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Файл:Professional English for Electrical Engineers. Part 1. Учебное пособие
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field, drawn such that the number of lines
relates to the magnetic field's strength
at a given point and the tangent of any curve
at a particular point is along the direction
of magnetic force at that point
rotation a soft iron rod in a coil or transformer that
provides a path for and intensifies the
magnetic field produced by the windings
eruption a property of magnets or electrically-charged
objects in which there are two possible
conditions (north and south for magnets,
positive and negative for electrical charges)
that describe an important characteristic
of the forces that they experience
vicinity a measure of field strength or of the energy
concentric noting or pertaining to a substance, as iron,
that below the Curie point can possess
magnetization in the absence of an external
magnetic field
wrap an influence (such as a force) that some
forms of matter produce, which extends
throughout the space that surrounds them
ferromagnetic a circular motion of a configuration about a
given point or line, without a change
in shape
core the lines of force of an electric or magnetic
field
Exercise 49. Transcribe the words from the previous exercise.
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Exercise 50. The following statements are not true to fact. Correct them.
Description
Unit of
Measure
Abbreviation
Description
Voltage
V
Amperes or
amps
That constant current that, if
maintained in two straight
parallel conductors of infinite
length, of negligible crosssection, and placed 1 meter apart
in a vacuum, would produce
between these conductors a force
equal to 2 × 10−7 newtons per
meter of length, or 1 coulomb of
charge moving past a point in 1
second
Resistance
The amount of resistance that
will produce a drop of 1 volt
when 1 amp flows through it
Energy
Joules, watthours, kilowatt
hours, or
megawatt hours
When electrons flow through a conductor, a magnetic field won’t be produced
around that conductor.
Theright-hand rule states that the magnetic flux lines produced by a current-carrying
wire won’t be oriented the same direction as the curled fingers of a person’s left hand (in
the “hitchhiking” position), with the thumb pointing in the direction of electron flow.
The magnetic field force produced by a current-carrying line can be greatly
increased by shaping the wire into a coil instead of a straight wire.
If wound in a coil shape, the magnetic field won’t be oriented along the axis of the
coil’s length.
The magnetic field force produced by an electromagnet (called the magnetomotive
force, or mmf), is proportional to the product (multiplication) of the voltage through the
electromagnet and the number of complete coil “turns” formed by the wire.
Exercise 51. Fill in the table.
Table 5
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Table 5 (continued)
Description
Unit of
Measure
Abbreviation
Description
W, kW, or
MW
1 joule per second
Exercise 52. Look through the following text, define its main idea.
It is questionable how people would exist without the knowledge of electricity and
magnetism nowadays.
It is widely known that the relationship between electricity and magnetism is one of
the most important in physics.
Electricity and magnetism are combined to form one of the fundamental forces of
the universe – electromagnetism.
Electromagnetism is one of the essential forces of the universe finding an
increasingly wide application nowadays.
Electricity and magnetism combine to form one of the fundamental forces of the
universe – electromagnetism. The two constantly interact, and the relationship between
them is one of the most important in physics. For example, an electric current passing
through a wire creates a magnetic field; and if the lines of force around a magnet are cut
by a passing object, an electric current will be produced.
Magnets are attracted to iron and to any material that contains iron. Magnets have
two poles, a north pole and a south pole. Unmagnetized iron and steel have magnetic
regions of atoms called domains that are jumbled up and point in lots of different
directions. When iron or steel becomes magnetized, the domains become aligned and they
all point in the same direction. One end of each domain points toward the magnetic north
pole.
Electronics is a new branch of physics, and one that plays an increasingly important
part in our lives. It is concerned with the use of electricity to produce signals that carry
information and control devices such as computers. These devices contain electric circuits
through which electric current flows. The controlling parts in a circuit are called
components, and these include diodes and transistors.
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Exercise 53.
a. Make an outline of the text “Magnetism. Electromagnetism”.
b. Retell the text “Magnetism. Electromagnetism” according to the written
outline.
Exercise 54. Did you know…?
Read the text and then make questions so that the words in bold provide
answers.
