Electric Current: Electric Current: Charges in MotionCharges in Motion

• Lightning was found to be an Lightning was found to be an electric discharge- a gigantic electric discharge- a gigantic spark but until 1800 not much spark but until 1800 not much usefulnessusefulness

• Allesandro Volta (1745 – 1827) Allesandro Volta (1745 – 1827) invented the electric battery- invented the electric battery- from this discovery a new era from this discovery a new era opened, Today's electrical opened, Today's electrical technology is based on electric technology is based on electric currentcurrent

Volta disproves Galvani’s animal Volta disproves Galvani’s animal electricity and invents the 1electricity and invents the 1stst

batterybattery

• Scientists recognized that batteries Scientists recognized that batteries produce electricity by transforming produce electricity by transforming chemical energy into electricalchemical energy into electrical

• electrodes- 2 dissimilar metals electrodes- 2 dissimilar metals (one can be carbon). These are (one can be carbon). These are immersed in a solution known as immersed in a solution known as the…the…

• Electrolyte: today’s dry cell – this is Electrolyte: today’s dry cell – this is a powdery pastea powdery paste

• Battery refers to multiple cellsBattery refers to multiple cells

Continued…Continued…

• Terminals: two electrodes Terminals: two electrodes that stick out of the that stick out of the electrolyteelectrolyte

• A chemical reaction occurs A chemical reaction occurs that causes electrons to flow that causes electrons to flow from the negative terminal from the negative terminal to the positive. When the to the positive. When the chemical reaction stops, we chemical reaction stops, we say the battery is “dead”say the battery is “dead”

Electric CurrentElectric Current• A circuit is produced when A circuit is produced when

a continuous path a continuous path connects the terminals of a connects the terminals of a batterybattery

++ --

SIGN FOR SIGN FOR BATTERYBATTERY

Image 18-6Image 18-6Pg. 530Pg. 530

FormulaFormula

II = = ΔΔQQ

ΔΔttCurrent (amperes)= Current (amperes)=

Charge(coulombs)Charge(coulombs)

Time Time (seconds)(seconds)

Sample ProblemsSample Problems• A steady current of 2.5 A steady current of 2.5

amps flows in a wire for 4 amps flows in a wire for 4 minutes.minutes.

a)a)How much charge passed How much charge passed through a point in the through a point in the circuit?circuit?

b)b)How many electrons How many electrons would this be?would this be?

Answer!Answer!A)A) ΔQ = I ΔtΔQ = I Δt

2.5C/s (240sec) = 2.5C/s (240sec) = 600C600C

B)B) Charge of 1 electron is Charge of 1 electron is 1.6x101.6x10-19-19C so 600C would C so 600C would be…be…600/ 1.6 x 10600/ 1.6 x 10-19-19CC= 3.8 x 10= 3.8 x 102121 electrons electrons

Current FlowCurrent Flow•Electrons actually flow Electrons actually flow

from negative terminal to from negative terminal to positive; when we are positive; when we are speaking of conventional speaking of conventional current, that is from current, that is from positive to negative. In positive to negative. In liquids and gasses, both liquids and gasses, both positive and negative positive and negative charges (ions) can move.charges (ions) can move.

OHMS LAWOHMS LAW• Current is proportional to Current is proportional to

voltagevoltage• water analogy:water analogy:

– 2 pipes2 pipes1 2

Which one will Which one will let the most let the most water flow??water flow??

OHMS LAWOHMS LAW• Resistance is opposite to the Resistance is opposite to the

flow of electricityflow of electricity• R OHMS (R OHMS (Ω)Ω)• I = V/RI = V/R • Current (amps)= Voltage (volts)Current (amps)= Voltage (volts)

Resistance Resistance (ohms)(ohms)

• V = IRV = IR

Sample Problem:Sample Problem:• A small flashlight bulb A small flashlight bulb

draws 300mA from its 1.5 draws 300mA from its 1.5 volt batteryvolt batterya)a)What is the resistance of What is the resistance of

the bulb?the bulb?b)b)If the voltage dropped to If the voltage dropped to

1.2V, how much would 1.2V, how much would the current change?the current change?

ANSWER!ANSWER!A)A)R = V/I R = V/I

1.5V/ .3A = 51.5V/ .3A = 5ΩΩ

B)B)If resistance stays If resistance stays constantconstantI = V/RI = V/R1.2V/ 51.2V/ 5ΩΩ.24A.24Aa drop of 60mAa drop of 60mA

All Appliances Have All Appliances Have ResistanceResistance

• Hair dryer, TV’s, Lightbulbs, Hair dryer, TV’s, Lightbulbs, electric motors, ect.electric motors, ect.

