Expansion of Solids, Liquids and Gases.

It is well known that the dimensions of all bodies are changed, more or less, by an increase or decrease in tem­perature, that is, upon heating or freezing.

According to the molecular theory the speed of mole­cular motion is increased when heat is added. It has been found that this increase in motion causes the molecules to collide with each other a little harder so that they move apart a little farther and occupy more space. That is why heated objects are usually expected to expand.

Solid bodies have the property of keeping their sha­pe without the support of a containing vessel. Hence, in the case of a solid bar we are able to measure the increa­se in length, width, or thickness due to a given rise of temperature. We can, thus, find the increase in length which a bar of 1 cm long would experience if it is heated through 1°C. This is defined as the coefficient of linear expansion of the substance the bar is composed of.

On the other hand, it is known that liquids and gases take the shape of the vessel which contains them. It would therefore be useless to make measurements of their linear dimensions. A certain mass of water, for instance, may be poured into vessels of various shapes and sizes, its linear dimensions being different every time. But in spite of all these variations the volume of the water will remain the same provided the temperature is not changed. Howe­ver, every change in temperature of the water will corre­spondingly change its volume.

In liquids the force of attraction between the molecu­les is less than in solids. They say, therefore, that heated liquids expand more than solids do when heated. Ex­periments show that this is really the case.

The expansion of a solid body or a liquid little depends on the pressure at which the expansion takes place. For example, the volume of a piece of iron or a given quanti­ty of water is known to be almost the same both under a pressure of 10 atm, and under a pressure of 1 atm.

It is quite different with gases. According to Boyle's Law the volume of a gas is inversely proportional to its pressure.

To find out the state of a definite mass of gas, one should know three things about it, namely, its volume, its pressure, and its temperature.

When the relation existing between these three variab­les is known, it is possible to determine one of them pro­vided the other two are given.

Suppose we have three variables and we want to deter­mine how they will be related in their effect on the state of a given mass of gas. It is evident that if all the three were allowed to vary at once, it would be impossible to determine the cause of any observed effect. One must, the­refore, allow only two of the variables to change simul­taneously while the third remains constant and conse­quently cannot produce the observed effects.

Thus, to determine the relation between pressure and volume, at constant temperatures, it is necessary to main­tain a given constant mass of gas at constant temperature and observe how the changing of pressure affects the volu­me to be occupied by this mass of gas.

 

Exercise 2. Answer the questions.

1. Do all bodies expend when heat is added?

2. When is the molecular motion increased?

3. What property of solids do you know?

4. What is the coefficient of linear expansion?

5. What shape do the gases take?

6. Is the force of attraction between the molecules in liquids more then in solids?

7. What does the state of given mass of gas depend on?

8. When is it possible to determine one of the three variables?

9. What is necessary for determining the relation between pressure and volume at constant temperatures?

 

Exercise 3. Complete the following sentences.

1. One can say that…

2. One should add that…

3. One must know that…

4. It is necessary to…

5. It must be remembered that…

 

Exercise 4. Give all the derivatives from the following words and translate them.

to apply, to use, to change, to differ, to determine, to convert, to change.

 

Exercise 5. Translate the following sentences paying attention to the words in bold type.

1. All bodies either radiate heat or absorb waves from other bodies

2. Dark surfaces are either good radiators when hot or good absorbers when cold.

3. The motion of the molecules of a solid is harmonic, the molecules moving between specific limits on either side of on average position and seldom passing outside those limits.

4. Have neither sire nor shape of their own the expected results.

 

Exercise 6. Write out the keywords and phrases and make up a plan.

Exercise 7. Make a short summary of the text.

TEXTS FOR ADDITIONAL READING.

OTHER SOURCES OF ENERGY.

People do not usually think of hydropower, or energy from mo­ving water as coming from the sun. Yet the sun does provide this energy by evaporating water, which then falls to Earth as rain and runs from high ground to low.

Modern power plants harness water power for electricity. Usually the builders of a hydroelectric power plant need to dam a stream or river. By holding water in a reservoir, the dam raises the wa­ter level behind it much higher than the level of the power plant at the bottom of the dam. This increases the gravitational po­tential energy of the water. As electricity is needed, the water is released to rush down with terrific force. The falling water strikes and turns the blades of turbines. In turn, the turbines spin the generators to make electricity.

The sun is also the source of wind energy. Wind is caused by the flow of air from areas of high pressure to areas of low pre­ssure. These high- and low-pressure areas are a result of un­even heating of Earth’s surface by the sun.

Wind was one of the first forms of energy harnessed by humans. Sailors first used it to move ships in ancient times. Later, pe­ople throughout Europe and the Middle East used windmills for pumping water and grinding grain. Some old windmills still ope­rate in places such as Netherlands and the island of Crete.

After 1973, as the price of fossil fuels rose, people became more concerned about supplies of petroleum. Interest in wind power was rekindled. Thousands of wind machines are now in use. Some power machinery directly. For instance, they may use water on a farm.

When many wind machines are grouped together in wind farms they can generate as much electricity as power plant.

 

ENERGY.

