Electric Potential and Electric Potential and Electric CircuitsElectric Circuits

Electric PotentialElectric Potential

Total electrical potential energy divided by Total electrical potential energy divided by the chargethe chargeElectric potential = Electric potential = Electric potential energyElectric potential energy

ChargeCharge

VOLTVOLT

SI unit for electric SI unit for electric potential – VOLTpotential – VOLT Named after Allesandro Named after Allesandro

Volta (1745-1827)Volta (1745-1827) 1 volt = 1 joule/coulomb1 volt = 1 joule/coulomb

VoltageVoltage

Same as voltsSame as voltsOne volt takes 1 joule of energy per One volt takes 1 joule of energy per coulomb to add it to the conductorcoulomb to add it to the conductorRub a balloon in your hairRub a balloon in your hair It may take several thousand voltsIt may take several thousand volts HOWEVER – it is very few coulombs – less HOWEVER – it is very few coulombs – less

than one millionth than one millionth SO – the energy it very lowSO – the energy it very low

CapacitorsCapacitors

Electrical storageElectrical storageUsesUses In computers to In computers to

store “1” and “0”store “1” and “0” In keyboardsIn keyboards Photoflash bulbPhotoflash bulb Giant lasersGiant lasers Electric fencesElectric fences

Simple Capacitors Simple Capacitors

Conducting plates separated by a small Conducting plates separated by a small distancedistanceConnected to battery which moves Connected to battery which moves electrons from battery to plateelectrons from battery to plateComplete when voltage on plates = Complete when voltage on plates = battery voltagebattery voltageAdvantage – Discharged in large quantity Advantage – Discharged in large quantity Can be dangerous – even when Can be dangerous – even when appliances are not powered.appliances are not powered.

FormulaFormula

C= Q/VC= Q/V C – capacitance (Farads or Coulombs/Volt)C – capacitance (Farads or Coulombs/Volt) Q – charge (coulombs)Q – charge (coulombs) V – voltage difference (volts)V – voltage difference (volts)

Electric TermsElectric TermsVoltage – electric Voltage – electric “pressure”“pressure”Current – flow of chargeCurrent – flow of chargeResistance – Restrains Resistance – Restrains the electron flowthe electron flowDC (Direct current) – DC (Direct current) – flows in one directionflows in one directionAC (Alternating current) – AC (Alternating current) – flows back and forthflows back and forthPower – Rate at which Power – Rate at which energy is transferredenergy is transferred

Flow of ChargeFlow of Charge

When ends of electric conductor When ends of electric conductor (example-wire) have different potential (example-wire) have different potential energy the charge flowsenergy the charge flowsFlow continues until it reaches the same Flow continues until it reaches the same potentialpotentialExamplesExamples Shock Shock FlashlightFlashlight

Electric CurrentElectric Current

Flow of electric chargeFlow of electric charge Electrons carry the chargeElectrons carry the charge Protons are bound to atomsProtons are bound to atoms In solutions, ions can carry the chargeIn solutions, ions can carry the charge

Measured in amperes (amps)Measured in amperes (amps) SI unit (A)SI unit (A) Amps = 1 coulomb/secondAmps = 1 coulomb/second

Example - 5 amps wire carries 5 coulombs of charge pass a Example - 5 amps wire carries 5 coulombs of charge pass a point in 1 secondpoint in 1 second

When there is no current, the wire has no When there is no current, the wire has no chargecharge

Formula for CurrentFormula for Current

I = I = ΔΔQ/Q/ΔΔtt I – current (amperes or amps)I – current (amperes or amps) Q – charge (coulombs)Q – charge (coulombs) T – time (seconds)T – time (seconds)

1 amp = 1 coulomb/second1 amp = 1 coulomb/second

Voltage SourcesVoltage Sources

Electron pumpElectron pumpNeeds to slowly release the Needs to slowly release the electronselectrons Not all at once – like in a shockNot all at once – like in a shockSteady flowSteady flow Dry cells – chemical energyDry cells – chemical energy Wet cells – chemical energyWet cells – chemical energy Generator – converts Generator – converts

mechanical energy mechanical energy Voltage provides “pressure” Voltage provides “pressure” to move electrons between to move electrons between terminalsterminals

