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Static Electricity

What does the term static mean?

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Not in motion

Electricity?

Involves electrons

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Atomic model positively charged nucleus (protons)

negatively charged electrons

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Outer Part Elementary Chargeelectrons e- –1

NucleusProtons p+ +1neutrons no 0

Neutral objects have same # p+ & e-.

Charged objects have net p+ or e-

Notation

When objects have excess or deficit of charge, can exert

electrostatic force.

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When objects have excess or deficit of charge, can exert electrostatic force (Fe).

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Charged objects can apply a Fnet.Proof?

What happens to the forces as the 2 objects separate?

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Decreases

Which graph do you think shows how Fe between 2 objects changes with distance.

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In solids,

Charge transferred by e- only.

How can we get positive charge object?

Loss of e-.

Uncharged objects can feel electrostatic force too:

by polarization

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PolarizationAtoms can be polarized by redistributing e-. Polarization is separation of charge not imbalance.

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Charged balloon causes wall to become polarized.

Pith ball polarization

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Concept Check: If 2 small objects are attracted to each other and move together, which of the following can be said with confidence?

• 1. They have opposite charges.• 2. They have the same charge.• 3. At least one is charged.• 4. None of the above.

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Conservation Law applies to charge

Although charge ( e-) can be transferred, charge cannot be created or destroyed.

Sum of charges in system remains the same. For polarization the system is the balloon and the wall.

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2 types of materials.

• Conductors – allow charges to move around – can be polarized.

• Insulators – hold excess charge in place – hard to polarize.

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Conductors – materials that allow e- to move freely often redistribute charge.

Metals are good conductors.

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Metal conductors distribute charge uniformly.

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Insulators – charges do not move freely. Tend to stay concentrated

in one spot on object.

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What’s happening here?

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Polarization produces only a surface charge. Try at home.

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3 ways of Charging Objects:

• 1. Friction – rub 2 neutral objects together.

• 2. Conduction - Contact with charged object.

• 3. Induction – by bringing charged object in vicinity of neutral conductor.

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Friction

Works well for insulators.

Do objects get same or opposite charge?

Opposite!

Conduction

• Charges transfer by touching charged object to neutral one.

• Good for conductors.

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Conduction: touch charged object to neutral object.

Do objects get same or opposite charge?

SAME!

Electroscope

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Why you get a shock.

• Charge yourself transfer e- either to or from your body to neutralize your charge. Always accompanied by E release.

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Static Electricity 9:15 min.

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http://www.youtube.com/watch?v=A893_7FGMHY&feature=relmfu

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Induction- – no touching of objects. Need to polarize & separate them.

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Charging by induction conductors only.

A ground can serve as an infinite source or sink of e-.

Earth, your hand, floor, wall.

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Charge an Electroscope by Induction

Charging By Induction 9:30 Min.

• http://www.youtube.com/watch?v=pJ36EtABLAk

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Sharing of Charge among conductors

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Conductors will share charge equally if they are in contact.

1. The elementary charge of each metal sphere below is shown. If they touch, and are then separated, what will be the resulting charge on each?

+3 -6 -9

Total charge 3 – 6 – 9 = - 12

They will share the total charge so divide:

- 12/3 spheres = -4.

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Why do the spheres need to touch. Why don’t charges jump from one to the other without

them touching?

+3 -6

Problem Set.

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Hwk Read Tx 17-1Answer pg 633 #1-2, 4-6

pg 654 #1-10 not 3Type or write it all including

questions.

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Determining Charge on electron.

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1909 Robert Millikan measured charge on e-

Millikan 1:15

http://www.youtube.com/watch?v=XMfYHag7Liw

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Robert Millikan found charge is quantized. There is a smallest unit of charge.

Charge can only exist in whole number integers of the charge on 1e-. Cannot have in between numbers.

Can an object have a charge of 3.53 x 10-19C?

• No.

• 3.53 x 10-19C ÷ 1.6 x 10-19 C = 2.2.

• Charges must be whole number integrals of 1.6 x 10-19 C .

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Units of charge = coulombs (C) Charge on e- is -1.6 x 10-19 C

Charge on p+ is +1.6 x 10-19 C

or can consider fundamental units e- has charge –1p+ has charge +1

Charge Units

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It takes 6.25 x 1018 elementary charges (e- or p+) to carry 1 C of charge.

Take the inverse of 1.6 x 10-19C.

2. What would be the charge on an object with 2.2 x 1015 excess electrons?

3.52 x 10-4 C

3. How many protons does it take to carry 0.001 C of charge?

6.25 x 1015 p+

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4. What is the total charge (in C) on 6.2 x 108 electrons?

• 9.9 x 10 -11 C

5. A metal sphere with an excess of 2 x 109 electrons is connected to a sphere with a deficit of 1 x 109 electrons.

• What is the charge in Coulombs on each sphere before they’re connected?

• What is the charge in Coulombs on each after they’ve been connected?

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Electrostatic Force

Charles Coulomb measured force exerted on one charged object by another.

He used torsion balance.

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Coulomb’s Torsion Balance

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Coulomb’s Law Relates Force btw. 2 charged objects.

