Part 28 (1/2)

The formulas for the laws of Boyle and Charles are sometimes combined into one expression as follows:

_PV/T = P'V'/T'_

or the product of the volume and pressure of a constant ma.s.s of gas is proportional to its absolute temperature.

Important Topics

1. Heat units; calorie, British thermal unit.

2. Three thermometer scales, fixed points on each.

3. Absolute zero, how determined. Its value on each scale.

4. Law of Charles, its meaning. Combination of laws of Boyle and Charles.

Exercises

1. Does ice melt at the same temperature at which water freezes? Express the temperature of freezing water on the three thermometer scales.

2. A comfortable room temperature is 68F. What is this temperature on the centigrade and absolute scales?

3. Change a temperature of 15C. to F.; 15F. to C.; -4C. to F.; -20F.

to C.

4. The temperature of the human body is 98.6F. What is this temperature on the absolute and centigrade scales?

5. The temperature of liquid air is -180C. What is it on the Fahrenheit scale?

6. Mercury is a solid at -40F. What is this on the centigrade scale?

7. How much heat will be required to raise the temperature of 8 lbs. of water 32F.; 5 lbs. 10F.?

[Ill.u.s.tration: FIG. 125.--A clinical thermometer used to take the temperature of the body.]

8. How much heat will be required to raise the temperature of 30 g. of water 43C.; 20 g., 50C.?

9. Compute the temperature of absolute zero on the Fahrenheit scale.

10. Take three basins of water, one hot, one cold, and one lukewarm. If one hand be placed in the hot water while the other is placed in the cold and after a few minutes both are placed in the lukewarm water, this water will feel cool to one hand and warm to the other. Explain.

11. If 200 ccm. of air at 200 absolute is heated to 300A. under constant pressure, what volume will the air occupy at the latter temperature?

12. How does one change a reading on the centigrade scale to a corresponding reading on the absolute scale?

(3) EXPANSION OF LIQUIDS AND SOLIDS

=146. Expansion of Gases.=--The law of Charles is found to apply to all gases. That is, all gases change in volume in proportion to the change of temperature provided the pressure remains constant. It is for this reason that we have the _gas thermometer_ (see Fig. 126) which gives in skillful hands more accurate temperature readings than the best mercurial thermometer. Galileo devised and used the first _air thermometer_ which consisted of a hollow bulb blown on a gla.s.s tube and inverted in a dish of water. (See Fig. 1.) The _water thermometer_ consists of a gla.s.s bulb filled with water which rises into a tube attached to the bulb. One disadvantage of the water thermometer is its limited range since it cannot be used below 0 or above 100. Why?

=147. Expansion of Liquids.=--The expansion of liquids differs from that of gases in several important respects:

(a) Liquids have a smaller rate of expansion than gases. The _rate_ of expansion per degree is called the _Coefficient of Expansion_. For example, the coefficient of expansion of a gas under constant pressure at 0C. is {1/273} of its volume per degree centigrade.