Chapter 9Chapter 9Static and Current Static and Current

ElectricityElectricity

Electricity and Electricity and MagnetismMagnetism

The electromagnetic force is what binds The electromagnetic force is what binds electrons to the nuclei of atomselectrons to the nuclei of atoms

Responsible for all of chemistryResponsible for all of chemistry When we liberate electrons from When we liberate electrons from

individual atoms, we can make them do individual atoms, we can make them do work for uswork for us

So, the topic is extremely important if So, the topic is extremely important if we are to understand the world around we are to understand the world around usus

Electricity and Electricity and MagnetismMagnetism

Many of the fundamental particles Many of the fundamental particles have something we call have something we call electric electric chargecharge

We don’t know what this is, we can We don’t know what this is, we can only describe the results when only describe the results when particles have the propertyparticles have the property

There are two kinds of charges There are two kinds of charges which we call which we call PositivePositive and and NegativeNegative

Electricity and Electricity and MagnetismMagnetism

We observe that We observe that like charges repellike charges repel We observe that We observe that unlike charges attractunlike charges attract The forces are inverse square law The forces are inverse square law

forces, just like gravity, but very much forces, just like gravity, but very much more powerfulmore powerful

AtomsAtoms have a have a positive nucleuspositive nucleus with with surrounding surrounding negatively charged negatively charged electronselectrons

Electricity and Electricity and MagnetismMagnetism

Each electron in an atom is identical to Each electron in an atom is identical to every other electron so they all have the every other electron so they all have the same mass and the same negative chargesame mass and the same negative charge

The nucleus is composed of positively The nucleus is composed of positively charged protons and uncharged neutronscharged protons and uncharged neutrons

All protons are identical and the charge All protons are identical and the charge of the proton is exactly the same size as of the proton is exactly the same size as the charge of the electron, but it is the charge of the electron, but it is oppositeopposite

Electricity and Electricity and MagnetismMagnetism

Normal atoms have identical Normal atoms have identical numbers of protons and numbers of protons and electronselectrons

Atoms with a missing electron Atoms with a missing electron are called are called positive ionspositive ions

Atoms with an extra electron are Atoms with an extra electron are called called negative ionsnegative ions

Coulomb’s LawCoulomb’s Law Relationship describing the force Relationship describing the force

between two charged particlesbetween two charged particles

Charge is measured in coulombsCharge is measured in coulombs Force in newtonsForce in newtons Distance in metersDistance in meters

221

rqqkF

Coulomb’s LawCoulomb’s LawGravitational Force

F GM pme

r 2

F 6.7 10 11 1.67 10 27 9.1 10 31

5 10 11 2

F 4 10 47 NCoulomb Force

F kqp qe

r 2

F 9 10 9 1.6 10 19 1.6 10 19

5 10 11 2

F 9.2 10 8 N

Charge PolarizationCharge Polarization Make a tiny difference in the Make a tiny difference in the

average positions of electrons in an average positions of electrons in an atomatom

Distorts electrons like tides on the earth and pulls the nucleus to the right

Electric FieldElectric Field How do things interact with each other How do things interact with each other

when they don’t physically touch?when they don’t physically touch? When we talked about gravity, we said that When we talked about gravity, we said that

objects appear to experience a force objects appear to experience a force attracting them to each other.attracting them to each other.

Place a charged particle in space and it Place a charged particle in space and it creates an creates an electric fieldelectric field

Using Coulomb’s Law, we can calculate the Using Coulomb’s Law, we can calculate the force on a test charge at every point in force on a test charge at every point in spacespace

E F / q

Electric FieldElectric Field

Electric PotentialElectric Potential Lift an object into the air and we Lift an object into the air and we

give it gravitational potential energygive it gravitational potential energy

PE WorkPE Force distancePE mg hPE mgh

Electric PotentialElectric Potential Separate a positive charge from a Separate a positive charge from a

negative charge and give it electric negative charge and give it electric potential energypotential energy

PE workPE force distancePE Eq hPE Eqh

Electric PotentialElectric Potential Define electric potential as the Define electric potential as the

electric potential energy per unit electric potential energy per unit chargecharge

Electric Potential =electric potential energy

amount of charge

1 Volt =1 Joule

Coulomb

Voltage SourcesVoltage Sources If we want to move charge from If we want to move charge from

one place to another, we must one place to another, we must apply a force to make it moveapply a force to make it move

Another way of thinking about Another way of thinking about this is to say that we must give this is to say that we must give the charges some potential the charges some potential energyenergy

Voltage SourcesVoltage Sources We can give an object gravitational We can give an object gravitational

potential energy by lifting it into the potential energy by lifting it into the airair

We have to do work on the object to We have to do work on the object to lift it into the airlift it into the air

Doing work uses energyDoing work uses energy Conservation of EnergyConservation of Energy

Voltage SourcesVoltage Sources

Voltage SourcesVoltage Sources We need an pump that pumps charges!We need an pump that pumps charges! The simplest charge pump is a batteryThe simplest charge pump is a battery It uses chemical reactions to separate It uses chemical reactions to separate

charges and thus create electrical charges and thus create electrical potential energypotential energy

More convenient to talk about More convenient to talk about PE/charge or electric potential PE/charge or electric potential measured in voltsmeasured in volts

So, a battery is a kind of electric pumpSo, a battery is a kind of electric pump

Voltage SourcesVoltage Sources Another kind of source is a Another kind of source is a

generatorgenerator A generator converts mechanical A generator converts mechanical

energy into electrical energyenergy into electrical energy In any case, what we need is a In any case, what we need is a

device to separate charges!!!device to separate charges!!!

