INTRODUCTION AND MEASUREMENT

(1) Introduction

1. Physics, an Explanation of Common Things.—Many students take up the study of physics expecting to see wonderful experiments with the "X" rays, wireless telegraphy, dynamos, and other interesting devices. Others are dreading to begin a study that to them seems strange and difficult, because they fear it deals with ideas and principles that are beyond their experience and hard to comprehend.

Each of these classes is surprised to learn that physics is mainly an explanation of common things. It is a study that systematizes our knowledge of the forces and changes about us; such as the pull of the earth, the formation of dew, rain and frost, water pressure and pumps, echoes and music, thermometers and engines, and many other things about us with which people are more or less familiar. Physics is like other school subjects, such as mathematics and language, in having its own peculiar vocabulary and methods of study; these will be acquired as progress is made in the course.

The most useful habit that the student of physics can form is that of connecting or relating each new idea or fact that is presented to him to some observation or experience that will illustrate the new idea. This relating or connecting of the new ideas to one's own personal experience is not only one of the best known means of cultivating the memory and power of association, but it is of especial help in a subject such as physics, which deals with the systematic study and explanation of the facts of our every-day experience.

2. Knowledge—Common and Scientific.—This leads to the distinction between common knowledge and scientific knowledge. We all possess common knowledge of the things about us, gained from the impressions received by our senses, from reading, and from the remarks of others. Scientific knowledge is attained when the bits of common knowledge are connected and explained by other information gained through study or experience. That is, common knowledge becomes scientific, when it is organized. This leads to the definition: Science is organized knowledge.

Common knowledge of the forces and objects about us becomes scientific only as we are able to make accurate measurements of these. That is, science is concerned not only in how things work, but even more in how much is involved or results from a given activity. For example, a scientific farmer must be able to compute his costs and results in order to determine accurately his net profits. The business man who is conducting his business with efficiency knows accurately his costs of production and distribution.

This book is written in the hope that it will make more scientific the student's common knowledge of the forces and changes in the world about him and will give him many ideas and principles that will help him to acquire the habit of looking from effects to their natural causes and thus tend to develop what is called the scientific habit of thought.

3. Hypothesis, Theory, and Law.—Three words that are frequently used in science may be mentioned here: hypothesis, theory, and law. An hypothesis is a supposition advanced to explain some effect, change, or condition that has been observed. For example, the Nebular Hypothesis of which many high-school students have heard, is an attempt to explain the origin of the sun, the earth, the planets, and other solar systems.

A theory is an hypothesis which has been tested in a variety of ways and which seems to fit the conditions and results so that it is generally accepted as giving a satisfactory explanation of the matter in question. The Molecular Theory of Matter which states that matter of all kinds is composed of very small particles called molecules (see Art. 6), is a familiar example of a theory.

A theory becomes a law when it may be definitely proved. Many laws are expressed in mathematical language, e.g., the law of gravitation. (See Art. 88.) Many of the laws of physics are illustrated by laboratory experiments, which show in a simple way just what the law means.

Exercises

Explain what is meant by the following terms and expressions:

1. Common knowledge.

2. Scientific knowledge.

3. Science.

4. Topics in physics.

5. Scientific habit of thought.

6. Value of relating new ideas to former experiences.

7. Hypothesis.

8. Theory.

9. Law.

(2) The States of Matter

4. Physics Defined.—In the study of any science or field of knowledge, it is helpful to have a basis for grouping or classifying the facts studied. In physics we are to study the objects, forces, and changes about us, to understand them and their relations to one another. Accordingly, physics, dealing with the material world about us, is often defined as the science of matter and energy, matter being anything that occupies space and energy the capacity for doing work. This definition of physics while not strictly accurate is sufficiently comprehensive for our present purpose.

5. The Three States of Matter.—Our bodies are matter since they occupy space. Further, they possess energy since they are able to do work. In beginning the study of physics it will simplify our work if we study one of these topics before the other. We will therefore begin with matter and consider first its three states.

