PASCAL'S LAW: Pressure exerted at any

these observations he concluded that for a

point upon an enclosed liquid is transmitted

constant temperature the product of the volume

undiminished in all directions.

and pressure of an enclosed gas remains constant.

This conclusion became Boyle's law.

B E R N O U L L I ' S PRINCIPLE: If an

You can demonstrate Boyle's law by confining

incompressible fluid flowing through a tube

a quantity of gas in a cylinder that has a tightly

reaches a constriction, or narrowing of the

fitted piston. Apply force to the piston so as to

tube, the velocity of fluid flowing through the

compress the gas in the cylinder to some specific

constriction increases and the pressure decreases.

volume. If you double the force applied to the

piston, the gas will compress to one half its

original volume (fig. 4-9).

Changes in the pressure of a gas also affect

the density. As the pressure increases, its volume

Jacques Charles, a French scientist, provided

decreases; however, there is no change in the

much of the foundation for the modern kinetic

weight of the gas. Therefore, the weight per unit

theory of gases. He found that, if the pressure

volume (density) increases. So it follows that the

is held constant, all gases expand and contract in

density of a gas varies directly with the pressure if

direct proportion to the change in the absolute

the temperature is constant.

temperature. Any change in the temperature of

a gas causes a corresponding change in volume.

Therefore, if a given sample of a gas were heated

while confined within a given volume, the pressure

Sir Isaac Newton was an English philosopher

should increase. Actual experiments found that

and mathematician who lived from 1642 to 1727

for each 1°C increase in temperature, the increase

A.D. He was the formulator of the basic laws of

in pressure was about l/273 of the pressure at

modern philosophy concerning gravity and

0°C. Thus, it is normal practice to state this

motion.

relationship in terms of absolute temperature. In

equation form, this part of the law becomes

law states that a body at rest tends to remain at

P1T2 = P2T1,

rest. A body in motion tends to remain in

motion. This law can be demonstrated easily in

where

everyday use. For example, a parked automobile

will remain motionless until some force causes it

P = pressure, and

to move-a body at rest tends to remain at rest.

The second portion of the law-a body in

T = temperature.

motion tends to remain in motion-can be

demonstrated only in a theoretical sense. The

In words, this equation states that with a

same car placed in motion would remain in

constant volume, the absolute pressure of an

enclosed gas varies directly with the absolute

temperature. Remember, when using this for-

mula, you must convert stated pressures and

temperatures to absolute values.

Compressibility is an outstanding charac-

teristic of gases. Robert Boyle, an English

scientist, was among the first to study this

characteristic. He called it the "springiness" of

air. He discovered that when the temperature of

an enclosed sample of gas was kept constant and

the pressure doubled, the volume was reduced to

half the former value. As the applied pressure was

decreased, the resulting volume increased. From

4-9