Physical SciencePhysical Science

Electricity and Electricity and MagnetismMagnetism

Slides subject to changeSlides subject to change

Electric ChargeElectric Charge

Charge is a fundamental quantity (just as Charge is a fundamental quantity (just as massmass is is a fundamental quantity).a fundamental quantity).

Positive and negative charges. Positive and negative charges. Charge is measured in coulombs (C).Charge is measured in coulombs (C). Elementary charge Elementary charge ee = 1.6x10 = 1.6x10––1919 coulombs. coulombs. At atomic levelAt atomic level

protons +protons +ee electrons electrons −−ee neutron neutron no chargeno charge

Atomic LevelAtomic Level

Most atoms have equal number of Most atoms have equal number of electrons and protons. electrons and protons.

Hydrogen 1 proton, 1 electronHydrogen 1 proton, 1 electron Uranium 92 protons, 92 electronsUranium 92 protons, 92 electrons If unbalanced number of charges, atom is If unbalanced number of charges, atom is

called an “ion.” called an “ion.”

NaNa++ ClCl––

Sodium ion, one less Sodium ion, one less electron than usualelectron than usual

Chlorine ion, one Chlorine ion, one more electron than more electron than

usualusual

Forms sodium Forms sodium chloride, “salt”chloride, “salt”

ExerciseExercise

If an object has 1 million more electrons If an object has 1 million more electrons than protons. What is the than protons. What is the netnet charge? charge?

GivenGiven FormulaFormula

NN = 1x10 = 1x1066 excess electronsexcess electrons qq = = NeNe

ee = charge per electron = charge per electron

= = ––1.6x101.6x10–19 –19 CC

qq = = NeNe = (1x10 = (1x1066)()(––1.6x101.6x10–19–19))

= –1.6x10= –1.6x10–13 –13 CC

Static ElectricityStatic Electricity

Excess charge.Excess charge. zzzzzt; crackle!zzzzzt; crackle!

Benjamin Franklin

May 10, 1752 Thomas-François Dalibard of France conducted Franklin's experiment using a 40-foot-tall iron rod instead of a kite, and he extracted electrical sparks from a cloud.

In 1750 Franklin proposed an experiment to prove that lightning is electricity—fly a kite in a storm that appeared capable of becoming a lightning storm.

Physics BackfiresPhysics Backfires

Prof. Georg Wilhelm Richmann electrocuted July 26, 1753, following Franklin's experiment.

Electrocuted in St. Petersburg, Russia, while "trying to quantify the response of an insulated rod to a nearby storm."

CurrentCurrent

When charge “When charge “qq” flows, we have current “” flows, we have current “II” ” measured in amperes (A):measured in amperes (A):

Current is the rate of flow of electric charge Current is the rate of flow of electric charge (units are coulombs/second or (units are coulombs/second or amperesamperes).).

Metal is good electrical conductor, other Metal is good electrical conductor, other materials such as glass are poor conductors materials such as glass are poor conductors (called “insulators”).(called “insulators”).

II = = q / tq / t

ExerciseExercise

If 0.16 coulombs of charge are released If 0.16 coulombs of charge are released across a wire in 0.10 s, what is the current?across a wire in 0.10 s, what is the current?

GivenGiven FormulaFormula q =q = 0.16 0.16 CC II = = qq//tt tt = 0.1 s = 0.1 s

II = = qq//tt = (0.16) / (0.10) = (0.16) / (0.10)

= 1.6 ampere= 1.6 ampere

Coulomb’s LawCoulomb’s Law

Force between charges:Force between charges:

F = F = kk qq11qq22

rr 22

kk = 9.0x10 = 9.0x1099 N-m N-m22/C/C22

Electric force is proportional to the charges on Electric force is proportional to the charges on the objects and inversely proportional to the the objects and inversely proportional to the square of the distance between them.square of the distance between them.

VoltageVoltage

Amount of work it takes to move a charge Amount of work it takes to move a charge of 1 coulomb between two points.of 1 coulomb between two points.

Measured in volts (V).Measured in volts (V). A battery supplies this work by electro-A battery supplies this work by electro-

chemical action.chemical action.

1.5 V1.5 V 9 V9 V 12 V12 V SchematicSchematic

ResistanceResistance

Opposes current flow.Opposes current flow. Metals offer little resistance, insulators Metals offer little resistance, insulators

great resistance, semi-conductors great resistance, semi-conductors moderate resistance.moderate resistance.