Some bodies are solid; as ice, iron, wax. Others are liquid; as water, mercury, oil. Still others are in the state of gas; as steam, air, and illuminating gas. Further we notice that the same substance may be found in any one of the three states. For example water may be either ice, water or steam; that is, either a solid, a liquid, or a gas.

Most persons have heard of liquid air and possibly some know of ice air, i.e., air cooled until it not only liquefies, but is solidified. On the other hand, iron may be melted and, if heated hot enough, may be turned into iron vapor. In fact most substances by heating or cooling sufficiently may be changed into any one of the three states.

Before defining the three states, let us consider the structure of matter. This may help us to answer the question: How is it possible to change a hard solid, such as ice, into a liquid, water, and then into an invisible gas like steam? This is explained by the molecular theory of matter.

6. The Molecular Theory of Matter.—It is believed that all bodies are made up of very small particles called molecules, and that these instead of being packed tightly together like square packages in a box, are, strange as it may seem, very loosely packed even in solids and do not permanently touch their neighbors. The size of these molecules is so minute that it has been estimated that if a drop of water could be magnified to the size of the earth, the molecules magnified in the same proportion would be in size between a baseball and a football. The air and all other gases are believed to be made up of molecules in rapid motion, striking and rebounding continually from one another and from any objects in contact with the gas.

7. States of Matter Defined.—These ideas of the structure of matter assist us in understanding the following definitions: A solid is that state of matter in which the molecules strongly cling together and tend to keep the same relative positions. (This of course follows from the tendency of a solid to retain a definite form.) A liquid is that state of matter in which the molecules tend to cling together, yet move about freely. Hence a liquid takes the form of any vessel in which it is placed. A gas is that state of matter in which the molecules move about freely and tend to separate indefinitely. Hence a gas will fill any space in which it is placed.

8. Effect of Heat on Matter.—It is further believed that when a body is heated, that the action really consists in making its molecules move or vibrate faster and faster as the heating progresses. This increase of motion causes the molecules to push apart from one another and this separation of the molecules causes an expansion of the body whether it be solid, liquid, or gas. Fig. 1 shows the expansion of air in an air thermometer. Fig. 2 shows the expansion of a solid on heating.

9. Physical and Chemical Changes. A change of state such as the freezing or boiling of water is called a physical change, for this change has not affected the identity of the substance. It is water even though it has become solid or gaseous. Heating a platinum wire red hot is also a physical change for the wire when on cooling is found to be the same substance as before. Further if salt or sugar be dissolved in water the act of solution is also a physical change since the identical substance (salt or sugar) is in the solution and may be obtained by evaporating the water.

If some sugar, however, is heated strongly, say in a test-tube, it is found to blacken, some water is driven off and on cooling some black charcoal is found in the tube instead of the sugar. This action which has resulted in a change in the nature of the substance treated is called a chemical change. To illustrate further, if some magnesium wire is heated strongly in a flame, it burns, giving off an intense light and when it cools one finds it changed to a light powdery substance like ashes. Chemical changes, or those that change the nature of the substance affected, are studied in chemistry. In physics we have to do only with physical changes, that is, with those changes that do not affect the nature of the substance.

Important Topics

1. Physics defined.

2. The three states of matter; solid, liquid, gas.

3. Molecular theory of matter.

4. Physical and chemical changes.

Exercises

Write out in your own words your understanding of:

1. The structure of matter.

2. Some of the differences between solids, liquids, and gases.

3. How to change solids to liquids and gases and vice versa.

4. The reason for the changes of size of a body on heating.

5. Why cooling a gas tends to change it to a liquid or a solid.

6. The actual size of molecules.

Which of the following changes are chemical and which physical?

Give reasons.

