Physics Part 3

Important Topics

1. The metric system; how originated.

2. Units; meter, liter, kilogram.

3. Metric tables.

4. Advantages of the metric system.

5. Equivalents.

6. The C.G.S. system.

Exercises

1. Which is cheaper, milk at 8 cents a quart or 8 cents a liter? Why?

2. Which is more expensive, cloth at $1.00 a yard or at $1.00 a meter?

Why?

3. Which is a better bargain, sugar at 5 cents a pound or 11 cents a kilogram? Why?

4. Express in centimeters the height of a boy 5 ft. 6 in. tall.

5. What is the length of this page in centimeters? In inches?

6. What is the ma.s.s of a liter of water? Of 500 ccm.? Of 1 ccm.?

7. From Chicago to New York is 940 miles. Express in kilometers.

8. A 10-gallon can of milk contains how many liters?

9. What will 100 meters of cloth cost at 10 cents a yard?

10. What will 4 kg. of beef cost at 15 cents a pound?

11. What will 5-1/2 lbs. of mutton cost at 40 cents a kilogram?

12. How can you change the state of a body? Give three methods.

13. Correct the statement 1 ccm. = 1 g.

14. How many liters in 32 quarts?

CHAPTER II

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) pa.s.s 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 0C.

and 76 cm. pressure has been calculated by Professor Rutherford to be 2.7 10?. 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 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 ill.u.s.trations 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 ill.u.s.trates the rapid diffusion of gases.

[Ill.u.s.tration: FIG. 6_a_.--Diffusion of gases.]

[Ill.u.s.tration: FIG. 6_b_.--Effusion of gases.]

Take two tumblers (see Fig. 6_a_), 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 pa.s.s through porous solids, cloth, unglazed earthenware, etc. The following experiment shows this fact strikingly. (See Fig. 6_b_.)

A flask containing water is closed by a rubber stopper through which pa.s.s the stem of a gla.s.s funnel and a bent gla.s.s 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 s.p.a.ce between the jar and the beaker. When hydrogen gas is allowed to flow into the s.p.a.ce 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 ill.u.s.trates 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 acc.u.mulates, 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 reenters. 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 quant.i.ty of gas is placed in a vacuum, the gas will expand immediately to fill the entire s.p.a.ce uniformly. This is shown by an experiment with the air pump. On raising the piston the air follows instantly to fill up the s.p.a.ce under it. As 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 s.p.a.ce in which it acts, but is due rather to the _motion_ of the molecules. The blow of a single molecule is imperceptible, but when mult.i.tudes 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 s.p.a.ce 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).

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.

[Ill.u.s.tration: 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 a.s.sumed 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.