Electric CurrentElectric Current The movement of charge is The movement of charge is

called a called a currentcurrent Metals have some electrons Metals have some electrons

that are not needed to bond the that are not needed to bond the atoms together in the solidatoms together in the solid

They are pretty free to just They are pretty free to just roam about the material and roam about the material and are not associated with any are not associated with any individual metal atomindividual metal atom

Electric CurrentElectric Current Compare to the flow of water in a Compare to the flow of water in a

hosehose

Electric CurrentElectric Current We measure the flow of water in We measure the flow of water in

gallons/minutegallons/minute We measure the flow of charges We measure the flow of charges

in coulombs/secin coulombs/sec 1 coulomb/sec = 1 ampere1 coulomb/sec = 1 ampere Remember this is 6.25 x 10Remember this is 6.25 x 101818

electrons moving past a point in electrons moving past a point in a wire per seconda wire per second

Electric CurrentElectric Current In a wire, the electrons actually move In a wire, the electrons actually move

quite slowly, less than 0.01 meters/secquite slowly, less than 0.01 meters/sec However, their electric field moves at However, their electric field moves at

the speed of light!!!the speed of light!!! So, we can send signals down a wire So, we can send signals down a wire

very quickly, because the information very quickly, because the information moves at the speed of the changing moves at the speed of the changing electric field which is at the speed of electric field which is at the speed of lightlight

Electric CurrentElectric Current

+ -

Electron flow

Electric CurrentElectric Current In the 1700’s people figured out that In the 1700’s people figured out that

charges could movecharges could move They had two choicesThey had two choices

Positive charge movesPositive charge moves Negative charge movesNegative charge moves

They guessed WRONG!!!!!They guessed WRONG!!!!! We know electrons moveWe know electrons move

Electric CurrentElectric Current Hook up a battery and electrons flow Hook up a battery and electrons flow

from minus terminal to plus terminalfrom minus terminal to plus terminal Ancients defined current as positive Ancients defined current as positive

charge flowcharge flow Direction of “current” is from plus to Direction of “current” is from plus to

minusminus

DC and ACDC and AC Hook up a battery and we have Hook up a battery and we have

direct currentdirect current, i.e. current flows , i.e. current flows ALWAYS in one directionALWAYS in one direction

Modern generators in power plants Modern generators in power plants reverse the polarity of the output reverse the polarity of the output terminals 60 times per secondterminals 60 times per second

This is called This is called alternating currentalternating current It is what comes out of the wall plugIt is what comes out of the wall plug

DC and ACDC and AC

Electrical ResistanceElectrical Resistance So, we hook up a wire between the So, we hook up a wire between the

terminals on a batteryterminals on a battery The question is, how much current The question is, how much current

flows?flows? We have lots of free electrons able to We have lots of free electrons able to

move in the metal composing the wiremove in the metal composing the wire So, how much charge moves past any So, how much charge moves past any

point in the wire per second?point in the wire per second?

Electrical ResistanceElectrical Resistance The amount of current that flows is The amount of current that flows is

determined by how much resistance determined by how much resistance there is to the flow of the chargesthere is to the flow of the charges

What makes this resistance?What makes this resistance? Collisions of the electrons with each Collisions of the electrons with each

other and with the “stationary” atoms other and with the “stationary” atoms of the metalof the metal

Atoms are actually vibrating in the Atoms are actually vibrating in the latticelattice

Electrical ResistanceElectrical Resistance Resistance depends on the structure of Resistance depends on the structure of

the material and the temperaturethe material and the temperature The higher the temperature, the more The higher the temperature, the more

the atoms vibrate and the more the atoms vibrate and the more electrons make collisions with the atomselectrons make collisions with the atoms

How much current flows was discovered How much current flows was discovered by George Ohm, and we honor him by by George Ohm, and we honor him by naming the unit of resistance an ohmnaming the unit of resistance an ohm

Ohm’s LawOhm’s Law Ohm’s discovery was that current is Ohm’s discovery was that current is

proportional to voltage and inversely proportional to voltage and inversely proportional to resistanceproportional to resistance

current voltage

resistance

amperes voltsohms

Ohm’s LawOhm’s Law Three forms are convenientThree forms are convenient

I VR

V IR

R VI

Electric ShockElectric Shock We all know not to stick our finger into a We all know not to stick our finger into a

light bulb socket or to drop an electrical light bulb socket or to drop an electrical appliance into our bath tubappliance into our bath tub

