)(06.1(VVRIV84.8 V
Compound (Complex) CircuitsAIVVAIVVRTTT06.18.8406.11203.11380803&23&23&21)(32)(8.84120VVVVVVVVseriesTparallelTWhat is the VOLTAGE DROP acrossthe 100 and 50 resistor?35.2 V Each!502.351002.35501003&21)(32)(IIIIIIIIseriesTparallelTWhat is the current across the100 and 50 resistor?0.352 A0.704 AAdd to1.06A
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