cp: electrostatics february 1, 2010. electrostatics – chap. 32

CP: ElectrostaticsCP: Electrostatics

February 1, 2010February 1, 2010

Electrostatics – Electrostatics – Chap. 32Chap. 32

ElectrostaticsElectrostatics

• Electricity at Electricity at restrest

• Electric charges Electric charges • Forces between Forces between

themthem• And how they And how they

actact

Charge ModelCharge Model

• 1. There are two types of charges: 1. There are two types of charges: Positive (+) and negative (-)Positive (+) and negative (-)


• 2. Particles that make up 2. Particles that make up matter have a charge.matter have a charge.


• 3. Like charges repel…3. Like charges repel…

while opposite charges attractwhile opposite charges attract


• 4. The force gets weaker with 4. The force gets weaker with increasing distance between the increasing distance between the charges.charges.


• 5. The force increases with 5. The force increases with increasing charge.increasing charge.

Conservation of ChargeConservation of Charge

• You can separate or join what you You can separate or join what you have, have, – but you can’t create charge.but you can’t create charge.

Conductors and InsulatorsConductors and Insulators

• Conductors allow charges to flow Conductors allow charges to flow through themthrough them

• Insulators allow charges to stay Insulators allow charges to stay put.put.

Three Charge States:Three Charge States:

• 1. Neutral = equal number of (+) 1. Neutral = equal number of (+) and (-), evenly distributed.and (-), evenly distributed.


• 2. Polarized = equal number of 2. Polarized = equal number of (+) and (-), unevenly distributed.(+) and (-), unevenly distributed.


• 3. Charged = unequal number of 3. Charged = unequal number of (+) and (-).(+) and (-).

Types of charge transferTypes of charge transfer

• Contact or frictionContact or friction• PolarizationPolarization• InductionInduction• But first we should mention…But first we should mention…

GroundingGrounding

• When we touch a surface to drain When we touch a surface to drain the charges the object becomes the charges the object becomes groundedgrounded..

• Why?Why?– The earth is a huge neutral object…The earth is a huge neutral object…– Equal + and - chargesEqual + and - charges

Contact and FrictionContact and Friction

• One object gains as much as the One object gains as much as the other loosesother looses

• Insulators vs conductorInsulators vs conductor– Which one will hold onto more of its Which one will hold onto more of its

chargecharge

PolarizationPolarization

• Electrostatic forces are “field” forcesElectrostatic forces are “field” forces– Contact is not necessaryContact is not necessary

• If you put a positively charged object If you put a positively charged object close to another objectclose to another object– It will repel the positive charges It will repel the positive charges – And attract the negative chargesAnd attract the negative charges

• What happens when you remove the What happens when you remove the charged rod?charged rod?

InductionInduction

• Polarize the chargePolarize the charge• Ground the objectGround the object

– Neutralizes the charge in an objectNeutralizes the charge in an object

• Remove the groundRemove the ground– Then remove the polarizing chargeThen remove the polarizing charge

LabLab

• Grounding is critical!Grounding is critical!• Between each step (except for #4)Between each step (except for #4)• Diagram and explain what is going Diagram and explain what is going

on for each step.on for each step.


Coulomb’s LawCoulomb’s Law


Review – 3 Charge StatesReview – 3 Charge States

• Neutral =Neutral =

• Polarized =Polarized =

• Charged = Charged =

Basics of charge modelBasics of charge model• 1. 1.

• 2.2.

• 3.3.

PolarizationPolarization

• When a charged object comes When a charged object comes close to a neutral objectclose to a neutral object

• They will be attracted They will be attracted – Whether the charged object is Whether the charged object is

negative or positive.negative or positive.

Insulators versus Insulators versus conductorsconductors

3 Ways to Charge3 Ways to Charge

• 1. Friction/contact1. Friction/contact

• 2. Polarization (temporary)2. Polarization (temporary)

• 3. Induction3. Induction

11stst way way

22ndnd way way

What does it mean to What does it mean to “ground” something?“ground” something?

What can we say about these objects?

