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Файл:Professional English for Electrical Specialties. Учебное пособие для СПО
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• In a single (one loop) circuit, the amount of …… at any point is the same as
Quantity
Symbol
Decimal
1 millivolt
1/1000 volt
1 V
1 kilovolt
1000 volts
the amount of ….. at any other point.
• If a circuit containing a voltage source is ….., the full voltage of that source
will appear across the points of the break.
• The +/- orientation of a voltage drop is called the ……. It is also relative
between two points.
Exercise 5. Define the omitted words and compose 8 sentences with them.
Exercise 6. Fill in the table.
Table 2
Exercise 7. Explain the following expressions. Discuss them with your partner,
choose one of the points and compose a dialogue.
1. Alternating voltage can be increased and decreased.
2. The unit of electric pressure is called the Volt.
3. Current can’t flow in a circuit without voltage.
Exercise 8.
a. Make an outline of the text “Voltage”.
b. Retell the text “Voltage” according to the written outline.
Exercise 9. Did you know…?
Read the text and then make questions so that the words in bold provide
answers.
Alessandro Volta (figure 14) (1745-1827), born in Como,
Italy, is best known for discovering current electricity and for
developing the voltaic pile, which became an invaluable tool in
electrochemistry.
Volta found that a current was produced when two
different metal disks such as silver and zinc were separated Figure 14
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by a moist conductor, such as paper soaked in salt water, and brought into contact by a
wire. By stacking a collection of silver-moist paper-zinc units, in effect forming a pile,
Volta determined that the current intensified. If someone touched the top of such a "voltaic
pile" (as this early battery was called) and put his or her other hand in a dish of salt water
that was connected to the bottom metal disk by a strip of metal, that person would feel a
continuous, if weak, shock.
Volta made his discovery of the current electricity–generating voltaic pile known
to the scientific community by 1800. His invention gave rise to new fields of scientific
inquiry, including electrochemistry, electromagnetism, and the modern applications of
electricity. The first chemists to use the voltaic pile were William Nicholson and Anthony
Carlisle, who built a pile and used it to decompose water. Humphry Davy (1778–1829)
used the voltaic pile to decompose many substances, such as potash and soda. Davy was
also able to isolate for the first time several elements, including calcium and magnesium,
using the voltaic pile.
The voltaic pile also had applications in other fields of science. William
Cruikshank discovered the process of electroplating while working with a voltaic pile.
Davy constructed the first crude electric light with the pile in 1820.
Exercise 10. Prepare a short report about A. Volta according to the plan:
• Biography;
• Scientific discoveries;
• The importance of his works.
2.2 Current
Before you start
1. What is current?
2. What is an electric circuit?
3. Can current flow in a circuit without voltage?
Exercise 11. Read and translate the text.
Current is the flow of electrons or the flow of electrical charges. It is what we
understand to be electricity. When a voltage is applied to a conductive material like
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copper, the electrons are pulled from their outer orbit and drift through the conductor. The
result is a transfer of energy from the source to the sink in the form of electrical current.
So long as the voltage source keeps “pushing” in the same direction, the electron flow will
continue to move in the same direction in the circuit. This single-direction flow of
electrons is called a Direct Current, or DC. When the direction of current switches back
and forth it is called an Alternating Current, or AC.
A voltage can exist without inducing a current, such as when a battery is not
connected to a load. In that case, there is no flow of electricity. In order for current to flow
there must be a closed path through which electrons can flow. A closed path that can
conduct electricity is referred to as a “complete” circuit or a “closed”
circuit (figure 15). Once a circuit is completed and a current starts
flowing, then and only then is there a flow of electricity. If there is a
break in a closed circuit that prevents the flow of electricity, it is
referred to as an “open” circuit. Figure 15
The amount of current that flows in an electrical circuit depends on the voltage. In
an electric circuit that always conducts equally
well, the current is directly proportional to the
applied voltage. If you double the voltage, you
double the current. If the voltage is cut in half, the
current is cut in half too. Figure 16 shows this
relationship as a graph in general terms. It assumes
that the power supply can provide the necessary
number of charge carriers.
Figure 16
Current is a measure of the rate at which charge carriers flow. The standard unit is
the ampere. This represents one coulomb (6,240,000,000,000,000,000) of charge carriers
flowing every second past a given point.
