Physics Part 6

3. Pressure increases the quant.i.ty 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.

(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 s.p.a.ce 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.

[Ill.u.s.tration: 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 ill.u.s.trates a characteristic of solids: that of changing shape when force is applied and of 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 pa.s.sed 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. Gla.s.s 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 ill.u.s.trated 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 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._"

[A] A law is a statement of a constant mode of behavior. It is often expressed in mathematical language.

=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 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. Gla.s.s 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 ill.u.s.tration 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?

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?