Molecular Forces And Motions


Chapter II. Molecular Forces And Motions.


MOLECULAR FORCES AND MOTIONS

(1) Evidences of Molecular Motion in Gases

14. Size of Molecules.—The difference between solids, liquids, and gases has been explained as due to the different behavior of molecules in the three states of matter. That is, in solids they cling together, in liquids they move freely, and in gases they separate. At this time we are to consider the evidences of molecular motion in gases. It must be kept in mind that molecules are exceedingly small. It has been said that if a bottle containing about 1 ccm. of ordinary air has pierced in it a minute opening so that 100,000,000 molecules (a number nearly equal to the population of the United States) pass out every second, it would take, not minutes or hours, but nearly 9000 years for all of the molecules to escape. The number of molecules in 1 ccm. of air at 0°C. and 76 cm. pressure has been calculated by Professor Rutherford to be 2.7 × 1019. It is evident that such minute particles cannot be seen or handled as individuals. We must judge of their size and action by the results obtained from experiments.
15. Diffusion of Gases.—One line of evidence which indicates that a gas consists of moving particles is the rapidity with which a gas having a strong odor penetrates to all parts of a room. For example, if illuminating gas is escaping it soon diffuses and is noticed throughout the room. In fact, the common experience of the diffusion of gases having a strong odor is such that we promptly[Pg 14] recognize that it is due to motion of some kind. The gas having the odor consists of little particles that are continually hitting their neighbors and are being struck and buffeted in turn until the individual molecules are widely scattered. When cabbage is boiled in the kitchen soon all in the house know it. Other illustrations of the diffusion of gases will occur to anyone from personal experience, such for instance as the pleasing odor from a field of clover in bloom.
The following experiment illustrates the rapid diffusion of gases.
Fig. 6a.—Diffusion of gases. Fig. 6b.—Effusion of gases.
Take two tumblers (see Fig. 6a), wet the inside of one with a few drops of strong ammonia water and the other with a little hydrochloric acid. Cover each with a sheet of clean paper. Nothing can now be seen in either tumbler. Invert the second one over the first with the paper between, placing them so that the edges will match. On removing the paper it is noticed that both tumblers are quickly filled with a cloud of finely divided particles, the two substances having united chemically to form a new substance, ammonium chloride.
On account of their small size, molecules of air readily pass through porous solids, cloth, unglazed earthenware, etc. The following experiment shows this fact strikingly. (See Fig. 6b.)
[Pg 15]
A flask containing water is closed by a rubber stopper through which pass the stem of a glass funnel and a bent glass tube that has been drawn out to a small opening (J). The funnel has cemented in its top an inverted porous clay jar (C), over the top of the latter is placed a beaker (B). A piece of flexible rubber tubing (H) leading from a hydrogen generator is brought up to the top of the space between the jar and the beaker. When hydrogen gas is allowed to flow into the space between C and B, the level of the water in W is seen to lower and a stream of water runs out at J spurting up into the air.
On stopping the flow of hydrogen and removing B, the water falls rapidly in J and bubbles of air are seen to enter the water from the tube. (The foregoing steps may be repeated as often as desired).
This experiment illustrates the fact that the molecules of some gases move faster than those of some other gases. Hydrogen molecules are found to move about four times as fast as air molecules. Hence, while both air and hydrogen molecules are at first going in opposite directions through the walls of C, the hydrogen goes in much faster than the air comes out. In consequence it accumulates, creates pressure, and drives down the water in W and out at J. On removing B, the hydrogen within the porous cup comes out much faster than the air reënters. This lessens the pressure within, so that air rushes in through J. This experiment demonstrates not only the fact of molecular motion in gases but also that molecules of hydrogen move much faster than those of air. (This experiment will work with illuminating gas but not so strikingly.)
Careful experiments have shown that the speed of ordinary air molecules is 445 meters or 1460 ft. per second; while hydrogen molecules move at the rate of 1700 meters or 5575 ft. or more than a mile per second.
16. Expansion of Gases.—Gases also possess the property of indefinite expansion, that is, if a small quantity of gas is placed in a vacuum, the gas will expand immediately to fill the entire space uniformly. This is shown by an experiment with the air pump. On raising the piston the air follows instantly to fill up the space under it. As[Pg 16] the air is removed from the receiver of an air pump the air remaining is uniformly distributed within.
17. How Gases Exert Pressure.—It is further found that air under ordinary conditions exerts a pressure of about 15 lbs. to the square inch. In an automobile tire the pressure may be 90 lbs. and in a steam boiler it may be 200 lbs. or more to the square inch.
How is the pressure produced? The molecules are not packed together solidly in a gas, for when steam changes to water it shrinks to about 1/1600 of its former volume. Air diminishes to about 1/800 of its volume on changing to liquid air. The pressure of a gas is not due then to the gas filling all of the space in which it acts, but is due rather to the motion of the molecules. The blow of a single molecule is imperceptible, but when multitudes of molecules strike against a surface their combined effect is considerable. In fact, this action is known to produce the pressure that a gas exerts against the walls of a containing vessel. Naturally if we compress twice as much gas into a given space there will be twice as many molecules striking in a given time, which will give twice as much pressure.
If gas is heated, it is found that the heat will cause a swifter motion of the molecules. This will also make the molecules strike harder and hence cause the gas to expand or exert more pressure.
17a. Brownian Movements.—Direct photographic evidence of the motion of molecules in gases has been obtained by studying the behavior of minute drops of oil suspended in stagnant air. Such drops instead of being at rest are constantly dancing about as if they were continually receiving blows from many directions. These motions have been called Brownian Movements (see Fig. 7).
[Pg 17]
It has been proved that these movements are due to the blows that these small drops receive from the swiftly moving molecules of the gas about them. If the drops are made smaller or the gas more dense, the movements increase in intensity. These effects are especially marked at a pressure of 0.01 of an atmosphere.
Fig. 7.—Photograph of Brownian movement. This record is prepared by the aid of Siedentopf's ultra-microscope and a plate moving uniformly across the field from left to right.

