General Physics I
.Electric field
By electric charge
.Magnetic field
By moving e. charge
·Light
By accelerating charge
Dr.Baba Basha
Objectives of the topic:
• Introduction of Electric charges
• Static Electricity
• Force between electric charges
• Coulomb’s law
• Insulators and conductors
Dr.Baba Basha
Electricity & Magnetism – Lecture 1
Electric Charge
Dr. Baba basha
Dr.Baba Basha
Electricity in Nature
• Most dramatic natural
electrical phenomenon
is lightning.
• Static electricity
(balloons, comb &
paper, shock from a
door knob)
• Uses—photocopying,
ink-jet printing
Dr.Baba Basha
Static electricity
• The electricity developed on objects , when
they are rubbed together, is known as
frictional electricity.
• The frictional electricity is also called static
electricity
Dr.Baba Basha
Static Charge
. How can I demonstrate static charge using an
inflated balloon?
A. Pop it. The sound it makes is due to static charge.
B. Rub it on cloth, rug, or hair, then it will stick to a wall.
C. Rub it on a metal surface, then use it to pick up bits
of paper.
D. Drop it and time its fall. If it falls slower than a rock,
it is affected by static charge.
E. Let the air out slowly. It will be larger than its
original size due to static charge.
Dr.Baba Basha
Charges
• There are two kinds of charges.
• The electric charge developed on glass when
rubbed with silk is different from the charge
developed on ebonite rod when rubbed with
fur.
• The algebraic signs, positive and negative ,
were first given by Benjamin Franklin.
• Like charges repel and unlike charges attract
each other
Dr.Baba Basha
ELECTRIC CHARGES
•
•
•
•
Two types of electric charges:
Protons: positive, +e
Electrons: negative, -e
The unit of an electric charge is Coulomb or
C.
• Basic charge: e = 1.60 x10-19C
Dr.Baba Basha
Electric charges
• Every neutral object contains an equal number
of electrons and protons.
• When the number of electrons and protons
are not equal, the object possesses a net
negative or positive charge.
Dr.Baba Basha
Demonstrations of Electrostatics
•
•
•
•
•
Balloon
Glass rod/silk
Plastic rod/fur
Electroscope
Van de Graaf Generator
Dr.Baba Basha
Glass Rod/Plastic Rod
• A glass rod rubbed with silk gets a positive charge.
• A plastic rod rubbed with fur gets a negative
charge.
• Suspend a charged glass rod from a thread, and
another charged glass rod repels it.
• A charged plastic rod, however, attracts it.
• This mysterious force is called the electric force.
• Many similar experiments of all kinds led
Benjamin Franklin (around 1750) to the
conclusion that there are two types of charge,
which he called positive and negative.
• He also discovered that charge was not created by
rubbing, but rather the charge is transferred from
the rubbing material to the rubbed object, or vice
versa.
Dr.Baba Basha
Forces Between Charges
• We observe that
Like charges repel each other
Opposite charges attract each other
Dr.Baba Basha
The Atom
•
We now know that all atoms are made of positive charges in the nucleus,
surrounded by a cloud of tiny electrons.
Electron
Proton
c
Neutron
Proton charge +e, electron charge
-e
where e = 1.60210-19 C
More accurate picture of the
atom—the Helium atom
Dr.Baba Basha
The Atom
•
We now know that all atoms are made of positive charges in the nucleus,
surrounded by a cloud of tiny electrons.
Electron
Proton
Neutron
Proton charge +e, electron charge -e
where e = 1.60210-19 C
Dr.Baba Basha
Atoms are normally neutral,
meaning that they have exactly the
same number of protons as they do
electrons.
The charges balance, and the atom
has no net charge.
2. Which type of
charge is easiest to
remove from an atom?
A.Proton
B.Electron
The Atom

In fact, protons are VASTLY more difficult to remove, and for all practical
purposes it NEVER happens except in radioactive materials. In this course, we
will ignore this case. Only electrons can be removed

3.If we remove an electron, what is the net charge on the atom?
A.Positive
B.Negative
If we cannot remove a proton, how do we
ever make something charged negatively? By
adding an “extra” electron.
Proton charge +e, electron charge -e
where e = 1.60210-19 C
Dr.Baba Basha
Glass Rod/Plastic Rod Again
We can now interpret what is happening
with the glass/plastic rod experiments.
 Glass happens to lose electrons easily, and
silk grabs them away from the glass
atoms, so after rubbing the glass becomes
positively charged and the silk becomes
negatively charged.
 Plastic has the opposite tendency. It
easily grabs electrons from the fur, so that
it becomes negatively charged while the
fur becomes positively charged.