An ampere is a comparatively large amount of current. The abbreviation is A. Often,
current is specified in terms of milliamperes, abbreviated mA, where 1 mA = 0.001 A, or a
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thousandth of an ampere. You will also sometimes hear of microamperes (μA), where 1
μA = 0.000001 A or 0.001 mA, which is a millionth of an ampere. It is increasingly
common to hear about nanoamperes (nA), where 1 nA = 0.001 μA = 0.000000001 A,
which is a thousandth of a millionth of an ampere.
A current of a few milliamperes will give you a startling shock. About 50 mA will
jolt you severely, and 100 mA can cause death if it flows through your chest cavity. An
ordinary 100-watt light bulb draws about 1 A of current in a household utility circuit. An
electric iron draws approximately 10 A; an entire household normally uses between 10 and
50 A, depending on the size of the house and the kinds of appliances it has, and also on the
time of day, week, or year.
Exercise 12. Define the meaning of the following words and expressions:
Conductor
Electric current
Wire
Single-direction flow
Drift
Pushing
Alternating current
Direct current
Induce
Battery
A closed circuit
An open circuit
Rate
The ampere
Shock
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Exercise 13. Insert the missing words.
Quantity
Symbol
Decimal
1 milliampere
1/1000 A
1 A or 1 amp
1 kiloampere
1000 amperes
• A ….. circuit is an electric circuit offering little or no resistance to the flow of
electrons. ….. circuits are dangerous with high voltage power sources because the high
currents encountered can cause large amounts of heat energy to be released.
• An open circuit is one where the continuity has been broken by an
interruption in the path for …….. to flow.
• A ……. circuit is one that is complete, with good continuity throughout.
• A device designed to open or close a circuit under controlled conditions is
called a …….
• The terms “open” and “closed” refer to switches as well as entire circuits. An
open switch is one without continuity: electrons …… flow through it. A closed switch is
one that provides a direct (low resistance) …… for electrons to flow through.
Exercise 14. Define the omitted words and compose7 sentences with them.
Exercise 15. Read the text, and label the picture
with the names of the circuit components.
An electric current flows through a pathway called
a circuit. It consists of a power source (e.g. a battery)
joined to an unbroken conductor (e.g. a loop of copper
wire) that connects the two ends of the power source
with opposite charges called poles or terminals. Figure 17
An electrical device (e.g. a light bulb) can be added to the circuit so that the electromotive
force in the circuit is transformed into other forms of energy such as light and heat. A
switch can be inserted to turn the current flow off and on.
Exercise 16. Fill in the table.
Table 3
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Exercise 17. Explain the following expressions.
1. Ampere determined the difference between the current and the static charges.
2. The electric current can flow through liquids and through gases.
3. The electrolytes change greatly when the current passes through them.
4. The flow of moving electrons is a form of the electric current.
5. The current is said to flow from the positive end of the wire to the negative end.
Exercise 18.
a. Make an outline of the text “Current”.
b. Retell the text “Current” according to the written outline.
Exercise 19. Did you know…?
Read the text and then make questions so that the words in bold provide
answers.
21 Golden Safety Rules
A safe work environment is not always enough to control all potential electrical
hazards. You must be very cautious and work safely. Safety rules help you control your
and others risk of injury or death from workplace hazards.
If you are working on electrical circuits or with electrical tools and equipment, you
need to use following golden safety rules:
Rule no. 1
Avoid contact with energized electrical circuits. Please don’t make fun of this
rule if you already know this and remember that if something bad occurs – you probably
won’t have second chance.
That’s not funny.
Rule no. 2
Treat all electrical devices as if they are live or energized.
You never know.
Disconnect the power source before servicing or repairing electrical equipment.
The only way to be sure.
Rule no. 3
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Rule no. 4
Use only tools and equipment with non-conducting handles when working on
electrical devices.
Easy to check.
Rule no. 5
Never use metallic pencils or rulers, or wear rings or metal watchbands when
working with electrical equipment. This rule is very easy to forget, especially when you
are showing some electrical part pointing with metallic pencil.
Always be aware.