• In many electronic devices In many electronic devices resistors are used to resistors are used to control current. They can control current. They can be less than 1 ohm to be less than 1 ohm to millions of ohmsmillions of ohms

• Symbol for resistance:Symbol for resistance:

Pg. 534

Pg. 534

Pg. 534

ResistivityResistivity• Resistance can change for Resistance can change for

a substancea substance

LowLow HighHigh

Thick Cross SectionThick Cross Section ThinThin

Short LengthShort Length LongLong Chart pg. 535Chart pg. 535

R = resistance (R = resistance (Ω)Ω)

L = length (m)L = length (m)

A = cross section area A = cross section area (m(m22))

P = Resistivity P = Resistivity (coefficient for (coefficient for

substance)substance)

R = P(L/A)R = P(L/A)

Temperature Affects Temperature Affects ResistivityResistivity

•Resistance increases Resistance increases with temperaturewith temperature

•At super low At super low temperatures, temperatures, resistance becomes resistance becomes zero- super zero- super conductingconducting

Electric PowerElectric Power• Electric energy is useful Electric energy is useful

because it can easily be because it can easily be converted into other forms of converted into other forms of energy.energy.

• Motors transform electricity Motors transform electricity into mechanical work, into mechanical work, heaters, stoves, toasters; all heaters, stoves, toasters; all transform it to thermal transform it to thermal energy, light bulbs convert it energy, light bulbs convert it to heat and light!to heat and light!

FormulasFormulasP = QV/t P = QV/t

OROR

P = IVP = IV

Power (watts)=Current Power (watts)=Current (Voltage)(Voltage)

BY SUBSTITUTIONBY SUBSTITUTION

P= IV = I (IR) = IP= IV = I (IR) = I22RR

=(V/R)V = V=(V/R)V = V22/R/R

SampleSample•Calculate the resistance Calculate the resistance

of a 40W headlight of a 40W headlight designed for a 12V designed for a 12V automobileautomobileR = R =

VV22/P/P= (12V)= (12V)22/ / 40W40W

= 3.6Ω= 3.6Ω

SampleSample• An electric heater draws 15A on An electric heater draws 15A on

a 120V line. How much power a 120V line. How much power does it use and how much does does it use and how much does it cost per month (30 days) if it it cost per month (30 days) if it operates 3 hours per day and operates 3 hours per day and the electric company charges the electric company charges 10.5 cents per kWh?10.5 cents per kWh?P = IV = 15A(120V) = P = IV = 15A(120V) =

1800Watts1800Watts1.8 kW1.8 kW

30 days (3hr/ day) = 90hrs30 days (3hr/ day) = 90hrs

1.8 kW (90hrs)($.105)= $171.8 kW (90hrs)($.105)= $17

SampleSample• A typical lighting bolt can A typical lighting bolt can

transfer 10transfer 1099J of energy across J of energy across a potential difference of 5x10a potential difference of 5x1077 Volts during a time interval .2 Volts during a time interval .2 seconds. Calculate charge seconds. Calculate charge transferred, current, average transferred, current, average power over the time interval.power over the time interval.Q = 10Q = 1099JJ

5x10 5x1077VV

= 20C= 20C

I = Q/tI = Q/t20C20C

.2sec.2sec=100A=100A

P = 10P = 1099JJ .2sec .2sec= 5x10= 5x1099WW5 Giga Watts5 Giga Watts

Power in Household Power in Household CircuitsCircuits

• SAFETY:SAFETY:– Thickness in wiresThickness in wires– FusesFuses– Circuit BreakersCircuit Breakers– ShortsShorts– FiresFires– GFCI outlet near a water GFCI outlet near a water

source in your homesource in your home

Will a Fuse Blow?Will a Fuse Blow?Find the total current drawn by Find the total current drawn by

all the devicesall the devices

Sum of Power = Sum of Power =

100 + 1800+ 350 + 1200100 + 1800+ 350 + 1200

Voltage = 120VVoltage = 120V

I = P/V 3450W/ 120=I = P/V 3450W/ 120=

28.8A28.8A

this would blow a 20 amp fuse!this would blow a 20 amp fuse!

AC DCAC DC• Batteries move electrons in Batteries move electrons in

one direction DCone direction DC• Electric generators at power Electric generators at power

plants produce alternating plants produce alternating current ACcurrent AC

• This type changes the This type changes the direction of electron flow direction of electron flow many times per second (In US many times per second (In US 60Hz – In Europe 50Hz)60Hz – In Europe 50Hz)

• The AC voltage is sinusoidalThe AC voltage is sinusoidal

Electrons Moving in a Electrons Moving in a WireWire

• When we flip a light switch- When we flip a light switch- there's light!there's light!