Ok…Ok…

• Enough review…Enough review…

Charles Augustin de Charles Augustin de CoulombCoulomb

Charles CoulombCharles Coulomb

• Looking at the force between 2 Looking at the force between 2 objectsobjects

• Two ideas:Two ideas:• #1 The force was proportional to #1 The force was proportional to

the two chargesthe two charges– If you double the charge on one object If you double the charge on one object

the force between them doubledthe force between them doubled

Charles Coulomb – idea Charles Coulomb – idea #2#2

• #2 The force was inversely #2 The force was inversely proportional to the distance in a proportional to the distance in a unique way…unique way…

• If he doubled the distance …If he doubled the distance …– The force decreased by 4 timesThe force decreased by 4 times

• Force is proportional to 1/dForce is proportional to 1/d22

• This is called an inverse square This is called an inverse square relationshiprelationship


• F is proportional to (qF is proportional to (q11 x q x q22)/d)/d22

• Or… F = k (qOr… F = k (q11 x q x q22)/d)/d22

– Where k is a constant that makes the Where k is a constant that makes the numbers “work”numbers “work”

• q is the symbol for chargeq is the symbol for charge– It’s units are CoulombsIt’s units are Coulombs

Coulomb’s Law:Coulomb’s Law:

What?


• If we increase the charge of one or If we increase the charge of one or both objects …both objects …– The force becomes stronger.The force becomes stronger.


• If we increase the distance If we increase the distance between the charges…between the charges…– The force becomes less.The force becomes less.

Electrostatic vs GravityElectrostatic vs Gravity

• Both are field forcesBoth are field forces– No touching necessaryNo touching necessary

• Both get smaller with distanceBoth get smaller with distance– ““inverse square”inverse square”

• Gravity gets stronger with more massGravity gets stronger with more mass– Electrostatic force …with more chargeElectrostatic force …with more charge

• Only one can repel, as well as attract…Only one can repel, as well as attract…

Conceptual PhysicsConceptual Physics


““Ouch!! Do it again…”Ouch!! Do it again…”AnonymousAnonymous

Today’s Plan:Today’s Plan:

• Check – HWCheck – HW• Electric fieldsElectric fields• Electrostatics Lab #2 - The Search Electrostatics Lab #2 - The Search

for Sparksfor Sparks

Demo Charge transfer…Demo Charge transfer…

• By contact / frictionBy contact / friction• By polarizationBy polarization• By inductionBy induction

Review of Coulomb’s LawReview of Coulomb’s Law

• If we increase magnitude of one or If we increase magnitude of one or both of the charges and leave the both of the charges and leave the distance the same the force…distance the same the force…

• Increases!Increases!


• If we increase the distance If we increase the distance between the charges the force between the charges the force gets…gets…

• Smaller Smaller – Do you remember the relationship?Do you remember the relationship?

• Inverse squareInverse square

• F = kqF = kq11qq22/d/d22

Electrical FieldsElectrical Fields

• Around every charge there is Around every charge there is space that is affected by the space that is affected by the charge. charge.

• We use vectors to show these We use vectors to show these force fieldsforce fields

Electric Field Line Rule #1Electric Field Line Rule #1• The lines always point in the The lines always point in the

direction of the force that would act direction of the force that would act on a positive test charge.on a positive test charge.

Electric Field Line Rule #1Electric Field Line Rule #1• The vectors always point in the The vectors always point in the

direction of the force that would act direction of the force that would act on a positive test charge.on a positive test charge.

Electrical Field Line Rule Electrical Field Line Rule #2#2

• The The number of linesnumber of lines is proportional to the is proportional to the sizesize of the charge! of the charge!

• We use We use lines of lines of forceforce. . • Farther apart = Farther apart =

weaker, close = weaker, close = strongerstronger

Electrical Field Line Rule Electrical Field Line Rule #3#3

• The lines don’t The lines don’t cross one anothercross one another

Charge DensityCharge Density• How tightly packed the charges are in How tightly packed the charges are in

an object. More charges = more an object. More charges = more forceful.forceful.

