basic electricty

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SECTION 1

Basic Electricity



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The purpose of this textbook is to provide the air-

conditioning and refrigeration technician with

knowledge of electricity. Electricity is an extremely

powerful force and should never be treated in a care-

less manner. The air-conditioning and refrigeration

technician commonly works with voltages that range

from 24 volts to 480 volts. One mistake can lead to

serious injury or death.

Never work on an energized circuit if it is pos-

sible to disconnect the power. When possible use a

three-step check to make certain that the power is

turned off. The three-step check is as follows:

1. Test the meter on a known live circuit to make

sure the meter is operating.

2. Test the circuit that is to be de-energized with the

meter.

3. Test the meter on the known live circuit again to

make certain that the meter is still operating.

Install a warning tag at the point of disconnection to

warn people not to restore power to the circuit, Fig-

ure SF–1.

GENERAL SAFETY RULES

Think

Of all the rules concerning safety, this one is prob-

ably the most important. No amount of safeguarding

or “idiot-proofing” a piece of equipment can protect

a person as well as the person taking time to think before acting. Many technicians have been killed by

supposedly “dead” circuits. Do not depend on circuit

breakers, fuses, or someone else to open a circuit.

Test it yourself before you touch it. If you are work-

ing on high voltage equipment, use insulated gloves

and meter probes designed to be used on the voltage

being tested. Your life is your own, so think before

you touch something that can take it away.

Certain pieces of equipment can be especially haz-

ardous if you are not aware of them. Some central air-

A Special Note on Safety

conditioning units use a main contactor that has only

one set of contacts to disconnect a 240-volt circuit,

Figure SF–2. The contactor operates on the principlethat a complete circuit must exist for current to flow.

If one line is broken or open, no current can flow to

the compressor. The hazard lies in the fact that one of

the 240-volt lines is still supplying power to the unit.

If a technician should touch the unbroken line and

ground, a 120-volt circuit is completed through his

body. Other contactors employ two load contacts to

break the circuit to the compressor, Figure SF–3. This

type of contactor is much safer and can prevent a

serious injury.

FIGURE SF–1 Warning tags warn people that the circuit

should not be turned back on.



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240-VOLTINPUT

TOCOMPRESSOR

FIGURE SF–2 Some main contactors use one set of load contacts to break a 240-volt connection to

the compressor.

240 VOLTINPUT

TOCOMPRESSOR

FIGURE SF–3 Contactors that employ two load contacts to break both sides of the 240-volt line are

much safer and can prevent a serious injury.



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Avoid Horseplay

Jokes and horseplay have a time and place, but

the time and place is not when someone is working

on an electric circuit or a piece of moving machinery.Do not be the cause of someone being injured or

killed, and do not let someone else be the cause of

your being injured or killed.

Do Not Work Alone

This is especially true when working in a haz-

ardous location or on a live circuit. Have someone

with you to turn off the power or give artificial respi-ration and/or cardiopulmonary resuscitation (CPR).

One of the effects of electrical shock is that it causes

breathing difficulties and can cause the heart to go

into fibrillation.

Work with One Hand When Possible

The worst case for electrical shock is when the

current path is from one hand to the other. Thiscauses the current to pass directly through the heart.

A person can survive a severe shock between the

hand and one foot that would otherwise cause death

if the current path was from one hand to the other.

Learn First Aid

Anyone working on electrical equipment should

make an effort to learn first aid. This is especiallytrue for anyone who must work with voltages above

50 volts. A knowledge of first aid and especially

CPR may save your life or someone else’s.

Effects of Electric Currenton the Body

Most people have heard that it is not the voltage

that kills but the current. Although this is a true state-ment, do not be misled into thinking voltage cannot

harm you. Voltage is the force that pushes the current

though the circuit. Voltage can be compared to the

pressure that pushes water through a pipe. The more

pressure available, the greater the volume of water

flowing through a pipe. Students often ask how much

current will flow through the body at a particular

voltage. There is no easy answer to this question. The

amount of current that can flow at a particular volt-

age is determined by the resistance of the current

path. Different people have different resistances.A body will have less resistance on a hot day when

sweating because salt water is a very good conduc-

tor. What you ate and drank for lunch can have an

effect on your body resistance. The length of the cur-

rent path can affect the resistance. Is the current path

between two hands or from one hand to one foot? All

of these factors affect body resistance.

The chart in Figure SF–4 illustrates the effects of

different amounts of current on the body. This chartis general and shows the effects on most people.

Some people may have less tolerance to electricity,

and others may have greater tolerance.

A current of 2 to 3 milliamperes will generally

cause a slight tingling sensation. The tingling sensa-

tion will increase as current increases and becomes

very noticeable at about 10 milliamperes. The tingling

sensation is very painful at about 20 milliamperes.Currents between 20 and 30 milliamperes generally

cause a person to seize the line and become unable to

let go of the circuit. Currents between 30 and 40 milli-

amperes cause muscular paralysis, and currents

between 40 and 60 milliamperes cause breathing dif-

ficulty. By the time the current increases to about

100 milliamperes, breathing is extremely difficult.

