PHYSICS: COURSE MATERIAL 1 and are insulators. In copper, the valence electrons are
ELECTRIC CHARGE essentially free and strongly repel each other. Any
external influence which moves one of them will cause a
Electric charge is the basic physical property of matter repulsion of other electrons which propagates, "domino
that causes it to experience a force when kept in an fashion" through the conductor. Simply stated, most
electric or magnetic field. An electric charge is associated metals are good electrical conductors, most nonmetals
with an electric field and the moving electric charge are not. Metals are also generally good heat conductors
generates a magnetic field. Combination of electric and while nonmetals are not.
magnetic fields is known as the electromagnetic field.
Interaction of the charges generates an electromagnetic
force which is the foundation of Physics.
The two types of electric charges are: Positive and
Negative, commonly carried by charge carriers’ protons
and electrons.
Examples of the types of charges are subatomic
particles or the particles of matter:
• Protons are positively charged
• Electrons are negatively charged Charging by Induction
• Neutrons have zero charge The process of charging an uncharged conductor by
bringing it near a charged conductor without any
Rules in action: physical contact is known as charging by induction
• like charges repel each other
• opposite charges attract each other
The easiest way to demonstrate this is involving a balloon
and rubbing it into your hair. The balloon will attract your
hair by some force. This occurs because of electric charge
which is displaced by rubbing together positively charged
(hair) and negatively charged (balloon).
Conductors and Insulators A positively charged glass rod is brought near the left side
In a conductor, electric current can flow freely, in an of the conducting sphere, attracting negative charge and
insulator it cannot. Metals such as copper typify leaving the other side of the sphere positively charged.
conductors, while most non-metallic solids are said to be Although the sphere is overall still electrically neutral, it
good insulators, having extremely high resistance to the now has a charge distribution, so it can exert an electric
flow of charge through them. force on other nearby charges. Furthermore, the
distribution is such that it will be attracted to the glass
"Conductor" implies that the outer electrons of the rod.
atoms are loosely bound and free to move through the
material. Most atoms hold on to their electrons tightly
THE BOOK LOUNGE PH | 1
, Coulomb’s Law PHYSICS: COURSE MATERIAL 2
Coulomb’s Law gives the force of attraction or repulsion
ELECTRIC POTENTIAL
between two-point charges. If two-point charges q1 and
q2 are separated by a distance r then the magnitude of
the force of repulsion or attraction between them is Introduction
We discussed electric forces and electric fields, and
leaned how to calculate them using Coulomb’s law.
We know that when a force acts on an object the
force may do mechanical work on the object. It
F = Force (N) should thus be no surprise that electric forces can
q = Charge (C) do work on a charged object.
r = distance between the charges (m)
Electrical work is related to electric potential
This is the magnitude of the force which each charge energy, which is analogous in many ways to
exerts on the other charge (recall Newton’s 3rd law). The gravitational potential energy. Using electric
symbol k as used here has to do with electrical forces; it potential energy, we will define a closely related
has nothing to do with any spring constants. If the quantity called the electric potential.
charges q1 and q2 are of the same sign (both positive or
both negative) then the force is mutually repulsive and
the force on each charge points away from the other
charge. If the charges are of opposite signs (one positive,
one negative) then the force is mutually attractive and
the force on each charge points toward the other one.
Electric Field and Force
Electric Potential Energy
When a charged particle with charge q′ at a point P is
acted upon by an electric force F, the electric field E at
Let’s begin by reviewing the relationship between force
that point is defined as
and work. The image shows a region of space in which
the electric field is constant so E has the same
magnitude and direction at all points. A point charge q
in this region experiences an electric force.
The test charge q can be either positive or negative. If it Suppose this charge moves a distance x, starting at
is positive, the directions of E and F are the same; if it is point A and end up in B, and for simplicity we assume
negative, they are opposite this displacement is parallel to electric force. The work
done by the electric force on the charge is
The electric force is conservative, so the work done on the
charge is independent of the path it takes to go from A to
B. We can now define the electric potential energy, which
we denote by PE.