• Do electrons move fast in a wire?Do electrons move fast in a wire?• - NO it only appears that way- NO it only appears that way

Add Add WaterWater

Over Over Flow Flow WaterWater

Electron drift speed is .05mm/sec or 5½ hrs to go Electron drift speed is .05mm/sec or 5½ hrs to go 1m1m

CHAPT. 19CHAPT. 19DC CircuitsDC Circuits

•Electric circuits are Electric circuits are everywhere- cars, everywhere- cars, homes, schools, homes, schools, businesses, radios, businesses, radios, TVs and computersTVs and computers

Basic SymbolsBasic SymbolsBatteryBattery

CapacitorCapacitor

ResistorResistor

Wire with negligible resistanceWire with negligible resistance

SwitchSwitch

Resistance in Series Resistance in Series and Paralleland Parallel

Series: one pathSeries: one path

Parallel: multiple Parallel: multiple pathspaths

Series CircuitSeries Circuit• RRtottot=R=R11+R+R22+R+R33+…+…

• The Current through each The Current through each resistance is the same: resistance is the same: I I

tottot=I=I11=I=I22=…=…

• The Voltage across each The Voltage across each resistance splits up proportionally resistance splits up proportionally among the R’s. among the R’s. VVtottot=V=V11+V+V22+V+V33……

Parallel CircuitParallel Circuit• Current from the source splits Current from the source splits

into separate branchesinto separate branchesWiring in businesses and houses are like Wiring in businesses and houses are like this- that way if one bulb burns out, the this- that way if one bulb burns out, the

rest stay on!rest stay on!

Parallel CircuitParallel Circuit Current splits up among the resistance Current splits up among the resistance

so they add…so they add…

IItottot = I = I11 + I + I22 + I + I33

• Voltage is the same for eachVoltage is the same for each resistorresistor VVtot=tot=VV1=1=VV2=2=VV33

• But I = V/RBut I = V/R

• SO resistors in parallel add this way:SO resistors in parallel add this way:

1/R1/Rtottot = 1/R = 1/R11 + 1/R + 1/R22 + 1/R + 1/R33

ExampleExample•Two 8Two 8Ω resistors Ω resistors in parallel and in parallel and have a Rhave a Rtottot of 4Ω of 4Ω

1/R1/Rtottot = 1/8Ω + 1/8Ω = 2/8Ω = = 1/8Ω + 1/8Ω = 2/8Ω = 1/4Ω1/4Ω

RRtottot = 4Ω = 4Ω

Water analogy Dual water Water analogy Dual water pipepipe

Water pipes in parallel- analogy to Water pipes in parallel- analogy to electric currents in parallelelectric currents in parallel

Example 19-1Example 19-1

EMFEMF• Electro Motive Force (not a true Electro Motive Force (not a true

force- not measured in Newtons)force- not measured in Newtons)• This is the potential difference This is the potential difference

across the terminals of a battery across the terminals of a battery (generator) when NO current (generator) when NO current flows into an external circuit.flows into an external circuit.

• Battery Voltage isn't constant Battery Voltage isn't constant (Start your car, headlights dim) (Start your car, headlights dim) Battery can’t supply current fast Battery can’t supply current fast enough to maintain the full EMFenough to maintain the full EMF

EMFEMF•Batteries have their own Batteries have their own

internal resistance internal resistance designated rdesignated r

•Terminal Voltage is what Terminal Voltage is what we measure across the we measure across the terminals of a battery when terminals of a battery when no circuit flowsno circuit flowsTerminal Voltage = VTerminal Voltage = Vabab = EMF - Ir = EMF - Ir

Kirchoff’s RulesKirchoff’s Rules• Deals with complicated circuitsDeals with complicated circuits• Junction RuleJunction Rule (conservation of (conservation of

charge): At any junction point, charge): At any junction point, the sum of all currents entering the sum of all currents entering the junction must equal the sum the junction must equal the sum of all currents leaving the of all currents leaving the junction.junction.

• Loop RuleLoop Rule (conservation of (conservation of energy): The sum of the changes energy): The sum of the changes in potential around any closed in potential around any closed path of a circuit must be zero.path of a circuit must be zero.

Electric HazardsElectric Hazards• The severity of electric shock The severity of electric shock

depends on the magnitude of depends on the magnitude of the current. Also how long it the current. Also how long it acts and through what part of acts and through what part of the body it passes.the body it passes.

• Heart and Brain are most Heart and Brain are most criticalcritical

• Electric current heats up Electric current heats up tissue and causes burnstissue and causes burns

Electric HazardsElectric Hazards• The severity of shock depends on The severity of shock depends on

the effective resistance of a body. the effective resistance of a body. Living tissue has quite low Living tissue has quite low resistance (cells contain ions) and is resistance (cells contain ions) and is a good conductor. However, the a good conductor. However, the outer layer of skin, when dry, offers outer layer of skin, when dry, offers a high resistance 10a high resistance 1044 to 10 to 1066 ohms. ohms.

• When the skin is wet, resistance When the skin is wet, resistance can be 10can be 1033 Ohms or less, this allows Ohms or less, this allows for more current to flow and a more for more current to flow and a more severe shocksevere shock

GroundingGrounding

A) An electric oven operating normally with 2 prong plug

B) Short to the case with ungrounded case: shock!

C) Short to the case with the case grounded with Third prong.

Circuit BreakersCircuit Breakers

Birds and squirrels Birds and squirrels don’t get shocked?don’t get shocked?