ELECTROSTATICS
Electrostatics (ie. static electricity) is the study of
electrical forces between stationary charges or
charges that are hardly moving.
Electric charge is a concept that is defined in terms of
the effect it produces. It has no mass, color, length,
or width. It is quantified based on multiples of a
fundamental charge e-.
Examples of electric charge are the nervous system of
the human body and lightning.
Atoms are electrically neutral. They consist of
protons, electrons, and neutrons.
Electrons in an atom occupy different orbits. Those in
orbits near the nucleus are tightly bound to it by their
electrical attraction. Those farthest from the nucleus
are loosely bound.
Since the outer electrons are loosely bound they may
be transferred to other atoms (chemically or by other
means).
If an atom has lost an electron, it becomes overall
positively charged and is called a cation.
If an atom has gained an electron, it becomes overall
negatively charged and is called an anion.
Transfer of Electric Charge
An atom holds on to its electrons by the force of
electrical attraction to its oppositely charged nucleus.
The electrostatic series table lists many of the
substances that can be charged by friction.
The substance that is higher in the table becomes
negatively charged, while the other substance becomes
positively charged.
Electrostatic Series
holds electrons tightly
sulphur
brass
copper
ebonite
paraffin wax
silk
lead
fur
wool
glass
+
holds electrons loosely
Separation of Charge
Induced charge separation is caused by the presence
of a negative or positive distribution of charge on an
object.
Negative distribution of charge:
pith ball
metal leaf electroscope
There is a repulsion between the object and the similar
charge on the far side. However, the strength of the
electric forces between similar and opposite charges
depends on the distance between the charges. As the
distance increases, the magnitude of the force of
attraction or repulsion decreases.
Charge by Contact
When a negatively charged object (ebonite rod) is
touched to a neutral object, some of the excess
electrons on the rod, repelled by their neighbouring
electrons, move over to the neutral object. Both
objects have some of the excess electrons and both
become negatively charged.
If the object is positively charged (glass rod), some of
the electrons on the neutral object are attracted onto
the glass rod to reduce its deficit of electrons. Both
objects share the deficit of electrons and both
become positively charged.
An object charged by contact has the same sign as the
charging rod.
Charge by Induction
An object charged by induction has a charge opposite
to that of the charging rod. (grounding)
Charge Conservation
•
•
•
if a glass rod is rubbed with silk, a positive charge
appears on the rod. Measurement shows that a
negative charge of equal magnitude appears on the
silk.
this suggests that rubbing does not create charge
but only transfers it from one body to another.
the hypothesis of charge conservation was put
forward by Benjamin Franklin and no exception has
been found either at the macroscopic or subatomic
level.
The existence of electric charge and of charge
transfer can be demonstrated in the lab or classroom.
These activities suggest the opposite charges attract
and like charges repel.
This is the basic law of electrostatics.
The SI unit of charge is called the Coulomb.
1 C = the charge on 6.25 x 1018 protons
therefore, the charge on one proton is:
e = 1.60 x 10-19 C
Coulomb used a torsion balance similar to that used by
Cavendish in his study of gravitational forces. Coulomb
would bring small charged spheres towards a charged
sphere on the torsion balance and it would rotate.
The magnitude of the electrical force that one charge
exerts on another charge was determined in 1788:
kq1q 2
Fe 
r2
This force is also known as the Coulomb force.
k = 9.0 x 109 Nm2/C2
q = charge in Coulombs
r = distance in m
Ex. An object A has a positive charge of 6.0 x 10-6 C.
Object B, carrying a positive charge of 3.0 x 10-6 C is
0.030 m away.
a) calculate the force on A.
b) what would be the force if the charge on B were
negative?
Ex. An object A with +6.0 x 10-6 C charge, has two
other charges nearby.
Object B, -3.0 x 10-6 C is 0.040 m to the right. Object
C, +1.5 x 10-6 C, is 0.030 m below. What is the net
force on A?
Electric Fields (p.643)
When you bring an ebonite rod close to a neutral pith
ball, the pith ball is attracted to the rod - charge
separation.
This is an example of an action - at - a - distance force.
SO, how does a charge become aware that there is
another charge near it?
Faraday (1792 - 1867) proposed the concept of a
FIELD.
A field is a property of space. An object influences
the space around it setting up an electric field. This
field in turn exerts a force on other objects located
within it.
The direction of the electric field at any point in space
is:
the direction of the force on a positive test charge at
that point.
Example.
P
+
P
An electric field is defined in terms of the electric
force that acts on a charged particle at a point in
space.
F
  
