Electrostatics
(Static Electricity)
Electrostatics can be explained by remembering the
structure of atoms from chemistry class:
Subatomic Particles
Proton
Neutron
In nucleus In nucleus
Electron
Outside
nucleus
Tightly
Bound
Tightly
Bound
Weakly
Bound
Positive
Charge
No
Charge
Negative
Charge
Since protons are tightly bound in the center of the atom, they NEVER leave the atom.
Electrons are loosely bound, so the OFTEN leave the atom.
Neutral vs. Charged Objects
• A neutral atom has the same number of
protons and electrons. Neutral objects
also have the same number of protons
and electrons.
• The electrons contained within some
objects are prone to move to other
objects. In fact, the process of an
electron leaving one object to reside
(perhaps only temporarily) in another
object is a common everyday occurrence.
Charged objects
• Electrons are often removed from and added to an
atom.
• Protons and neutrons are not removable by usual
everyday methods.
• If something gets a negative charge, how did that
happen?
• If something gets a positive charge, how did that
happen?
Check your understanding:
True or False?
1. An object that is positively charged
contains all protons and no electrons.
2. A positively charged object is an object
that has an excess of positive electrons.
3. A positively charged object has more
protons than electrons.
4. Protons have a positive charge.
Charging Objects by Friction
Electric Charge
When a rubber rod is rubbed against fur, electrons
are removed from the fur and deposited on the rod.
Electrons
move from
fur to the
rubber rod.
negative
- positive
- ++++
The rod becomes negatively charged because of an
excess of electrons. The fur becomes positively
charged because of a deficiency of electrons.
Glass and Silk
When a glass rod is rubbed against silk, electrons are
removed from the glass and deposited on the silk.
glass
silk
Electrons
move from
glass to the
silk cloth.
positive
+ +
negative
+ +
- - - -
The glass becomes positively charged because of a
deficiency of electrons. The silk becomes negatively
charged because of a excess of electrons.
Check your understanding:
During a physics lab, a plastic strip was rubbed with
cotton and became positively charged. The correct
explanation for why the plastic strip becomes
positively charged is that ...
a. the plastic strip acquired extra protons from the
cotton.
b. the plastic strip acquired extra protons during the
charging process.
c. protons were created as the result of the charging
process.
d. the plastic strip lost electrons to the cotton during
the charging process.
How do you know which becomes
negative and which becomes positive?
The First Law of Electrostatics
Like charges repel; unlike charges attract.
Neg
Pos
Neg
Pos
Neg Pos
Conductors and Insulators:
Conductors are materials that permit
electrons to flow freely from atom to atom
and molecule to molecule. (metals)
Insulators are materials that impede the
free flow of electrons from atom to atom
and molecule to molecule. (plastic, rubber,
glass)
Conductors and Insulators
If any part of a conductor gets a charge, that
charge will quickly spread out through the entire
object.
If charge is transferred to an insulator at a given
location, the excess charge will remain at the initial
location of charging.
Check your understanding:
A conductor differs from an insulator in that a conductor ______.
a. has an excess of protons
b. has an excess of electrons
c. can become charged and an insulator cannot
d. has faster moving molecules
e. does not have any neutrons to get in the way of electron flow
f. none of these
Check your understanding:
One of these charged spheres is copper and the other is rubber.
The diagram below depicts the distribution of excess negative
charge over the surface of two spheres.
Which sphere is copper, and which sphere is rubber?
Check your understanding:
Suppose that a metal sphere is charged positively by some
method. The charge is initially deposited on the left side of the
sphere. Yet because the metal is a conductor, the charge spreads
uniformly throughout the surface of the sphere. The uniform
distribution of charge is explained by the fact that ____.
a. the charged atoms at the location of charge move throughout
the surface of the sphere
b. the excess protons move from the location of positive charge
to the rest of the sphere
c. excess electrons from the rest of the sphere are attracted
towards the excess protons
What about neutral objects –
are they attracted or repelled
by anything?
• Any charged object - whether positively
charged or negatively charged - will have an
attractive interaction with a neutral object.
Positively charged objects and neutral
objects attract each other; and negatively