Fe = kq1q2 r2

k = constant 8.99 x 109 N m2/C2. q charge on obj in Coulombs (C)r is dist in meters.F is force (N)

Ex 1: An alpha particle is a nucleus with 2 protons and 2 neutrons. It is near a proton.

1. What is the charge in Coulombs of each?

2. They are separated by a distance of 3 nm. What is the force between them?

3. Is the force repulsive or attractive?53

• nucl = 3.2 x 10-19 C.• p+ = 1.6 x 10-19 C.

• F = 5.11 x 10-11 N

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2: Two protons are 0.025 m apart. Calculate:

a) the gravitational attraction between them.

B) the electrostatic force between them.

C) what is the ratio between the forces.

D) What do you think the sign + or – indicates for electrostatic force?

Hwk read text 17 -2 and pg 634 – 636

• Do pg 636 #1-4 and pg 654 #1, 2, 6, 10.

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Mech Universe “Static Electricity”

Electric Fields E

region of space around charged object where a “test charge” feels an electrostatic force.

Write questions & Answers separate sheet.

• Hwk Review Book. p206 #1-7, 21 – 25, 29.

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Electric Field (E) defined as:The force and direction a small positive “test” charge feels in presence of field created by a larger charge Q.

E = F/q.

E = Electric Field (N/C)F is force on test charge (N).q is amt of charge on test charge (C).

Ex 1: A charge of 2 C feels a force of 10 N in an electric field. What is the field strength at that point.

E = F/q. = 10 N

2 C

E = 5 N/C

 Ex 2: How much force does a test charge with + 0.4 C feel in a field of 8 N/C?

• E = Fe/q

• Fe = qE

• 0.4 C x 8 N/C = 3.2 N.

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Ex 3. An electron is placed in a field of 100 N/C.

a. What is the force on the electron?

b. What is the acceleration of the electron?

• qE = F• 1.6 x 10-17 N• a = Fnet/m

• 1.8 x 1013 m/s2.63

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Electric Field Strength is Inversely Proportional to Distance Around a

Point Charge.

Field Lines represent electric fields.

Electric field lines show the force that a small positive test charge feels in a field created by a much larger charge. They represent the strength and direction of the field.

Phet Charges & Fields.

• http://phet.colorado.edu/en/simulation/charges-and-fields

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Sketch vectors to show force magnitude & direction on a + test charge at each point.

+

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• Suppose you bring a small positive test charge to various points (a,b,c etc) in space around the sphere below. Sketch vector arrows at each point to show the magnitude and direction of the force on the test charge at each point.

A B C

D

E

F

G

H

Sketching E fields.

I

J

K

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Field around positive object.

Sketch the field around a negatively charges sphere.

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- - -- - - -

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The denser the field lines are, the stronger the field.

Stronger field near charge.

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What are the field lines now?

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What if field was formed between opposite charged parallel plates? Sketch it.

++++

----

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Field Between Parallel Plates

How would the strength of the field vary if a charge moves from the + to the – plate?

Rules:

• Fields have strength/intensity and direction.

• Density/spacing of lines shows strength.

• Direction arrows determined by an imaginary + test charge.

• Electric Field lines don’t touch, cross, or angle sharply.

• Lines start on + end on neg.76

Electric field due to more than one charge.

Field is stronger near the larger charge. Density of lines show the increased

strength.

E field due to more than one charge.

Force due to more than one charge is the vector sum of all the forces on a charged particle.

Electrostatic Equilibrium

Fields produced by more that a single charge will have spots where the forces on a charge in the field will be balanced.

F net = 0.

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• Review Elec Field youtube lesson kahn.• http://www.youtube.com/watch?v=vaDT4Gw

AZ2I&feature=relmfu

Hwk elect field wksht andAnd Rev book 206 #2, 4, 7,9,

10-15, 24-25, 30-34

Work & EnergyElectric Potential

Intro to Potential Difference /Voltage Difference.

• 1. Define gravitational PE.

• 2. How is Energy related to work. Explain.

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• Coulomb’s Law between 2 charged objects:

Fe = kq1q2.

r 2

• As region of space.• Electric Field

Fe = qE

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We discussed Force due to charges in 2 ways.

• What is the definition of Energy?• Things that have E can do work.• When work is done on an object it gains

PE (+W) .• When work is done by an object it loses

PE (-W) .

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Do Charged particles in an electric field can have Energy?

Gravitational FieldEvery Height associated with dif PEg.

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Voltage Potential

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Every point associated with dif PEe.

It takes work to move charges in a field.

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Where does a positive test charge have more PE – close to or far from a large positive sphere?

++

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The amt of work done on every coulomb of charge moving it is called electric potential, V.

V = W/q. W work in Jq is charge in C.V is Volts = J/C.

V defines the potential at P at a point.

P is like a particular height in a gravity field.

Ex 1. It takes 150 x 10-6 J to move a 2.0 mC charge to point P. What is the electric potential (voltage) at P?

V = W/q = 150 x 10-6J = 75 V

2 x 10-6C

How much PE did every C of charge gain?