Rule no. 6
When it is necessary to handle equipment that is plugged in, be sure hands are
dry and, when possible, wear nonconductive gloves, protective clothes and shoes with
insulated soles.
Remember: gloves, clothes and shoes (figure 18).
Figure 18
Rule no. 7
If it is safe to do so, work with only one hand, keeping the other hand at your side or
in your pocket, away from all conductive material. This precaution reduces the likelihood
of accidents that result in current passing through the chest cavity.
If you ever read about current passing through human body you will know, so
remember – work with one hand only.
If you don’t clue about electric current path through human body, read more in
following technical articles:
• Do You Understand What Is Electric Shock?
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• What psychological effect does an electric shock?
Rule no. 8
Minimize the use of electrical equipment in cold rooms or other areas where
condensation is likely. If equipment must be used in such areas, mount the equipment on
a wall or vertical panel.
Rule no. 9
If water or a chemical is spilled onto equipment, shut off power at the main switch
or circuit breaker and unplug the equipment.
Very logical. NEVER try to remove water or similar from equipment while
energized. After all, it’s stupid to do so.
Rule no. 10
If an individual comes in contact with a live electrical conductor, do not touch the
equipment, cord or person. Disconnect the power source from the circuit breaker or pull
out the plug using a leather belt.
Tricky situation and you must be very calm in order not to make the situation even
worse.
Like in previous rules – Always disconnect the power FIRST.
Equipment producing a “tingle” should be disconnected and reported promptly for
Figure 19
Rule no. 11
repair.
Rule no. 12
Do not rely on grounding to mask a defective circuit nor attempt to correct a fault
by insertion of another fuse or breaker, particularly one of larger capacity.
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Rule no. 13
Drain capacitors before working near them and keep the short circuit on the
terminals during the work to prevent electrical shock.
Rule no. 14
Never touch another person’s equipment or electrical control devices unless
instructed to do so.
Don’t be too smart. Don’t try your luck.
Rule no. 15
Enclose all electric contacts and conductors so that no one can accidentally come
into contact with them.
If applicable do it always, if not be very careful.
Rule no. 16
Never handle electrical equipment when hands, feet, or body are wet or perspiring,
or when standing on a wet floor.
Remeber: Gloves and shoes
Rule no. 17
When it is necessary to touch electrical equipment (for example, when checking for
overheated motors), use the back of the hand. Thus, if accidental shock were to cause
muscular contraction, you would not “freeze” to the conductor.
Rule no. 18
Do not store highly flammable liquids near electrical equipment.
Rule no. 19
Be aware that interlocks on equipment disconnect the high voltage source when a
cabinet door is open but power for control circuits may remain on.
Read the single line diagram and wiring schemes – know your switchboard.
Rule no. 20
De-energize open experimental circuits and equipment to be left unattended.
Rule no. 21
Do not wear loose clothing or ties near electrical equipment.
Act like an electrical engineer, you are not on the beach.
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Exercise 20. Compose a dialogue between a professor and a student using the
words from the text “21 Golden Safety Rules”.
2.3 Resistance
Before you start
1. What is resistance?
2. What is the definition of an ohm?
3. What good conductors do you know?
Exercise 21. Read and translate the text.
Electrical resistance is the opposition to the flow of electric current. A perfect
conductor is one in which there is no resistance. In the real world, under normal
circumstances, there is no such thing as a perfect conductor; every material has some
element of resistance.
The characteristic resistance of a material is a function of its atomic structure and
how many electrons are in its outer orbit. In the real world, the total resistance of cable and
wire increases with length, increasing temperature, and decreasing cross-sectional
diameter of the conductor. How the resistance in a circuit affects the applied voltage and
the amount of current, power, and energy consumed is of great importance to the electrical
engineer, electrician, and technician.
For example, when 1 V is placed across 1 Ω of resistance, assuming that the power
supply can deliver an unlimited number of charge carriers, there is a current of 1 A. If the
resistance is doubled to 2 Ω, the current decreases to 0.5 A. If the resistance is cut by a
factor of 5 to 0.2 Ω, the current increases by the
same factor, to 5 A. The current flow, for a
constant voltage, is said to be inversely
proportional to the resistance. Figure 20 is a graph
that shows various currents, through various
resistances, given a constant voltage of 1 V across
the whole resistance. Figure 20
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