Transcript Static Electricity - Red Hook Central Schools

Static Electricity
1
What does the term static mean?
Not in motion
Electricity?
Involves electrons
2
Atomic model positively charged
nucleus (protons)
negatively charged electrons
3
Notation
Outer Part
electrons eNucleus
Protons p+
neutrons no
Elementary Charge
–1
+1
0
Neutral objects have same # p+ & e-.
Charged objects have net p+ or e4
When objects have excess or
deficit of charge, can exert
electrostatic force.
5
When objects have excess or deficit of
charge, can exert electrostatic force.
6
Charged objects supply a force.
Proof?
7
In solids,
Charge transferred by e- only.
How can we get positive charge object?
Loss of e-.
8
Uncharged objects can feel
electrostatic force too:
by polarization
9
Polarization
Atoms can be polarized by
redistributing e-. Polarization is
separation of charge not imbalance.
10
Charged balloon causes wall to become
polarized.
11
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.
12
2 types of materials.
• Conductors – allow charges to move
around – can be polarized.
• Insulators – hold excess charge in
place – hard to polarize.
13
Conductors – materials that allow e- to
move freely often redistribute charge.
Metals are good conductors.
14
Metal conductors distribute
charge uniformly.
15
Insulators – charges do not move
freely. Tend to stay concentrated
in one spot on object.
16
What’s happening here?
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Water Stream
• Try at home.
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Polarization produces only a
surface charge.
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Charging Objects:
• 1. Friction – rub 2 neutral objects
together.
• Conduction - Contact with charged object.
• Induction – by bringing charged object in
vicinity of neutral conductor.
20
Friction
Works for insulators and conductors.
Do objects get same
or opposite charge?
Opposite!
21
Why you get a shock.
• Charge yourself transfer e- either to or from
your body to neutralize your charge. Always
accompanied by E release.
22
Conduction: touch charged object to
neutral object.
Do objects get same or
opposite charge?
SAME!
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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.
25
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
28
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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How do we measure amount of
charge?
1. Elementary Charge
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Conductors will share elementary charges
equally if they are allowed to touch.
Example:
1. The elementary charge of each metal sphere
below is shown. If they are allowed to touch,
and are then separated, what will be the
charge on each?
+3
-6
-9
Total charge 3 – 6 – 9 = - 12
They will share the total charge so 12/3 spheres = -4 charges each.
Determining Charge on electron.
33
1909 Robert Millikan measured
charge on e-
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Millikan 1:15
http://www.youtube.com/watch?v=X
MfYHag7Liw
35
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.
36
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 .
37
Charge Units
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 –1
p+ has charge +1
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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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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+
4. What is the total charge (in C)
on 6.2 x 108 electrons?
• 9.9 x 10 -11 C
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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?
Electrostatic Force
Charles Coulomb measured force
exerted on one charged object by
another.
He used torsion balance.
44
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)
46
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?
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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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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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Textbook Probs
Hwk read 17 -2 and pg 634 – 636
Read 634 – 636. Peruse example prb.
Do:
• Pg 636 # 1 – 4 Show equations and work.
• & pg 655 #16-19.
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Mech Universe “Static Electricity”
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Electric Fields
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Electric Fields
region of space around charged
object where a “test charge”
feels an electrostatic force.
Electric Fields-Charge alters space
around it. Charged objects feel a force.
Either repulsion or attraction.
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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
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.
58
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?
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Electric Field Strength is Inversely
Proportional to Distance Around a
Point Charge.
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Sketching Electric Fields
• Do Now
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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.
Sketch vectors to show force
magnitude& direction on a + test
charge at each point.
+
63
Field around positive object.
64
65
The denser the field lines are, the
stronger the field.
Stronger field
near charge.
66
What are the field lines now?
67
68
What if field was formed between
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?
70
Fields have strength and direction.
Density of lines shows strength.
Direction shown as arrows.
Direction is determined by a + test charge.
Electric Field lines don’t touch or cross.
Lines start on + end on neg.
71
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.
Film Mech Universe E fields.
15 minutes.
75
Hwk elect field wksht and
And Rev book 206 #2, 4, 7,9,
10-15, 24-25, 30-34
• Review Elec Field youtube lesson kahn.
• http://www.youtube.com/watch?v=vaDT4Gw
AZ2I&feature=relmfu
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Work & Energy
Electric Potential
Do Now.
• 1. Define gravitational PE.
• 2. How is Energy related to work. Explain.
78
We discussed Force due to charges in 2 ways?
• Coulomb’s Law between 2 charged objects:
Fe = kq1q2.
r2
• As region of space.
• Electric Field
Fe = qE
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Do Charged particles in an electric field
can have Energy?
• What is the definition of Energy?
• Things that have E can do work.
• When work is done on an object it gains
energy.
80
Voltage Potential
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It takes work to move charges in a field.
Where does a positive test charge have
more PE – close to or far from a large
positive sphere?
+
+
82
The amt of work done on every coulomb of
charge moving it is called electric potential, V.
V = W/q.
W work in J
q 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.
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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 – Joules
Q – Coulombs
V = Volts
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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.
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Potential Difference
Volage Dif betw 2 points in field ~ Dheight
Energy Dif involves moving charge in field = qDV
pd = 28V – 13V = 15 V.
B = 28 V
A = 13 V
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.
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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
It helps to
assume 1 plate
is 0, the other
is 250 Volts.
250 V
W = q DV = (5 x 10-6C)(250 J/C - 0)
= 1.25 x 10-3J.
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
• Work done by field will accelerate
charge:
W = DKE = qV.
• So:
•
before
ET
=
after
ET .
Is acceleration between parallel plates
uniform? Explain.
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Uniform Electric Fields
Parallel Plates
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There is a potential difference – voltage
between the two plates based on their
charge & distance between them.
A +q near the positive plate is at a high
potential (E).
What is the potential of a proton stuck to
the negative plate?
(0)
94
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.
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Prove that V = Ed in uniform
field
V = W/q = Fd/q but F/q = E
So
V = Ed
For parallel plates.
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Hwk
• Reg Prob set “Intro to Voltage”.
Old but good Voltage Clip 9:36 min.
http://www.youtube.com/watch?v=F1
p3fgbDnkY&list=UUN7TW2MSHtVLT04z_-t2rg&index=13
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Define
•
•
•
•
Gravitational Field
Electric Field
Electric Potential
Electric Potential Difference
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The electron-volt: 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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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.
100
What if 3e- move across 12 V?
36 eV
To find eV (# elm charges) (voltage)
101
7. 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 8. 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
103
Ex 9: 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
104
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.
105
Charges set lose in E fields
will accelerate!
The average lightning bolt
contains 5 coulombs
106
Some typical voltages
107
Can calculate acceleration of
charges in E fields & through
Voltages.
Set PE elc = KE
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Kahn Elec Potential
Elec Potential & Potential E
109
Prove that for parallel plates
E= V
d
•
V=
W = Fd but
q
q
• Rearranging
F =E
q
V = Ed
E = V
d
110
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.
111
Electric PE review youtube.
Kahn
http://www.youtube.com/watch?v=w
T9AsY79f1k
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