Charge is invisibly small

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Charge is invisibly small
but sparks are visible!
What are they made of? Why can we see them?
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The sudden cascade of many
electrons leaping across the gap
produce the visible flash of light
Light is NOT made up of electrons. When electrons
jump they create electromagnetic wavesof light.
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mproton 0.000 000 000 000 000 000 000 000 001
6748 kg melectron0.000 000 000 000 000 000
000 000 000 0009 kg
1836?melectron
mproton
Despite the mass difference, they carry exactly
equal, but opposite, electric charges.
qe electric charge on 1 electron
qP electric charge on 1 proton
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1e ? qe 1.60210?10-19 Coulomb
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1e ? qe 1.60210?10-19 Coulomb
How many electrons are there in 1 Coulomb of
negative charge? How many protons are there
in 1 Coulomb of positive charge?
6,241,800,000,000,000,000 protons
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Two uniformly charged spheres are firmly
fastened by insulated stands to frictionless
pucks which are set on an air table.
The charge on sphere 2 is 3? the charge on sphere
1.
Which force diagram below correctly shows the
magnitude and direction of the electrostatic
forces?












1












2












3












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F
? Q1Q2
electro- static
1 R2
F ?
electro- static
Charles Coulomb (1736 1806)
Q1Q2 R2
F k
electro- static
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Q1Q2 R2
F k
electro- static
k 8.9875 ? 109
Nm2 C2
which we can compare to
m1m2 R2
F G
grav
G 6.67 ? 10 -11
Nm2 kg2
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What force exists between two concentrations of
charge, each 1 coulomb separated by a
center- to-center distance of 1 meter?
(1 C)(1 C) (1 m2)
? 0.2248 lb/N
8.9875?109 N
2,020,390,000 lbs 1,010,195 tons
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The charge holding our balloons apart
STRINGS TENSION
?0.8 grams each
ELECTROSTATIC REPULSION
W ? 0.008 N
BALLOONS WEIGHT
F
T
W
q ? 1 or 2 ?10-7 Coulomb
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4 discrete charges of magnitude, q, and the
indicated (/-) signs, occupy the corners of a
rectangle as shown. The direction of the
electric field, E , at the point shown
is 1) up 2) left 3) right 4) down 5) other
- q
q
1
2
3
4
q
- q
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4 discrete charges of magnitude, q, and the
indicated (/-) signs, occupy the corners of a
rectangle as shown. The direction of the
electric field, E , at the point shown
is 1) up 2) left 3) right 4) down 5) other
- q
q
1
2
3
4
q
- q
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4 discrete charges of magnitude, q, and the
indicated (/-) signs, occupy the corners of a
rectangle as shown. The direction of the
electric field, E , at the point shown
is 1) up 2) left 3) right 4) down 5) other
- q
q
1
q
2
3
- q
4
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2q
-3q
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Similarly, the earth can be thought of as
surrounded by a gravitational field, or g-field.
Locally, near its surface, the g-field is
constant and points down everywhere.
Its constant value (g 9.8 m/sec2) is
represented by the uniform spacing between field
lines.
Note the weight of any object is determined by
multiplying its mass by the local value of g.
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Further away from earths surface the g-field
weakens.
This decrease with distance is seen in the
spreading out of field lines!
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Recall
Mobject g
mass ? acceleration
Similarly
For a localized concentration of charge,Q
q
Q
R
q E
The electric field Force per coulomb of charge
describes the effective strength of a charge
in terms of its potential influence on any other
charge that enters the space surrounding it.
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A spherical conductor carries a charge
uniformly distributed across its surface


E
D
F

C
G



H
J

I















The E-field at the indicated position, points
1.(A)out of the page. 2.(B)into the
page. 3.(C)left into sphere. 4.(D)diagonally
left/up. 5.(E)points into page. 6.(F)diagonally
right/up. 7.(G)points away. 8.(H)diagonally
right/down. 9.(I)points down.
10.(J)diagonally left/down.
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The electric field at the very CENTER of this
spherical conductor


E ?

• points out 4. points left
• points in 5. points right
• points up 6. points down
• 7. is zero

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What is the electric field at the very CENTER of
this spherical conductor?






















E 0!!
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The electric field at this off-CENTER point
within the spherical conductor


E ?

• points out 4. points left
• points in 5. points right
• points up 6. points down
• 7. is zero

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The electric field at this off-CENTER point
within the spherical conductor


The electric field due to charge directly above
this position


will be countered exactly by the that due to
the equal amount of charge just as far away below.


















In fact a ring of equally distant charge will all
have their influence at this point cancelled!
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The electric field at this off-CENTER point
within the spherical conductor




Nearby charges create a strong electric field.














Much farther away, individual charges have a
much smaller effect, but there are much more of
them!




All of this balances beautifully and the electric
field even at off-center points (in fact,
EVERYWHERE) within the conductor is zero!
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Thats why fan motors or transformers (which can
produce fluctuating electric fields) are often
shielded from the more sensitive parts of
circuits by cans of conducting metal.
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Conducting panels when screwed in place provide a
surrounding shield against stray electric fields!
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QUESTION 1
5) other
QUESTION 2
At the very center the electric field due to any
single charge is exactly opposite to the field
created by the charge at the opposite corner!
1) up
QUESTION 3

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Net electric field is up.
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2) left
QUESTION 4

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Net electric field is left.
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G. points away.
QUESTION 5
For a spherical collection of charge the field
lines are all directed along radial lines (for
positive charge pointing straight out from the
center).