Charge DensityCharge Density• The The charge densitycharge density of objects that of objects that

are touching is the same are touching is the same • The total amount of charge will be The total amount of charge will be

greater in the larger object.greater in the larger object.

Charge increases in Charge increases in “corners” of objects“corners” of objects

• No repelling charge No repelling charge to balance out the to balance out the force.force.

Lightning RodsLightning Rods

• Serve 2 purposes:Serve 2 purposes:• 1. Attract lighting by providing 1. Attract lighting by providing

access to “ground”access to “ground”• 2. Provide a sharp point 2. Provide a sharp point

– Charge concentrates thereCharge concentrates there– Actually allows grounding before the Actually allows grounding before the

charge gets too large!charge gets too large!

Electric ShieldingElectric Shielding

• Occurs when all electrons mutually Occurs when all electrons mutually repel each other, so the charge inside repel each other, so the charge inside of a conductor is normally zero.of a conductor is normally zero.

+

Conductor

Electric Shielding:Electric Shielding:• Explains why it is safe to be in a car Explains why it is safe to be in a car

struck by lightning, electrical struck by lightning, electrical components are housed in metal boxes, components are housed in metal boxes, etc.etc.

ElectroscopesElectroscopes

HomeworkHomework

• Review …Review …– Questions?Questions?

LightningLightning

• Any good lightning stories?Any good lightning stories?

What you need to learn What you need to learn from the video:from the video:

• How is lightning formed?How is lightning formed?• What are the characteristics of What are the characteristics of

lightning?lightning?– Size, temperature, how does it move…Size, temperature, how does it move…

• What is the purposeWhat is the purposess of a lightning of a lightning rod?rod?

• How do you study lightning?How do you study lightning?

Video (cont’d)Video (cont’d)

• Where are you safe in a lightning Where are you safe in a lightning storm and what should you avoid?storm and what should you avoid?

• What is a fulgurite?What is a fulgurite?

Quiz…Quiz…

• You are sitting on the hood of your car You are sitting on the hood of your car during a lightning stormduring a lightning storm– Is this a good idea? (Why/why not?)Is this a good idea? (Why/why not?)

• The truck you are sitting on gets hit by The truck you are sitting on gets hit by lightning and has an electrical charge on lightning and has an electrical charge on it. Will there be more charge:it. Will there be more charge:– On the flat hoodOn the flat hood– On the edge of the truck bedOn the edge of the truck bed– It’s all the sameIt’s all the same

Electrical Potential EnergyElectrical Potential Energy

• Remember gravity?Remember gravity?– And gravitational potential energy?And gravitational potential energy?– ““PE = mgh”PE = mgh”

• The higher you go the more PE you The higher you go the more PE you have…have…

• Consider a ladder 10 meters high. Consider a ladder 10 meters high. – What’s the PE of an object with a mass of 5 What’s the PE of an object with a mass of 5

kg?kg?• How about 10 kg?How about 10 kg?

Electrical PEElectrical PE

• Electrical energy is very similarElectrical energy is very similar• If we take a “+” charge and pull it into If we take a “+” charge and pull it into

away from a “-” chargeaway from a “-” charge– We do “work” on it (force x distance)We do “work” on it (force x distance)

• We create potential energyWe create potential energy

• If you let it goIf you let it go– Smaaackkk…Smaaackkk…– It flies towards the “-” chargeIt flies towards the “-” charge– Making kinetic energyMaking kinetic energy

Same idea …Same idea …

• If you push a “+” into a “+” charge If you push a “+” into a “+” charge force field…force field…

• It acts like a spring…It acts like a spring…– You work to get it in there…You work to get it in there…– Let it go and…Let it go and…

• WHEEEE!!!WHEEEE!!!

