Electric Force
• One of the four fundamental forces
• Responsible for much of our technology
• Governs chemistry which deals with
interactions of the outer electrons between
atoms or groups of atoms
• Interesting topic because it also involves the
concept of magnetism
Electric Force
• We are all familiar with static electricity
• When the humidity is low, we can walk
across a carpet wearing leather-soled shoes
and give someone a big awakening
• What is happening here?
• Transferring some entity into our fingertips
Electric Charge
• We now know that elementary particles like
the electron and proton have a quality
known as charge.
• Objects that accumulate charge have either
gained or lost electrons
• These objects experience large forces as a
result of being “charged”
Electric Charge
• There are two kinds of charges which we
have called positive and negative
• We have defined the charge of the electron
to be negative and the charge of the proton
to be positive
• What it is interesting is that “Like charges
repel and unlike charges attract!”
Electric Charge
• Earlier we studied some conservation laws that
turned out to be very important
• The conservation of energy, linear momentum and
angular momentum have major consequences for
the way the world behaves
• Electric charge is also a conserved quantity and
for every positive charge there seems to be a
corresponding negative charge
Conservation of Charge
• In any process, the net amount of electric
charge produced is zero!
• This means that if we add up all the positive
charges and all the negative charges moving
around in a process, the total charge will
sum to zero
• So we can neither create nor destroy charge
Electric Charge
• Charged objects are composed of atoms, a
few of which have either gained or lost an
electron
• Those special atoms are called ions
• On a macroscopic level, ions eventually
gain or lose an electron from contact with
something else and return to neutral status
Insulators and Conductors
Insulators and Conductors
• Materials such as metals conduct electricity
• This means that charges can move freely
through the material (electrons do the
moving)
• Other materials, such as wood or glass do
not conduct electricity
• Electrons cannot move freely in these
materials
Insulators and Conductors
• A few materials fall somewhere between
these two extremes and are called
semiconductors
• Examples are silicon and germanium
• The key is whether or not the electrons in
the material are tightly bound to individual
atoms
Insulators and Conductors
• In metals, not all the electrons are required
for bonding the atoms into a lattice
• At least one electron per atom is free to
move around at will in the material
• It is almost like the molecules of air can
move freely anywhere in a room
Insulators and Conductors
• Since the metals have free electrons, they
can easily allow charges to move
• In insulators, the electrons are tightly bound
to individual atoms and are not free to move
• The result is they do not conduct electricity
• In semiconductors, only a small fraction of
the electrons are able to move. We’ll learn
more next semester
Induced Charge
Touch a neutral
metal rod with a
charged metal rod
and you will transfer
some charges to the
neutral rod
Induced Charge
We have induced charges at the
ends of the neutral rod.
Bring a charged
metal rod close to a
neutral metal rod
and you can
separate charges on
the neutral rod.
Note charge
conservation is at
work.
Induced Charge
The earth is large and
can conduct. So, when
you separate the charges
and allow the electrons
to flow to the earth, you
can leave a charged
object behind.
Coulomb’s Law
By sharing charges
between two objects,
Coulomb was able to
make charges that were
1/2, 1/4, etc. of the
original charge on a rod.
Q1Q2
F k 2
r
He then could measure
the forces between two
charged objects.
Coulomb’s Law
• The standard unit of charge is the Coulomb
• We’ll define it later in terms of magnetic
fields, but for now all we need is the
constant k in SI units.
• We want force in Newtons, distance in
meters and charge in Coulombs
• K = 8.988 x 109 N m2/C2
Coulomb’s Law
• The Coulomb is a HUGE charge
• The electron has a charge 1.6 x 10-19 C and
is designated as e.
• The net charge on any object is some
multiple of this fundamental charge
• We say charge is quantized because there is
a smallest value, namely e
Coulomb’s Law
• Strictly speaking, Coulomb’s Law applies
only to point charges (no finite extent in
space)
• However, for spheres, can consider all the
charge concentrated at a point in the center
• Just like gravity applies to a sphere (like the
earth)
Coulomb’s Law
• Compare electrical to gravitational force in
a hydrogen atom
1.6 10   8.2 10
0.5310 
19
Felectrical  9 10
9
Fgravity  6.67 10
2
8
10 2
27
31
1.67
10

9.1110
11
0.5310 
10 2
 3.6110 47
Coulomb’s Law
• The electric force is a vector!
• Superposition of forces Fnet = F1 + F2 + …
Coulomb’s Law
• The electric force is a vector!
• Superposition of forces Fnet = F1 + F2 + …