Static Electricity occurs when an excess of
electrons builds up on an object.
Opposite charges will attract
Similar charges repel
It is not necessary for the charges to make contact. The force that
repels or attracts, called the electric force, acts at a distance.
Electric force is the force between two charges.
Atoms will accept or discard electrons as they move
toward a more stable state.
Insulators
Conductors
Do not give up electrons
easily.
Give up their electrons
easily.
Other electrons do not
move through them very
well.
Other electrons flow
through them easily.
Plastic, cloth, glass and
dry air are good
insulators.
Most metals are good
conductors
Humid air and people also
conduct electricity.
Electroscopes are used to detect
static electricity.
Charge Separation
1) a rod with (-) charge is brought near a neutral electroscope.
2) Electrons located at the top of the electroscope will be repelled from the
incoming rod. They will flow down into the leaves, the farthest place from
the (-) source.
3) Since the leaves now have excess electrons, they have an
overall (-) charge.
4) The leaves will repel each other.
http://www.shep.net/resources/curricular/physics/P30/Unit2/electroscope.swf
Charging by Induction
Objects that gain/lose a charge to another object without physical
contact are charged by induction.
Example:
1) Starts like charge separation.
2) While the (-) rod is present, an object touches the electroscope and the
"excess" e- are drained.
3) When the rod is taken away, the electroscope is left with an
overall (+) charge.
The object that touches the electroscope is called a "ground".
http://www.shep.net/resources/curricular/physics/P30/Unit2/electroscope.swf
Charging by Conduction
When objects gain or lose a charge through physical contact with another
object, they are charged by conduction.
Example:
1) A (-) rod is brought to a neutral electroscope.
2) As the rod approaches, e- on the electroscope are repelled toward the
leaves. This leaves the top with an overall (+) charge.
3) The (-) rod makes contact with the head and transfers e-. This is
possible because charge separation left the top of the electroscope with a
(+) charge.
4) When the rod is removed, the electroscope has an overall (-) charge.
http://www.shep.net/resources/curricular/physics/P30/Unit2/electroscope.swf
Grounding can also be used as a source of electrons.
If a positive rod is brought near the knob of a grounded electroscope,
electrons will be attracted from the ground, and the electroscope will
obtain a negative charge.
When this process is employed, the charge induced on the electroscope
is opposite that of the object used to charge it.
Because the rod never touches the electroscope, its charge is not
transferred, and it can be used many times to charge objects by
induction.
Grounding is a process of removing excess
charge from an object by making contact
with the Earth (usually through some other
object).
Since the Earth is a very large sphere,
the charges can be distributed without
changing the overall charge on the
surface.
Coulomb's Law
F=
q1q2
k 2
r
+ indicates repulsion
- indicates attraction
The sign on the force ( + or -)
is related to whether the
charges are attracted or
repelled.
F =
the force on each charge
k =
q1 =
q2 =
r =
the electrostatic constant, 8.99 x 109 N x m2/C2
the quantity of charge 1 measured in Coulombs
the quantity of charge 2 measured in Coulombs
the radius of separation from the center of one
charge to the center of the other. (meters)
Using Coulomb's Law:
A point charge of -1.60 x 10-19 Coulombs is separated from another charge
of +1.60 x 10-19 Coulombs by 5.3 x 10-11m.
What is the magnitude of the force between them?
(kc = 8.99 x 109 N x m2/C2)
Any two charged objects will create a force on each
other.
Opposite charges will produce an attractive force while
similar charges will produce a repulsive force.
+
positive charges have
field lines extending
outward
-
negative charges have
field lines directed
inward
These differences in field
direction explain why
similar charges repel, but
opposite charges attract.
weaker
-
stronger
-
The number of field lines corresponds to the
strength of the charge.
Websites to check out:
http://www.shep.net/resources/curricular/physics/P30/Unit2/electroscope.swf
http://www.colorado.edu/physics/2000/waves_particles/wavpart2.html
http://www.colorado.edu/physics/PhysicsInitiative/Physics2000.05.98/applets/nforcefield.html