75 J

To find PE or work done by E field:

Since W = DPE and V = W/q:

PE elc = qV also W = qV.

PE – JoulesQ – CoulombsV = Volts

Ex 2. The electric potential at point P is 12.0 V.

A 3C charge is placed at P. What is the PE of q at P?

PE = W = qV

(3 C)(12 V) = 36 J

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Ex 2b. If q = -2 C is moved to a point P = 12 V, What is the PE of q?

DPE = qV

• (-2 C)(12 V) = -24 J

• q lost PE, the field did work on it. Think of the charge as falling.

Potential DifferenceVolage Dif betw 2 points in field ~ DheightEnergy Dif involves moving charge in field = qDV

A = 13 VB = 28 V

pd = 28V – 13V = 15 V.

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Potential Difference in a Uniform Field.

The field intensity F/q between plates is constant, the work done (Fd) to move a charge

between plates is constant, the potential difference (voltage) is constant.

Ex 3: What work must be done to move a +5 mC charge from the – to the + plate in the 250 V pd across plates?

W = q DV = (5 x 10-6C)(250 J/C - 0)

= 1.25 x 10-3J.

250 V

It helps to assume 1 plate is 0, the other is 250 Volts.

250 0 V

Hwk Rev Book

• Voltage Read Rev Book 203 - 206 Do pg 208# 12 - 20, 26 - 28, 30 - 34., 39-42. Write out question and

•answer (show sketch or calculation).

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Natural Potential Difference “Lightning”

Potential Dif/Voltage

• Provides “push” to move the charge, q.• Moving charge q, in field changes its PEelc.

• To move q to higher PE, do work on charge from outside.

• To move q, to lower PE, work done by E on charge.

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Lightening caused by p.d. cloud / ground. Cloud bottom becomes neg, polarizes ground, creates E field V push until charges accelerate!

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Which way will the field go?

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Charges set loose in E fields will accelerate!

The average lightning bolt contains 5-10 coulombs

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Lightening bolt physics 1 min.

• http://www.youtube.com/watch?v=C3y289F1eQ4

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Define

• Gravitational Field • Electric Field• Electric Potential • Electric Potential Difference

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Energy of Moving Charges in Fields.

• As a charge moves thru a field, its total E (the SE) is constant. By consv of Energy.

• If a charges “falls” toward the oppositely charged plate its PEelc decreases, What increases?

KE

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Is acceleration between parallel plates uniform? Explain.

• Work done by field will accelerate charge: W = DKE = qV.

• So: before after• ET = ET .

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How can I calculate PE of a mass in a gravity field?

• PEg = mgh

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How can I calculate PE of a charge q in an Electric field?

• PEelc = qV.

New Energy unit electron-volt

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The electron-volt: tiny unit of work & E.

For very small changes in PEelc (on the order of 10-19J) unit eV is used.

The electron-volt, eV, is the work & E required to push 1 e- (or p+) through a voltage of 1V.

W = qV = (1.6 x 10-19 C)(1V) = 1.6 x 10-19 J = eV.

1.6 x 10-19 J = eV

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If 1 e- is pushed across 1V then (1e)(1V)= 1 eV of work is done.

If a charge of 2e- is pushed across a 1V pd then (2e )(1V) = 2eV.

If 2e- pushed across 6V then work is 12 eV.

To find eV given elementary charges:

(# e )(# V ) = eV.

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36 eV

What if 3e- move across 12 V?

To find eV (# elm charges) (voltage)

1. How many joules of energy are represented by 6.9 x 1029 eV.

6.9 x 1029 eV x 1. 6 x 10-19 J. = 1.1 x 1011 J

eV

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Ex 2. A field does 3.3 x 10-7 J of work on an e-. How many eV is that?

• 3.3 x 10-7 J x 1eV = 2.1 x 1012 eV

• 1.6 x 10-19 J

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Ex 3: A proton is accelerated in a 100 V pd. How much work is done in eV?

• W = qV but if we use elem charge, we can just multiply by the voltage.

• (1 p+)(100 V) = 100 eV

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Summery Voltage or Electric Potential

V = Wk per Coulomb to bring a charged particle to point in field from infinity.

Potential / Voltage difference Wk per Coulomb to move charge between two

points at different potentials.

Charges in field have PEelc.High PE charge near point with same charge.

Low PE charge near point with opposite charge.

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Some typical voltages

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Hwk Rev Book

• Voltage Read Rev Book 203 - 206 Do pg 208# 12 - 20, 26 - 28, 30 - 34., 39-42. Write out question and

•answer (show sketch or calculation).

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Can calculate acceleration of charges in E fields & through

Voltages.

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Set PE elc = KE

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Prove that for parallel plates

E = Vd

•V = W = Fd but F = E V = Ed

q q q

• Rearranging E = V

d

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Plates with battery

AC Delco 12 volts

d

- +d = 1 cm

A B

Batteries are meant to maintain the potential difference.

ABV Ed

AB /E V d12 / 0.01E 12000 N/CE

Electric PE review youtube. Kahn

http://www.youtube.com/watch?v=wT9AsY79f1k

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