Joseph Henry (figure 30) (1797-1878), American physicist,
who did his most important work in electromagnetism. He was
born in Albany, New York, and educated at Albany Academy. He
was appointed professor of mathematics and natural philosophy at
Albany Academy1 in 1826 and professor of natural philosophy at
Princeton University in 18322. The foremost American
physicist of his day, he discovered the principle of electromagnetic. Figure 30
induction before the British physicist Michael Faraday announced his discovery of
electromagnetically induced currents, but Faraday published his findings first and is
credited with the discovery. The discovery of the phenomenon of self-inductance, which
Henry announced in 1832, is, however, attributed to him3, and the unit of inductance is
named the henry in his honor. Henry experimented with and improved the
electromagnet, which had been invented in 1823 by the Briton William Sturgeon. By
1829 he had developed electromagnets of great lifting power and efficiency and essentially
of the same form used later in dynamos and motors. He also developed electromagnets
that were capable of magnetizing iron at a distance from the source of current, and in
1831 he constructed the first practical electromagnetic telegraph. Henry also devised
and constructed one of the first electric motors. In 1842 he recognized the oscillatory
nature of an electric discharge.
In 1846 Henry was elected secretary and director of the newly formed Smithsonian
Institution, and he served in those positions until his death. Under his direction, the
institution stimulated activity in many fields of science. He organized meteorological
studies at the Smithsonian and was the first to use the telegraph to transmit weather
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reports, to indicate daily atmospheric conditions on a map, and to make weather forecasts
from meteorological data. The meteorological work of the Smithsonian led to the
creation of the U.S. Weather Bureau. Henry was a founder of the American Association
for the Advancement of Science and president (1868-78) of the National Academy of
Sciences.
Exercise 55. Prepare a short report about J. Henry according to the plan:
• Biography;
• Scientific discoveries;
• The importance of his works.
2.7 Check Yourself
Refer to the texts in this unit if necessary. A good score is at least 13 correct
answers out of these 15 questions. The answers are listed in the back of this book
(Appendix A).
1. A positive electric pole
(a) has a deficiency of electrons.
(b) has fewer electrons than the negative pole.
(c) has an excess of electrons.
(d) has more electrons than the negative pole.
2. An EMF of 1 V
(a) cannot drive much current through a circuit.
(b) represents a low resistance.
(c) can sometimes produce a large current.
(d) drops to zero in a short time.
3. The volt is the standard unit of
(a) current.
(b) charge.
(c) electromotive force.
(d) resistance.
4. If an EMF of 1 volt is placed across a resistance of 2 ohms, then the current is
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(a) half an ampere.
(b) 1 ampere.
(c) 2 amperes.
(d) impossible to determine.
5. A potentially lethal electric current is on the order of
(a) 0.01 mA.
(b) 0.1 mA.
(c) 1 mA.
(d) 0.1 A.
6. A current of 25 A is most likely drawn by
(a) a flashlight bulb.
(b) a typical household.
(c) a utility power plant.
(d) a small radio set.
7. A piece of wire has a conductance of 20 S. Its resistance is
(a) 20 Ω.
(b) 0.5 Ω.
(c) 0.05 Ω.
(d) 0.02 Ω.
8. A resistor has a value of 300 Ω. Its conductance is
(a) 3.33 mS.
(b) 33.3 mS.
(c) 333 μS.
(d) 0.333 S.
9. A span of wire 1 km long has a conductance of 0.6 S. What is the conductance of
a span of this same wire that is 3 km long?
(a) 1.8 S
(b) 0.6 S
(c) 0.2 S
(d) More information is necessary to determine this.
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10. In a battery, chemical energy can sometimes be replenished by
(a) connecting it to a light bulb.
(b) charging it.
(c) discharging it.
(d) no means known; when a battery is dead, you must throw it away.
11. Of the following energy units, the one most often used to define electrical
energy is
(a) the Btu.
(b) the erg.
(c) the foot-pound.
(d) the kilowatt-hour.
12. A low voltage, such as 12 V,
(a) is never dangerous.
(b) is always dangerous.
(c) is dangerous if it is ac, but not if it is dc.
(d) can be dangerous under certain conditions.
13. A fluctuating magnetic field
(a) produces an electric current in an insulator.
(b) magnetizes the earth.
(c) produces a fluctuating electric field.
(d) results from a steady electric current.
14. Which of the following units can represent magnetic flux density?
(a) The volt-turn
(b) The ampere-turn
(c) The gauss
(d) The gauss-turn
15. A ferromagnetic material
(a) concentrates magnetic flux lines within itself.
