Part c.

Exercise 1. Read and translate the text.

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.