Important Topics

It is assumed that air and all gases are made up of molecules in rapid motion; that this motion is dependent upon temperature and pressure. Evidence of this is shown by (a) diffusion, (b) expansion, (c) pressure. Brownian Movements.

Questions

1. What is the molecular (kinetic) theory of gases?
2. What three kinds of evidence help to confirm the theory?
3. What have you seen that seems to show that a gas consists of molecules in motion?
4. How many meters long is a 10-ft. pole?
5. A 50-kg. boy weighs how many pounds?
6. What are three advantages of the metric system?
7. What will 12 qts. of milk cost at 8 cents a liter?
8. A cube 1 meter each way will contain how many cubic centimeters? How many liters? What will a cubic meter of water weigh?
[Pg 18]

(2) Molecular Motion in Liquids

18. Diffusion of Liquids.—From the evidence given in Arts. 14-17, (a) of diffusion of odors, (b) of the continued expansion of air in the air pump, and (c) of the pressure exerted by a gas in all directions, one may realize without difficulty that a gas consists of small particles in rapid motion. Let us now consider some of the evidence of molecular motion in liquids. If a little vinegar is placed in a pail of water, all of the water will soon taste sour. A lump of sugar in a cup of tea will sweeten the entire contents. This action is somewhat similar to the diffusion of gases but it takes place much more slowly. It is therefore believed that the motion of liquid molecules is much slower than that of gas molecules.
Again, if a dish of water is left standing in the open air in fine weather, within a few days the dish will become dry though no one has taken anything from it. We say the water has evaporated. What was liquid is now vapor. If we were to observe carefully any dish of water we would find that it continually loses weight on dry days. That is, there is a constant movement of the molecules of water into the air. This movement of the molecules is explained as follows. There appear to be in the dish of water some molecules that by moving back and forth acquire a greater velocity than their neighbors; when these reach the surface of the liquid, some vibration or movement sends them flying into the air above. They are now vapor or gas molecules, flying, striking, and rebounding like the air molecules. Sometimes on rebounding, the water molecules get back into the water again. This is especially apt to happen when the air is damp, i.e., when it contains many water molecules. Sometimes the air over a dish becomes saturated, as in the upper part of a corked bottle[Pg 19] containing water. Although molecules are continually leaving the surface of the water they cannot escape from the bottle, so in time as many molecules must return to the water from the space above as leave the water in the same time. When this condition exists, the air above the water is said to be saturated. On very damp days the air is often saturated. The explanation above shows why wet clothes dry so slowly on such a day (See Arts. 166-7 on Saturation.)
19. Cooling Effect of Evaporation. We have seen that warming a gas increases its volume. This expansion is due to the increased motion of the warmed molecules. Now the molecules that escape from a liquid when it evaporates are naturally the fastest moving ones, i.e., the hottest ones. The molecules remaining are the slower moving ones or colder molecules. The liquid therefore becomes colder as it evaporates, unless it is heated. This explains why water evaporating on the surface of our bodies cools us. In evaporating, the water is continually losing its warm, fast moving molecules. The cooling effect of evaporation is, therefore an evidence of molecular motion in liquids.
Fig. 8.—Osmosis Shown by carrot placed in water.
20. Osmosis.—If two liquids are separated by a membrane or porous partition, they tend to pass through and mix. This action is called osmose, or osmosis.
Such a movement of liquid molecules in osmosis may be illustrated by filling a beet or carrot that has had its interior cut out[Pg 20] to form a circular opening (see Fig. 8) with a thick syrup. The opening is then closed at the top with a rubber stopper through which passes a long glass tube.
If the carrot is immersed in water, as in Fig. 8, a movement of water through the porous wall to the interior begins at once. Here, as in the experiment of the hydrogen and air passing through the porous cup, the lighter fluid moves faster. The water collecting in the carrot rises in the tube. This action of liquids passing through porous partitions and mingling is called osmosis.
Gases and liquids are alike in that each will flow. Each is therefore called a fluid. Sometimes there is much resistance to the flow of a liquid as in molasses. This resistance is called viscosity. Alcohol and gasoline have little viscosity. They are limpid or mobile. Air also has some viscosity. For instance, a stream of air always drags some of the surrounding air along with it.

