I семестр
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I. ELECTRON THEORY AND CURRENT
1. Electric Currents
Electricity, like mechanics, may be divided into two main branches, namely, static electricity and current electricity, corresponding to the mechanical ideas of statics and dynamics, respectively. Static electricity is the electricity at rest, or in a stationary condition, while current electricity treats electricity in motion.
Until recently, (1) nothing was really known about the nature of electricity, although its effects had been the subject of considerable study. It was accepted that electricity was some kind of fluid, but there was no experimental evidence to support the theory. In fact, (2) it was known that matter was built up from atoms, and an atom was defined as the smallest part of a chemical element that can take part in a chemical reaction. Originally, it was believed that the atom was a tiny but solid mass, but, later on, it was shown that the atom, far from being solid,(3) is more like a miniature solar system, with small bodies rotating around a central core or nucleus. These tiny bodies behave much as the planets in the solar system, for they travel in definite orbits, and the whole system is held together (4) by forces of attraction to the central nucleus. It was also found that these miniature planets are extremely small charges of negative electricity, and they were therefore named electrons.
Some substances contain not only planetary electrons, i.e. electrons which are confined to the orbit of an atom, but also free electrons. These electrons can move freely between the atoms or from atom to atom. Such substances are called electric conductors. In such substances under certain circumstances we can get a flow or migration of electrons through the substance, and we then speak of an electric current.
Notes:
1. until recently – совсем еще недавно, до недавнего времени
2. in fact – на самом деле
3. far from being solid – будучи далеко не сплошной массой
4. is held together – удерживаться (около ядра)
Questions to answer:
1. How may electricity be divided?
2. What is static electricity?
3. What is current electricity?
4. What did people first think of the nature of electricity?
5. What is atom like?
6. What was found about electrons?
7. Why are some electrons called planetary?
8. What substances are called electric conductors?
9. When do we speak of electric current?
2. Resistance. Ohm’s Law
Georg Ohm, a German physicist, was the first to notice that, when using a cell with a constant voltage, the amount of current would change when different loads were connected across it.
For instance, Ohm noticed that more current would flow through a copper wire than would flow through an iron wire of the same size and that more current would flow through a thick wire than through a thin wire of the same material. Georg Ohm concluded that some types of materials tend to resist the flow of current more than others. Iron has greater resistance than copper.
A thin wire has greater resistance than a thick wire. To resist means to hold back. Resistance tends to reduce the amount of current that is flowing through a circuit.
If Ohm used a larger cell but kept the voltage and the resistance in the load the same, would more current flow? No. The size of the cell does not effect the amount of current delivered. Only voltage and resistance control this.
Ohm then connected a cell with a higher emf (1) (voltage) to the same load, and he discovered that more current flowed into the circuit. The unit used to measure resistance was later named after its discoverer. The basic unit of resistance is the ohm. An ohm is defined as the amount of resistance that will allow 1 ampere of current to flow at an electromotive force of 1 volt.
Georg Ohm discovered that different types, shapes, and quantities of materials subject to the same emf (2) tend to resist the flow of current to varying degrees. Assuming that voltage is constant, if the current in a circuit increases, it is because the resistance in the circuit decreases.
A good way to see the type of relation that exists between current, voltage and resistance is in division:
15(numerator)/3(denominator) = 5 (quotient)
As we increase the numerator, the quotient will become larger. As we increase the denominator, the quotient (current) will become smaller. Georg Ohm saw this same relationship between arithmetic and electricity and stated it in a law (now called Ohm's law). Since current increases as voltage increases and decreases as voltage decreases, current equals voltage divided by resistance.
I is the letter symbol for current, E is the letter symbol for electromotive force, and R stands for resistance. Ohm's law is expressed by
I = E/R
Notes:
1. emf = electromotive force – электродвижущая сила (э. д. с.)
2. subject to the same emf – зд. под действием одной и той же э. д. с.
Questions to answer:
1. What was Georg Ohm the first to notice?
2. What did he notice about a copper wire and an iron wire?
3. Does a thin wire have greater resistance than a thick wire?
4. What does it mean to resist?
5. What does resistance tends to do?
6. Does not the size of the cell affect the amount of current delivered?
7. What controls the amount of current?
8. What is the basic unit of resistance?
9. How is the ohm defined?
10. What relationship between arithmetics and electricity did Ohm notice?
11. How is Ohm's law expressed?
II СЕМЕСТР
1. MAGNETS AND MAGNETISM
Magnetism is a property of certain substances that allow them to attract bits of iron and other ferrous materials. The action of a magnet is exerted throughout the space surrounding it. This space around a magnet is called a magnetic field.
Hundreds of years ago, man discovered a natural substance called lodestone or magnetite, which could attract other pieces of the same substance and pieces of iron. If a bit of it was suspended to turn freely, it would always come to rest with one end pointing north. This natural substance came to be used as a direction finder.(1) The lodestone was a natural magnet. Man also learned how to make magnets himself. These are called artificial magnets. Artificial magnets are subdivided into temporary and permanent magnets.