Units are called ohms (Units are called ohms (ΩΩ).).

SchematicSchematic

Ohm’s LawOhm’s Law

Voltage Voltage VV equals the current times the equals the current times the resistance.resistance.

VV = = IIRR

RRVV

II

Ohm’s LawOhm’s Law

Voltage Voltage VV equals current equals current II times resistance times resistance R:R:

GivenGiven FormulaFormula

RR = 3.0 = 3.0 ohms.ohms. VV = = IIRR

VV = 12.6 volts = 12.6 volts

V = IRV = IR = 12.6 = = 12.6 = II (3.0) (3.0)

II = 4.2 amps = 4.2 ampsR=3.0 R=3.0 ΩΩ

VV

II

Add a SwitchAdd a Switch

““Open circuit” if switch Open circuit” if switch is open – no current is open – no current flows.flows.

““Closed circuit” if Closed circuit” if switch closed, current switch closed, current flows.flows.

RRVV

II = 0 = 0

Car HeadlightsCar Headlights A typical car alternator generates up to 60 A typical car alternator generates up to 60

amps at 14 volts.amps at 14 volts. ““Open circuit” if switch is open – no Open circuit” if switch is open – no

current.current. ““Closed circuit” if switch closed, current Closed circuit” if switch closed, current

flows.flows.

VVVV

PowerPower Power Power PP equals current equals current II times the voltage times the voltage V:V:

PP = = IIVV

Electrical power units same as with Electrical power units same as with mechanical energy, mechanical energy, watts (W)watts (W)..

Car headlight (halogen) Car headlight (halogen) II = = 4.2 A, 4.2 A, VV = 13 V. = 13 V.

PP = = IIVV = (13)(4.2) = (13)(4.2) PP = 55 W = 55 W

Joule’s LawJoule’s Law

Light BulbLight Bulb

Original commercial bulb: carbonized Original commercial bulb: carbonized filament.filament.

Today: tungsten filament in an inert gas.Today: tungsten filament in an inert gas. Thomas Edison – in his lab in 1880 tested Thomas Edison – in his lab in 1880 tested

over 6,000 various carbon filaments before over 6,000 various carbon filaments before he got a carbonized cotton thread to last 15 he got a carbonized cotton thread to last 15 hours before burning out.hours before burning out.

Edison Electric Light Company Edison Electric Light Company → GE in → GE in 1892.1892.

Car Battery PowerCar Battery Power

A typical car battery has voltage of 12.6 V, A typical car battery has voltage of 12.6 V, maximum “cold cranking amps” of 500 A. How maximum “cold cranking amps” of 500 A. How much power is available to start your car?much power is available to start your car?

PP = = IIVV = 500 x 12.6 = 6,300 W = 500 x 12.6 = 6,300 W

Recall 1 hp = 746 WRecall 1 hp = 746 W Therefore, PTherefore, P = 8.4 hp = 8.4 hp ““Turns over” the engine.Turns over” the engine.

Connect Elements in SeriesConnect Elements in Series Both lamp elements have same Both lamp elements have same currentcurrent..

RRseriesseries= = RR1 1 + + RR22

What if one light burns out? No current, open What if one light burns out? No current, open circuit!circuit!

RR11

RR22RR22

RR11

Connect Elements in SeriesConnect Elements in Series Both lamp elements have same Both lamp elements have same currentcurrent.. Let Let RR1 1 = 30 = 30 ΩΩ

RR22 = 30 = 30 ΩΩ

Find equivalent series resistance –Find equivalent series resistance – RRseriesseries = = RR1 1 + + RR22

= 30 + 30= 30 + 30

= 60 = 60 ΩΩ

RR11

RR22

Connect Elements in ParallelConnect Elements in Parallel Both lamp elements have same Both lamp elements have same voltagevoltage..

1/R1/Rparallelparallel= 1/= 1/RR1 1 + 1/+ 1/RR22

What if one light burns out? No problem!What if one light burns out? No problem!