Important Topics

1. Liquids behave as if they were composed of small particles in motion.
2. This is shown by (1) Diffusion, (2) Solution, (3) Evaporation, (4) Expansion, (5) Osmosis.

Exercises

1. Give an example or illustration of each of the five evidences of molecular motion in liquids.
2. When is air saturated? What is the explanation?
3. Why does warming a liquid increase its rate of evaporation?
4. Air molecules are in rapid motion in all directions. Do they enter a liquid with a surface exposed to the air? Give reason.
5. What are some of the inconveniences of living in a saturated atmosphere?
6. Fish require oxygen. How is it obtained?
[Pg 21]

(3) Molecular Forces in Liquids

21. Cohesion and Adhesion.—In liquids "the molecules move about freely yet tend to cling together." This tendency of molecules to cling together which is not noticeable in gases is characteristic of liquids and especially of solids. It is the cause of the viscosity mentioned in the previous section and is readily detected in a variety of ways. For instance, not only do liquid molecules cling together to form drops and streams, but they cling to the molecules of solids as well, as is shown by the wet surface of an object that has been dipped in water. The attraction of like molecules for one another is called cohesion, while the attraction of unlike molecules is called adhesion, although the force is the same whether the molecules are alike or unlike. It is the former that causes drops of water to form and that holds iron, copper, and other solids so rigidly together. The adhesion of glue to other objects is well known. Paint also "sticks" well. Sometimes the "joint" where two boards are glued together is stronger than the board itself. The force of attraction between molecules has been studied carefully. The attraction acts only through very short distances. The attraction even in liquids is considerable and may be measured. The cohesion of water may be shown by an experiment where the force required to pull a glass plate from the surface of water is measured.
Fig. 9.—The water is pulled apart.
Take a beam balance and suspend from one arm a circular glass plate, Fig. 9. Weigh the plate and its support. Adjust the glass plate so that it hangs horizontally and just touches the surface of clean water, the under side being completely wet. Now find what additional weight is required to raise the glass plate from the water.
[Pg 22]
Just as the plate comes from the water its under side is found to be wet. That is, the water was pulled apart, and the plate was not pulled from the water. The cohesion of the water to itself is not so strong as its adhesion to the glass.
The cohesion of liquids is further shown by the form a drop of liquid tends to take when left to itself. This is readily seen in small drops of liquids. The spherical shape of drops of water or mercury is an example. A mixture of alcohol and water in proper proportions will just support olive oil within it. By carefully dropping olive oil from a pipette into such a mixture, a drop of the oil, an inch or more in diameter suspended in the liquid, may be formed. It is best to use a bottle with plane or flat sides, for if a round bottle is used, the sphere of oil will appear flattened.
Figs. 10 a and b.—Surface tension of a liquid film.
22. Surface Tension.—The cohesion of liquids is also indicated by the tendency of films to assume the smallest possible surface. Soap bubble films show this readily. Fig. 10 a represents a circular wire form holding a film in which floats a loop of thread. The tension of the[Pg 23] film is shown in Fig. 10 b by the circular form of the loop after the film within it has been pierced by a hot wire, Fig. 11 shows a rectangular wire form with a "rider." The tension in the film draws the rider forward.
Fig. 11.—The rider is drawn forward.
Fig. 12.—Surface tension causes the pointed shape.
A soap bubble takes its spherical shape because this form holds the confined air within the smallest possible surface. A drop of liquid is spherical for the same reason. Many illustrations of the tension in films may be given. Users of water colors notice that a dry camel's-hair brush is bushy. (Fig. 12 A). When in water it is still bushy. (Fig. 12 B.) But when it is taken from the water and the excess is shaken from it, it is pointed as in Fig. 12 C. It is held to the pointed shape by the tension of the liquid film about the brush.
Fig. 13.—A needle depresses the surface when floating.