q
ε: the electric field (N/C or V/m)
F: the force (N)
q+: a small imaginary test charge (C)
The electric field is a property of the space that
surrounds the charge.
Electrical Fields Inside and Surrounding a Conductor
Conductor: materials through which charged particles
move readily.
Charges on a conductor are spread apart as far as
possible until they come to rest in electrostatic
equilibrium. The result is that all the charges are on
the surface of a conductor and there is no net field on
the inside of the conductor.
If the net electric field inside the conductor is zero,
there will be no net electric forces acting on the
charges along the surface.
The electric field will be perpendicular to the surface
at all points. If the shape of the surface is not
symmetrical, the charges will be concentrated near the
surfaces that are more pointed. The electric fields
are greater at these points. If the fields become very
large, the molecules of air near the sharp edge become
ionized and a spark occurs.
Electrical Potential Energy
Two or more like charges have more electric potential
energy when pushed together than when they are apart
because work has been done to bring them together.
Oppositely charged particles have more electric
potential energy when pulled apart than when they are
close together.
Electric potential energy:
Ee 
kq1q2
r
units: Nm (J)
Electric potential difference, V, is the change in
electrical potential energy that occurs when a test
charge, q+ is in the field of any other charge.
Ee kq
V 
q
r
Units - J/C or V (scalar)
1 volt is the change in electrical potential difference
that occurs when 1 Coulomb of charge experiences a
change in electrical potential energy of 1 Joule.
If A and B are two points in an electric field and a test
charge moves from A to B, then, regardless of the
actual path taken by q,
the change in electric
=
potential energy of q, in
moving from A to B
the work required to
move q from A to B,
against the electric field
E e  qV B  qV A
W  Fd
V

d
This is the expression for the magnitude of the
electric field at any point in the space between two
large parallel plates. The electric field direction is
from the + plate to the - plate, in the direction of
decreasing potential.
Volta (1800) developed the voltaic pile.
 A zinc disk and silver disk were used with a piece
of cardboard soaked in salt water.
 When a wire is connected to the bottom zinc disk
to the top silver disk, the pile produced repeated
sparks.
Before the pile, these sparks were created by friction
(this involved work).
The voltaic pile provided a continuous source of charge
flow.
Volta's pile is widely regarded as the first battery.
Batteries generate a difference in electric potential
between two points.
The potential difference is the difference in voltage.
IE. A 12 V battery generates an electric field usually
via a chemical process. The potential difference
V+  VBetween the positive and negative terminals is 12 V.
In order to move a positive charge of 1 C from the to the + terminal of the battery, 12 J of work must be
done against the electric field.
Conventional current

Electron flow 
Current flows from the positive terminal to the
negative terminal.
Electrons flow from the negative terminal to the
positive terminal.
e- : negative, therefore, it attracts to the positive
terminal.
When Ben Franklin arbitrarily chose + and - charges, he
also determined the direction of current.
Current flows from regions of + charge to regions of charge. This is the convention followed to this day.
When JJ Thomson discovered the electron, more was
known about electric charge. Only e- flow in a current
carrying wire.
The directions of current and electron flow are
OPPOSITE!
Current Electricity
When a conductor acquires an excess or deficit of
electrons, we say it has an electric charge. For this to
happen, electrons must be able to move.
When electric charges move from one place to another,
we say that they constitute an electric current.
In metals, electric current is defined as the amount of
charge that moves past a given point in a conductor per
second.
When a total charge, Q, flows through an area in a
time of t, the electric current, I, through the wire is:
I 
q
t
Units - Amperes (A)
1 A is the electric current when 1 C of charge moves
past a point in a conductor in 1 s.
Electric Potential
Electrons move through a conductor and thus
constitute and electric current. What causes them to
move through the conductor? What pushes or pulls
them?
 Voltage pushes the current though the wire.
If there are two oppositely charged particles near one
another, work must be done on the particle to
overcome the electric force and pull it away from the
oppositely charged particle.
Charged particles moving in the presence of an electric
field and converting electric potential energy into some
other form of energy constitute an electric current.
The amount of work that is done on an electric charge
to move it through an electric field is equal to the
increase of electric potential energy, E, of the charge.
V
W
q
Electric potential difference, V, is often referred to
as "voltage".
It is easier to measure current, I, through a potential
difference, V, for a time, t.
Therefore, electrical potential energy, E is
represented by:
E  qV
q  It
E  VIt
To measure the potential difference, V, a voltmeter is
used.
As charges move from one point to another through a
conductor, they lose energy. This is the electric
potential difference.
The charges experience a decrease in electric potential
difference.