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Static Electricity
1
Free Associate – the terms
• Static
• Electricity
Senior Physics Conductors and Insulators 9:45
https://www.youtube.com/watch?v=qUhxmXZwPmg
2
Atomic model positively charged nucleus (protons)
negatively charged electrons
Neutral objects have
same # p+ & e-.
Charged objects have net
p+ or e-
3
When objects have charge imbalance, can
exert electrostatic force.
What happens when to objects Fnet
applied?
4
When objects have excess or deficit of
charge, can exert electrostatic force (Fe).
5
Charged objects can apply a Fnet.
Proof?
6
What happens to the forces as the 2 objects
separate?
Decreases
7
Which graph do you think shows how Fe
between 2 objects changes with distance.
8
Charge Notation
Amount of Charge
Outer Part
electrons eNucleus
Protons p+
neutrons no
Elementary Charge
–1
+1
0
9
In solids,
Charge transferred by e- only.
How can we get positive charge object?
Loss of e-.
10
• Bozeman Science 6 min. Good Phet Demos.
• https://www.youtube.com/watch?v=zHJkJG
BdvwE
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Uncharged objects can feel
electrostatic force too:
by polarization
12
Polarization
Atoms can be polarized by
redistributing e-. Polarization is
separation of charge not imbalance.
13
Charged balloon causes wall to become
polarized.
14
Pith ball polarization
15
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.
16
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.
17
2 types of materials.
• Conductors – allow charges to move
around – can be polarized.
• Insulators – hold excess charge in
place – hard to polarize.
18
Conductors – materials that allow e- to
move freely often redistribute charge.
Metals are good conductors.
19
Metal conductors distribute
charge uniformly.
20
Insulators – charges do not move
freely. Tend to stay concentrated
in one spot on object.
21
What’s happening here?
22
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!
25
Conduction
• Charges transfer by touching charged object
to neutral one.
• Good for conductors.
Conduction: touch charged object to
neutral object.
Do objects get same or
opposite charge?
SAME!
27
Electroscope
28
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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29
Static Electricity 9:15 min.
http://www.youtube.com/watch?v=A
893_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
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Charging By Induction 9:30 Min.
• http://www.youtube.com/watch?v=pJ36EtA
BLAk
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Sharing of Charge among
conductors
Conductors will share charge equally if they are in
contact.
35
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.
2. If these 2 spheres touch, what will be the charge on
each?
Why do the spheres need to touch. Why don’t charges
jump from one to the other without them touching?
+3
-7
Total Charge = -4
Each Sphere = -2
Air is an insulator so charge does not easily
travel through it.
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Problem Set.
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Hwk Read Tx 17-1
Answer pg 633 #1-2, 4-6
pg 654 #1-10 not 3
Type 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-
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Millikan 1:15
http://www.youtube.com/watch?v=X
MfYHag7Liw
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Charge is quantized. There is a smallest unit of
charge.
• Charge can only exist in whole number integers of
the charge on 1e- or p+
• Cannot have fractional numbers.
• New Unit - Charge can be measured in Coulombs.
• Use your table to find the smallest unit of charge
which is 1 elementary charge e.
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Charge Units
Units of charge = coulombs (C)
e- is -1.6 x 10-19 C
p+ is +1.6 x 10-19 C
or can consider elementary or fundamental units
e- has charge –1
p+ has charge +1
44
2. 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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3. How many electrons carry a charge of 1-C?
• Take the inverse.
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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.
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4. What would be the charge on an object
with 2.2 x 1015 excess electrons?
3.52 x 10-4 C
5. How many protons does it take
to carry 0.001 C of charge?
6.25 x 1015 p+
6. What is the total charge (in C)
on 6.2 x 108 electrons?
• 9.9 x 10 -11 C
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7. 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?
• - 3.2 x 10-10 C
• What is the charge in
Coulombs on each
after they’ve been
connected?
• - 0.8x 10-10 C
• + 1.6 x 10-10 C
Electrostatic Force
Charles Coulomb measured
force exerted on one charged
object by another.
He used torsion balance.
52
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)
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Force vs. Distance
Inverse square Fe and Fg
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8: 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?
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• 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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9. 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 about Fe?
• Fg = 3 x 10-61 N
• Fe = 3.7 x 10-25 N
• Fe is 1036 x stronger than Fg.
• pos is repulsive, neg is attractive
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Hwk: Finish Static Prc 2
• read text 17 -2 and pg 634 – 636
• Do pg 636 #1-4 and pg 654 #1, 2, 6, 10.
Mech Universe “Static Electricity”
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Force
• 2 objects involved
Gravitational Field, g
• The force and direction “felt” by a small mass (N/kg).
Same as acceleration but dif units.
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Electric Fields E
region of space around charged
object where a “test charge”
feels an electrostatic force.
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
2C
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.
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Electric Field Strength is Inversely
Proportional to Distance Around a
Point Charge.
68
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/charg
es-and-fields
70
Sketching E fields.
•
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.
G
D
E
A
B
C
F
J
H
K
I
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Sketch vectors to show force
magnitude & direction on a + test
charge at each point.
+
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Field around positive object.
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Sketch the field around a
negatively charges sphere.
- - - - -
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The denser (close spacing) the field
lines are, the stronger the field.
Stronger field
near charge.
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What are the field lines now?
76
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What if field was formed between opposite
charged parallel plates? Sketch it.
+
+
+
+
-
Field Between Parallel Plates
How would the strength of the field vary if a charge
moves from the + to the – plate?
79
Rules:
• Fields are vectors, the have strength/intensity and
direction.
• Lines start on + end on neg.
• Direction arrows determined by an imaginary +
test charge.