The surface of water acts as if covered by a film which coheres more strongly than the water beneath it. This is shown by the fact that a steel needle or a thin strip of metal may be floated upon the surface of water. It is supported by the surface film. (See Fig. 13.) If the film breaks the needle sinks. This film also supports the little water bugs seen running over the surface of a quiet pond in[Pg 24] summer. The surface film is stronger in some liquids than in others. This may be shown by taking water, colored so that it can be seen, placing a thin layer of it on a white surface and dropping alcohol upon it. Wherever the alcohol drops, the water is seen to pull away from it, leaving a bare space over which the alcohol has been spread. This indicates that the alcohol has the weaker film. The film of greasy benzine is stronger than the film of the pure material. If one wishes to remove a grease spot and places pure benzine at the center of the spot, the stronger film of the greasy liquid will pull away from the pure benzine, and spread out, making a larger spot than before, while if pure benzine is placed around the grease spot, the greasy liquid at the center pulls away from the pure benzine, drawing more and more to the center, where it may be wiped up and the grease entirely removed.
Fig. 14.—The molecule at A is held differently from one within the liquid.
23. Explanation of the Surface Film.—Beneath the surface of a liquid each molecule is attracted by all the other molecules around it. It is attracted equally in all directions. Consequently the interior molecules move very easily over each other in any direction. A molecule at the surface, as at A, Fig. 14, is not attracted upward by other liquid molecules. Its freedom of motion is thereby hindered with the result that a molecule at the surface[Pg 25] behaves differently from one beneath the surface. The surface molecules act as if they form an elastic skin or membrane upon the liquid surface.
Fig. 15.—Capillary attraction in tubes.
24. Capillarity.—A striking action of the surface film of a liquid is seen in the rise of liquids in tubes of small bore when the liquid wets them. If the liquid does not wet the tube, as when mercury is placed in glass, the liquid is depressed. It is found in general that: Liquids rise in capillary tubes when they wet them and are depressed in tubes which they do not wet; the smaller the diameter of the tube the greater the change of level. (See Fig. 15.) This action is explained as follows: The molecules of a liquid have an attraction for each other and also for the sides of a tube. The former is called "cohesion for itself," the latter is called "adhesion for the sides of the containing vessel." If the cohesion for itself is greater than the adhesion for the side of the containing vessel, the liquid is pulled away from the side and is depressed. If the adhesion is greater, the liquid is elevated. This action is called "capillary action" from the Latin word (capillus) signifying hair, since it shows best in fine hairlike tubes.
There are many common illustrations of capillary action: oil rising in a wick; water rising in a towel or through clothes; ink in a blotter, etc. The minute spaces between the fibers composing these objects act as fine tubes. If cloth is treated with a preparation which prevents water from adhering to its fibers, the material will not be wet when water is poured upon it, because[Pg 26] the water will not run in between the fibers; a surface film spreads over the cloth so that no water enters it. Cravenette cloth has been treated in this way and hence is waterproof.
The action of this film may be shown by the following experiment. Dip a sieve of fine copper gauze in melted paraffin, thus coating each wire so that water will not adhere to it. Water may now be poured into the sieve, if a piece of paper is first laid in it to break the force of the water. On carefully removing the paper the surface film of the water will prevent the passage of the water through the sieve.
25. Capillary Action in Soils.—The distribution of moisture in the soil depends largely upon capillary action. When the soil is compact the minute spaces between the soil particles act as capillary tubes, thus aiding the water to rise to the surface. As the water evaporates from the surface more of it rises by capillary action from the damper soil below. Keeping the soil loose by cultivation, makes the spaces between the particles too large for much capillary action, thus the moisture is largely prevented from rising to the surface.
In the semi-arid regions of the West "dry farming" is successfully practised. This consists in keeping the surface covered with a "dust mulch" produced by frequent cultivation. In this way the moisture is kept below the surface, where it can be utilized during the hot dry summer by the roots of growing plants.

