LECTURE # 1

 Charge and Its Types

 Forces Applied in Charges
 Electrical lines, Electric Field and Electric Field Intensity
 Current and Its Types
 An atom is the smallest particle of an
element that retains the characteristics of
that element. Each of the known 118
elements has atoms that are different
from the atoms of all other elements.
This gives each element a unique atomic
structure.
 According to the classic Bohr model, an
atom is visualized as having a planetary
type of structure that consists of a central Figure 1. The Bohr model of an atom showing
electrons in circular orbits around the nucleus.
nucleus surrounded by orbiting electrons. The “tails” on the electrons indicate they are in
motion.
 The nucleus consists of positively charged particles called protons
and uncharged particles called neutrons. The basic particles of
negative charge are called electrons.

 Each type of atom has a certain number of protons that

distinguishes it from the atoms of all other elements.
 For example, the simplest atom is that of hydrogen, which has one
proton and one electron. As another example, the helium atom, has
two protons and two neutrons in the nucleus and two electrons
orbiting the nucleus.

Figure 2. The two simplest atoms, hydrogen and helium

 All elements are arranged in the periodic table of the elements in
order according to their atomic number. The atomic number equals
the number of protons in the nucleus.

 For example, hydrogen has an atomic number of 1 and helium has

an atomic number of 2.

 In their normal (or neutral) state, all atoms of a given element have
the same number of electrons as protons; the positive charges
cancel the negative charges, and the atom has a net charge of zero,
making it electrically balanced.
 As you have seen in the Bohr model,
electrons orbit the nucleus of an
atom at certain distances from the
nucleus and are restricted to these
specific orbits. Each orbit
corresponds to a different energy
level within the atom known as a
shell.

 The shells are designated 1, 2, 3,

and so on, with 1 being closest to
the nucleus. Electrons further from
the nucleus are at higher energy Figure 3. Energy Levels in Hydrogen
levels.
 Electrons that are in orbits farther from the nucleus have higher
energy and are less tightly bound to the atom than those closer to the
nucleus.

 This is because the force of attraction between the positively charged

nucleus and the negatively charged electron decreases with increasing
distance from the nucleus.

 Electrons with the highest energy levels exist in the outermost shell of
an atom and are relatively loosely bound to the atom. This outermost
shell is known as the valence shell, and electrons in this shell are
called valence electrons.

 These valence electrons contribute to chemical reactions and bonding

within the structure of a material, and they determine the material’s
electrical properties.
 If an electron absorbs a photon with sufficient energy, it escapes from
the atom and becomes a free electron.

 Any time an atom or group of atoms is left with a net charge, it is

called an ion.

 When an electron escapes from the neutral hydrogen atom (designated

H), the atom is left with a net positive charge and becomes a positive
ion (designated H+).

 In other cases, an atom or group of atoms can acquire an electron, in

which case it is called a negative ion.
 Copper is the most commonly used metal in electrical applications.

 The copper atom has 29 electrons that orbit the nucleus in four shells.

 The number of electrons in each shell follows a predictable pattern

according to the formula, 2N2 ,where N is the number of the shell.

 The first shell of any atom can have up to two electrons, the second
shell up to eight electrons, the third shell up to 18 electrons, and the
fourth shell up to 32 electrons.
 The fourth or outermost shell, the valence shell,
has only one valence electron.
 The inner shells are called the core.
 When the valence electron in the outer shell of the
copper atom gains sufficient thermal energy, it can
break away from the parent atom and become a
free electron.
 In a piece of copper at room temperature, a “sea”
of these free electrons is present. These electrons
are not bound to a given atom but are free to move
in the copper material.
 Free electrons make copper an excellent conductor
and make electrical current possible. Figure 4. The Copper Atom
 Conductors
 Semiconductors
 Insulators
Conductors
 Conductors are materials that readily allow current. They have a large
number of free electrons and are characterized by one to three valence
electrons in their structure.

 Most metals are good conductors. Silver is the best conductor, and
copper is next. Copper is the most widely used conductive material
because it is less expensive than silver. Copper wire is commonly used
as a conductor in electric circuits.
Semiconductors
 Semiconductors are classed below the conductors in their ability to
carry current because they have fewer free electrons than do
conductors.

 Semiconductors have four valence electrons in their atomic structures.