The change in potential energy is associated with a
particular conservative force is equal to -W, where W is
the work done by that force. So, if the electric force does
THE BOOK LOUNGE PH | 2
ELECTRIC CHARGE essentially free and strongly repel each other. Any
external influence which moves one of them will cause a
Electric charge is the basic physical property of matter repulsion of other electrons which propagates, "domino
that causes it to experience a force when kept in an fashion" through the conductor. Simply stated, most
electric or magnetic field. An electric charge is associated metals are good electrical conductors, most nonmetals
with an electric field and the moving electric charge are not. Metals are also generally good heat conductors
generates a magnetic field. Combination of electric and while nonmetals are not.
magnetic fields is known as the electromagnetic field.
Interaction of the charges generates an electromagnetic
force which is the foundation of Physics.
The two types of electric charges are: Positive and
Negative, commonly carried by charge carriers’ protons
and electrons.
Examples of the types of charges are subatomic
particles or the particles of matter:
• Protons are positively charged
• Electrons are negatively charged Charging by Induction
• Neutrons have zero charge The process of charging an uncharged conductor by
bringing it near a charged conductor without any
Rules in action: physical contact is known as charging by induction
• like charges repel each other
• opposite charges attract each other
The easiest way to demonstrate this is involving a balloon
and rubbing it into your hair. The balloon will attract your
hair by some force. This occurs because of electric charge
which is displaced by rubbing together positively charged
(hair) and negatively charged (balloon).
Conductors and Insulators A positively charged glass rod is brought near the left side
In a conductor, electric current can flow freely, in an of the conducting sphere, attracting negative charge and
insulator it cannot. Metals such as copper typify leaving the other side of the sphere positively charged.
conductors, while most non-metallic solids are said to be Although the sphere is overall still electrically neutral, it
good insulators, having extremely high resistance to the now has a charge distribution, so it can exert an electric
flow of charge through them. force on other nearby charges. Furthermore, the
distribution is such that it will be attracted to the glass
"Conductor" implies that the outer electrons of the rod.
atoms are loosely bound and free to move through the
material. Most atoms hold on to their electrons tightly
THE BOOK LOUNGE PH | 1
, Coulomb’s Law PHYSICS: COURSE MATERIAL 2
Coulomb’s Law gives the force of attraction or repulsion
ELECTRIC POTENTIAL
between two-point charges. If two-point charges q1 and
q2 are separated by a distance r then the magnitude of
the force of repulsion or attraction between them is Introduction
We discussed electric forces and electric fields, and
leaned how to calculate them using Coulomb’s law.
We know that when a force acts on an object the
force may do mechanical work on the object. It
F = Force (N) should thus be no surprise that electric forces can
q = Charge (C) do work on a charged object.
r = distance between the charges (m)
Electrical work is related to electric potential
This is the magnitude of the force which each charge energy, which is analogous in many ways to
exerts on the other charge (recall Newton’s 3rd law). The gravitational potential energy. Using electric
symbol k as used here has to do with electrical forces; it potential energy, we will define a closely related
has nothing to do with any spring constants. If the quantity called the electric potential.
charges q1 and q2 are of the same sign (both positive or
both negative) then the force is mutually repulsive and
the force on each charge points away from the other
charge. If the charges are of opposite signs (one positive,
one negative) then the force is mutually attractive and
the force on each charge points toward the other one.
Electric Field and Force
Electric Potential Energy
When a charged particle with charge q′ at a point P is
acted upon by an electric force F, the electric field E at
Let’s begin by reviewing the relationship between force
that point is defined as
and work. The image shows a region of space in which
the electric field is constant so E has the same
magnitude and direction at all points. A point charge q
in this region experiences an electric force.
The test charge q can be either positive or negative. If it Suppose this charge moves a distance x, starting at
is positive, the directions of E and F are the same; if it is point A and end up in B, and for simplicity we assume
negative, they are opposite this displacement is parallel to electric force. The work
done by the electric force on the charge is
The electric force is conservative, so the work done on the
charge is independent of the path it takes to go from A to
B. We can now define the electric potential energy, which
we denote by PE.
The change in potential energy is associated with a
particular conservative force is equal to -W, where W is
the work done by that force. So, if the electric force does
THE BOOK LOUNGE PH | 2
