At one time, students used to be told: "We don't know what electricity is, we don't know how electricity goes through a solid wire," etc. The electron theory explains these things clearly and simply. In addition, il explains the true meaning of voltage, resistance, 5 etc. Therefore, an understanding of the electron theory is basic to the
understanding of the electrical and electronic theory.
Scientists now agree that our universe is basically dependent on two factors, one of which is matter, the other, energy. Matter is anything that occupies space and has weight. It can exist in any of the 10 three forms: solid, liquid or gas.
Matter is composed of protons, neutrons and electrons. The proton has a positive charge. This component has very little weight. The neutron has no charge, but it supplies almost all the weight of matter. The electron has a negative charge. It also has very little weight. If we 15 could look at the structure of a piece of copper, we would find that it consists of a specific number of protons, neutrons, and electrons arranged in some particular way. In a piece of iron, a certain number of protons, neutrons, and electrons are arranged in a different way. The proton of iron is identical to that of copper and other elements. 20 They are all made up of the same components. It is the arrangement of
these components that makes them different.
The electrons of an atom are arranged in shells around the nucleus. The electrons in the last shell are called 'valence' electrons and the electrical properties of a material are dependent on the number of such 25 electrons. Atoms with less than four valence electrons give up one or
more electrons, and the fewer the valence electrons, the easier this becomes. Atoms with more than 4 electrons in their last shell take one or more additional electrons. The conduction of electricity is made possible by the free electrons in the outer shell.
30 Metals are good conductors of electricity as they have less than 4 valence electrons. These electrons aren't strongly attached to the nucleus, but the ones in the inner shell are. Therefore, in a metal, they can move easily from one nucleus to another.
HYDROPONICS
Hydroponics is the technique of growing plants in water. It is generally thought that plants need soil for growing. In fact, what they need is the nutrients (vitamins and minerals) and moisture contained in the soil and these can be supplied through water, as well as through 5 soil.
Hydroponics is not a new process. As long ago as the 1690's, an English physician tried growing plants in water in a laboratory experiment. However, it was not until the 1800's that German researchers used this method to develop many of the formulas for 10 plant nutrient solutions still in use today.
About a generation ago, hydroponics moved out of the research laboratory into practical use. In the past 40 years, hydroponic farming has progressed in a number of areas, especially in those where water is in short supply and temperatures are too extreme for ordinary 15 agriculture. This is because hydroponic farming is the only
economical solution in such desert areas.
Each year, more than 2.7 million kilograms of vegetables and fruit arc produced by hydroponic farming. These are mostly tomatoes but cucumbers, lettuce and melons are also grown. On hydroponic farms, 20 each tomato plant produces an average of 6 kilograms of fruit twice a year - a total of 12 kilograms every year. An ordinary soil-grown plant, on the other hand, produces only a total of 9 kilograms per year.
In hydroponic farming, plants are grown in greenhouses. The greenhouses measure 8 by 39 metres and consist of steel frames 25 covered with strong transparent plastic that is resistant to weather and
lets in a maximum amount of light. The plants are fed by inorganic nutrients dissolved in water which is supplied by a plastic pipeline. The feeding and watering system is automated. Electric sensing devices (sensors) determine when the plants are hungry or thirsty. The 30 sensors send messages which automatically start the water and
nutrient delivery system. When the sensors 'know' that the plant have had enough, the system turns off automatically.
Nothing is left to chance within the greenhouses. Temperature, humidity and air circulation are carefully controlled. Air conditioning 35 and heating equipment keep the temperature at 29°C by day and 18°C
by night.
In recent years, hydroponic farming has expanded to many parts of the world. An application of the method has occurred in Italy, for example, where the largest hydroponic installation - 50,000 square
35 expansion of such a bar, due to heating, may be used to operate switches and valves.
In a gas refrigerator, there is a reservoir containing ammonia water. When the lower gas flame is burning, the ammonia water rises through the tube to the 'generator'. The upper gas flame drives off the 40 ammonia gas, which passes into the 'condenser'. The cold air around
the condenser rapidly brings down the temperature of the gas. Then the cooled gas, now condensed into a liquid, passes into the 'evaporator', which contains hydrogen. In the evaporator, the ammonia expands rapidly, especially since its expansion in hydrogen is greater 45 than it would be in air. This rapid expansion greatly lowers its
temperature. It is the cooling of the gas in the evaporator which lowers the temperature of the whole refrigerator and freezes the water in the ice-cube trays.
A. What do the following refer to? 1. 'it' (line 4):
2. 'they' (line 11): 3. 'doing so' (line 11):
4. 'do* (line 13):
5. 'one' (line 18): 6. 'Many' (line 19): 7. MV (line 31): 8. 'if (line 32):
9. 'such a bar" (line 35): B. Mark the best choice.
1. In selecting a good refrigerant, we must choose one that . a) evaporates quickly
b) is cheap
c) is not explosive d) All of the above.
2. When a substance in gaseous form is allowed to pass through a small hole, a) unlike the molecules of evaporating liquids, it absorbs heat energy from its
surroundings
b) like the molecules of evaporating liquids, it absorbs heat energy from its surroundings
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