# electric force

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15 Νοε 2013 (πριν από 4 χρόνια και 7 μήνες)

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Concept Summary

Batesville High School Physics

Forces

By the early 19th century, physicists
forces in nature to just 3 kinds:

Gravitational

force

Electric

force

Magnetic

force

Forces

By the end of the 19th century, they had
narrowed the list to just 2 forces:

Gravitational

force

Electromagnetic

force (We will see in Ch
36 how the electric and magnetic forces
are really just 2 aspects of the same force.)

Forces

The 20th century first added two new
forces to this list that are observed only
inside the atomic nucleus:

Gravitational

force

Electromagnetic

force

Weak

force

Strong

force

Forces

And then found theoretical links that
narrowed the list back to 2 kinds of
forces:

Gravitational

force

Strong/Electroweak

force

The Electric Force

We will turn our attention to the
electric
force
, which is a force between objects
with
charge
, just as the gravitational
force is a force between objects with
mass.

Electrostatics

Electrostatics
is the study of
electric
charge at rest
.

(Or more or less at rest, in contrast with
current electricity.)

Electrical Charges

Electric charge

is a
fundamental
property

of matter.

Two types

of electric charges

Positive
charge
-

every proton has a single
positive charge.

Negative

charge
-

every electron has a single
negative charge.

Electrical Charge

An object with an
excess of electrons

is
negatively charged
.

An object with
too few electrons

(too
many protons) is
positively charged
.

An object with the
same number

of
electrons and protons is
neutral
.

Electrical Forces

Like

charges
repel
.

Opposite

charges
attract
.

Elementary Charges

Protons

carry the smallest positive
charge.

Protons and uncharged neutrons
generally reside in an atom’s
nucleus
.

Protons are held in the nucleus by the
strong force
.

Elementary Charges

The smallest negative charge is the
charge on the
electron
.

In normal atoms, electrons orbit the
nucleus.

The
electric force

between electrons
and protons supplies the centripetal
force to keep electrons in the atom.

Elementary Charges

The
charges
carried by the proton and
electron are
equal in size
.

The
mass

2000
times

the mass of the electron.

Units of Charge

The SI unit of charge is the
Coulomb
.

1 Coulomb = the charge of 6.24 x 10
18

electrons

Charge is Conserved

Electric charge is conserved
-

Electric charge moves

from one place to
another
-

no case of the net creation or
destruction of electric charge has ever
been observed.

In solids, only electrons can move.

In liquids, gasses, and plasmas, both positive
and negative ions are free to move.

Conductors & Insulators

Materials in which
charges are free

to
conductors
.

Materials in which
charges are
not
free

insulators
.

Semiconductors

Semiconductors

are materials which
are good insulators in pure form, but
their
conducting properties can be

over a wide range by
introducing very small amounts of
impurities.

Silicon, germanium, etc.

Transistors, computer chips, etc.

Superconductors

Superconductors

are materials that
lose all resistance

to charge
movement at temperatures near
absolute zero (0 K or about
-
273
o
C).

Recently, “high temperature” (above
100 K) superconductors have been
discovered.

“Creating” an Electric Charge

When you “create” an electric charge
(by rubbing your feet on a carpet) you
are
actually separating existing
charges
-

not creating charges
.

One object ends up with an excess of
electrons (
-

charge), and the other a
deficit of electrons (+ charge).

Charging by Friction

If one neutral material has more affinity
for electrons than another (neutral)
material, it will attract electrons from the
other.

One material becomes negatively
charged, the other positively charged.

Charging by Contact

If a charged object is brought in contact
with a neutral object, charges will be
repelled from (or attracted to) the
charged object.

The neutral object will gain a charge of
the same sign as the charged object.

Grounding

Providing a path from a charged object
to the Earth is called
grounding

it.

Charges will be attracted from (or
repelled to) the Earth by the charged
object.

Since the Earth is so large, both the
charged object and the Earth are
neutralized.

Electrical Forces

The
electrical force

between 2 charges
depends on
:

The
size

of each charge

More charge means more force.

The
distance

between the charges

More distance means less force.

Electrical Forces

The electrical force between 2 charges
is:

Directly
proportional to

each
charge
.

Inversely proportional to

the
square

of
the
distance

between the charges.

where k = 9.0 x 10
9

N m
2
/C
2

Coulomb’s Law

F
electric

=

k

Q q

r
2

Polarization

Bringing a charged object
near (but not
touching)

a neutral object
polarizes

(
temporarily separates
) the charge of
the neutral object.

Like charges in the neutral object are
repelled by the charged object.

Unlike charges in the neutral object are
attracted by the neutral object.

The neutral object returns to normal
when the charged object is removed.

Electric Dipoles

An object that is
electrically neutral

overall, but
permanently polarized
, is
called an
electric dipole
.

Example: H
2
0 molecule

Charging by Induction

1.
Bring a charged object
near (but not
touching)

a neutral object.

2.
Ground the neutral object.

3.
Remove the ground.

4.
Remove the charged object

5.
The neutral object now has a charge
opposite

to the charged object.

The End