Concept Summary
Batesville High School Physics
Forces
 By
the early 19th century, physicists had
classified the apparent myriad of forces
in nature to just 3 kinds:
 Gravitational
 Electric
force
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 of the proton is about 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 1018
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
move about are called conductors.
in which charges are not
free to move about are called
insulators.
 Materials
Semiconductors
 Semiconductors
are materials which
are good insulators in pure form, but
their conducting properties can be
adjusted 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 -273oC).
 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

More charge means more force.
 The

size of each charge
distance between the charges
More distance means less force.
Electrical Forces
 The
electrical force between 2 charges
is:
 Directly
proportional to each charge.
proportional to the square of
the distance between the charges.
 Inversely
Coulomb’s Law
Felectric = k
•
Qq
r2
where k = 9.0 x 109 N m2/C2
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:
H20 molecule
Charging by Induction
1.
2.
3.
4.
5.
Bring a charged object near (but not
touching) a neutral object.
Ground the neutral object.
Remove the ground.
Remove the charged object
The neutral object now has a charge
opposite to the charged object.
The End