Electrostatics
From our previous discussions in class you
do remember the four fundamental
interactions in the universe,
Can you name them?
Four Kinds of Forces
Strong Nuclear Force:
A strong force that holds the
particles of the nucleus of an atom
together. Short range attractive
force that is much larger in
magnitude to the gravitational or the
electromagnetic forces.
Weak Force:
Force involved in transmutation of particles within
the nucleus. Only observed/viewed in radioactive
decay. Stronger only than the gravitational force.
electron
neutron
proton
Four Kinds of Forces
Gravitational Force:
Attractive force that exists
between all objects. The
gravitational force between the
Earth and the moon keeps the
moon in orbit. It may be the
most evident but it is the
weakest of all the forces.
Electromagnetic Force:
Charged particles at rest or in motion exert electric
forces on each other. They give materials their
strength, their ability to bend, squeeze, stretch or
shatter. When charged particles are in motion they
produce magnetic forces on each other. Electric and
magnetic forces are both considered to be aspects of
this single force.
Electromagnetic Force
We are going to study electromagnetic force and its effects and importance on our world for
the next couple of months
To start we look at electric charges at rest called……….
Electrostatics
Study of properties and results of electric
charges at rest
Atom is Neutrally
Charged
Atom– Positive
charge on the
nucleus is exactly
balanced by the
negative charge of
electrons
Electrons
An Electron can be removed
from an atom to create a
positive ion
Freed electron can be:
Unattached and Free – Creating a
negative charged particle
Attached to an atom – Creating a
negative Ion
Electric Forces
• There are two kinds of electric charges:
– Positive – protons – p+
– Negative – electrons – e-
• Charges exert a force on other charges
over a distance.
• Like charges repel.
• Unlike charges attract.
• Electroscope = Instrument to determine
charge.
Materials can be of three kinds:
Insulators – Materials that inhibit
the flow of free charged particles
Examples: wood, air & rubber
Conductors – Materials that allow the
flow of free charged particles
Examples: metal & water
Semiconductors – Intermediate
class, conduction between an
insulator and conductor
Charging of Objects
An object can be charged either
through conduction or induction
Charging by
Conduction
Charging a
neutral body by
touching it with
a charged body
-
-
-
-
-
-+
+
-
Charging of Objects
An object can be charged either
through conduction or induction
Induction
-
Charging a neutral
object by bring a
charged body
close to but not
touching the
object.
-
-
-+
+
+
-
Charging of Objects
An object can be charged either
through conduction or induction
-
Charging by
Induction
-
- +++
-
+
-
+-
+
Charging of Objects
An object can be charged either
through conduction or induction
-
Charging by
Induction
-
-
--
++
+
+
- -
Charles Coulomb
Charles Coulomb (1738-1806)
was a French physicist and
military engineer. Because of his
expertise with simple machines,
he was able to build an apparatus
to measure the electrical force
between two charged objects.
He derived a law, Coulomb’s Law,
which gives the relationship
between charges, their
separation and the electrical
force of attraction or repulsion.
Coulombs Law
Coulombs Law states that the size
of the electric force between two
charged particles depends on the
qsize
is aof
unit
of charges
charge measured
the
and the in
coulombs
“C”
distance between
them.
r is the distance between the charged
objects
2 x 109 Nm2/C2
K is a constant = 9.0
Kqq '
F
r
Coulombs Law
The charge on one electron or one
proton is called an elementary charge
-19
10
e- = -1.60 x
C
-19
p+ = +1.60 x 10 C
One coulomb of charge has
6.25 x 1018 electrons or protons
One lightning bolt may have 10 C of charge
Millikan Oil Drop Experiment
Robert Millikan
1868-1953
American physicist who determined the charge
on an electron using charged oil drop
experiments in 1909.
Millikan Oil Drop Experiment
What Millikan did was to put a charge on a tiny drop of oil, and measure how
strong an applied electric field had to be in order to stop the oil drop from falling.
Since he was able to work out the mass of the oil drop, and he could calculate the
force of gravity on one drop, he could then determine the electric charge that the
drop must have. By varying the charge on different drops, he noticed that the
charge was always a multiple of
-1.6 x 10 -19 C, the charge on a single electron. This meant that it was electrons
carrying this unit charge.
The Millikan oil-drop experiment was far superior to previous determinations
of the charge of an electron. Where other workers had attempted to
measure the quantity by observing the effect of an electric field on a cloud
of water droplets, Millikan used single drops, first of water and then, when he
found these evaporating, of oil. The experiment had broader significance than
a simple refinement of a number. Millikan emphasized that the very nature of
his data refuted conclusively the minority of scientists who still held that
electrons (and perhaps atoms too) were not necessarily fundamental, discrete
particles. And he provided a value for the electronic charge which, when
inserted in Niels Bohr's theoretical formula for the hydrogen spectrum,
accurately gave the Rydberg constant—the first and most convincing proof of
Bohr's quantum theory of the atom.