Electrostatics The Shocking Truth February 1/2, 2010.

ElectrostaticsElectrostatics

The Shocking TruthThe Shocking Truth

February 1/2, 2010February 1/2, 2010

Unit SummaryUnit Summary

• Chapter 17 - ElectrostaticsChapter 17 - Electrostatics– Electric chargeElectric charge– Electric forceElectric force– Electric field strengthElectric field strength– Lightning!Lightning!

Today Today

• Electric ChargeElectric Charge• Conductors and insulatorsConductors and insulators• Charging byCharging by

– ContactContact– InductionInduction– PolarizationPolarization

Fun with balloons…Fun with balloons…

• Anyone want to sacrifice your Anyone want to sacrifice your hairstyle for science?hairstyle for science?

• What is going on?What is going on?

Electrostatic chargesElectrostatic charges

• Electrostatic charges Electrostatic charges – PositivePositive– NegativeNegative

• Opposites attractOpposites attract– Like charges repelLike charges repel

• No kidding!No kidding!

Conservation of chargeConservation of charge

• You might recall from chemistry…You might recall from chemistry…– positive and negative chargespositive and negative charges

• Some atoms LOVE electronsSome atoms LOVE electrons– ElectronegativityElectronegativity

• Different materials have different Different materials have different electronegativityelectronegativity– Stronger or weaker hold on electronsStronger or weaker hold on electrons

What happens when…What happens when…

• You rub two objects together?You rub two objects together?• You put some energy into the You put some energy into the

system…system…– Electrons get excited! Electrons get excited!

• May allow one material to pull May allow one material to pull electrons off of another!electrons off of another!

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.

• How would you illustrate this?How would you illustrate this?

Conductors and insulatorsConductors and insulators

• Why are electric cords coated in plastic?Why are electric cords coated in plastic?• What is electric wire made of?What is electric wire made of?• InsulatorsInsulators

– Materials that do Materials that do notnot transfer charge easily transfer charge easily

• ConductorsConductors– Materials that transfer charge easilyMaterials that transfer charge easily

• Examples? Examples?

You’re grounded!You’re grounded!

• What does it mean?What does it mean?• The earth is a big bank.The earth is a big bank.

– Supply or receive large number of Supply or receive large number of electronselectrons

• Grounding neutralizes the charge Grounding neutralizes the charge in an objectin an object– Through contactThrough contact

Types of charge transferTypes of charge transfer

• ContactContact– Friction/rubbing (slightly different)Friction/rubbing (slightly different)

• PolarizationPolarization• InductionInduction

ContactContact

• High concentration to low High concentration to low concentrationconcentration

• 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 about a neutral object?What about a neutral object?• What happens when you remove the What happens when you remove the

charged object?charged object?

Grounding and InductionGrounding and Induction

• What happens when you ground an What happens when you ground an object that has a polarized charge?object that has a polarized charge?– It becomes charged!It becomes charged!

CFUCFU

• ……

LabLab

• Read the questions on the backRead the questions on the back• Lucite and blanciteLucite and blancite

– Wool and blanciteWool and blancite– Silk and Lucite (or glass)Silk and Lucite (or glass)

• Ground the pith ball and the rods Ground the pith ball and the rods before each stepbefore each step

• Be patient – use lots of chargeBe patient – use lots of charge



Sorry… no pun today.Sorry… no pun today.

Today Today

• Millikan’s ExperimentMillikan’s Experiment• Coulomb’s LawCoulomb’s Law

– Electrostatic ForceElectrostatic Force

• Finish Lab 1Finish Lab 1

Back to the lab!Back to the lab!

• 10 minutes10 minutes• If you’re done – work on completing If you’re done – work on completing

the in-class worksheet from last the in-class worksheet from last time.time.

R. Millikan (in 1909)R. Millikan (in 1909)

• Oil drops??Oil drops??• Measured the Measured the

charge on the charge on the drops…drops…

Millikan’s machine…Millikan’s machine…

The point?The point?

• Charges are “quantitized”Charges are “quantitized”– Always a multiple of a very small Always a multiple of a very small

number…number…

• qqee = 1.60 x 10 = 1.60 x 10-19-19 Coulombs (C) Coulombs (C)

• Charge on an electronCharge on an electron– And a proton…And a proton…

So…So…

• How many “free electrons” does an How many “free electrons” does an object with a charge of 5.00 object with a charge of 5.00 coulombs have?coulombs have?