1. Melting of ice.

2. Burning of a candle.

3. Production of steam.

4. Falling of a weight.

5. Drying of clothes.

6. Making an iron casting.

7. Decay of vegetables.

8. Sprouting of seeds.

9. Flying an aeroplane.

10. Growth of a plant.

11. Grinding of grain.

12. Sawing a board.

13. Pulverizing stone.

14. Making toast.

15. Sweetening tea or coffee with sugar.

16. Burning wood or gas.

(3) The Metric System

10. The Metric System.—In order to study the three states of matter with sufficient exactness it is necessary to employ a system of measurement. The system universally employed by scientists is called The Metric System. In many respects it is the most convenient for all purposes. Every student should therefore become familiar with it and learn to use it. At the present time, not only do scientists everywhere use it, but many countries have adopted it and use it in common measurements. It was legalized in the United States in 1866. The metric system was originated by the French Academy of Sciences during the latter part of the 18th century. There were so many different systems of weights and measures in use, each country having a system of its own, that commerce was much hindered. It was therefore decided to make a system based upon scientific principles. The length of the earth's quadrant passing from the equator to the pole was determined by surveying and computation. One-ten-millionth of this distance was selected as the unit of length and called a meter. Accurate copies of this meter were made and preserved as standards.

Later surveys have shown that the original determination of the earth's quadrant was not strictly accurate; so that after all the meter is not exactly one-ten-millionth of the earth's quadrant.

11. The Standard Meter.—The standard unit of length in the metric system is the meter. It is the distance, at the temperature of melting ice, between two transverse parallel lines ruled on a bar of platinum (see Fig. 3), which is kept in the Palace of the Archives in Paris. Accurate copies of this and other metric standards are also kept at the Bureau of Standards at Washington, D. C. Fig. 4 shows the relation between the inch and the centimeter (one-hundredth of a meter).

12. Units and Tables in the Metric System.—The metric unit of area commonly used in physics is the square centimeter.

The standard unit of volume or capacity is the liter. It is a cube one-tenth of a meter on each edge. It is equal to 1.057 quarts. It corresponds, therefore, to the quart in English measure.

The standard unit of mass is the kilogram. It is the mass of 1 liter of pure water at the temperature of its greatest density, 4°C. or 39.2°F.

The three principal units of the metric system, the meter, the liter, and the kilogram, are related to one another in a simple manner, since the liter is a cube one-tenth of a meter in each dimension and the kilogram is the mass of a liter of water. (See Fig. 5.)

The metric system is a decimal system that is, one unit is related to another unit in the ratio of ten or of some power of ten. This is indicated by the following tables:

The measures commonly used are the centimeter, meter and kilometer.

The masses commonly used are the milligram, gram and kilogram.

Notice in these tables the similarity to 10 mills equal 1 cent, 10 cents equal 1 dime, 10 dimes equal 1 dollar, in the table of United States money.

Other tables in the metric system are built upon the same plan. Learn the prefixes in order thus: milli, centi, deci, deka, hecto, kilo, myria. The first three prefixes are Latin numerals and represent divisions of the unit. The last four are Greek numerals and represent multiples. In these tables, milli means 1/1000, centi means 1/100, deci means 1/10, deka means 10, hecto, 100, kilo, 1000, myria, 10,000. Two other prefixes are sometimes used, micro which means 1/1,000,000; as microfarad or microvolt, and meg which means 1,000,000, as megohm meaning 1,000,000 ohms.

13. Advantages of the Metric System.—First, it is a decimal system; second, the same form and prefixes are used in every table; third, the standards of length (meter), volume (liter), and mass (kilogram) bear a simple relation to one another. This simple relation between the three standard units may be given thus: first, the liter is a cubic decimeter, and second, the kilogram is the mass of a liter of water. (See Fig. 5) Since the liter is a cubic decimeter, the length of one side is 10 cm. The liter therefore holds 1000 ccm. (10 × 10 × 10). Therefore, 1 liter = 1 cu. dm. = 1000 ccm. and since 1 liter of water has a mass of 1 kg. or 1000 g., then 1000 ccm. of water has a mass of 1000 g., or 1 ccm. of water has a mass of 1 g.

The following table of equivalents gives the relation between the most common English and metric units. Those marked (*) should be memorized.

The c. g. s. system. Scientists have devised a plan for expressing any measurement in terms of what are called the three fundamental units of length, mass, and time. The units used are the centimeter, the gram and the second. Whenever a measurement has been reduced to its equivalent in terms of these units, it is said to be expressed in C.G.S. units.

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 mass 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?

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