Electric Forces and Fields
Static electricity is the accumulation of electrical charges on the
surface of a material, usually an insulator or non-conductor of
electricity. It is called “static” because there is no current flowing
1
Typically, two materials are involved in static electricity, with
one having an excess of electrons or negative (−) charges
on its surface and the other material having an excess of
positive (+) electrical charges.
Atoms near the surface of a material that have lost one or
more electrons will have a positive (+) electrical charge.
2,3
As the neutrally charged person walks across the wool carpet,
his leather soled shoes have less desire for electrons than the
wool carpet. As a result, electrons get stolen from the shoe by
the carpet. With every step the person becomes more and
more positively charged. That charge distributes itself over the
body.
When the positively charged person gets near the metal door
he will actually attract charges from the door which jump in the
form of a spark. Notice how only the negative charges
(electrons) are free to move.
The process causes electrons to be pulled from the
surface of one material and relocated on the surface of
the other material. It is called the triboelectric effect or
triboelectric charging.
The material that loses electrons ends up with an
excess of positive (+) charges. The material that gains
electrons ends up an excess of negative (−) charges
on its surface.
4,5
A substance that is lower on the list TAKES electrons
(becomes negatively charged) from a substance that
is higher on the series.
7
If two items from the list are rubbed together, then the item that
is higher on the list will end up more positive and the lower
one will end up more negatively charged.
For example, if leather were rubbed with wool, the leather
becomes positive and the wool negative. Yet if rubber is rubbed
with wool, the rubber becomes negative and the wool positive.
It is important to note that this series is true only if the
samples are clean and dry. The presence of moisture, dirt,
or oils may cause some of the items to interact differently.
8
Rub a balloon filled with air on a
wool sweater or on your hair. Then
hold it up to a wall. The balloon will
stay there by itself.
When you rub the balloon it picks
up extra electrons from the
sweater or your hair and becomes
slightly negatively charged.
The negative charges in the single balloon are
attracted to the positive charges in the wall.
Tie strings to the ends of two
balloons. Now rub the two balloons
together, hold them by strings at
the end and put them next to each
other. They'll move apart.
Rubbing the balloons gives them
static electricity.
The two balloons hanging by strings both have
negative charges.
There are two kinds of electric charge:
Positive and negative.
A basic law of the universe is that like
charges repel and unlike charges attract.
A negative and a positive charge will attract
each other.
Two positive charges or two negative
charges will repel each other.
9
Electric charge is conserved.
If you rub a balloon on your hair, some of your hair’s
electrons are transferred to the balloon. The balloon
gains a certain amount of electrical charge while
your hair loses an equal amount of negative charge.
10
The positive charge on your hair is equal in
magnitude to the negative charge on the balloon.
Charges can be transferred from one object to
another; however, electric charge is conserved in
this process; no charge is created or destroyed.
11
Electric charge is quantized.
In 1909, Robert Millikan (1886 – 1953) performed
his famous oil drop experiment at the University
of Chicago in which he observed the motion of tiny
oil droplets between two parallel metal plates.
After repeating this process for thousands of
drops, Millikan found that when an object is
charged, its charge is always a multiple of a
fundamental unit of charge, symbolized by the
letter e.
12, 13, 14
Other experiments by Millikan’s time demonstrated that the
electron has a charge of –e and the proton has a charge of
+e.
The value of e (a single electrical charge unit) has since been
determined to be 1.60219 x 10-19 C.
15, 16, 17
The Coulomb (C) is the SI unit of charge.
Materials in which electric charge moves freely, such as copper
and aluminum (most metals) are called conductors.
The animation is showing a neutrally charged conductor and
its response to charged objects being brought near it.
18
As the positively charged rod is brought near the conductor, the
electrons are attracted toward the charged rod. This causes a
force of attraction to be created between the rod and the
conductor.
As the negatively charged rod is brought near the conductor, the
electrons are repelled away from the charged rod. This causes a
force of attraction to be created between the rod and the
conductor.
Notice that only the
electrons are free to move.
