Phy 102: Fundamentals of Physics II Chapter 14 Lecture Notes

Irish-born, English Chemist The first prominent scientist to carry out controlled experiments –Used rigorous experimental and quantitative methods –Published results in detail Main Scientific Contributions: –Proposed the first modern definition of a chemical element Ironically he did not believe in the physical reality of atoms –Designed the first vacuum pump –The first to use color indicators to test acidity Robert Boyle ( ) Considered by many to be the “Father of Modern Chemistry”

Gases Matter that has no definite shape and no definite volume –Take the shape of their container –Take the volume of the container Gases are fluids Gas particles have very little interaction with each other (except collisions) Since gases take the volume of their container, the density is not constant –Depends on temperature and pressure

Atmospheric Pressure Varies with elevation –decreases as you increase in elevation –decreases as air density decreases Note: Air Pressure = weight density x depth or P =  x H H is measured from the top down (it is depth not height) However: –Air density (  ) decreases as you increase in elevation –At sea level, 1 cubic meter has a mass of 1-and-¼ kilograms (  air ~ 12.5 N/m 3 )

Barometers Devices that measure pressure of the atmosphere A typical mercury barometer: –Closed end (P = 0) –Open end exposed to atmosphere –The mercury in the closed end drops until its weight is balanced by the atmospheric pressure The mercury column (the meter) will –increase as atmospheric pressure increases –decrease as atmospheric pressure decreases

Boyle’s Law The relationship between pressure & volume for an enclosed gas at constant temperature Presented by Boyle (but not discovered by him!) For an enclosed gas the product of pressure times volume is a constant value, or Pressure 1 x Volume 1 = Pressure 2 x Volume 2 or P 1 V 1 = P 2 V 2 Increases/decreases in volume result in inversely proportional change in pressure (vice versa) Valid only when the temperature is kept constant.

Example Pressure vs. Volume Curve

Bernoulli’s Principle When the speed of a fluid increases its pressure decreases Ever ask yourself: –Why do golf balls have dimples? –Why does a pitcher scuff the baseball before throwing it? –Why do loose papers fly out the window when you are driving down the road? –Why don’t airplanes fall out of the sky (at least not most of the time…)? The answers to these questions, my friends, lie in Bernoulli’s Principle… Pressure Difference =  P = ½ Dv 2