RR11 RR22RR11 RR22

Connect Elements in ParallelConnect Elements in Parallel Both lamp elements have same Both lamp elements have same voltagevoltage.. Let Let RR1 1 = 30 = 30 ΩΩ

RR22 = 30 = 30 ΩΩ

Find equivalent parallel resistance –Find equivalent parallel resistance – 1/R1/Rparallelparallel = 1/= 1/RR1 1 + 1/+ 1/RR22

= 1/30 + 1/30= 1/30 + 1/30

= 1/15= 1/15 Therefore, RTherefore, Rparallelparallel = 15 = 15 ΩΩ

RR11 RR22

Resistors in parallel Resistors in parallel have lower equivalent have lower equivalent resistance than resistance than resistors in series.resistors in series.

MagnetismMagnetism

North pole, South pole.North pole, South pole. Like poles repel, unlike poles attract.Like poles repel, unlike poles attract. Always appear as pairs.Always appear as pairs. Invisible magnetic Invisible magnetic

fieldfield

Source of MagnetismSource of Magnetism

Source of magnetic field − moving Source of magnetic field − moving electrons, and in some cases, “spinning” electrons, and in some cases, “spinning” electrons.electrons. Electric current in a wire creates a Electric current in a wire creates a

magnetic field.magnetic field. Magnetic materials. A few elements, Magnetic materials. A few elements,

ironiron, , nickelnickel, and , and cobalt, cobalt, and their alloys, and their alloys, have a strong magnetic effect.have a strong magnetic effect.

Magnetic MaterialsMagnetic Materials

Ordinarily, countless electrons in a material Ordinarily, countless electrons in a material are oriented in different directions, leaving are oriented in different directions, leaving no effect.no effect.

In an iron bar magnet the electrons are In an iron bar magnet the electrons are aligned in the same direction, so they act aligned in the same direction, so they act cooperatively, creating a net magnetic field.cooperatively, creating a net magnetic field.

Every electron, by its nature of Every electron, by its nature of spinning, is a small magnet. spinning, is a small magnet.

Earth’s Magnetic FieldEarth’s Magnetic Field

Earth's magnetic field can be modeled as a Earth's magnetic field can be modeled as a simple bar magnet.simple bar magnet.

The direction in which a compass points is The direction in which a compass points is known as known as magnetic northmagnetic north..

The Earth's North Magnetic Pole is a The Earth's North Magnetic Pole is a wandering point on the Earth's surface, with wandering point on the Earth's surface, with evidence that it flips about every 300,000 evidence that it flips about every 300,000 years.years.

Drifting at about 35 miles per year.Drifting at about 35 miles per year.

82.7° N 114.4° W in 2005.82.7° N 114.4° W in 2005.

81.3° N 110.8° W. in 20081.3° N 110.8° W. in 20011..

885.15.1° N 1° N 135.635.6° W in 20° W in 201212..

DeclinationDeclination

The direction of magnetic north from true The direction of magnetic north from true north is called “declination.”north is called “declination.”

Needed to make corrections in navigation.Needed to make corrections in navigation. In Los Angeles, magnetic north is about In Los Angeles, magnetic north is about

1212° east of true north.° east of true north. NN

EE

SS

WW

Magnetic Magnetic northnorth

ElectromagnetismElectromagnetism Michael Faraday (1791–1867) studied the Michael Faraday (1791–1867) studied the

magnetic field around a conductor magnetic field around a conductor carrying a direct currentcarrying a direct current

Established the basis for the “magnetic Established the basis for the “magnetic field” concept in physics.field” concept in physics.

Moving a magnet Moving a magnet through a static loop through a static loop of wire creates of wire creates electric current in the electric current in the wire.wire.

GeneratorGenerator

Moving wire loop in a static magnetic field Moving wire loop in a static magnetic field also produces an electric current.also produces an electric current.

Generator converts Generator converts mechanical energy mechanical energy to electrical to electrical energy.energy.

History of Local Power

Pomona College’s first president Cyrus Baldwin helped found San Antonio Light and Power Company in 1891.

It was the first generating plant to provide electricity in the Pomona and San Gabriel Valleys, using a waterfall in San Antonio Canyon. Transformers built by George Westinghouse provided for transmission of 10,000 volts from this plant to Pomona.

The source of water proved to be undependable, so the project failed to provide the expected results.

Hoover Dam PowerHoover Dam Power8 of 17 turbines8 of 17 turbines

Mutual InductionMutual Induction

Two insulated coils of wire.Two insulated coils of wire.

Passing a current Passing a current through one coil, a through one coil, a momentary current is momentary current is induced in the other induced in the other coil.coil.

