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Is For Electricity
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fundamentals that drive our world
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Charge It!!!What is evidence of charge?What are the two kinds of charge?What are the three ways for and object or
material to become charged?
HW: Learn 194-201 At least 6 talking to the text points.
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The Mighty AtomSince there arean equal
numberof electrons
andprotons, the net charge onthis atom is 0.
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The Mighty AtomThe only way for it
to become charged is to gain or lose electrons.
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Two Types of ChargeNEGATIVE (-) : More electrons than protons or
an excess of electrons.POSITIVE (+): Fewer electrons than protons or
a deficit of electrons.NEUTRAL – Is a state of charge, NOT A TYPE
OF CHARGE!Neutral is equal amounts + and -.
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Quick CheckWhat is the charge on each atom?
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Charging Can Be Done By..FRICTION:Rubbing two objects together to remove or
deposit electrons.CONTACT:Touching one charged object to a neutral to
remove or deposit charge.INDUCTION:Bringing one charged object near a neutral so
charges polarize or separate.
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The Fundamental ConceptNo matter what – the net charge in any process
is always zero.Charge is always conserved.One object might lose charge to another, but
the pair of objects still has a net charge of zero.
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Sign Convention
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How Charges Interact
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How Objects Interact
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Proof Is In The Pudding
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Proof Is In The Pudding
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Quick Check
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InsulatorsRubbing wool over
the R, tape reacted near the R but not near the N.
Charge was localized (stuck) near the R and couldn’t move over to the N.
NR
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Conductors
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The Pie PlatesThe key to getting a
net charge on the pie plate is to touch it so electrons rush from the plate leaving it with a net + charge.
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Conductors & InsulatorsCONDUCTORS:Allow charges to pass through them.Induction causes charges to separate.Charges will become evenly distributed on the
surface of a conductor.
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Conductors & InsulatorsINSULATORS:Do not allow charges to pass through them.Induction causes atoms/molecules to polarize.Charges will become stuck in certain regions of
the material.
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Quick Check
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Polarization of Insulators
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Polarization of Conductors
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Neutrals Will Be Attracted!
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Quick CheckCharged rubber rods are placed near a
neutral conducting sphere, causing a redistribution of charge on the spheres. Which of the diagrams below show the proper distribution of charge on the spheres? List all that apply.
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Charge It!As we have seen that charges behave
differently in different materials due to the nature of the bonds involved.
Metallic bonds allow electrons to flow freely whereas covalent bonds do not.
The dividing line is the metalloids on the Periodic Table.
(Thought you were done with chem, eh?!?!)
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Excuse Me Mr. Coulomb, But Would You Like To Charge That?Like gravity, charges exert forces over a
distance. So perhaps we can use gravity as an analog to forces created by charges.
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Coulomb’s Force PendulumA positively charged metallic
sphere is hung from a non conducting thread.
An equally charged negative sphere is brought near by.
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Coulomb’s Force PendulumA positively charged metallic
sphere is hung from a non conducting thread.
An equally charged negative sphere is brought near by.
The other sphere shifts towards it.The angle is proportional to the
relationship of Fe , mg and separation distance r.
+
mg
Fe
θ
d
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Coulomb’s Force PendulumWe know the equation of
gravity
F = G
r
mg
Fe
θ
m1m2
d2
L
d
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Coulomb’s Force PendulumWhich brings us to this for
small angles:We can use similar triangles to
relate the two kinds of forces.Fe/d =mg/LFe/mg = d/LWhich says that gravity and the
electric force are directly related.
r
But what about r? mg
Fe
θL
d
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Coulomb’s Force Pendulum
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Some Simulationshttp://webphysics.davidson.edu/physlet_resources/bu_semester2/index.html
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Coulomb’s Force PendulumSince they are directly related
we conclude both follow the inverse square law.
Fe = k r
mg
Fe
θ
Lq1 q2
r2
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Coulomb’s Law
Where q is charge measured in coulombs which must be noted as + or - charges
r is separation distance in metersk is a constant 9 x 109 N·m2/C2
Fe = kq1 q2
r2
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How Big Is A Coulomb?The charge on an electron (e-)−1.602176487×10−19 CAnd a proton (p+)+1.602176487×10−19 CIt takes 6.2414 ×1018 electrons to = 1CFor singly ionized water that’s .000187 gramsImagine the amount of charge a few grams
would hold and you understand a lightning bolt!
A mole of e- = 96,484 C. Holy @#&*$!
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Field of Dreams, Part ICharges and charged objects generate electric
fields.These fields can be thought of as vectors
passing through empty space.How the field vectors interact is how forces of
attraction and repulsion are transferred.We can’t see these fields, only observe their
interaction with other objects.Perhaps a look at gravity would help.
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Electric Field Strength
Coulombs Law
Fg
= G
Gravity
Fe = kq1 q2
d2
m1 m2
d2
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Electric Field Strength
Coulombs Law
Fg
= G
When these terms are collected we get g the acceleration due to gravity, 9.81 m/s2 leaving us with the familiar F = mg.
Fe = kq1 q2
d2
m1 m2
d2
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Electric Field Strength
These terms collect to form a notation of field strength called the Electric Field Intensity, E, which is a vector field, + or -.
F = EqWhere E is measured in N/C.
Fe = kq1 q2
d2
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Electric Field NotationTo note a field we typically show a line (or
plate) with a vectors as field lines.
+
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Projectile Motion LinkShoot a charge through the field and it will
curve.
+ -
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Fields Created By Chargeshttp://www.falstad.com/emstatic/
http://www.falstad.com/vector2de/
http://www.falstad.com/vector3de/
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Force Related to DistanceLift an object and do work against gravity…..Hey that sounds familiar!!!!!!!!!!!It’s work against gravity, or PE.Wonder if it works for moving a charge in an
Electric field?
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Work Done On ChargesPE = mgh
E is similar to gh is similar to r or dm is similar to q
Bowling Ball
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Work Done On ChargesI can say that I did so
many joules of work, total.
PE =mgh W = FdOr I could say I did so
many joules of work on the ball.
PE = Joules/ BB
Bowling Ball
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Work Done On ChargesEach ball has the same PE.The more balls the greater
the potential to wreak havoc on the floor.
Total PE = 3 x (J/BB) Or 3 J/BB
Bowling Ball
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Work Done On ChargesIf I imagine that each BB is
equal to one coulomb of charge we can write this:
PE = J/C
Volt = J/C
Bowling Ball
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Work Done On ChargesVolt = J/CWhich is called POTENTIAL.
Bowling Ball
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A Return to Gravity
Coulombs Law
F = G F = mg
Fe = k F = Eqq1 q2
d2
m1 m2
d2
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Coulombs Law In New WayFe = k (I)
F = Eq E = F/q (II) subbing II into I
E = kq/r2 (III)
q1 q2
d2
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Work Done To Move A Chargehttp://webphysics.davidson.edu/physlet_resources/bu_semester2/index.html
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Work On A ChargeW = Fd …………. W = Fed (I)
Fe = Ed E = Fe/q E = k q/r2 (II)
Working a number of substitutions of the these 5 expressions brings some more
W = qV V = k q/r