Energy in great quantities is necessary to power civilization and meet human needs. Fossil fuels, the primary energy source, exist in limited supplies. However, energy is available from a number of natural sources. Humans are developing the technolo­gy to generate and harness energy for their work using alter­nate sources,

As the 1990s opened, fossil fuels provided most of the energy consumed world-wide. Of the alternative sources available in 1991, only nuclear power had the potential to replace fossil fuels on a large scale. Yet utility companies are discouraged by the costs and the public is varied of the hazards. Supplies for hydropower are limited and solar, wind and geothermal sources need more rese­arch to make them more efficient and practical for wide-scale use. During the 1980s, governments and industry devoted less re­search and fewer resources than in the 1970s to developing al­ternative sources of energy. The reason for this lack of inte­rest goes beyond change in administration: petroleum was inex­pensive relative to most alternative sources. During an era of relatively low oil prices, insufficient industry and consumer interest existed in the use of these fuels. In a crisis situa­tion, petroleum cannot be replaced immediately.

Vehicle manufacturers must plan three to five years in advance planning and construction of a coal-fired electrical plant takes seven years; a nuclear facility twelve years; a solar plant one to five years; and a geothermal facility three, wind farms can be constructed in six months, provided that the sites, supp­liers, constructors, and funds are in place.

THE IMPORTANCE OF ENERGY.

Energy is important for a number of reasons, we depend on it to power civilization and maintain our life-style. Harnessing en­ergy from natural forces allows us a level of comfort and achie­vement beyond the dreams of our ancestors. From the design of the house you live in to the shape of your family car, the effec­tive use of energy is a key factor in shaping your world. Daily life is filled with countless marvels beyond the imagination of anyone living 300 years ago. People today take these develop­ments for granted, yet they are in danger of losing them. Vehic­les, appliances and machines all need energy to run, and most run on fuels that exist in limited quantities.

Everyone must understand the nature of thermal energy and how it interacts with matter, how it can be produced and controlled, and the risks and benefits connected with using it.

The state of the world in 10, 20, or 100 years will depend upon decisions that you and others will make about the ways in which energy will be used. Your priorities, concerns, needs, and purchases will help shape the world in which we live.

The search for energy resources and their development and use has become an all-important theme in today's world. Our attached to a ready supply of energy.

SOLAR ENERGY.

1. Most of the energy that we use on the Earth even in some converted form such as coal and oil stems from the Sun. Indeed, oil, coal and natural gas are fuels that release energy received from the Sun millions of years ago. When we use wood in a fire it is necessary to remember that the Sun supplied the energy for the growth of trees.

2. Hydroelectric energy is known to be electricity produced from the energy of falling water. This is actually stored solar energy, the water being lifted from the sea in the course of the hydrological cycle which is driven by the Sun. Hydro electric energy has several advantages over other ways of producing electricity. No fuel is required, since the energy comes from the Sun.

3. The Sun is also important to us as a laboratory in which we can study hot gases in a magnetic field. The knowledge we are gathering from the studies of the solar gas enables us to control fusion processes on the Earth. If we could build magnetic "bottles" to contain hydrogen undergoing fusion at temperatures of millions of degrees and would use the hydrogen in the oceans we should obtain unexhausted sources of energy for millions of years.

4. It is clear today that the supply of coal, oil and natural gas will soon become inadequate for our needs. It is natural that scientists began their search for new sources of energy. There is an increasing interest in obtaining energy from the Sun.

5. There are devices that give off an electrical signal when struck by sunlight. The device employed for converting solar energy into useful power is the solar cell. In the solar cell the junction consists of two different kinds of semiconductors. The cell is energized not by heat but by light. But solar cells are still expensive to use for general commercial purposes. They proved to be an ideal source of power for artificial satellites.

6. There is now considerable research centering on finding ways of conversion solar radiation into heat and electricity. Man has learned to obtain electric power directly from the Sun at present. Architects have built houses to be heated by solar radiation due to applying suitably designed roofs and using suitable construction materials, the latter tend to retain heat obtained from the Sun. Under suitable conditions solar radiation can raise the temperature of the air to 300° F.

THERMAL ENERGY.

Thermal, kinetic, nuclear, and chemical energy are all availab­le for human work. People obtain them from fuel, solar radiation, moving water, geothermal facilities or nuclear fission.

Today, however, most energy comes from burning fuels. Burning releases the chemical energy stored in the bonds between atoms and molecules of the fuels. The thermal energy released from the fuels burning is the result of a chemical reaction called oxidation. Oxi­dation is the chemical combination of oxygen with another element. The elements such as carbon, sulphur and nitrogen combined with oxy­gen to produce such oxides as carbon dioxide and nitrogen oxide, releasing thermal energy in the process. This is the reason fuels such as wood and coal lose so much mass in the burning process. Oxidation releases the energy in stored chemical bonds.

Valuable as oxidation is in providing energy for human work; its by-products affect the atmosphere, climate and the health of liv­ing organisms. Carton monoxide, sulphur dioxide and nitrogen oxi­de are all injurious to human health; oxides or sulphur and nitro­gen become acids that living and nonliving matter; and car­bon dioxide, a greenhouse gas, is instrumental in global warming. The destructive effects of oxidation, together with dwindling supp­lies of fossil fuels provide the impetus for development of al­ternative sources.