When a lodestone, or artificial magnet, is suspended so that it can turn freely, one end always comes to rest (2) pointing north. This end is called the north-seeking pole, or simply, the north pole. The other end is called the south pole.
There is a law of magnetic forces that is very much like (3) the law of electrical charges. The laws of magnetic forces states that like magnet poles will repel each other and unlike poles attract each other.
When a sheet of paper is placed over a magnet and iron filings are dropped on it, they will arrange themselves in a pattern. This shows that magnetic force acts in a definite direction at every point around a magnet. This direction might be shown symbolically by the lines of force or flux lines. The three characteristics of flux lines (lines of force) are: (a) they never cross each other; (b) they pass through almost any material; (c) they are elastic and stretch or tighten like rubber bands.
Flux density (4) is greatest at the poles of a magnet. This simply means that there are more lines of force per square inch at the ends of poles. The force with which poles repel or attract each other depends not only on the strength of the poles, but on the distance between them.
The opposition that a material offers to magnetization is called reluctance. Permeability is the relative ease (5) with which flux lines can be established in a substance. Air has the permeability of 1. Magnetic substances have permeability thousands of times greater than air. The greater the permeability, the greater is the magnetic flux density.
Notes:
1.came to be used as a direction finder – начали использовать в качестве указателя направления
2.comes to rest – устанавливается
3.is very much like – очень похож на
4.Flux density – Плотность потока
5.relative ease – относительная легкость
Questions to answer:
1.What is magnetism?
2.How is magnetism exerted?
3.What is a magnetic field?
4.What is a natural magnet?
5.What is an artificial magnet?
6.How are artificial magnets subdivided?
7.What does the law of magnetic forces state?
8.How can the direction of magnetic force be shown symbolically?
9.What are the characteristics of flux lines?
10.How is the opposition that a material offers to magnetization called?
11.What is permeability?
2. ELECTRICITY AND MAGNETISM
Electricity and magnetism are very closely interrelated. Both have many similar features. In this section we will discuss some of the ways in which magnetism is related to electricity.
When an electric current flows through a wire, a magnetic field surrounds the wire. The lines of force which surround a current-carrying wire have direction, just as in a bar magnet. The direction of the lines of force is dependent on the direction of current flow. The direction in which current flows is called current polarity. The direction of the lines of force can be related to the direction of current flow (1) by the left-hand rule.(2) The left-hand rule states that if you grasp the current-carrying wire in your left hand with your thumb pointing in the direction of electron-current flow, your fingers will point in the direction of the flux lines. As you know, a current going through a wire creates a magnetic field and, therefore, flux lines. The number of flux lines around the wire and the distance from the wire at which their magnetic influence is felt is directly proportional to the amount of current flowing. The number of flux lines increases as current increases.
A loop of wire as well as a straight piece of wire can carry a current, but the magnetic field around a loop is strengthened. The lines of force in a loop tend to combine and strengthen the field. When current is carried in a loop of wire, the magnetic field is stronger and the loop is considered to have n – s poles.
Notes:
1.The direction of the lines of force can be related to the direction of current flow – Соотношение между направлением силовых линий и направлением движения тока может быть получено
2.left-hand rule – правило левой руки (магн.)
Questions to answer:
1.What is this text about?
2.What happens when an electric current flows through a wire?
3.What is the direction of the lines of force dependent on?
4.What is current polarity?
5.How can the direction of the lines of force be related to the direction of current flow?
6.How is the number of flux lines related to the amount of cur¬rent flowing? Why is it so?
7.What magnetic field is stronger that of a loop or a straight piece of wire?
8.Why is magnetic field of a loop stronger?
3. ELECTROMAGNETS
A loop of wire which is carrying a current, concentrates the magnetic lines of force that surround a wire and has polarity just like a regular magnet. A magnet that consists of a loop of wire, or a series of loops (coils), is called an electro¬magnet. Unlike a permanent magnet, an electromagnet has the unique feature of being turned on and off (1) by controlling the current that passes through it. The coil which is a series of wire loops represents a simple electromagnet.
Electromagnets are used as relays, solenoids, motors, and other devices. In making such magnets, the proper field strength should be maintained. The field strength of an electromagnet depends upon some factors. There are three factors that determine the field strength of an electromagnet: 1) the field strength of electromagnet is directly proportional to the permeability of the core; 2) the field strength of a coil is also proportional to current; 3) the field strength of a coil is proportional to the number of turns of the coil. The field strength of electromagnet is equal to the product of current multiplied by the number of turns and expressed as ampere-turns.
The magnetic polarity of a coil can be found by the left-hand coil rule. The coil is grasped with the left hand with the fingers pointing in the direction of current flow. Your thumb¬nail then will point in the direction of the coil's north pole.
One of the most widely used applications of electromagnets is the relay. Another widely used application of electro-magnetism is the solenoid. Solenoid is an open coil of wire, its length is great compared with its diameter. The magnetic field intensity at the centre of the solenoid might be given by a specific formula.
Notes:
1.the unique feature of being turned on and off – характерное свойство включаться и выключаться
Questions to answer:
1.How is a loop of wire carrying a current similar to a regular magnet?