What causes the damage to our bodies?What causes the damage to our bodies? It is the amount of current that flows It is the amount of current that flows

through the body that can cause problemsthrough the body that can cause problems Human skin has resistance ranging from Human skin has resistance ranging from

100 ohms to 500,000 ohms (wet to dry)100 ohms to 500,000 ohms (wet to dry)

Electric ShockElectric Shock We can use Ohm’s Law to calculate We can use Ohm’s Law to calculate

current based on the size of the applied current based on the size of the applied voltagevoltage

You can feel 0.001 ampere (1 milliampere)You can feel 0.001 ampere (1 milliampere) 0.005 amperes hurts0.005 amperes hurts 0.010 amperes causes muscle spasms0.010 amperes causes muscle spasms 0.015 amperes loss of muscle control0.015 amperes loss of muscle control 0.070 amperes disrupts heart rythyms 0.070 amperes disrupts heart rythyms

(fatal)(fatal)

Electric ShockElectric Shock There must be a potential difference There must be a potential difference

between one part of your body and between one part of your body and anotheranother

You must become a conductor of You must become a conductor of electricityelectricity

OK for birds to sit on a 5000 V OK for birds to sit on a 5000 V transmission line as long as no part of transmission line as long as no part of the bird touches something elsethe bird touches something else

Squirrels get across the transformersSquirrels get across the transformers

Electric CircuitsElectric Circuits A circuit is a path where a current can A circuit is a path where a current can

flowflow If the flow is to be continuous, the can If the flow is to be continuous, the can

be no gaps in the pathbe no gaps in the path Intoduce gaps in the form of switches, Intoduce gaps in the form of switches,

so we can control completing a circuit so we can control completing a circuit Most circuits have more than one Most circuits have more than one

device that we want to provide with device that we want to provide with electrical energyelectrical energy

Electric CircuitsElectric Circuits There are two ways to connect There are two ways to connect

multiple devices to a voltage sourcemultiple devices to a voltage source One is called One is called seriesseries The other is called The other is called parallelparallel Each has unique properties which Each has unique properties which

we now examinewe now examine

Series CircuitsSeries Circuits

Series CircuitsSeries Circuits A single pathway through the circuitA single pathway through the circuit The current is the same everywhere in The current is the same everywhere in

the circuitthe circuit Each device provides resistance and Each device provides resistance and

total resistance is the sum of the total resistance is the sum of the devicesdevices

Voltage divides among the devicesVoltage divides among the devices Voltage drop across each device is IrVoltage drop across each device is Irdevicedevice

Parallel CircuitsParallel Circuits

Parallel CircuitsParallel Circuits Each device connects to the voltage Each device connects to the voltage

sourcesource Voltage is the same across each deviceVoltage is the same across each device Current from source divides into devicesCurrent from source divides into devices Total current is the sum of device Total current is the sum of device

currentscurrents Current in each device is just V/RCurrent in each device is just V/R Add devices, lower total resistanceAdd devices, lower total resistance

CircuitsCircuits Ignore resistance of wires for Ignore resistance of wires for

calculationscalculations Just as we ignored air resistanceJust as we ignored air resistance Ohm’s Law answers all questionsOhm’s Law answers all questions Let’s do some sample calculationsLet’s do some sample calculations

Series Circuit Series Circuit CalculationCalculation

12 Volt

10 ohm 20 ohm 30 ohm

Parallel Circuit Parallel Circuit CalculationCalculation

12 Volt

10 ohm

20 ohm

30 ohm

Electric PowerElectric Power Moving charges do workMoving charges do work We can heat the filament in a light bulbWe can heat the filament in a light bulb We can turn the rotor in a motorWe can turn the rotor in a motor The rate at which work is done is powerThe rate at which work is done is power Electric Power = current x voltageElectric Power = current x voltage Units are watts = joules/sec = amps x Units are watts = joules/sec = amps x

voltsvolts

Electric PowerElectric Power

P VI

P IR

I I 2 R

P VVR

V 2

R

Power CalculationPower Calculation

12 Volt

10 ohm 20 ohm 30 ohm

Power CalculationPower Calculation

12 Volt

10 ohm

20 ohm

30 ohm

Energy Loss in Power Energy Loss in Power LinesLines

Wires have some resistanceWires have some resistance To transport energy from Point A to To transport energy from Point A to

Point B, we connect wiresPoint B, we connect wires Each wire has resistanceEach wire has resistance Energy Loss in one second is IEnergy Loss in one second is I22RR Make I small to minimizeMake I small to minimize Transformers convert AC VoltagesTransformers convert AC Voltages

FusesFuses Limit the current that runs through Limit the current that runs through

wires in your housewires in your house These wires have some resistanceThese wires have some resistance Energy loss by IEnergy loss by I22R converts to heatR converts to heat Hot wires can start a fireHot wires can start a fire Limit the current with a fuse or Limit the current with a fuse or

circuit breakercircuit breaker