Charge is invisibly small but sparks are visible! What are they made of? Why can we see them?

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 manyelectrons leaping across the gapproduce the visible flash of light

Light is NOT made up of electrons.When electrons jump they create“electromagnetic waves”of light.

mproton = 0.000 000 000 000 000 000 000 000 001 6748 kg

melectron=0.000 000 000 000 000 000 000 000 000 0009 kg

mproton =

1e qe =1.602101019 Coulomb

1836melectron

Despite the mass difference, they carryexactly equal, but opposite, electric charges.

qe

electric chargeon 1 electron

qP

electric chargeon 1 proton

=

How many electrons are there in 1 Coulomb of negative charge?

How many protons are there in 1 Coulomb of positive charge?

1e qe =1.602101019 Coulomb

eC

C

/1060210.1

119−×

electrons18102418.6 ×=

protons18102418.6 ×

6,241,800,000,000,000,000 protons

Two uniformly charged spheres are firmly fastened by insulated stands to frictionless

pucks which are set on an air table.

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Which force diagram below correctly shows the magnitude and direction of the electrostatic forces?

The charge on sphere 2 is 3 the charge on sphere 1.

Felectro-static

Q1·Q2

F 1R2electro-

static

Charles Coulomb(1736 – 1806)

F = kQ1·Q2

R2electro-static

F = kQ1·Q2

R2electro-static

k = 8.9875 109 N·m2

C2

which we can compare to:

F = Gm1·m2

R2grav

G = 6.67 10 11 N·m2

kg2

What force exists between two concentrations of charge, each

1 coulomb separated by a center-to-center distance of 1 meter?

)/109875.8( 29

221

CNm

R

qqkF

2×=

=

(1 C)(1 C)

(1 m2)

= 2,020,390,000 lbs

= 1,010,195 tons

0.2248 lb/N= 8.9875109 N

0.8 grams each

W 0.008 NBALLOON’S

WEIGHT

STRING’STENSION

ELECTROSTATICREPULSION

T

F

W

q 1 or 2 107 Coulomb

The charge holding our balloons apart:

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) up2) left 3) right4) down5) other

+q

+q

q

q

1

2 3

4

+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.

Further away from earth’s surface the g-field weakens.

This decrease with distance is seenin the spreading out of field lines!

Recall 2R

MMGF earthobject

grav=

= Mobject g

mass acceleration

2R

MGg earth=

Similarly: For a localized concentration of charge,Q

2R

qQkF

electric= Q R

q

= 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:

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.

E

C G

I

D F

HJ

The E-field at the indicated position, points:

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The electric field at the very CENTER of this spherical conductor

E = ?

1. points out 4. points left2. points in 5. points right3. 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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E = ?

1. points out 4. points left2. points in 5. points right3. points up 6. points down 7. is zero

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

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will be countered exactlyby the that due to the

equal amount of chargejust as far away below.

The electric field due to charge directly above this position

In fact a ring of equally distant charge will all have their influence at this point cancelled!

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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 theelectric field even at off-center points

(in fact, EVERYWHERE) within the conductor is zero!

That’s why fan motors or transformers(which can produce fluctuating electric fields)

are often shielded from the more sensitiveparts of circuits by “cans” of conducting metal.

Conducting panels when screwed in place providea surrounding shield against stray electric fields!

5) otherQUESTION 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) upQUESTION 3

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Net electric field is up.

2) leftQUESTION 4+

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Net electric field is left.

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3

QUESTION 1

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).

Charge is invisibly small but sparks are visible! What are they made of? Why can we see them?

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Transcript of Charge is invisibly small but sparks are visible! What are they made of? Why can we see them?