2.What is an electromagnet?
3.What is the difference between an electromagnet and a regular magnet?
4.How are electromagnets used?
5.What are the three factors determining the field strength of an electromagnet?
6.What is the field strength of electromagnet equal to?
7.How is the field strength of electromagnet expressed?
8.How can the magnetic polarity of a coil be found?
III СЕМЕСТР
ELECTRIC MOTORS
Motors are used for converting different forms of energy into mechanical energy.
The main part of a motor is a coil or armature. The armature is placed between the poles of a powerful magnet. When a motor is put into operation current starts flowing through the coil (armature) and the armature starts rotating.
Electric motors are used practically in every branch of industry, transport, and agriculture. Naturally, they are produced in many different designs. They are used in industrial plants, and operate under different conditions.
Each motor is supplied with a nameplate which bears machine ratings: output power, voltage, the rated current, the starting current, the power factor, the efficiency, and the rated torque.
These motor ratings should be taken into consideration since they are necessary for the users. On them depends the length of motors' service life, which is normally equal to about 10 years, provided that the operating conditions are normal. Naturally, under abnormal conditions the service life becomes much shorter: motors operate poorly and may have different faults.
Complete the sentences using the correct variant:
1.Motors are used
a) for transmitting energy.
b) for converting energy.
2.Motor's main part is
a) the frame.
b) the armature.
the stator.
3.The armature is placed
a) between the poles of the magnet.
b) about the poles of the magnet.
4.Motors' service life becomes shorter
a) under normal conditions,
b) under abnormal conditions.
5.Faulty motors operate
a) normally.
b) poorly.
Questions to answer:
1.What are motors used for?
2.What is the motor's main part?
3.Where is the armature placed?
4.What ratings does the nameplate of a motor bear?
FAULTS OF MOTORS AND WAYS OF THEIR REPAIR
Motors may have different faults. A faulty motor does not start, or, when it is started, it operates at an excessive speed.
Its brushes may spark and its windings and the commutator may be overheated and burnt. Besides, a motor may produce an abnormal noise, etc. All these and other faults should be detected and repaired.
In case the motor does not start it may have different faults:
Possible causes of faults
1. Fuses are faulty.
2. Motor is overloaded.
3. Circuit in armature winding has an open.
Ways of repair
1. Replace the fuses.
2. Reduce motor load.
3. Repair the armature winding.
In case the motor, when started, stops:
Possible causes of faults
1. Rheostat is shorted.
2. Rheostat switches from one position to another.
Ways of repair
1. Check the rheostat and repair it.
2. Slow down operation of rheostat handle.
Brushes may spark in case:
Possible causes of faults
1. Motor is overloaded.
2. Brushes are in poor condition.
3. Pressure is low.
4. Pressure is excessive.
Ways of repair
1. Reduce the load and remove overload.
2. Replace the brushes.
3. Adjust the pressure.
4. Adjust the pressure.
In case the armature winding is overheated:
Possible causes of faults
1. Motor is overloaded.
2. Ventilation fails to operate properly.
Ways of repair
1. Remove the overload.
2. Check for slowing down the speed of the motor.
In case of abnormal motor speed:
Possible causes of faults
1. Motor is overloaded.
2. Rotor circuit has poor contact.
Ways of repair
1. Reduce the load.
2. Repair the shorting mechanism.
In case rotor brushes against stator:
Possible causes of faults
Rotor brushes against stator.
Ways of repair
Adjust air gap.
Complete the sentences using the correct variant:
1. A motor with a fault
a) operates normally.
b) operates poorly.
2. Motor brushes spark in case
a) they are in normal conditions.
b) they are in poor conditions.
3. Burnt commutator should be
a) replaced.
b) repaired.
4. Brushes may spark in case
a) pressure is low.
b) pressure is excessive.
5. Air gap is adjusted in case
a) the rotor brushes against the stator.
b) the stator brushes against the rotor.
Questions to answer:
1.When does a motor operate poorly?
2.What should be done in case the motor is overloaded?
3.What should be done in case the fuses are faulty?
4.What should be done in case the rheostat is shorted?
5.What should be done in case the brushes spark?
6.What should be done in case the pressure is low?
7.What should be done in case the ventilation does not operate?
8.What should be done in case the rotor brushes against stator?
VI СЕМЕСТР
ELECTRIC POWER
Read and translate the text.
Electric power is generated by converting heat, light, chemical energy, or mechanical energy to electrical energy. Most electrical energy is produced in large power stations by the conversion of mechanical energy or heat. The mechanical energy of falling water is used to drive turbine generators in hydroelectric stations, and the heat derived by burning coal, oil, or other fossil fuels is used to operate steam turbines or internal-combustion engines that drive electric generators. Also, the heat from the fissioning of uranium or plutonium is used to generate steam for the turbine generator in a nuclear power plant.
Electricity generated by the conversion of light or chemical energy is used mainly for portable power sources. For example, a photoelectric cell converts the energy from light to electrical energy for operating the exposure meter in a camera, and a lead-acid battery converts chemical energy to electrical energy for starting an automobile engine.