chapter 9 static and current electricity
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Chapter 9 Static and Current Electricity. Electricity and Magnetism. The electromagnetic force is what binds electrons to the nuclei of atoms Responsible for all of chemistry When we liberate electrons from individual atoms, we can make them do work for us - PowerPoint PPT PresentationTRANSCRIPT
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