• Electric Field lines never touch, cross, or angle
sharply.
• Density/spacing of lines shows strength.
80
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.
Challenge: What would a positive charge feel at z?
z
small
negative
• large
positive
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Hwk Watch link “Electric Field
Introduction” Handout
• Watch Elec Field youtube lessons kahn.
• https://www.youtube.com/watch?v=laGSICm_agMhttp://ww
w.youtube.com/watch?v=vaDT4GwAZ2I&feature=relmfu
• And https://www.youtube.com/watch?v=puTZvhOFpRA
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Work & Energy
Electric Potential
Intro to Potential Difference /Voltage
Difference.
•
•
•
•
•
1. Define gravitational PE.
2. What are the units of PE?
3. How is Energy related to work. Explain.
4. What are the units of work?
5. What is the equation for work?
87
Gravitational Field
Every Height associated with dif PEg = mgDh
Takes work to lift mass to height.
h
88
It takes work to push q around in E field.
• Each point called potential.
• Like a height
• Charge has PEelc in E.
• Do work to push q in E field, q gains
PE.
•
q released in field, q loses PE, to KE.
89
It takes work to move charges in a field.
W = DPE.
• Where does a +test charge have more PE – point A or B?
• It takes more work to push +q to B.
• A is at higher potential, V.
• High potential q feels big push.
• Voltage, V = work done/C to push
charge between points in
E field.
• Units J/C
Voltage
Work done on every coulomb of charge moving it is
called electric potential/difference, Voltage.
V = W/q.
W work in J
q is charge in C.
V is Volts = J/C.
V is the electric potential at P at a point.
P is like a particular height in a gravity field.
Equals PE gained or lost per C charge, q.
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 =
2 x 10-6C
75 V or 75 J/C
How much PE did every C of charge gain?
75 J
Rearrange to find DPE of charge or work done by
E field:
W = DPE and V = W/q:
PE elc = qV
W = qV.
PE, W – Joules
Q – Coulombs
V = 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
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.
95
Potential/Voltage Difference
DV simply difference between 2 points in field.
DV pd = 28V – 13V = 15 V.
DV = 15 V
B = 28 V
A = 13 V
If q above is 2C, what is the work
done moving it from A to B?
W = qDV
= 2C(15V) = 30 J
Potential Difference in a Uniform Field.
•
•
•
•
•
Constant on charge between plates
force
E,
Voltage
Work must be done to move charge between plates.
Charge gains or loses PE.
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?
250
0V
250 V
W = q DV = (5 x 10-6C)(250 J/C - 0)
= 1.25 x 10-3J.
4. What physical value is the slope of the line
below? Write the equation.
• Voltage.
• W = qV
99
Natural Potential Difference
“Lightning”
Lightening caused by p.d. cloud /
ground. Cloud bottom becomes
neg, polarizes ground, creates E
field V push until charges
accelerate!
5. Which
way will the
field go?
101
Charges set loose in E
fields will accelerate!
The average lightning bolt
contains 5-10 coulombs
102
Lightening bolt physics 1 min.
• http://www.youtube.com/watch?v=C3y289
F1eQ4
• Senior Physics Voltage
• https://www.youtube.com/watch?v=YLeRb
TlKkJY
103
Label where PE is high and low
-
+
-
+
104
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
Hi PEelc
Low PEelc
105
• Work done by field will accelerate charge:
W = DKE = qV.
• So:
•
•
•
Ebefore
ET
PE
qV
=
=
=
=
E after
ET
KE
½ mv2.
6. Is acceleration between parallel plates uniform?
Explain.
7. An electron is accelerated through a 150-V
pd. What is the maximum speed it can attain?
•
•
•
•
•
Electrice PE lost
=
KEgained.
qV
=
½ mv2.
(1.6 x 10-19C) (150V) = ½ (9.11 x 10-31kg)v2.
v = 2.4 x 10-17 =
4.555 x 10-31v2 .
7.3 x 106 m/s
107
•
•
•
•
•
Prove that
V = Ed for parallel plates
V = Voltage
E = electric field
d = distance between plates.
108
Summery Voltage or Electric Potential
• Potential / V = Energy per Coulomb of a charge at a point
(potential).
•
Potential / Voltage difference / pd
W/C to move q between 2 points at different potentials.
• Charges in field have PEelc.
• When charge accelerates in field PE lost = KE gained.
109
Hwk
• Watch the following clips. Kahn
• http://www.youtube.com/watch?v=wT9AsY79f1k
• https://www.youtube.com/watch?v=Aq31mjWYdJ8
• Optional but good.
• https://www.youtube.com/watch?v=LkIai_KXGxg
110
New Energy unit electron-volt
111
How can I calculate PE of a mass in a
gravity field?
• PEg = mgh
How can I calculate PE of a charge q
in an Electric field?
• PEelc = qV.
112
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
113
To find eV given elementary charges:
(# e )(# V ) = eV.
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.
114
What if 3e- move across 12 V?
36 eV
To find eV (# elm charges) (voltage)
115
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
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
117
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
118
Summery Voltage or Electric Potential
V = Wk per Coulomb to push a charged particle
to point (potential).
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.
119
Some typical voltages
120
Can calculate acceleration of
charges in E fields & through
Voltages.
Set PE elc = KE
121
Plates with battery
d = 1 cm
-
+
AC Delco
12 volts
DVAB  Ed
E  DVAB / d
A
B
E  12 / 0.01
E  12000 N/C
d
Batteries are meant to maintain the potential difference.
122
Electric PE review youtube.
Kahn
http://www.youtube.com/watch?v=w
T9AsY79f1k
123
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