 However, because of their unique characteristics, certain

semiconductor materials are the basis for electronic devices such as
the diode, transistor, and integrated circuit.

 Silicon and germanium are common semi conductive materials.

Insulators
 Insulators are nonmetallic materials that are poor conductors of
electric current; they are used to prevent current where it is not
wanted.

 Insulators have no free electrons in their structure. The valence

electrons are bound to the nucleus and not considered “free.”
 The charge of an electron and that of a proton are equal in magnitude.

 Electrical charge is an electrical property of matter that exists because

of an excess or deficiency of electrons. Charge is symbolized by the
letter Q.

 A point charge is a hypothetical charge located at a single point in

space.
 Materials with charges of opposite polarity are attracted to each other,
and materials with charges of the same polarity are repelled. A force
acts between charges, as evidenced by the attraction or repulsion. This
force, called an electric field.

Figure 5. Attraction and Repulsion of electrical charges.

 A force acts between charges, as evidenced by the attraction or
repulsion. This force, called an electric field.

Figure 6. Electric field between two oppositely charged

surfaces as represented by lines of force
Electric Field Intensity
The electric field intensity at a point is the force experienced by a unit
positive charge placed at that point.
 Electrical charge (Q) is measured in coulombs, symbolized by C.

 One coulomb is the total charge possessed by 6.25 × 1018 electrons.

 A single electron has a charge of 1.6 × 10-19 C.

 The total charge Q, expressed in coulombs, for a given number of

electrons is stated in the following formula:
number of electrons
Q =
6.25 × 1018 electrons/C
 Consider a neutral atom that is, one that has the same number of
electrons and protons and thus has no net charge.

 When a valence electron is pulled away from the atom by the

application of energy, the atom is left with a net positive charge (more
protons than electrons) and becomes a positive ion.

 If an atom acquires an extra electron in its outer shell, it has a net

negative charge and becomes a negative ion.
 The amount of energy required to free a valence electron is related to
the number of electrons in the outer shell.

 An atom can have up to eight valence electrons. The more complete

the outer shell, the more stable the atom and thus the more energy is
required to remove an electron.
Figure 7. Example of the formation of positive and negative ions
Figure 7. Example of the formation of positive and negative ions
 As you have seen, a force of attraction exists between a positive and a
negative charge.

 A certain amount of energy must be exerted, in the form of work, to

overcome the force and move the charges a given distance apart.

 All opposite charges possess a certain potential energy because of the

separation between them.

 The difference in potential energy per charge is the potential

difference or voltage.

 Voltage is the driving force, sometimes called electromotive force or

emf, in electric circuits and is what establishes current.
 Voltage, symbolized by V, is defined as energy or work per unit
charge.
V=W/Q
Where
V is voltage in volts (V),
W is energy in joules (J),
Q is charge in coulombs (C).

 Some sources use E instead of V to symbolize voltage.

 As you Know, free electrons are available in all conductive and semi
conductive materials. These outer-shell electrons drift randomly in all
directions, from atom to atom, within the structure of the material.

 These electrons are loosely bound to the positive metal ions in the
material, but because of thermal energy, they are free to move about
the crystalline structure of the metal

Figure 16. Random motion of free electrons in a material

 If a voltage is placed across a conductive or semi conductive material,
one end becomes positive and the other negative.

 The repulsive force produced by the negative voltage at the left end
causes the free electrons (negative charges) to move toward the right.

 The attractive force produced by the positive voltage at the right end
pulls the free electrons to the right. The result is a net movement of
the free electrons from the negative end of the material to the positive
end.

Figure 17. Electrons flow from negative to positive

 The movement of these free electrons from the negative end of the
material to the positive end is the electrical current, symbolized by I.

 Electrical current is the rate of flow of charge.

 Current in a conductive material is determined by the number of

electrons (amount of charge) that flow past a point in a unit of time.
I=Q/t

 Where I is current in amperes (A), Q is charge in coulombs (C), and t is

time in seconds (s).
Figure 18. Illustration of 1 A of current (1 C/s) in a material
Direct current (DC)
In a direct current, the electrons flow in one direction. Batteries create a
direct current because the electrons always flow from the 'negative' side
to the 'positive' side.

Alternating Current (AC)

Alternating Current pushes the electrons back and forth, changing the
direction of the flow several times per second.