Download Chapter 1- Fundamentals of Electricity

Fundamentals of Electricity
OBJECTIVES
After
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completing this chapter, the student will be able to:
Define atom, matter, element, and molecule.
List the parts of an atom.
Define the valence shell of an atom.
Identify the unit for measuring current.
Draw the symbol used to represent current flow in a circuit.
Describe the difference between conductors, insulators, and semiconductors.
Define difference of potential, electromotive force, and voltage.
Draw the symbol used to represent voltage.
Identify the unit used to measure voltage.
Define resistance.
Identify characteristics of resistance in a circuit.
Identify the unit for measuring resistance.
Draw the symbol used to represent resistance in a circuit.
See accompanying
Chapter 1.
CD for interactive
Everything, whether natural or man-made, can
be broken down into either an element or a compound. However, the smallest part of each of
these is the atom.
The atom is made up of protons, neutrons,
and electrons. The protons and neutrons group
together to form the center of the atom called the
presentations
and tutorials
relating to
nucleus. The electrons orbit the nucleus in shells
located at various distances from the nucleus.
When appropriate external force is applied to
electrons in the outermost shell, they are knocked
loose and become free electrons. The movement
of free electrons is called current. The external
force needed to create this current is called voltage.
3
SECTION 1 DC CIRCUITS
As it travels along its path, the current encounters
some opposition, called resistance.
This chapter looks at how current, voltage,
and resistance collectively form the fundamentals
of electricity.
11II MATTER,
ELEMENTS,
AND COMPOUNDS
Matter is anything that occupies space and has
weight. It may be found in anyone
of three
states: solid, liquid, or gas. Examples of matter include the air we breathe, the water we drink, the
clothing we wear, and ourselves. Matter may be
either an element or a compound.
An element is the basic building block of nature. It is a substance that cannot be reduced to a
simpler substance by chemical means. There are
now over 100 known elements (Appendix 2). Examples of elements are gold, silver, copper, and
oxygen.
The chemical combination of two or more elements is called a compound (Figure 1-1). A compound
can be separated by chemical but not by physical
means. Examples of compounds are water, which
consist of hydrogen and oxygen, and salt. which
consists of sodium and chlorine. The smallest part of
the compound that still retains the properties of the
compound is called a molecule. A molecule is the
chemical combination of two or more atoms. An
atom is the smallest particle of an element that retains the characteristic of the element.
The physical combination of elements and
compounds is called a mixture. Examples of mixtures include air, which is made up of oxygen, nitrogen, carbon dioxide, and other gases, and salt
water, which consists of salt and water.
FIGURE 1-1
The chemical combination of two or more elements is called a compound. A molecule is the chemical combination of two or more
atoms. Examples are water (H20) and salt (NaC!).
ELEMENT
COMPOUND
1
ELEMENT 2
CHAPTER 1
"1-1 QUESTIONS
1. In what forms can matter be found?
2. What is a substance called that cannot be
reduced to a simpler substance by
chemical means?
3. What is the smallest possible particle that
retains the characteristic of a compound?
4. What is the smallest possible particle that
retains the characteristic of an element?
1m
A CLOSER LOOK
AT ATOMS
As previously stated, an atom is the smallest particle of an element. Atoms of different elements differ from each other. If there are over 100 known
elements, then there are over 100 known atoms.
Every atom has a nucleus. The nucleus is located at the center of the atom. It contains positively charged particles called protons and
uncharged particles called neutrons. Negatively
charged particles called electrons orbit around the
nucleus (Figure 1-2).
FIGURE 1 2
Parts of an atom.
ELECTRON
PROTON
NEUTRON
ORBIT
NUCLEUS
FUNDAMENTALS
OF ELECTRICITY
The number of protons in the nucleus of the
atom is called the element's atomic number. Atomic
numbers distinguish one element from another.
Each element also has an atomic weight. The
atomic weight is the mass of the atom. It is determined by the total number of protons and neutrons in the nucleus. Electrons do not contribute
to the total mass of the atom; an electron's mass is
only 1/1845 that of a proton and is not significant
enough to consider.
The electrons orbit in concentric circles about
the nucleus. Each orbit is called a shell. These shells
are filled in sequence; K is filled first, then L, M,
N, and so on (Figure 1-3). The maximum number of electrons that each shell can accommodate
is shown in Figure 1-4.
The outer shell is called the valence shell and
the number of electrons it contains is the valence.