GeneratorsGenerators

Average home outlets are Average home outlets are ACACPressure of 120 voltsPressure of 120 volts This means there is 120 This means there is 120

joules of energy forcing the joules of energy forcing the coulombs through the wirecoulombs through the wire

Voltage does not FLOW Voltage does not FLOW through the wirethrough the wire The electrons are pushed by The electrons are pushed by

the voltagethe voltage

Electric ResistanceElectric Resistance

Current depends onCurrent depends on VoltageVoltage ResistanceResistance

Resistance – slows the flowResistance – slows the flow Depends onDepends on

Conductivity of material – silver bestConductivity of material – silver bestThickness – more thick, less resistanceThickness – more thick, less resistanceLength of wire - longer, more resistanceLength of wire - longer, more resistanceTemperature – hotter, more resistanceTemperature – hotter, more resistance

Drawing CurrentsDrawing Currents

Ohm’s LawOhm’s Law

Current in a circuit is directly Current in a circuit is directly proportional to the voltage and proportional to the voltage and inversely proportional to the resistanceinversely proportional to the resistance Current = voltage/resistance Current = voltage/resistance

I = V/RI = V/R Units Units 1 ampere = 1 volt/ohm 1 ampere = 1 volt/ohmDouble the voltage – double the Double the voltage – double the currentcurrentIncrease the resistance – decrease Increase the resistance – decrease the currentthe current

Typical resistanceTypical resistanceCord – less than 1 ohmCord – less than 1 ohm100 W light bulb – 100 ohms100 W light bulb – 100 ohmsIron – 15 ohmsIron – 15 ohmsElectric toaster – 15-20 ohmsElectric toaster – 15-20 ohms Low resistance = larger current which Low resistance = larger current which

produces lots of heatproduces lots of heat

Ohm’s Law FormulaOhm’s Law Formula

V=IRV=IR V – voltage (volts)V – voltage (volts) I – Current (amps)I – Current (amps) R – resistance (ohms – R – resistance (ohms – ΩΩ))

ResistorsResistors

Radios and TV electronicsRadios and TV electronicsCurrent is regulated by resistorsCurrent is regulated by resistors Ranges from few ohms to millionsRanges from few ohms to millions

QuestionQuestion

What is the resistance of an electric frying What is the resistance of an electric frying pan that draws a 12 ampere current when pan that draws a 12 ampere current when connected to a 120-volt circuit?connected to a 120-volt circuit?R = V/I = 120 V/12 A = 10 R = V/I = 120 V/12 A = 10 ΩΩHow much current is drawn by a lamp that How much current is drawn by a lamp that has a resistance of 100 has a resistance of 100 ΩΩ when a voltage when a voltage of 50 V is impressed across it?of 50 V is impressed across it?I = V/R = 50 V/ 100 I = V/R = 50 V/ 100 ΩΩ = 0.5 amps = 0.5 amps

Electric ShockElectric Shock

What effects us – current or voltage?What effects us – current or voltage?Damage comes from current through the Damage comes from current through the bodybody Current depends on voltage and resistance in Current depends on voltage and resistance in

bodybody Resistance varies from 100 Resistance varies from 100 ΩΩ (covered in salt (covered in salt

water) to 500,000 water) to 500,000 ΩΩ (very dry skin) (very dry skin)Usually can’t feel 12 volts and 24 volts Usually can’t feel 12 volts and 24 volts would tingle would tingle If wet – 24 volts could be very uncomfortableIf wet – 24 volts could be very uncomfortable

Effect of Various Electric CurrentEffect of Various Electric CurrentCurrent in AmpsCurrent in Amps EffectEffect