Yes, it’s a ForceYes, it’s a Force

• First day – 2 x 2 and the charged First day – 2 x 2 and the charged plastic rodplastic rod– Electrostatics are field forcesElectrostatics are field forces– Do not require contact to be “felt”Do not require contact to be “felt”

• What makes a force a force?What makes a force a force?• F = maF = ma

– When it causes a change in velocity of When it causes a change in velocity of an objectan object

Charles Augustin de Charles Augustin de CoulombCoulomb

Charles CoulombCharles Coulomb

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

the product of the two chargesthe product of the 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 proportional to the distance squaredsquared

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

• This is called an “inverse square This is called an “inverse square relationship”relationship”

Coulomb’s LawCoulomb’s Law

• 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”

– k = 9.0 x 10k = 9.0 x 1099 N m N m22/C/C22

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

So we can calculate…So we can calculate…

• F = k (qF = k (q11 x q x q22)/d)/d22

• 2 charges are 0.30 meters away 2 charges are 0.30 meters away from each other from each other

• Q1 has a charge of 12.0 x 10Q1 has a charge of 12.0 x 10-9 -9 CC• Q2 has a charge of –18 x 10 Q2 has a charge of –18 x 10 -9 -9 CC• FF1212 = -2.2 x 10 = -2.2 x 10 -5 -5 NN• What does a negative force imply?What does a negative force imply?

Calculating forcesCalculating forces

• 1 Coulomb is a large charge1 Coulomb is a large charge– 1x101x10-6 -6 C 1x10C 1x10-9 -9 C or is more typicalC or is more typical

• μC or nCμC or nC

• You will need to be adept at using You will need to be adept at using scientific notationscientific notation

• The sign matters!The sign matters!– Opposite sign – attractingOpposite sign – attracting– Same sign - repellingSame sign - repelling



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

Anonymous Anonymous

Today Today

• Review HomeworkReview Homework• More Electrostatic ForceMore Electrostatic Force• Electrostatic FieldsElectrostatic Fields

HomeworkHomework

633633

2. Charge is 2. Charge is quantitizedquantitized

3. 6.25 x 103. 6.25 x 101919 electronselectrons

6366361.1. -230 N-230 N2.2. a) -2.2 x 10a) -2.2 x 10-5-5 N N

b) 9.0 x 10b) 9.0 x 10-7-7 N N

3.3. a ) 16 Na ) 16 Nb) Attractb) Attractc) 2.7 x 10c) 2.7 x 101313 e- e-d) 3.8 x 10d) 3.8 x 101313 e- e-

4.4. 0.393 m0.393 m

What do you think?What do you think?

• Does it make a difference? Does it make a difference? – If you are standing barefoot in water… If you are standing barefoot in water… – or on rubber soled shoes?or on rubber soled shoes?

• Can you ground an insulator by touching Can you ground an insulator by touching one end?one end?

• Which can you induce a charge in? Which can you induce a charge in? – An insulator or a conductor?An insulator or a conductor?

Review Coulomb’s LawReview Coulomb’s Law

• F = k (qF = k (q11 x q x q22)/d)/d22

• Bigger the charge (q)Bigger the charge (q)– Bigger the forceBigger the force

• Larger the distanceLarger the distance– Smaller the forceSmaller the force– Inverse squareInverse square

What happens…What happens…

ExampleExample

• 3 objects in sitting in a line3 objects in sitting in a lineQ1 = 4.0 x 10Q1 = 4.0 x 10-6-6 C at x = 0 m C at x = 0 mQ2 = 3.0 x 10Q2 = 3.0 x 10-6-6 C at x = 1.2 m C at x = 1.2 mQ3 = -1.2 x 10Q3 = -1.2 x 10-6-6 C at x = 2.0 m C at x = 2.0 m

• Find the total force acting on Q2Find the total force acting on Q2• FF1212 = 0.075 N = 0.075 N• FF3232 = 0.051 N = 0.051 N• FFnetnet = 0.126 N = 0.126 N

Remember…Remember…

• If I put a charged objectIf I put a charged object– between 2 like-charged objectsbetween 2 like-charged objects

• The forces push (or pull) against The forces push (or pull) against each other. each other.