The farther the valence shell is from the nucleus,
the less attraction the nucleus has on each valence
electron. Thus the potential for the atom to gain
or lose electrons increases if the valence shell is
not full and is located far enough away from the
FIGURE 1 3
The electrons are held in shells around the nucleus.
SECTION I DC CIRCUITS
FIGURE 1-4
FIGURE 1-6
The number of electrons each shell can accommodate.
Copper has a valence of 1.
TOTAL
NUMBEAOF
SHE;LL
DESIGNATION
ELECTRONS
K
2
L
8
M
18
N
32
o
18
P
12
Q
2
nucleus. Conductivity of an atom depends on its
valence band. The greater the number of electrons in the valence shell, the less it conducts. For
example, an atom having seven electrons in the
valence shell is less conductive than an atom having three electrons in the valence shell.
Electrons in the valence shell can gain energy.
If these electrons gain enough energy from an external force, they can leave the atom and become
free electrons, moving randomly from atom to
atom. Materials that contain a large number of
free electrons are called conductors. Figure 1-5
NUCLEUS
compares the conductivity of various metals used
as conductors. On the chart, silver, copper, and
gold have a valence of 1 (Figure 1-6). However,
silver is the best conductor because its free electron is more loosely bonded.
Insulators, the opposite of conductors, prevent
the flow of electricity, Insulators are stabilized by absorbing valence electrons from other atoms to fill
their valence shells, thus eliminating free electrons.
FIGURE 1-7
Insulation properties of various materials used as insulators.
FIGURE 1-5
Conductivity of various metals used as conductors.
Mica
High
Glass
Silver
High
Teflon
Copper
Paper (Paraffin)
Gold
Rubber
Aluminum
Bakelite
Tungsten
Oils
Iron
Procelain
Nichrome
Low
Air
Low
CHAPTER
Materials classified as insulators are compared in
Figure 1-7. Mica is the best insulator because it has
the fewest free electrons in its valence shell. A perfect insulator will have atoms with full valence
shell. This means it cannot gain electrons.
Halfway between conductors and insulators
are semiconductors. Semiconductors are neither good
conductors nor good insulators but are important
because they can be altered to function as conductors or insulators. Silicon and germanium are
two semiconductor materials.
An atom that has the same number of electrons and protons is said to be electrically balanced. A balanced atom that receives one or more
electrons is no longer balanced. It is said to be negatively charged and is called a negative ion. A balanced atom that loses one or more electrons is said
to be positively charged and is called a positive ion.
The process of gaining or losing electrons is called
ionization. Ionization is significant in current flow.
1-2 QUESTIONS
l. What atomic particle has a positive
charge and a large mass?
2. What atomic particle has no charge at all?
3. What atomic particle has a negative
charge and a small mass?
4. What does the number of electrons in the
outermost shell determine?
5. What is the term for describing the
gaining or losing of electrons?
lIB CURRENT
Given an appropriate external force, the movement
of electrons is from negatively charged atoms to
positively charged atoms. This flow of electrons is
called current (I). The symbol I is used to represent
current. The amount of current is the sum of the
charges of the moving electrons past a given point.
1 FUNDAMENTALS
OF ELECTRICITY
An electron has a very small charge, so the
charge of 6.24 X 1018 electrons is added together
and called a coulomb (C). When one coulomb of
charge moves past a single point in one second it
is called an ampere (A). The ampere is named for a
French physicist named Andre Marie Ampere
(1775-1836). Current is measured in amperes.
1-3 QUESTIONS
l. What action causes current in an electric
circuit?
2. What action results in an ampere of current?
3. What symbol is used to represent current?
4. What symbol is used to represent the
unit ampere?
lID VOLTAGE
When there is an excess of electrons (negative
charge) at one end of a conductor and a deficiency
of electrons (positive charge) at the opposite end,
a current flows between the two ends. A current
flows through the conductor as long as this condition persists. The source that creates this excess of
electrons at one end and the deficiency at the
other end represents the potential. The potential is
the ability of the source to perform electrical work.
The actual work accomplished in a circuit is a
result of the difference of potential available at the
two ends of a conductor. It is this difference of potential that causes electrons to move or flow in a
circuit (Figure 1-8). The difference of potential is
referred to as electromotive force (emf) or voltage.
Voltage is the force that moves the electrons in the
circuit. Think of voltage as the pressure or pump
that moves the electrons.
