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Файл:Professional English for Electrical Engineers. Part 1. Учебное пособие
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Resistance has another property. If there is a
current flowing through a resistive material, there is
always a potential difference across the resistive
component (called a resistor). This is shown in Figure
21. In general, this voltage is directly proportional to the
current through the resistor. This behavior of resistors is
useful in the design of electronic circuits, as you will
learn later in this book. Figure 21
Electrical circuits always have some resistance. There is no such thing as a perfect
conductor. When some metals are chilled to temperatures near absolute zero, they lose
practically all of their resistance, but they never become absolutely perfect, resistance-free
conductors. This phenomenon, about which you might have heard, is called
superconductivity.
Just as there is no such thing as a perfectly resistance-free substance, there isn’t a
truly infinite resistance, either. Even air conducts to some extent, although the effect is
usually so small that it can be ignored. In some electronic applications, materials are
selected on the basis of how “nearly infinite” their resistance is.
The standard unit of resistance is the ohm. This is sometimes symbolized by the
uppercase Greek letter omega (Ω). You’ll sometimes hear about kilohms (symbolized k or
kΩ), where 1 kΩ = 1000 Ω, or about megohms (symbolized M or MΩ), where 1 MΩ =
1000 kΩ = 1,000,000 Ω.
Electric wire is sometimes rated for resistivity. The standard unit for this purpose is
the ohm per foot (ohm/ft or Ω/ft) or the ohm per meter (ohm/m or Ω/m). You might also
come across the unit ohm per kilometer (ohm/km or Ω/km).
Exercise 22. Match the words to their synonyms.
opposition impedance
perfect cool down
decrease defiance, resistance
resistance transport
deliver flawless, superlative
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carry belongings
inversely boundless
property reciprocally
chill transfer, carry
infinite grow less or make less
Exercise 23. Compose 7 sentences using the words from the previous exercise.
Each sentence should contain 2 synonyms.
Exercise 24. Put the words into the correct order.
1. In circumstances thing real, under, there no a perfect the; every conductor such
normal material some has element is of world as resistance.
2. a characteristic of resistance is function a of how its atomic material structure and
outer electrons The are in many its orbit.
3. is there a through a resistive is material, there always difference If across the
current potential resistive component a flowing.
4. In decreasing world, the of cable wire increases with, increasing temperature
total, and the resistance real cross-sectional diameter and length of conductor the.
5. metals chilled to When near absolute become zero, lose all of are resistance, some
but they they never their absolutely temperatures perfect, resistance-free practically
conductors.
6. Just is no such isn’t perfectly as a either substance, there a resistance-free truly
infinite as thing resistance, there.
7. the in circuit the and amount current, power, and energy a of resistance applied
consumed is of great voltage How importance to the affects electrical the, electrician, and
engineer technician.
8. When V supply is across of, that the power A can an resistance unlimited
assuming 1 Ω number of charge, there of carriers is a placed 1 deliver current 1.
called.
9. unit standard this ohm meter purpose is the per ohm foot The or the per for.
10. which phenomenon, superconductivity about might you This heard, is have
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Exercise 25. Fill in the table.
Quantity
Symbol
Decimal
1 ohm
1 ohm
1kW
1 megohm
1,000,000 ohms
Table 4
Exercise 26. Translate the sentences below.
1. Свободные электроны в проводнике, перемещаясь по цепи, сталкиваются с
атомами, которые в свою очередь препятствуют потоку электронов, тем самым
уменьшая значение электрического тока.
2. Электрическое сопротивление — физическая величина, характеризующая
свойства проводника препятствовать прохождению электрического тока и равная
отношению напряжения на концах проводника к силе тока, протекающего по нему.
3. 1 Ом — это сопротивление, которое оказывает току ртутный столбик
высотой 106,3 см и сечением 1 мм кв. при температуре 0 градусов по Цельсию.
4. Сопротивление металлических проводников при повышении температуры
увеличивается, сопротивление электролитов (жидких проводников), угля и
некоторых твердых веществ, наоборот, уменьшается.
5. Чем большим сопротивлением обладает проводник, тем меньшую он имеет
проводимость, тем хуже он проводит электрический ток, и, наоборот, чем меньше
сопротивление проводника, тем большей проводимостью он обладает, тем легче
току пройти по проводнику.
6. В 1911 г. Голландский физик Камерлинг-Оннес провел опыты с ртутью,
обнаружив, что удельное сопротивление ртути при температуре 4,2 K (около -269
°C) резко упало до такой малой величины, что его практически стало невозможно
измерить. Это явление обращения электрического сопротивления в нуль КамерлингОннес назвал сверхпроводимостью.