charged objects and neutral objects attract
each other.
How can neutral objects be
attracted to charged objects?
• Recall that electrons are loosely held in some atoms
(especially metal atoms.)
• In conducting objects, electrons are so loosely
bound that they may be induced into moving from
one portion of the object to another portion of the
object.
• To get an electron in a conducting object to get up
and go, all that must be done is to place a charged
object nearby the conducting object.
Inducing charge separation in a conductor
•
If a negatively charged balloon is brought near a metal can, the
electrons within the can will experience a repulsive force. The
repulsion will be greatest for those electrons that are nearest the
negatively charged balloon.
•
Many of these electrons will be induced into moving away from the
repulsive balloon. Being present within a conducting material, the
electrons are free to move from atom to atom.
•
This electron movement leaves atoms on the balloon's side of the
can with a shortage of electrons; they become positively charged.
And the atoms on the side opposite of the can have an excess of
electrons; they become negatively charged. The two sides of the
aluminum pop can have opposite charges.
Charge polarization
Polarization is the process of separating opposite
charges within an object. The positive charge
becomes separated from the negative charge. By
inducing the movement of electrons within an object,
one side of the object is left with an excess of
positive charge and the other side of the object is left
with an excess of negative charge.
The polarization process always involves the use of a
charged object to induce electron movement or
electron rearrangement.
Check your understanding:
Which of the diagrams below best
represents the charge distribution on a
metal sphere when a positively charged
plastic tube is placed nearby?
Charge polarization in insulators
But what if the object being polarized is an
insulator? Electrons are not free to move
across the surface of an insulator. How can
an insulator such as a wooden wall be
polarized?
Charge polarization in insulators
In a conducting object, electrons are
induced into movement from one side of
the object to the opposite side.
In an insulator, electrons merely
redistribute themselves within the atom or
molecules nearest the outer surface of the
object.
Charge polarization in insulators
In insulators, the atoms themselves
become polarized:
Example:
• Remember from the computer lab, the
negatively charged balloon was attracted
to the wall?
One more thing…
Polarization is not charging! When an object
becomes polarized, there is simply a
redistribution of the centers of positive and
negative charges within the object.
Yet, there are still equal numbers of positive
charges (protons) and negative charges
(electrons) within the object. While there is
a separation of charge, there is NOT an
imbalance of charge. When neutral objects
become polarized, they are still neutral
objects.
Charging by induction
• We’ve already seen that objects can be
charged by friction (rubbing different
objects together.)
• Induction charging is a method used to
charge an object without actually
touching the object to any other charged
object.
Charging Spheres by Induction
Induction
--- - +
+
++
Uncharged Spheres
--- - +
-+
++
-Isolation of Spheres
- - Electrons
-- Repelled
Separation of Charge
+
+ +
+
-
-
-
-
Charged by Induction
Induction for a Single Sphere
--- - ---+
+
++ ---Uncharged Sphere
--- - ---+ +
++ -Electrons move
to ground.
Induction
Separation of Charge
+
+ +
+
Charged by Induction
The Quantity of Charge
The quantity of charge (q) can be defined in
terms of the number of electrons, but the
Coulomb (C) is a better unit for later work.
One definition is given below:
The Coulomb: 1 C = 6.25 x 1018 electrons
Which means that the charge on a single electron is:
1 electron: e- = -1.6 x 10-19 C
Example If 16 million electrons are
removed from a neutral sphere, what is
the charge on the sphere in coulombs?
1 electron: e- = -1.6 x 10-19 C
-19

-1.6
x
10
C
6 q  (16 x 10 e ) 

1e


+ +
+ + +
+ + + +
+ + +
+ +
q = -2.56 x 10-12 C
Since electrons are removed, the charge
remaining on the sphere will be positive.
Final charge on sphere:
q = +2.56 x 10-12 C
Coulomb’s Law
The force of attraction or repulsion between two
point charges is directly proportional to the product
of the two charges and inversely proportional to the
square of the distance between them.
- q
F
F
r
q’
q
q’
-
-
+
F
Coulumb’s Law
Practice problem:
What is the electrostatic force between two
metal spheres, each having 5.0 C of
charge? The spheres are 0.30 m apart.
CONCLUSION:
Electrostatics