0.0010.001 Can be feltCan be felt

0.0050.005 PainfulPainful

0.0100.010 SpasmsSpasms

0.0150.015 Loss of muscle controlLoss of muscle control

0.0700.070 Probably fatal (if through the Probably fatal (if through the heart) for more than one heart) for more than one

secondsecond

QuestionsQuestions

If resistance of your body were 100,000 If resistance of your body were 100,000 ohms, what would be the current in your ohms, what would be the current in your body when you touched the terminals of a body when you touched the terminals of a 12-volt battery?12-volt battery?Current = Current = V/R = 12 v/100,000 V/R = 12 v/100,000 ΩΩ = 0.00012 A = 0.00012 AIf your skin were wet, your resistance If your skin were wet, your resistance would be 1000 would be 1000 ΩΩ, what would you feel on , what would you feel on a 24 volt battery?a 24 volt battery?Current = 24 V/ 1000 Current = 24 V/ 1000 ΩΩ = 0.024 A = 0.024 A

ShockedShocked

120 V in house120 V in house normally our shoes provide resistancenormally our shoes provide resistance Standing barefoot in bathtub – very little Standing barefoot in bathtub – very little

resistance to ground – don’t use appliances in resistance to ground – don’t use appliances in bathtub!!bathtub!!

Birds sitting on wireBirds sitting on wire Same potential on both sides of bodySame potential on both sides of body Only get it if they touch wires with different Only get it if they touch wires with different

potentialpotential

GroundedGrounded

Most appliances are groundedMost appliances are grounded Helps prevent potential differencesHelps prevent potential differences All ground wires are connected togetherAll ground wires are connected together Provides a direct route to the groundProvides a direct route to the ground

What causes shock – current or voltage?What causes shock – current or voltage? The initial cause is the voltage, but the current The initial cause is the voltage, but the current

does the damagedoes the damage

Types of currentTypes of current

DC – Direct currentDC – Direct current Flow of charge is always in one directionFlow of charge is always in one direction Source – batteries (wet or dry)Source – batteries (wet or dry) Move from negative terminal to positiveMove from negative terminal to positive

AC – Alternating currentAC – Alternating current Charge moves back and forth in the lineCharge moves back and forth in the line In North America – frequency is 60 cycles per In North America – frequency is 60 cycles per

second – 60 Hzsecond – 60 Hz

VoltageVoltage

North America North America Small appliances – 110-120 voltsSmall appliances – 110-120 volts Large appliances – 220 voltsLarge appliances – 220 volts

Obtain by “combining voltage”Obtain by “combining voltage”

Europe – 220 volts (more efficient)Europe – 220 volts (more efficient)Electrons do NOT travel from power Electrons do NOT travel from power plants, they oscillate in placeplants, they oscillate in place They sell energy, you provide the electrons in They sell energy, you provide the electrons in

your wiringyour wiring

DiodeDiode

One way valve in a circuitOne way valve in a circuitOnly allows electrons to move one wayOnly allows electrons to move one way Can be used to convert AC to DC currentCan be used to convert AC to DC current

PowerPower

Rate at which electric energy is converted Rate at which electric energy is converted to another energy formto another energy formElectric power = current x voltageElectric power = current x voltage Units – 1 watt = 1 amp x 1 voltUnits – 1 watt = 1 amp x 1 volt 1 kilowatt = 1000 watts1 kilowatt = 1000 watts 1 kilowatt hour = amount of energy consumed 1 kilowatt hour = amount of energy consumed

in 1 hour at rate of 1 kilowattin 1 hour at rate of 1 kilowatt

CalculationsCalculations

How much power is used by a calculator How much power is used by a calculator that operates on 8 volts and 0.1 amps? If that operates on 8 volts and 0.1 amps? If it is used for one hour, who much energy it is used for one hour, who much energy does it use?does it use?Power = A x V = 0.1 A x 8 V = 0.8 WPower = A x V = 0.1 A x 8 V = 0.8 WEnergy = Power x time = 0.8 W x 1 hour = Energy = Power x time = 0.8 W x 1 hour = 0.8 watt-hours or 0.0008 kilowatt-hours0.8 watt-hours or 0.0008 kilowatt-hoursFYI – Speed of e- in wire = 0.01 cm/sFYI – Speed of e- in wire = 0.01 cm/s

Lighting a bulb – which one works?Lighting a bulb – which one works?

Must have a closed loop!!