(b) increases the total magnetomotive force around a current-carrying wire.
(c) causes an increase in the current in a wire.
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(d) increases the number of ampere-turns in a wire.
3 Unit 3. Measuring Devices
3.1 What is a Meter?
Before you start
1. What kinds of meters do you know?
2. Why do many measuring devices work?
Exercise 1. Read and translate the text.
A meter is any device built to accurately detect and display an electrical quantity
in a form readable by a human being. Usually this “readable form” is visual: motion of a
pointer on a scale, a series of lights arranged to form a “bargraph,” or some sort of
display composed of numerical figures.
Most modern meters are “digital” in design, meaning that their readable display is
in the form of numerical digits. Older designs of meters are mechanical in nature, using
some kind of pointer device to show quantity of measurement. In either case, the
principles applied in adapting a display unit to the measurement of (relatively) large
quantities of voltage, current, or resistance are the same.
The display mechanism of a meter is often referred to as a movement, borrowing
from its mechanical nature to move a pointer along a scale so that a measured value may
be read. Though modern digital meters have no moving parts, the term “movement” may
be applied to the same basic device performing the display function.
Most mechanical movements are based on the principle of electromagnetism: that
electric current through a conductor produces a magnetic field perpendicular to the axis
of electron flow. The greater the electric current, the stronger the magnetic field
produced. If the magnetic field formed by the conductor is allowed to interact with
another magnetic field, a physical force will be generated between the two sources of
fields. If one of these sources is free to move with respect to the other, it will do so as
current is conducted through the wire, the motion (usually against the resistance of a
spring) being proportional to strength of current.
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The first meter movements built were known as galvanometers, and were usually
designed with maximum sensitivity in mind. A very simple galvanometer may be made
from a magnetized needle (such as the needle from a
magnetic compass) suspended from a string, and
positioned within a coil of wire. Current through the
wire coil will produce a magnetic field which will
deflect the needle from pointing in the direction of
earth’s magnetic field. An antique string
galvanometer is shown in the following photograph
(figure 31). Figure 31
Now, the term “galvanometer” usually refers to any design of electromagnetic
meter movement built for exceptional sensitivity, and not necessarily a crude device
such as that shown in the photograph.
Practical electromagnetic meter movements
can be made now where a pivoting wire coil
is suspended in a strong magnetic field,
shielded from the majority of outside
influences. Such an instrument design is
generally known as a permanent-magnet,
moving coil, or PMMC movement (figure 32).
Figure 32
While most mechanical meter movements are based on electromagnetism
(electron flow through a conductor creating a perpendicular magnetic field), a few are
based on electrostatics: that is, the attractive or repulsive force generated by electric
charges across space. This is the same phenomenon exhibited by certain materials (such
as wax and wool) when rubbed together. If a voltage is applied between two conductive
surfaces across an air gap, there will be a physical force attracting the two surfaces
together capable of moving some kind of indicating mechanism. That physical force is
directly proportional to the voltage applied between the plates, and inversely
proportional to the square of the distance between the plates. The force is also
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irrespective of polarity, making this a polarity-insensitive
type of meter movement (figure 33).
Whatever the type of meter or size of meter
movement, there will be a rated value of voltage or current
necessary to give full-scale indication. In electromagnetic
movements, this will be the “full-scale deflection current” Figure 33
necessary to rotate the needle so that it points to the exact end of the indicating scale. In
electrostatic movements, the full-scale rating will be expressed as the value of voltage
resulting in the maximum deflection of the needle actuated by the plates, or the value of
voltage in a cathode-ray tube which deflects the electron beam to the edge of the
indicating screen. In digital “movements,” it is the amount of voltage resulting in a “full-
count” indication on the numerical display: when the digits cannot display a larger
quantity.
The task of the meter designer is to take a given meter movement and design the
necessary external circuitry for full-scale indication at some specified amount of voltage
or current. Most meter movements (electrostatic movements excepted) are quite
sensitive, giving full-scale indication at only a small fraction of a volt or an amp. This is
impractical for most tasks of voltage and current measurement. What the technician
often requires is a meter capable of measuring high voltages and currents.
Exercise 2. Define the following words and expressions.
Detect
Readable
Scale
A digital meter
Value
Mechanical movements
A galvanometer
Sensitivity
A magnetized needle
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