It is also important to notice
that when no charged
object is near the
conductor, the electrons
evenly distribute
themselves within the
conductor.
Charging a conductor by induction.
Induction is the process of charging a conductor by
bringing it near another charged object and
grounding the conductor.
Charged object does not touch
the electroscope.
Electroscope ends up oppositely
charged to the object used to charge it.
The first charge is strong and stays
strong each time the electroscope
is recharged. (This is due to the
original object not losing any charge
in the process.)
19
Conduction or Induction
A
B
Induction
Polarization
Materials in which electric charges do not move
freely , such as glass, rubber, silk and plastic are
called insulators.
20
This animation is showing a neutrally charged insulator and
its response to a charged object being brought near it.
In an insulator (such as plastic, rubber, glass, etc) the
electrons are not free to move around the entire object.
They are generally restricted to moving only around the atom
they are attached to.
They can move from one side of the atom to the other but are
unable to leave the atom. As a result, we say that charges
stay where you put them on an insulator.
21,22
Inducing surface charge on
insulators
by polarization
In the animation, the electrons are evenly distributed but are
attached to only one of the positive charges. As the negatively
charged rod is brought near the insulator, the electrons move to
the other side of the positive charges but are unable to move
completely to the far side of the object.
This results in polarization where more positive charges are on
one side of the molecule than on the other.
You should notice that the upper side of the insulator becomes
more positive and feels a force of attraction to the the charged
object. This would also be true if the object was positively
charged. Therefore, a neutral insulator will always be attracted
23,2
to a charged object.
Semiconductors are a third class of materials
characterized by electrical properties that are
somewhere between those of insulators and
conductors.
In their pure state, semiconductors are insulators.
But the carefully controlled addition of specific atoms
as impurities can dramatically increase a
semiconductor’s ability to conduct electric charge.
25, 26, 27
Silicon and Germanium are two well-known
semiconductors that are used in a variety of
electronic devices.
28
Superconductors become perfect conductors when
they are at or below a certain temperature.
29
Insulators and conductors can be charged when
objects come into direct contact with each other.
this process is known as charging by contact.
Examples of charging by contact:
Rubbing a balloon in your hair.
Rubbing a glass rod with silk.
The two rods become oppositely charged and attract
one another.
30, 31
Conductors can be charged by induction. Induction
is the process of charging a conductor by bringing
it near another charged object and grounding the
conductor.
When a conductor is connected to Earth by means
of a conducting wire or copper pipe, the conductor
is said to be grounded.
The Earth can be considered an infinite reservoir for
electrons because it can accept or supply an
unlimited number of electrons.
32, 33, 34
Michael Faraday made his discovery of electromagnetic
induction in 1831, he hypothesized that a changing
magnetic field is necessary to induce a current in a nearby
circuit.
To test his hypothesis he made a coil by wrapping a paper
cylinder with wire. He connected the coil to a galvanometer,
and then moved a magnet back and forth inside the cylinder.
Faraday confirmed that a moving magnetic field is
necessary in order for electromagnetic induction to occur.
35, 36
Charles Coulomb established
The inverse square law for
electric force between two
charges.
He used a torsion balance like
the one shown here in his
experiment.
Coulomb’s experiment showed that electric repulsion obeys a law having
the same form as Newton’s law of gravity. The device, a torsion balance,
measured extraordinarily small forces, relying on a single filament of silk
suspended from a pure silver wire thin as a hair.
38, 39
Because like charges repel, the spheres on the torsion balance twist away
from the other spheres. By knowing the distance between the spheres, the
force needed to twist them (the torque from which the torsion balance gets it
name), and the charges on the spheres, Coulomb could figure out a formula.
Coulomb's law states that
The electrical force between two charged objects is directly
proportional to the product of the quantity of charge on the
objects and inversely proportional to the square of the separation
distance between the two objects. In equation form, Coulomb's
law can be stated as
40, 41
FE represents the electrical force between two objects.