TransformerTransformer

Transformers are passive devices.Transformers are passive devices. TransformTransform alternating current alternating current from from

one circuit into another through one circuit into another through electromagnetic induction. electromagnetic induction.

Number of windings in secondary Number of windings in secondary determines “step up” or “step down” determines “step up” or “step down” voltage.voltage.

VVss = (N = (Nss/N/Npp) V) Vpp

Ratio of windingsRatio of windings

TransformerTransformer

MotorsMotors

Current in a conducting wire experiences a Current in a conducting wire experiences a force from magnetic field.force from magnetic field.

MotorMotor

Split-ring commutator changes direction of Split-ring commutator changes direction of the current.the current.

Transforms Transforms electrical energy electrical energy to mechanical to mechanical energy. energy.

No Motor

Suspend, guide, and propel a train using electromagnets.

Magnetic levitation (Maglev) trains.

19-mile 19-mile Shanghai Maglev train 268 mi/hr.

Utility-Scale PowerUtility-Scale Power

Turns a turbine and generator.Turns a turbine and generator.

Utility-Scale Energy SourcesUtility-Scale Energy Sources

Fossil energy − Fossil energy − burn fossil fuels, heat water, burn fossil fuels, heat water, and create steam, turn turbine.and create steam, turn turbine.

NuclearNuclear – heat water, create steam, turn – heat water, create steam, turn turbine.turbine.

GeothermalGeothermal – steam from deep in Earth – steam from deep in Earth (6000 feet – Hawaii), turn turbine.(6000 feet – Hawaii), turn turbine.

Gas turbineGas turbine – hot gases direct into turbine. – hot gases direct into turbine. WindWind – wind turns turbine. – wind turns turbine. HydroelectricHydroelectric – water pressure turns turbine. – water pressure turns turbine.

U.S. Sources of EnergyU.S. Sources of Energy

US DOE, 2006US DOE, 2006

Historical ViewHistorical ViewEnergy Information Administration /

Annual Energy Review 2007

http://www.eia.doe.gov/cneaf/electricity/epa/epa_sum.htmlhttp://www.eia.doe.gov/cneaf/electricity/epa/epa_sum.html

Iron and Permanent MagnetsIron and Permanent Magnets

Soft ironSoft iron Very strong magnetic field when an Very strong magnetic field when an

electromagnetic field applied.electromagnetic field applied. Iron’s magnetic field disappears Iron’s magnetic field disappears

after field removed.after field removed. Hard ironHard iron

Retains magnetic field, “permanent Retains magnetic field, “permanent magnet”magnet”

Will lose permanent field if heated or Will lose permanent field if heated or struck.struck.

Toy Train TransformerToy Train Transformer

Fewer windings on secondary side creates Fewer windings on secondary side creates a a step-downstep-down (lower voltage) transformer. (lower voltage) transformer.

Primary Primary coilcoil

Secondary Secondary coilcoil

VVpp = 120 V ac = 120 V ac

NNpp = 1,000 windings = 1,000 windings

NNs s = 160 windings. = 160 windings.

What is VWhat is Vss??

VVss = (N= (Nss/N/Npp) V) Vpp

VVss = (160/1000) 120= (160/1000) 120

= 19 V= 19 V

Light BulbLight Bulb

Typical 100-watt light bulb Typical 100-watt light bulb VV = 120 volts = 120 volts PP = 100 watts = 100 watts What is the current?What is the current?

PP = = IIVV 100 = 100 = II (120) (120) II = 0.83 amperes = 0.83 amperes

Analyze Simple FlashlightAnalyze Simple Flashlight

1.0 W flashlight bulb, 3.0 V battery. 1.0 W flashlight bulb, 3.0 V battery. What What is current?is current?

Joules’ Law: PJoules’ Law: P = = IV = IV = 1 =1 = I I (3) (3) II = 1.0/3.0 = 0.33 A = 1.0/3.0 = 0.33 A

VV VV

Continue Simple FlashlightContinue Simple Flashlight

What is the bulb resistance?What is the bulb resistance? From previous slide, V = 3.0 V, and From previous slide, V = 3.0 V, and

I I = 0.33 A = 0.33 A Ohm’s Law: VOhm’s Law: V = = IR, 3IR, 3 = = 0.33R, 0.33R, RR = 10 = 10 ΩΩ

VV VV