Important Topics

1. Attractive forces between liquid molecules.
2. Cohesion (like molecules); adhesion (unlike molecules).
3. Special effects of this force are classified as (a) capillary action, and (b) surface tension.
[Pg 27]

Exercises

1. What evidence of capillary action have you seen outside of the laboratory?
2. What is the explanation for capillary action?
3. Where are surface films found?
4. What are three common effects of surface films?
5. Explain why cravenette cloth sheds water.
6. If a circular glass disc 10 cm. in diameter requires 50 grams of force to draw it from the water, what is the cohesion of water per square centimeter?
7. What is the weight in grams of 1 ccm. of water? of a liter of water?
8. Name five examples of adhesion to be found in your home.
9. Under what conditions will a liquid wet a solid and spread over it?
10. When will it form in drops on the surface?
11. Explain the proper procedure for removing a grease spot with benzine.
12. What difference is there between a liquid and a fluid?
13. Why cannot a "soap bubble" be blown from pure water?
14. Which are larger, the molecules of steam or those of water? Why?
15. Why is the ground likely to be damp under a stone or board when it is dry all around?
16. Why does any liquid in falling through the air assume the globule form?
17. Give three examples of capillary attraction found in the home. Three out of doors.
18. Why does cultivation of the soil prevent rapid evaporation of water from the ground?

(4) Evidences of Molecular Forces in Liquids and Solids

26. Solutions.—A crystal of potassium permanganate is placed in a liter of water. It soon dissolves and on shaking the flask each portion of the liquid is seen to be colored red. The dissolving of the permanganate is an illustration[Pg 28] of the attraction of the molecules of water for the molecules of the permanganate. We are familiar with this action in the seasoning of food with salt and sweetening with sugar.
Water will dissolve many substances, but in varying degrees, i.e., of some it will dissolve much, of others, little, and some not at all. Further, different liquids have different solvent powers. Alcohol will dissolve resin and shellac, but it will not dissolve gum arabic, which is soluble in water. Benzine dissolves grease. Beeswax is not dissolved by water, alcohol or benzine, but is soluble in turpentine.
It is found that the temperature of the liquid has a marked effect upon the amount of substance that will dissolve. This is an indication that the motions of the molecules are effective in solution. It appears that dissolving a solid is in some respects similar to evaporation, and just as at higher temperatures more of the liquid evaporates, because more of the molecules will escape from the liquid into the air above, so at higher temperatures, more molecules of a solid will detach themselves through greater vibration and will move into the liquid.
Further, just as an evaporating liquid may saturate the space above it so that any escape of molecules is balanced by those returning, so with a dissolving solid, the liquid may become saturated so that the solution of more of the solid is balanced by the return of the molecules from the liquid to the solid condition.
27. Crystals and Crystallization.—This return from the liquid to the solid state, of molecules that are in solution, is especially noticeable when the solution is cooling or evaporating and hence is losing its capacity to hold so much of the solid. On returning to the solid, the molecules attach themselves in a definite manner to the solid[Pg 29] portion, building up regular solid forms. These regular forms are crystals. The action that forms them is called crystallization.
Each substance seems to have its own peculiar form of crystal due to the manner in which the molecules attach themselves to those previously in place. The largest and most symmetrical crystals are those in which the molecules are deposited slowly with no disturbance of the liquid. Beautiful crystals of alum may be obtained by dissolving 25 g. of alum in 50 ccm. of hot water, hanging two or three threads in the solution and letting it stand over night. The thread fibers provide a foundation upon which crystals grow.
When a solution of a solid evaporates, the molecules of the liquid escape as a gas, the molecules of the solid remain accumulating as crystals. This principle has many uses: (a) sea water is purified by evaporating the water and condensing the vapor, which of course forms pure water. (b) water is forced down to salt beds where it dissolves the salt. The brine is then raised and evaporated, leaving the salt in the evaporating pans.
28. Absorption of Gases by Solids and Liquids.—If a piece of heated charcoal is placed in a test-tube containing ammonia gas, inverted in mercury, the ammonia is seen to disappear, the mercury rising to take its place. The ammonia has been absorbed by the charcoal, the gas molecules clinging closely to the solid. The charcoal being very porous presents a large surface to the action of the gas.
This experiment indicates that attraction exists between gas molecules and other molecules. Many porous substances have this power of absorbing gases. We have all noticed that butter has its flavor affected by substances placed near it.
That liquids absorb gases is shown by slowly heating[Pg 30] cold water in a beaker. Small bubbles of air form on the sides and rise before the boiling point is reached. Ammonia gas is readily absorbed in water, the bubbles disappearing almost as soon as they escape into the water from the end of the delivery tube. Household ammonia is simply a solution of ammonia gas in water. On warming the solution of ammonia the gas begins to pass off; thus, warming a liquid tends to drive off any gas dissolved in it.
Soda water is made by forcing carbon dioxide gas into water under strong pressure. When placed in a vessel open to the air the pressure is lessened and part of the gas escapes. The dissolved gas gives the characteristic taste to the beverage.