Currents from 100 to 200 milliamperes generally

cause death because the heart usually goes into fibril-

lation. Fibrillation is a condition in which the heart

begins to “quiver” and the pumping action stops.

Currents above 200 milliamperes generally cause the

heart to squeeze shut. When the current is removed the

heart will typically return to a normal pumping action.

This is the principle of operation of a defibrillator. It is

often said that 120 volts is the most dangerous voltage

to work with. The reason for this is that 120 volts gen-erally cause a current flow between 100 and 200 mil-

liamperes through the bodies of most people. Large

amounts of current can cause severe electrical burns.

Electrical burns are usually very serious because the

burn occurs on the inside of the body. The exterior of

the body may not look seriously burned, but the inside

may be severely burned.



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0.100–0.200 AMPERES (DEATH) THIS RANGE GENERALLY CAUSESFIBRILLATION OF THE HEART. WHEN THEHEART IS IN THIS CONDITION, IT VIBRATESAT A FAST RATE LIKE A “QUIVER” AND

CEASES TO PUMP BLOOD TO THE RESTOF THE BODY.

0.060–0.100 AMPERES (EXTREME DIFFICULTY IN BREATHING)

0.040–0.060 AMPERES (BREATHING DIFFICULTY)

0.030–0.040 AMPERES (MUSCULAR PARALYSIS)

0.020–0.030 AMPERES (UNABLE TO LET GO OF THE CIRCUIT)

0.010–0.020 AMPERES (VERY PAINFUL)

0.009–0.010 AMPERES (MODERATE SENSATION)

0.002–0.003 AMPERES (SLIGHT TINGLING SENSATION)

FIGURE SF–4 The effects of electric current on the body.



6

OBJECTIVES: After studying this unit the student should be able to:

s Discuss basic atomic theory

s Name the basic parts of an atom

s Discuss the law of charges

s Discuss centrifugal force

s Define electricity

s Discuss the differences between conductors and insulators

UNIT

Atomic Structure1

To understand electricity, it is necessary to start

with the study of atoms. The atom is the basic build-

ing block of the universe. All materials are made

from a combination of atoms. An atom is the small-est part of an element. The three principal parts of an

atom are the electron, the neutron, and the proton.

Figure 1–1 illustrates these parts of the atom. Notice

that the proton has a positive charge, the electron

has a negative charge, and the neutron has no charge.

The neutron and proton combine to form the nucleus

of the atom. The electron orbits around the outside of

the nucleus. Notice that the electron is shown to be

larger than the proton. Actually, the electron is about

three times larger than a proton, but the protonweighs about 1,840 times more than an electron. It is

like comparing a soap bubble to a piece of buckshot.

This means that the proton is a very massive particle

in comparison to the electron.

To understand atoms, it is necessary to first under-

stand two basic laws of physics. One of these laws is

the law of charges that states that opposite charges

attract and like charges repel. Figure 1–2 illustrates

this principle. In Figure 1–2, charged balls are sus-pended from strings. Notice that the two balls that

contain opposite charges are attracted to each other.

The two positively charged balls and the two nega-

tively charged balls are repelled from each other.

Since the proton has a positive charge and the electron

has a negative charge, they are attracted to each other.

The second law that must be understood is the law

of centrifugal force. This law states that a spinningobject will pull away from its centerpoint. The faster

an object spins, the greater the centrifugal force

becomes. Figure 1–3 shows an example of this prin-

ciple. If an object is tied to a string, and the object

is spun around, it will try to pull away from you.

The faster the object spins, the greater the force is

that tries to pull the object away. Centrifugal force

keeps the electron from falling into the nucleus of the

atom. The faster an electron spins, the farther away

from the nucleus it will be.

ELECTRON

NEUTRON

PROTON

FIGURE 1–1 Principal parts of an atom.



Atoms have a set number of electrons that can be

contained in one orbit or shell. The number of elec-

trons that can be contained in any one shell is found

by the formula (2 N 2). The letter “ N ” represents the

number of the orbit or shell. For example, the first

orbit can hold no more than two electrons: 2 × (1)2=

2×

1=

2. The second orbit can hold no more than8 electrons: 2 × (2)

2= 2 × 4 = 8. The third orbit can

contain not more than 18 electrons: 2 × (3)2=

2 × 9 = 18. The fourth orbit cannot hold more than

32 electrons: 2 × (4)2= 2 × 16 = 32.

The outer shell of an atom is known as the

valence shell. Any electrons located in the outer

shell of an atom are known as valence electrons.