7. Движение электронов в металле, находящемся в состоянии
сверхпроводимости, является до такой степени упорядоченным, что электроны,
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перемещаясь по проводнику, почти не испытывают соударений с атомами и ионами
решетки.
8. Измерительные мосты упомянутого типа могут быть выполнены с
автоматическим уравновешиванием, т. е. в виде так называемых автоматических
мостов, в которых ток IG в гальванометре вызывает срабатывание реверсивного
двигателя, изменяющего отношение R1/R2 до тех пор, пока оно не станет равным
нулю.
9. Резистор (англ. resistor, от лат. resisto - сопротивляюсь), структурный
элемент электрической цепи, основное функциональное назначение которого
оказывать известное (номинальное) сопротивление электрическому току с целью
регулирования тока и напряжения.
10. Измеряемое сопротивление Rx можно сравнить с сопротивлением Rn
эталонного резистора изменением отношения R1/R2 до тех пор, пока показание
нуль- гальванометра G не станет равным нулю.
Exercise 27.
a. Make an outline of the text “Resistance”.
b. Retell the text “Resistance” according to the written outline.
Exercise 28. Did you Know…?
Read the text and then make questions so that the words in bold provide
answers.
Conductance
Electricians and electrical engineers sometimes talk about the conductance of a
material, rather than about its resistance. The standard unit of conductance is the siemens,
abbreviated S. When a component has a conductance of 1 S, its resistance is 1 Ω. If the
resistance is doubled, the conductance is cut in half, and vice versa. Therefore,
conductance is the reciprocal of resistance.
If you know the resistance of a component or circuit in ohms, you can get the
conductance in siemens: divide 1 by the resistance. If you know the conductance in
siemens, you can get the resistance: divide 1 by the conductance. Resistance, as a variable
quantity, is denoted by an italicized, uppercase letter R. Conductance, as a variable
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quantity, is denoted as an italicized, uppercase letter G. If we express R in ohms and G in
siemens, then the following two equations describe their relationship:
G = 1/R
R = 1/G
Units of conductance much smaller than the siemens are often used. A resistance of
1 kΩ is equal to 1 millisiemens (1 mS). If the resistance is 1 MΩ, the conductance is one
microsiemens (1 μS). You’ll sometimes hear about kilosiemens (kS) or megasiemens
(MS), representing resistances of 0.001 Ω and 0.000001 Ω (a thousandth of an ohm and a
millionth of an ohm, respectively). Short lengths of heavy wire have conductance values in
the range of kilosiemens. Heavy metal rods can have conductance in the megasiemens
range.
Determining conductivity is tricky. If wire has a resistivity of 10 Ω/km, you can’t
say that it has a conductivity of 1/10, or 0.1, S/km. It is true that a kilometer of such wire
has a conductance of 0.1 S, but 2 km of the wire has a resistance of 20 Ω (because there is
twice as much wire). That is not twice the conductance,
but half. If you say that the conductivity of the wire is
0.1 S/km, then you might be tempted to say that 2 km
of the wire has 0.2 S of conductance. That would be a
mistake! Conductance decreases with increasing wire
length.
Figure 22 illustrates the resistance and
conductance values for various lengths of wire having
a resistivity of 10 Ω/km. Figure 22
Exercise 29. Discuss the following points with your partner.
1. Do all substances conduct the electric current easily?
2. What is a conductor?
3. What does conductance depend upon?
4. What materials are the best conductors of electricity?
5. Does temperature influence the conductor's resistance?
6. How to calculate conductivity?
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7. How is conductivity measured?
8. What is a conductive solution?
9. What is conductance reference value?
10. How are conductivity and conductance related?
2.4 Power
Before you start
1. What is power?
2. What is the watt?
3. What formulas for power do you know?
Exercise 30. Read and translate the text.
Power is the rate at which work is being done. In physics, work is done when a
force is applied over a distance. The rate at which it is applied, that is, the magnitude of
the force and the speed at which the distance is covered, determines the amount of power
involved.
In the case of electric power, the electromotive force (EMF) does work on the
negatively charged electrons to move them through a distance. The applied voltage
determines the strength of the electric field, and the amount of current is an indicator of
how much work is being done. The power, then, is determined by the magnitude of the
voltage and current; it is, in fact, the product of the instantaneous voltage and the
instantaneous current.
Understanding the power requirements in any given situation is critical to the
success of an event. Before the first case is loaded off the truck and before the first rigging
point is hung, someone on the crew should have already calculated the power requirements
to make sure there is enough power feeding into the building to handle the event and that
the power distribution system is able to safely handle the load. It takes a skilled person to
understand the power requirements well enough to make that determination.
In the SI system, power is measured in watts, and one watt is defined as one joule
per second. The watt is named after James Watt (1736-1819), a Scottish inventor whose
improvements to the steam engine helped usher in the Industrial Revolution.