Important Topics

1. The solution of solids is increased by heating.
2. The solution of gases is decreased by heating.
3. Pressure increases the quantity of gas that can be dissolved in a liquid.
4. The attraction (cohesion) of molecules of a dissolved solid for each other is shown by crystallization.

Exercises

1. How do fish obtain oxygen for breathing?
2. Why does warming water enable it to dissolve more of a salt?
3. Why does warming water lessen the amount of a gas that will stay in solution?
4. Will water absorb gases of strong odor? How do you know?
5. Name three solvents. Give a use for each.
6. What liquids usually contain gases in solution? Name some uses for these dissolved gases.
7. What is the weight of a cubic meter of water?
8. Name three substances obtained by crystallization.
9. How is maple sugar obtained?
10. Name five crystalline substances.
[Pg 31]

(5) Evidence of Molecular Forces in Solids

29. Differences between Solids and Gases.—In studying gases, it is seen that they behave as if they were composed of small particles in rapid motion, continually striking and rebounding, and separating to fill any space into which they are released. This action indicates that there is practically no attractive force between such molecules.
Between the molecules of a solid, however, the forces of attraction are strong, as is shown by the fact that a solid often requires a great force to pull it apart; some, as steel and iron, show this property in a superlative degree, a high-grade steel rod 1 cm. in diameter requiring nearly 9 tons to pull it apart. Tests show that the breaking strengths of such rods are directly proportional to their areas of cross-section. That is, twice the area has twice the breaking strength.
Fig. 16.—Elasticity of bending.
30. Elasticity.—Fully as important as a knowledge of the breaking strengths of solids, is the knowledge of what happens when the forces used are not great enough to break the rods or wires.
Take a wooden rod (as a meter stick) and clamp one end to the table top, as in Fig. 16. At the other end hang a weight. Fasten a wire to this end so that it projects out in front of a scale. Add successively several equal weights and note the position of the wire each time. Remove the weights in order, noting the positions as before. The rod will probably return to the first position.
This simple experiment illustrates a characteristic of solids: that of changing shape when force is applied and of[Pg 32] returning to the original shape when the force is removed. This property is called elasticity.
Tests of elasticity are made by subjecting wire of different materials but of the same dimensions to the same tension. The one changing least is said to have the greatest elastic force or elasticity. If greater forces are applied to the wire and then removed, one will finally be found that will permanently stretch the wire so that it will not return exactly to the former length. The wire has now passed its elastic limit and has been permanently stretched.
Just as there are great differences between the elastic forces of different substances, so there are great differences in the limits of elasticity. In some substances the limit is reached with slight distortion, while others are perfectly elastic even when greatly stretched. India rubber is an example of a body having perfect elasticity through wide limits. Glass has great elastic force but its limit of elasticity is soon reached. Substances like India rubber may be said to have great "stretchability," but little elastic force. In physics, elasticity refers to the elastic force rather than to ability to endure stretching.
31. Kinds of Elasticity.Elasticity may be shown in four ways: compression, bending or flexure, extension or stretching, twisting or torsion. The first is illustrated by squeezing a rubber eraser, the second by an automobile spring, the third by the stretching of a rubber band, the fourth by the twisting and untwisting of a string by which a weight is suspended.
There are two kinds of elasticity: (1) elasticity of form or shape; (2) elasticity of volume. Gases and liquids possess elasticity of volume, but not of shape, while solids may have both kinds. Gases and liquids are perfectly elastic because no matter how great pressure may be applied, as soon as the pressure is removed they regain[Pg 33] their former volume. No solid possesses perfect elasticity, because sooner or later the limit of elasticity will be reached.