The valence shell of an atom cannot hold more than

eight electrons. The valence electrons are of primary

concern in the study of electricity, because these

UNIT 1 ATOMIC STRUCTURE 7

electrons explain much of electrical theory. A con-

ductor, for instance, is made from a material that

contains one or two valence electrons. When an atom

has only one or two valence electrons, they are

loosely held by the atom and are easily given up for

current flow. Silver, copper, and aluminum all con-

tain one valence electron. Although all of these mate-rials contain only one valence electron, silver is a

better conductor than copper, and copper is a better

conductor than aluminum. The reason for this is that

an atom of silver is larger than an atom of copper,

and an atom of copper is larger than an atom of alu-

minum. Since an atom of silver is larger than an atom

of copper, it contains more orbits than an atom of

copper. This means that the valence electron of silveris farther away from the nucleus than an atom of

copper. Since the speed an electron spins is deter-

mined by its distance from the nucleus, the valence

electron of silver is spinning around the nucleus at a

faster speed than the valence electron of copper.

Therefore, the valence electron of silver contains

more energy than the valence electron of copper.

When the valence electron of silver is knocked out of

orbit, it simply contains more energy than the

valence electron of copper, and therefore, makes a

FIGURE 1–2 The law of charges states that opposite

charges attract and like charges repel.

FIGURE 1–3 Centrifugal force causes an object to pull away.



better conductor of electricity. Copper is a betterconductor of electricity than aluminum for the same

reason. Figure 1–4 shows an atom of silver and an

atom of copper.

Electricity is the flow of electrons. It is produced

by knocking the electrons of an atom out of orbit by

another electron. Figure 1–5 illustrates this action.

When an atom contains only one valence electron, it

is easily given up when it is struck by another elec-

tron. The striking electron gives its energy to the

electron being struck. The striking electron settles

into orbit around the atom, and the electron that was

struck moves off to strike another electron. This

same action can often be seen in the game of pool.

If the moving cue ball strikes a stationary ball

exactly right, the energy of the cue ball is given to the

stationary ball. The stationary ball then moves off

with most of the energy of the cue ball, and the cue

ball stops moving. Figure 1–6 illustrates this condi-tion. Notice that the stationary ball did not move off

8 SECTION 1 BASIC ELECTRICITY

VALENCEELECTRON

SILVER ATOM

VALENCEELECTRON

COPPER ATOM

FIGURE 1–4 Silver and copper atoms.

FIGURE 1–5 An electron knocked out of orbit by another

electron.

FIGURE 1–6 The cue ball gives energy to the stationary

ball.



with the same energy of the cue ball. It moved off

with most of the energy of the cue ball. Some of the

energy of the cue ball was lost to heat when it struck

the stationary ball. This is true when one electron

strikes another also. This is the reason that a wire

heats when current flows through it. If too much

current flows through a wire, it will overheat and

damage the wire or become a fire hazard.

If an atom that contains two valence electrons is

struck by a moving electron, the energy of the strik-

ing electron is divided between the two valence elec-

trons. Figure 1–7 shows this action. If the valence

electrons are knocked out of orbit, they will contain

only half the energy of the striking electron. This

action can also be seen in the game of pool. If a mov-

ing cue ball strikes two stationary balls at the same

time, the energy of the cue ball is divided between

the two stationary balls. Both of the stationary balls

will move, but with only half the energy of the cue

ball. Materials that are made from atoms that contain

seven or eight valence electrons are known as insula-

tors. Insulators are materials that resist the flow of

electricity. Some good examples of insulator materi-

als are rubber, plastic, glass, and wood. Figure 1–8

illustrates what happens when a moving electron

strikes an atom that contains eight valence electrons.

The energy of the moving electron is divided so

many times that it has little effect on the atom.

UNIT 1 ATOMIC STRUCTURE 9

FIGURE 1–7 Energy is divided between two valence

electrons.

FIGURE 1–8 Energy is divided among eight electrons.

SUMMARY

1. The three major parts of an atom are the electron, proton, and neutron.

2. Electrons have a negative charge, protons have a positive charge, and neutrons have

no charge.

3. The nucleus of an atom contains protons and neutrons.

4. An electron is about three times larger than a proton, but the proton weighs about

1,840 times more than an electron.

5. The law of charges states that opposite charges attract and like charges repel.

6. Centrifugal force is the force that causes a spinning object to pull away from itscenter or axis point.

7. Centrifugal force is proportional to the mass of the object and its speed.



10 SECTION 1 BASIC ELECTRICITY

8. Valence electrons are the electrons located on the outermost shell or orbit of an

atom.

9. Electron impact can be used to knock an electron out of the orbit of an atom.

10. Conductors are materials that conduct electricity very easily.

11. The best conductors are made from materials that generally contain one or two

valence electrons.

12. Insulators are materials that do not conduct electricity very easily.

13. Insulators are made from materials that generally contain seven or eight valence

electrons.

KEY TERMS

atom insulators nucleus

conductor law of centrifugal force proton

electricity law of charges valence electrons

electron neutron valence shell

REVIEW QUESTIONS

1. What are the three subatomic parts of atoms and what charge does each carry?

2. How many times larger is an electron than a proton?

3. The weight of a proton is how many times heavier than that of an electron?

4. State the law of charges.

5. What force keeps the electron from falling into the nucleus of the atom?

6. Materials that make the best conductors contain how many valence electrons?

7. Materials that make the best insulators contain how many valence electrons?

8. What is electricity?

basic electricty

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