2Circuit Basics All electrical circuits require three elements. A source voltage, that is, an electron pump usuallya battery or power supply.[ ENERGY IN](2) A conductor to carry electrons from and to the voltagesource (pump). The conductor is often a wire.[ENERGY TRANSFER](3) A load or resistance. A point where energy is extractedform the circuit in the form of heat, light, motion, etc.[ENERGY OUT]
3THE MOST BASIC ELECTRICAL CIRCUIT LOAD(RESISTANCE)[ENERGY OUT]ELECTRONSINTOLOADELECTRONSOUT OF LOADCONDUCTORCONDUCTORELECTRONSOUT OF SOURCEELECTRONPUMP(SOURCE VOLTAGE)[ENERGY IN]ELECTRONSBACK TOSOURCEHIGHER ENERGY ELECTRONSLOWER ENERGY ELECTRONS
4Potential Changes of Current in a Circuit Resistance(Potential Drop)LowEnergycurrentHighEnergycurrentVoltage Source(Potential Rise)LowEnergycurrentHighEnergycurrent
5Potential Rise Across a Power Source voltscurrentcurrentBatteryElectrons haveMore EnergyElectrons haveLess EnergyElectrons getAn energy boost
6Potential Drop Across a Resistor voltscurrentResistorElectrons haveLess EnergyElectrons haveMore EnergyEnergy is lostIn the resistor
7MEASUREABLE QUANTITIES THAT CAN BE OBTAINED FROM AN ELECTRICAL CIRCUIT (1) VOLTAGE RISE – MEASURES THE ENERGY GIVEN TO ELECTRONS ASTHEY LEAVE A VOLTAGE SOURCE. IT IS MEASURED IN VOLTS (+)(2) VOLTAGE DROP – MEASURES THE ENERGY LOST BY TO ELECTRONSWHEN THEY LEAVE A RESISTANCE. IT IS MEASURED IN VOLTS (-)(3) CURRENT – MEASURES THE FLOW RATE THROUGH A CONDUCTOR.IT IS MEASURED IN AMPERES (AMPS)(4)RESISTANCE – MEASURES THE OPPOSITION TO CURRENT FLOWTHROUGH A CONDUCTOR OR RESISTORIT IS MEASURED IN OHMS (ITS SYMBOL IS OMEGA )
8ELECTRICAL QUANTITIES 6.25 x 10 18electrons1 coulomb1 coulomb1 second1 amp = 1 coul / sec1 volt = 1 joule / coul1 joule1 coulomb of charge
9Electrical Meters volts Ammeters measure current in amperes and are alwayswired in series inthe circuit.AMPSVoltmeters measurepotential in voltsand are alwayswired in parallelin the circuit.volts
12Electrical Symbols A V battery + - junction wiring terminal voltmeter AC generatorammeterAVariableresistanceresistanceVariablecapacitorcapacitor
13Relationships Among Electrical Quantities in a Circuit Measure electrical quantities in an electricalCircuit are related to eachother by:OHM’S LAWOHM’S LAW says, if the source voltage remains constant,increasing the resistance in a circuit will cause a decreasein current flow in that circuit.In mathematical terms it tells us that current flow isinversely proportional to resistance.In equation form it says:Voltage (V) = Current (I) x Resistance (R)orV(in volts) = I (in amps) x R (in ohms)
14Ohm's Law V = I R Potential In volts (joules / coul) Current In amperes(coul / second)ResistanceIn ohms(volts / amp)Drop across aresistanceCurrent passingThrough theresistor
15Voltage vs Current for a Constant Resistance (volts)Current (I) in ampsIvThe slope of theLine gives theresistanceR = v /Ior rearrangedv = I x R
16Voltage Sources and Internal Resistance All voltage sources contain internal resistance, thatis resistance that is part of the voltage producingdevice itself which cannot be eliminated.The voltage that the device (battery for example)could produce if no internal resistance was presentis called its EMF.EMF stands for electromotive force – the forcethat moves the electrons.The useable voltage which is available to the circuitafter the internal resistance consumes its shareof the EMF is called The terminal voltage.
17Electromotive Force (EMF) and Terminal Voltage riseVoltagedropconventionalcurrentflowEMF of SourceInternalresistance+-Terminalvoltage
18terminal voltage = EMF (+) + Internal resistance loss (-) Since voltage rise across a source orVoltage drop across a resistance can beCalculated by Ohm’s LawV= IRAnd the above equation becomesV terminal = EMF – I x R internalNote that if R internal is very small then a largePercentage of the EMF is available to the circuit.Also note that if I, the current is very large thena large percentage of the EMF is consumed withinthe battery itself which can cause overheating and failure.This is generally the result of a short circuit.
19Internal Resistance and EMF Net VoltageRise of cell(Terminal VoltagevoltscurrentcurrentBatteryRiEMF-V+VVoltage dropDue to internalresistanceVoltage riseDue to EMF