• These forces can be balanced if the These forces can be balanced if the middle object is in the right spot…middle object is in the right spot…

So, what do I mean?So, what do I mean?

+1 +3-2

F12 = F23?

Are these charges going to stay where they are?

Example…Example…

• Two “+” charges are 2.2 meters apart.Two “+” charges are 2.2 meters apart.Q1 = 2.0 x 10Q1 = 2.0 x 10-6-6 C C Q2 = 1.5 x 10Q2 = 1.5 x 10-6-6 C C

• A third charge “-” is put between themA third charge “-” is put between themQ3 = -1.2 x 10Q3 = -1.2 x 10-6-6 C C

• Where does it need to be if it is in Where does it need to be if it is in equilibrium?equilibrium?

New Tools - LabNew Tools - Lab

• ElectroscopeElectroscope• ElectrophorusElectrophorus



Electric fieldsElectric fields

Electroscope labElectroscope lab

HomeworkHomework

639 639 • #1) q1: –46.8 N; q2: 158 N; #1) q1: –46.8 N; q2: 158 N;

q3: -111 Nq3: -111 N

641 641 • #1) x = 0.62 m#1) x = 0.62 m• #2) 24.5 cm from q1 or #2) 24.5 cm from q1 or

• 15.5 cm from q215.5 cm from q2

Lightnin’ videoLightnin’ video

• How does lightning form?How does lightning form?• Grauple particles?Grauple particles?• What does a lightning rod do?What does a lightning rod do?• Fulgurite?Fulgurite?


February 12/16February 12/16

Electric fieldsElectric fields

Review WorksheetsReview Worksheets

• #1 and #2#1 and #2

Electric field?Electric field?

• Consider the “potential” force of a Consider the “potential” force of a charged objectcharged object– The bigger the chargeThe bigger the charge– The closer it is…The closer it is…

• E = k qE = k q11/d/d22

– qq11 is the charge on the object is the charge on the object – And the distance “d” is the location of And the distance “d” is the location of

the field strength of interestthe field strength of interest

So…So…

• The strength of the field is The strength of the field is determined by determined by – the strength of qthe strength of q11 (WHAT IS q (WHAT IS q11?) ?)

– the distance (squared)the distance (squared)

ExampleExample

• An electrical field around a charged An electrical field around a charged object is 5.95 x 10object is 5.95 x 1066 N/C at a N/C at a distance of 10.0 cm.distance of 10.0 cm.

• Find the charge on the object.Find the charge on the object.

• E = k qE = k q11/d/d22

• q = 6.61 x 10q = 6.61 x 10-6-6 C C

Field LinesField Lines

• We use field lines to visualize the We use field lines to visualize the direction and strength of electric direction and strength of electric fieldsfields

A few rulesA few rules

• Lines begin at positive chargesLines begin at positive charges• Lines end at negative charges Lines end at negative charges • The # of lines is proportional to the The # of lines is proportional to the

size of the charge.size of the charge.• Field lines are perpendicular to Field lines are perpendicular to

surfacesurface• Lines cannot crossLines cannot cross

Let’s look at some…Let’s look at some…

More…More…

• The The number of number of lineslines is is proportional to proportional to the the sizesize of the of the charge! charge!

• Farther apart = weakerFarther apart = weaker

• close = strongerclose = stronger

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.

Conductors in EquilibriumConductors in Equilibrium

• Electric Field = 0 inside a Electric Field = 0 inside a conductorconductor

• Excess charge goes to the outsideExcess charge goes to the outside– It’s repelled by like chargesIt’s repelled by like charges

Other odds and ends…Other odds and ends…

• Field lines perpendicular to surfaceField lines perpendicular to surface• Accumulates at the pointy endsAccumulates at the pointy ends

– Like lightning rods…Like lightning rods…

• Pointy ends alsoPointy ends also– More likely to “leak” chargeMore likely to “leak” 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

ReviewReview

• labslabs

Electrostatics The Shocking Truth February 1/2, 2010.

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Transcript of Electrostatics The Shocking Truth February 1/2, 2010.