Q1 represents the quantity of charge on object 1 (in Coulombs)
Q2 represents the quantity of charge on object 2 (in Coulombs)
d represents the distance of separation between the two objects
(in meters).
k is a proportionality constant known as the Coulomb's law
constant. The value of this constant is dependent upon the
medium that the charged objects are immersed in. In the case
of air, the value is approximately 8.99 x 109 N • m2 / C2.
42
Electric Fields
A Van de Graaff generator is an Electrostatic generator
which uses a moving belt to accumulate very high
electrostatically stable voltages on a hollow metal globe on
the top of the stand.
A pulley drives an
insulating belt by a
sharply pointed metal
comb which has
been given a positive
charge by a power
supply.
Electrons are
removed from the
belt, leaving it
positively charged. A
similar comb at the
top allows the net
positive charge* to
spread to the dome.
When you approach a Van de Graaf generator, you can
sense the electrical charge surrounding the dome. The
hair on your arms stand up, just a tiny bit if you are more
than a meter away and more if you are closer.
It is obvious that the space surrounding the dome is altered
somehow by the electric charges.
The space is said to
contain an electric
field.
A charged object sets up an electric field in the space
around it.
For example, an electric force holds an electron in orbit
around a proton. In this case, there is no contact between
the objects and the forces are acting “at a distance”.
Here, the electron is interacting
within the electric field of the
proton.
The force that one electric
charge exerts on another can
be described as the interaction
between on charge and the
electric field set up by another.
An electric field is a region in space around a charged
object in which a stationary charged object
experiences an electric force because of its charge.
Electric field is a vector quantity and has both
magnitude and direction.can be represented by a
vector arrow.
Electric field strength depends on charge and
distance.
For any given location, the arrows point in the direction of the
electric field and their length is proportional to the strength
of the electric field at that location.
Such vector arrows are shown in the diagram below. Note that
the length of the arrows are longer when closer to the source
charge and shorter when further from the source charge.
Electrical Field
• Michael Faraday (1791 – 1867)
developed an approach to
discussing fields
• An electric field is said to exist in
the region of space around a
charged object
– When another charged object
enters this electric field, the field
exerts a force on the second
charged object
Electric Field Strength
The only way we can tell if a field exists is to place a test charge
at that spot and see if it feels a force. (In other words, it takes
one to know one.)
When a test charge is brought in, a force is present on that
charge and so it shows evidence of a field being present. The
closer the test charge is brought to the stationary charge the
greater the force. The greater the force on the test charge, the
stronger the field is.
Electric Field Strength
Field Strength is described as the ratio of Force to the amount
of charge.
The field intensity for an electric field is measured in Newtons per
Coulomb [N/C]. This describes the amount of force present for
every coulomb of charge used as a test charge.
The field strength equation (on the next slide) has no way of
specifying the direction of the field, therefore you should ignore
any negative signs that get created in your answer.
Electric Field
• A charged particle, with
charge Q, produces an
electric field in the region
of space around it
• A small test charge, qo,
placed in the field, will
experience a force
Electric Field Definition
ke Q
F
• Mathematically, E 
 2
qo
r
• SI units are N / C
• The electric field is a vector quantity
• The direction of the field is defined to be the
direction of the electric force
that would
be exerted on a small positive test charge
placed at that point.
Electric Field due to a Positive
Spherical or Point Charge
• The electric field produced by a
positive charge is directed away
from the charge
– A positive test charge would be
repelled from the positive
source charge
• The magnitude of the electric
field produced by the positive
charge is
More About a Test Charge and
The Electric Field
• The test charge is required to be a
small charge
– It can cause no rearrangement of the
charges on the source charge
• The electric field exists whether or not
there is a test charge present
Electric Field due to a Negative
Spherical or Point Charge
• The electric field produced by a
negative charge is directed
toward the charge
– A positive test charge would be
attracted to the negative source
charge
• The magnitude of the electric
field produced by the negative
charge is
A charged object
sets up an electric
field around it.
You can show
electric field
patterns using:
Electric
Field
Lines
The concept of electric field lines was introduced by Michael
Faraday.
Although electric field lines don’t really exist, they offer a
useful means of analyzing fields.