Electric CircuitsElectric Circuits

NeedNeed PathwayPathway Voltage source (battery or AC)Voltage source (battery or AC) Resistance (light, fan, bell, etc)Resistance (light, fan, bell, etc) Optional Optional

switch – stops and starts currentswitch – stops and starts currentCapacitor – hold a large voltage for quick releaseCapacitor – hold a large voltage for quick releaseDiode – one-way valveDiode – one-way valveTransformer and capacitors– controls current flowTransformer and capacitors– controls current flow

Types of CircuitsTypes of Circuits

Series – single Series – single pathway for all pathway for all electrons to flowelectrons to flow

Parallel – branched Parallel – branched pathway for electrons pathway for electrons to flowto flow

Series circuitSeries circuit

When switch is closed, current runs When switch is closed, current runs through all lamps in same paththrough all lamps in same pathOpen – when one break occurs, all flow Open – when one break occurs, all flow stopsstops Break from switch, broken wire, burned out Break from switch, broken wire, burned out

light-bulblight-bulb

Series Circuit Features Series Circuit Features

Single pathway – same currentSingle pathway – same currentTotal resistance = sum of individual Total resistance = sum of individual resistanceresistanceCurrent = voltage/ total resistance (Ohms Current = voltage/ total resistance (Ohms law)law)Voltage drop across each device depends Voltage drop across each device depends on resistance of the deviceon resistance of the device

Disadvantage of SeriesDisadvantage of Series

If one device fails, the whole circuit ceasesIf one device fails, the whole circuit ceases

Parallel CircuitsParallel Circuits

Devices are connected to the same points Devices are connected to the same points in the circuitin the circuitEach resistor has its own path to the Each resistor has its own path to the voltage sourcevoltage sourceIf one device fails, it does not interupt the If one device fails, it does not interupt the other devicesother devices

Features of Parallel CircuitsFeatures of Parallel Circuits

Each device connects directly to the voltage Each device connects directly to the voltage source. Therefore the voltage is the same source. Therefore the voltage is the same across each device.across each device.Total current divides between devices. The one Total current divides between devices. The one with the lowest resistance gets the most current with the lowest resistance gets the most current (Ohm’s law)(Ohm’s law)Total current = sum of current in the devicesTotal current = sum of current in the devicesAs number of devices increases, resistance As number of devices increases, resistance decreasesdecreases

Schematic DiagramsSchematic DiagramsResistance

Battery (2)

http://www.rkm.com.au/ANIMATIONS/animation-electrical-circuit.html

Combined Compound CircuitsCombined Compound Circuits

Calculating Current in Compound Calculating Current in Compound CircuitsCircuits

When resistors are inWhen resistors are in SeriesSeries

The resistance equal sum of devicesThe resistance equal sum of devices -----^^^^^-----^^^^^^------- = -----^^^^^-----------^^^^^-----^^^^^^------- = -----^^^^^------

88ΩΩ 8 8ΩΩ 16 16 ΩΩ Parallel (2 devices with same resistance)Parallel (2 devices with same resistance)

The resistance equal half of one deviceThe resistance equal half of one device ----^^^^^------ = ----^^^^----- ----^^^^^------ = ----^^^^-----

88ΩΩ 4 4 ΩΩ ----^^^^^-----------^^^^^------- 88ΩΩ

Another ExampleAnother Example ----^^^^^---------^^^^^-----

88ΩΩ ----^^^^^--------^^^^^---- ----^^^^^-----^^^^^-- ----^^^^^-----^^^^^--

88ΩΩ 88ΩΩ 4 4ΩΩ ----^^^^^---------^^^^^----- 88ΩΩ

---^^^^^----- ---^^^^^----- 1212ΩΩ

OverloadingOverloading

The more devices on a line, the more The more devices on a line, the more current it draws as resistance lowerscurrent it draws as resistance lowersOverloaded – line carrying more than a Overloaded – line carrying more than a safe amount of currentsafe amount of currentFuses – put in a current in SERIESFuses – put in a current in SERIES Cuts the line if it overheatsCuts the line if it overheats Overheating caused byOverheating caused by

Short circuit (cuts the resistance)Short circuit (cuts the resistance)Too many devices on a lineToo many devices on a line