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Again, notice that power is not the same as energy, and it is very important to
understand the difference between the two. Power is an instantaneous measurement of
how much work is being done, while energy is a measure of how much force is applied
over a distance. Power is usually represented in an equation by the letter P.
Suppose we call the voltage E and the current I, in volts (V) and amperes (A),
respectively. Then the power in watts dissipated by the resistance, call it P, is the product
of the voltage in volts and the current in amperes:
P = EI
If the voltage E across the resistance is caused by two flashlight cells in series,
giving 3 V, and if the current I through the resistance (a light bulb, perhaps) is 0.1 A, then
E = 3 V and I = 0.1 A, and we can calculate the power P in watts as follows:
P = EI = 3 × 0.1 = 0.3 W
Suppose the voltage is 117 V, and the current is 855 mA. To calculate the power, we
must convert the current into amperes: 855 mA = 855/1000 A = 0.855 A. Then:
P = EI = 117 × 0.855 = 100 W
Sometimes you need to use the power equation to find currents or voltages. Then
you should use I = P/E to find current, or E = P/I to find voltage. Always remember to
convert, if necessary, to the standard units of volts, amperes, and watts before performing
the calculations.
Exercise 31. Match the words to their antonyms.
strength decode
instantaneous incongruously
convert dangerously
define weakness
respectively doubt
requirement collection
determination slow
distribution empty
safely hide
load nonessential
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Exercise 32. Match the parts of the sentences below. Define their sequence
according to the text.
The rate at which it is applied, that is, while energy is a measure of how
much force is applied over a
distance.
The applied voltage determines and the amount of current is an
the strength of the electric field, indicator of how much work
is being done.
Before the first case is loaded off the truck to move them through a distance.
and before the first rigging point is hung, in volts and the current in amperes.
someone on the crew should have already
calculated the power requirements to make sure
The watt is named after James Watt (1736-1819), whose improvements to the steam
a Scottish inventor engine helped usher in the
Industrial Revolution.
Power is an instantaneous measurement of is the product of the voltage
how much work is being done,
The power, then, is determined by the the magnitude of the force and the
magnitude of the voltage and current; speed at which the distance is
covered, determines the amount of
power involved.
In the case of electric power, the electromotive it is, in fact, the product of the
force (EMF) does work on the instantaneous voltage and
negatively charged electrons the instantaneous current.
Then the power in watts dissipated there is enough power feeding into
by the resistance, call it P, the building to handle the event
and that the power distribution
system is able to safely handle the
load.
48
Exercise 33. Complete the sentences without referring to the text just
presented.
1. The rate at which work is done is called _________ and is measured in ________.
2. There are three common formulas used for calculating power. List the formulas
here: P = _____________ P = _______________ P=
________________.
3. Power is measured in watts (or kilowatts)
and equals _______________.
4. Use one of the formulas for power to
calculate the power consumed in the following
circuit (figure 23). Figure 23
5. True or false? A 100-watt light bulb costs more to use than a 50-watt light bulb
because more current flows through the 100-watt light bulb and more power is consumed.
6. A kilowatt-hour (kWh) is equivalent to ___________________ watts consumed
in ___________ _________.
Exercise 34. Match the words to their definitions.
Alternating current Unit of current.
Ampere This theory states that electrons flow from positive (+)
to negative (-).
Conductor A material that permits a very free
exchange/movement of electrons from one atom to
another.
Conventional flow Voltage forces electrons to flow in one direction and
then quickly alternate to the opposite direction.
Current This theory states that electrons flow from negative (-)
to positive (+).
Direct current Voltage forces the electrons to flow continuously in
one direction.
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Electromagnets Unit of force applied to a conductor to free electrons,
to cause electrical current flow.
Electron flow The force applied to a conductor to free electrons,
causing electrical current to flow.
Ohm The basic unit of power, indicating the amount of work
accomplished when one volt causes one ampere to pass
through a circuit.
Resistance Do not retain their magnetism after a magnetizing force
is removed.
Volt The flow of electrons in the same direction from atom
to atom.
Voltage A device to measure voltage.
Voltmeter The restriction to the flow of electrons.
Watt Unit of resistance.
Exercise 35.
a. Make an outline of the text “Power”.
b. Retell the text “Power” according to the written outline.
Exercise 36. Did you Know…?
Read the text and then make questions so that the words in bold provide
answers.
James Watt (figure 24) (1736-1819), Scottish inventor and
mechanical engineer, renowned for his improvements of the steam
engine. Watt was born on January 19, 1736, in Greenock, Scotland. He
worked as a mathematical-instrument maker from the age of 19 Figure 24
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