Electric field lines reveal information about the direction and
the strength of an electric field within a region of space.
This is useful because the field at each point is often the result
of more than one charge.
If the charge that sets up the field is positive, the field
points away from the charge.
If the charge that sets up the field is negative,
the field points toward the charge.
For a single isolated charge, the lines extend to
infinity.
Electric Field Lines
Rules for Drawing Electric Field Lines
1. The lines must originate on a positive charge (or
infinity) and end on a negative charge (or
infinity).
2. The number of lines drawn leaving a positive
charge or approaching a negative charge is
proportional to the magnitude of the charge.
3. No two field lines can cross each other.
4. The line must be perpendicular to the surface of
the charge
Where the electric field lines are farther apart,
the field is weaker.
The higher the density of the electric field lines
around a charge, the greater the quantity of charge
will be.
The three objects above reveal, that the quantity of
Charge on C is greater than the quantity of charge
on B which is greater than the quantity of charge on A.
• An electric dipole consists of two equal and
opposite charges.
• The high density of lines between the charges
indicates the strong electric field in this region.
Electric Field Line Patterns
Electric Field Line Patterns
• Two equal but like point charges
• The bulging out of the field lines between the charges
indicates the repulsion between the charges
• The low field lines between the charges indicates a
weak field in this region
Electric Field Patterns
• Unequal and unlike
charges
• Note that two lines
leave the +2q
charge for each line
that terminates on -q
The number of electric field lines is proportional
to the electric field strength.
Conductors in Electrostatic
Equilibrium
• When no net motion of charge occurs within a
conductor, the conductor is said to be in
electrostatic equilibrium
• An isolated conductor in electrostatic
equilibrium has four important properties
Conductors – Property 1
When a conductor is in electrostatic equilibrium, only
points on the surface have net charge.
The electric field is zero everywhere inside the
conducting material.
Property 1
• The electric field is zero everywhere inside the
conducting material
– Consider if this were not true:
• If there were an electric field inside the conductor, the
free charge there would move and there would be a flow
of charge
• If there were a movement of charge, the conductor would
not be in equilibrium
Conductors – Property 2
When a conductor is in electrostatic equilibrium, the
charges are concentrated at regions of greater
curvature.
Property 2
• On an irregularly
shaped conductor, the
charge accumulates at
locations where the
radius of curvature of
the surface is smallest
(that is, at sharp points)
• Why a lightning rod
works
Conductors – Property 3
When a conductor is in electrostatic equilibrium, the
Electric field at the surface is perpendicular.
• The electric field just outside a charged
conductor is perpendicular to the
conductor’s surface
Consider what would happen if it
this was not true
– The component along the
surface would cause the charge
to move
– It would not be in equilibrium
Property 4
• Any excess charge on an
isolated conductor resides
entirely on its surface
– A direct result of the 1/r2 repulsion
between like charges in Coulomb’s Law
– If some excess of charge could be
placed inside the conductor, the
repulsive forces would push them as far
apart as possible, causing them to
migrate to the surface
Experiment to Verify Properties of Charges
Michael Faraday used a metal ice pail as a
conducting object to study how charges distribute
themselves when a charged object was placed
inside the pail.
• Faraday’s Ice-Pail Experiment
– A charged object suspended inside a
metal container causes a
rearrangement of charge on the
container so that the sign of the
charge on the inside surface of the
container is opposite the sign of the
charge on the suspended object
– Any charge transferred to a conductor
resides on its surface in electrostatic
equilibrium
Electrostatic spray painting (Powder-coating)
The atomized particles are made to be electrically charged,
thereby repelling each other and spreading themselves
evenly as they exit the spray nozzle.
When a spray gun is charged, the electric field around the
sharp points can be strong enough to produce a corona
around the gun. Air molecules in this area are ionized and
paint droplets pick up negative charges from these
molecules as they pass through the ionized air.
This method also means that paint covers hard to reach
areas. Car body panels and bike frames are two examples
where electrostatic spray painting is often used.
Electrosatic Spray Painting
The End