The symbol E is used in electronics to represent voltage. The unit for measuring voltage is
the volt (V), named for Count Alessandro Volta
(1745-1827), inventor of the first cell to produce
electricity.
SECTION 1 DC CIRCUITS
FIGURE 1-8
Electrons flow in a circuit because of the difference of
potential.
1"'-
I
I
I
I
1-
.~-V
I
DIFFERENCE
OF POTENTIAL
+
-..=
-
I
I
_...J
LOAD
ously mentioned, silver, copper, gold, and aluminum are examples of good conductors.
Materials with a high resistance are called insulators. Insulators have few free electrons and offer a high resistance to current flow. As previously
mentioned, glass,rubber, and plastic are examples
of good insulators.
Resistance is measured in ohms, a unit named
for the German physicist George Simon Ohm
(1787-1854). The symbol for the ohm is the
Greek letter omega (Q).
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I
L
1-4
QUESTIONS
1. What force moves electrons in a circuit?
2. What is the term that represents the
potential between the two ends of a
conductor?
3. What symbol is used to represent voltage?
4. What symbol is used to represent the
unit volt?
IlIJ RESISTANCE
As the free electrons move through the circuit,
they encounter atoms that do not readily give up
electrons. This opposition to the flow of electrons
(the current) is called resistance (R).
Every material offers some resistance or
opposition to current flow. The degree of resistance of a material depends on its size, shape,
and temperature.
Materials with a low resistance are called conductors. Conductors have many free electrons and
offer little resistance to current flow. As previ-
1-5
QUESTIONS
1. What is the term used to describe
opposition to current flow?
2. What is the main difference between
conductors and insulators?
3. What is the symbol used to represent
resistance?
4. What is the symbol used to represent the
unit of resistance?
SUMMARY
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• Matter is anything that occupies space.
• Matter can be an element or compound.
• An element is the basic building block of
nature.
• A compound is a chemical combination of
two or more elements.
• A molecule is the smallest unit of a
compound that retains the properties of
the compound.
• An atom is the smallest unit of matter that
retains the structure of the element.
• An atom consists of a nucleus, which
contains protons and neutrons. It also has
one or more electrons that orbit around the
nucleus.
• Protons have a positive charge, electrons
have a negative charge, and neutrons have
no charge.
CHAPTER 1 FUNDAMENTALS
• The atomic number of an element is the
number of protons in the nucleus.
• The atomic weight of an atom is the sum
of protons and neutrons.
• The orbits of the electrons are called shells.
• The outer shell of an atom is called the
valence shell.
• The number of electrons in the valence
shell is called the valence.
• An atom that has the same number of
protons as electrons is electrically
balanced.
• The process by which atoms gain or lose
electrons is called ionization.
• The flow of electrons is called current.
• Current is represented by the symbol I.
• The charge of 6,240,000,000,000,000,000
(or 6.24 X 1018) electrons is called a
coulomb.
• An ampere of current is measured when
one coulomb of charge moves past a given
point in one second.
• Ampere is represented by the symbol A.
• Current is measured in amperes.
• An electric current flows through a
conductor when there is an excess of
CHAPTER
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OF ELECTRICITY
electrons at one end and a deficiency at the
other end.
A source that supplies excess electrons
represents a potential or electromotive
force.
The potential or electromotive force is
referred to as voltage.
Voltage is the force that moves electrons in
a circuit.
The symbol E is used to represent voltage.
A volt (V) is the unit for measuring
voltage.
Resistance is the opposition to current flow.
Resistance is represented by the symbol R.
All materials offer some resistance to
current flow.
The resistance of a material is dependent
on the material's size, shape, and
temperature.
Conductors are materials with low
resistance.
Insulators are materials with high
resistance.
Resistance is measured in ohms.
The Greek letter omega (Q) is used to
represent ohms.
1 SELF-TEST..
'~
~~,
i ~J'
1. What criteria determine whether an atom is a good conductor?
2. What determines whether a material is a conductor, semiconductor, or insulator?
3. Why is it essential to understand the relationship between conductors, semiconductors, and
insulators?
4. Explain the difference between current, voltage, and resistance.
5. Describe how the resistance of a material is determined.
6. Name one standard that can be a resource to you in evaluating the safety compliance of an
electric drill?
7. Where can you determine what health hazard, if any, solder in your lab poses?
8. In comparing electronics laboratory practices with applicable MSDS information (such as
soldering practices), identify changes that could be made to improve safety in the lab.