The ability to gain or lose electrons through
rubbing is called Triboelectricity.
Tribo means rubbing
Dr.Baba Basha
Insulators and Conductors
Both insulators and conductors can be charged.
Insulator
 The difference is that



On an insulator charges are not able to move from
place to place. If you charge an insulator, you are
typically depositing (or removing) charges only from
the surface, and they will stay where you put them.
On a conductor, charges can freely move. If you try
to place charge on a conductor, it will quickly spread
over the entire conductor.
Dr.Baba Basha
Metals and Conduction
Notice that metals are not only good electrical conductors, but they are also
good heat conductors, tend to be shiny (if polished), and are malleable (can
be bent or shaped).
 These are all properties that come from the ability of electrons to move
easily.
This iron atom (26 protons, 26 electrons) has two electrons in its outer shell,
which can move from one iron atom to the next in a metal.

Path of
electron
in a
metal
Dr.Baba Basha
Van de Graaf Generator

Rubber band steals electrons from
glass
Dr.Baba Basha
Electric Force and Coulomb’s Law

We can measure the force of attraction or repulsion between charges, call
them q1 and q2 (we will use the symbol q or Q for charge).
r
 When we do that, we find that the force is proportional to the each of the
charges, is inversely proportional to the distance between them, and is
directed along the line between them (along r).
q q
In symbols, the magnitude of the force is F  k 1 2
where k is some
2
constant of proportionality.
r
 This force law was first studied by Coulomb in 1785, and is called
Coulomb’s Law. The constant k = 8.98755109 N m2/C2 is the Coulomb
constant.

Dr.Baba Basha
Electric Force and Coulomb’s Law


Although we can write down a vector form for the force, it is easier to simply
use the equation for the magnitude, and just use the “like charges repel,
opposites attract” rule to figure out the direction of the force.
Note that the form for Coulomb’s Law is exactly the same as for gravitational
force between two masses
Gk
mq
Note BIG difference,
There is only one “sign”
of mass, only attraction.
Note also that the mass is an intrinsic property of matter. Likewise, charge is also
an intrinsic property. We only know it exists, and can learn its properties, because
of the force it exerts.
Full form of Coulomb’s Law
 Because it makes other equations easier to write, Coulomb’s constant is actually
written
k
1
F
4e 0
1
q1 q2
4e 0
r2
where e0 = 8.8510-12 C2/N-m2 is called the permittivity constant.
Dr.Baba Basha
Spherical Conductors
Because it is conducting, charge on a metal sphere will go everywhere over
the surface.
 You can easily see why, because each of the charges pushes on the others
so that they all move apart as far as they can go. Because of the symmetry
of the situation, they spread themselves out uniformly.
 There is a theorem that applies to this case, called the shell theorem, that
states that the sphere will act as if all of the charge were concentrated at
the center.

Note, forces are equal and opposite
These two situations are the
same
Dr.Baba Basha
Summary
• Charge is an intrinsic property of matter.
• Charge comes in two opposite senses, positive and negative.
• Mobil charges we will usually deal with are electrons, which can be
removed from an atom to make positive charge, or added to an atom to
make negative charge. A positively charged atom or molecule can also be
mobil.
• There is a smallest unit of charge, e, which is e = 1.60210-19 C. Charge
can only come in units of e, so charge is quantized. The unit of charge is
the Coulomb.
• Charge is conserved. Charge can be destroyed only in pairs (+e and –e can
annihilate each other). Otherwise, it can only be moved from place to
place.
• Like charges repel, opposite charges attract. F  1 q1 q2
• The electric force is give by Coulomb’s Law:
4e0 r 2
• Materials can be either conductors or insulators.
• Conductors and insulators can both be charged by adding charge, but
charge can also be induced.
• Spherical conductors act as if all of the charge on their surface were
concentrated at their centers.
Dr.Baba Basha