Let’s go back to the 10-m Let’s go back to the 10-m ladderladder

• So we could say that the potential due So we could say that the potential due to gravity at 10 meters…to gravity at 10 meters…– Is equal to 10 m/sIs equal to 10 m/s2 2 x 10 m x the massx 10 m x the mass

• If we wanted to consider this 10 meter If we wanted to consider this 10 meter height…height…– The PE = 100 mThe PE = 100 m22/s/s22 x whatever mass you x whatever mass you

havehave

• The gravitational “potential” is equal to The gravitational “potential” is equal to 100 J per 1 kg of mass100 J per 1 kg of mass

Electrostatics – the big Electrostatics – the big copy catcopy cat

• If you look at the potential energy per If you look at the potential energy per unit charge…unit charge…– PE/# chargesPE/# charges

• In units of Joules per CoulombIn units of Joules per Coulomb

• This is the Electric PotentialThis is the Electric Potential– NOT Potential ENERGYNOT Potential ENERGY– Just PotentialJust Potential

• For every Coulomb of charge at some For every Coulomb of charge at some locationlocation– You get so many Joules of potential energyYou get so many Joules of potential energy

Wow! Watt’s it called?Wow! Watt’s it called?

• Named after a strange ItalianNamed after a strange Italian– Whose name was Antonio…Whose name was Antonio…

• Volta!Volta!

• Note that a VoltNote that a Volt– Doesn’t tell us how much energy is Doesn’t tell us how much energy is

presentpresent– Just how much energy per unit of Just how much energy per unit of

chargecharge

Volts don’t killVolts don’t kill

• Flow of charge through your heart Flow of charge through your heart killskills

• Volts just tell you how far those Volts just tell you how far those charges will “fall” when you let charges will “fall” when you let them go!them go!

Volts don’t killVolts don’t kill

• If you have a penny on top of the If you have a penny on top of the schoolschool– It has a lot of gravitational potentialIt has a lot of gravitational potential– This is like voltageThis is like voltage

• But not much massBut not much mass– That would be like the chargeThat would be like the charge

Contrast with your carContrast with your car

• Let’s say you jack your car up to change Let’s say you jack your car up to change a tirea tire– It’s gravitational potential is big or small?It’s gravitational potential is big or small?– It’s potential energy is big or small?It’s potential energy is big or small?

• If you’re unsure imagine which you’d If you’re unsure imagine which you’d rather have fall on yourather have fall on you– The penny or the car?The penny or the car?

Amps kill…Amps kill…

• The mass of the car is like a huge The mass of the car is like a huge electrical chargeelectrical charge– Even if its “potential” is lowEven if its “potential” is low– It packs a lot of energy…It packs a lot of energy…

Who can tell me…Who can tell me…

• The difference between The difference between – Electrical Potential and Electrical Potential and – Electrical Potential Energy?Electrical Potential Energy?

Capacitors:Capacitors:

• Devices that that can store electrical Devices that that can store electrical energy. They are not batteries.energy. They are not batteries.

CapacitorsCapacitors• Two conducting plates that are Two conducting plates that are

oppositely charged with space or oppositely charged with space or some insulator separating them. some insulator separating them.

Capacitors:Capacitors:• The stored electrical energy can be The stored electrical energy can be

discharged if a conducting path is discharged if a conducting path is created between the two plates. created between the two plates.

Capacitors:Capacitors:

• If you have If you have – Large plates (surface area)Large plates (surface area)– Close togetherClose together– Attached to a large voltage source Attached to a large voltage source

• Then…your capacitor has a large stored Then…your capacitor has a large stored charge.charge.

Review Day!Review Day!

• Positive and negativePositive and negative• Positive and positivePositive and positive• 3 charges sitting side by side…3 charges sitting side by side…• Can a neutral balloon be attracted Can a neutral balloon be attracted

to a charged balloon?to a charged balloon?• Does a negative charged object Does a negative charged object

have any positive charges?have any positive charges?

More…More…

• Insulators and conductorsInsulators and conductors• How are charges transferred?How are charges transferred?• Explain and draw how induction Explain and draw how induction

works.works.– Contact transferContact transfer

• States of charge?States of charge?

Review - 3Review - 3

• Field force…Field force…• Field strength…Field strength…• Field lines…Field lines…• Coulomb’s LawCoulomb’s Law• Lightning rodLightning rod• Electric potentialElectric potential

– Vs electric potential energy…Vs electric potential energy…

cp: electrostatics february 1, 2010. electrostatics – chap. 32

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