32. Hooke's Law.[A]—On examining the successive movements of the end of the rod in Art. 30, we find that they are approximately equal. Carefully conducted experiments upon the elasticity of bodies have shown that the changes in shape are directly proportional to the forces applied, provided that the limit of elasticity is not reached. This relation, discovered by Robert Hooke, is sometimes expressed as follows: "Within the limits of perfect elasticity, all changes of size or shape are directly proportional to the forces producing them."
33. Molecular Forces and Molecular Motions.—If a solid is compressed, on releasing the pressure the body regains its former shape if it has not been compressed too far. This indicates that at a given temperature the "molecules of a solid tend to remain at a fixed distance from each other, and resist any attempt to decrease or increase this distance." This raises the question, Why does not the cohesion pull the molecules tightly together so that compression would be impossible? The reason is that heat affects the size of solid bodies. On lowering the temperature, bodies do contract, for as soon as the temperature is lowered the vibration of the molecule is lessened. On raising the temperature the molecules are pushed farther apart.
The size of a body, then, is the result of a balance of opposing forces. The attractive force between the molecules pulling them together is cohesion, while the force which pushes them apart is due to the motions of the molecules. Raising the temperature and thus increasing[Pg 34] the motion causes expansion; lowering the temperature decreases the molecular motion and so causes contraction. If an outside force tries to pull the body apart or to compress it this change of size is resisted by either cohesion or molecular motion.
34. Properties of Matter.—Many differences in the physical properties of solids are due to differences between the cohesive force of different kinds of molecules. In some substances, the attraction is such that they may be rolled out in very thin sheets. Gold is the best example of this, sheets being formed 1/300,000 of an inch thick. This property is called malleability. In other substances the cohesion permits it to be drawn out into fine threads or wire. Glass and quartz are examples of this. This property is called ductility. In some, the cohesion makes the substance excessively hard, so that it is difficult to work or scratch its surface. The diamond is the hardest substance known. Some substances are tough, others brittle. These are tested by the ability to withstand sudden shocks as the blow of a hammer.

Important Topics

1. Molecular forces in solids; (a) adhesion, (b) cohesion.
2. Elasticity, Hooke's Law.
3. Contraction on cooling.
4. Malleability, ductility, hardness, brittleness, etc.

Exercises

1. Give an illustration of Hooke's Law from your own experience.
2. What devices make use of it?
3. Do solids evaporate. Give reasons.
4. When iron is welded, is cohesion or adhesion acting?
5. When a tin basin is soldered, is cohesion or adhesion acting?
6. Sometimes a spring is made more elastic by tempering and made soft by annealing. Look up the two terms. How is each accomplished?
[Pg 35]
7. Review the definitions: solid, liquid, and gas. Why do these definitions mean more to you now than formerly?
8. If a wire is stretched 0.3 cm. on applying 4 kg. of force, what force will stretch it 0.75 cm? Explain.
9. How long will it take under ordinary conditions for a gas molecule to cross a room? Give reasons for your answer.
10. What is meant by the elastic limit of a body?
11. Without reaching the elastic limit, if a beam is depressed 4 mm. under a load of 60 kg., what will be the depression under a load of 400 kg.? Of 600 kg.?
12. Name three substances that possess elasticity of volume.
13. Give three examples of each; elasticity of (1) compression, (2) stretching, (3) torsion, (4) flexure.

Review Outline: Introduction and Molecules

Physics; definition, topics considered, physical and chemical changes.
Science; hypothesis, theory, law. Knowledge; common, scientific.
Matter; three states, molecular theory. Mass, weight, volume.
Metric system; units, tables, equivalents, advantages.
Evidences of molecular motions; gases (3), liquids (5), solids (3).
Evidences of molecular forces; liquids (3), solids (many) special properties such as: elasticity, tenacity, ductility, hardness, etc.
Hooke's law; applications.

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