AIR PRESSURE

What is pressure? A force that is applied over an area.

What is air pressure? Balloon example – what about the particles is causing pressure in the balloon? Air pressure is caused by the particles of air colliding with sides of the container (in this case, the balloon)

What is atmospheric pressure? What is the Earth’s atmosphere made up of? What force pulls the Earth’s atmosphere close to the surface?

Gravity and Atmospheric Pressure The stronger the gravity, the more gas is held by the world and the greater the weight of atmospheric pressure on a point.

Atmospheric Pressure Atmospheric pressure – the amount of force (gravity) exerted on 1 meter2 column of air.

High altitudes = lower pressure Air pressure can be thought of as the column of air rising above us. As we go up in altitude, we get closer to the top of the column. Thus there are fewer molecules of air above us to be pulled down by gravity, so the air “weighs” less. Therefore, pressure always decreases as one goes up. Low altitudes = higher pressure

What would be the units of pressure? Pressure is a force that is exerted on an area. Units of Pressure: Atmosphere (atm.) pounds per square inch or psi (1 atm. = 14.7 pounds per square inch) mm or inches of mercury (1 atm.= 760 mm or 29.92 inches of Hg) torrs (1 torr = the pressure exerted by 1 cm2 of mercury) Pascal or Pa(1 newton per square meter) 1 kPa = 1000 Pa and 1 atm. = 101.325 kPa

World’s Longest Straw - Veritasium

What happens when you draw a liquid through a straw into your mouth? When you suck on a straw, you remove some of the particles of air in the straw. Since there are more particles of air striking the liquid in the cup than are striking the liquid in the straw, the effect of these extra collisions is to push the liquid up into your mouth. So actually, you are not sucking the liquid into your mouth, the atmosphere is pushing it there.

Evangelista Torricelli (1606-1647) Torricelli wanted to figure out why he could not pump liquid up to the top floor of his villa with a pump. He reasoned that even if the pump could remove all the air above the column of liquid, the pressure exerted by the atmosphere could only support a column of liquid so high (~34 ft). As the story goes, he replaced the top portion of the pipe with a glass tube and noted that the water level fluctuated depending on the weather. He found that he could more conveniently study this behavior if he used a liquid with a greater density. He filled the tube with mercury (d = 13.6 g/mL) and found that atmosphere could pressure could support a column of mercury varying from 28-31 inches. Such a device is called a barometer. Torricelli didn’t actually build a barometer, but he gave detailed instructions on how to build one, so he is given credit for the invention. He was actually trying to prove the existence of a vacuum. Many scientists in his day didn’t believe that a vacuum could exist, hence the phrase, “nature abhors a vacuum.”

Torricelli’s barometer used a glass column suspended in a bowl of mercury. The pressure of the air molecules pushed the mercury up into the glass tube. The weight of the mercury in the tube was equal to the weight of the air pressing down on the mercury in the dish. The abbreviation “Hg” is the chemical symbol for mercury. Some kinds of pressure reading instruments, including some barometers, use the abbreviation “mmHg,” meaning “millimeters of mercury.” 760 mmHg is considered the standard “normal” atmospheric pressure at sea level. This unit is called a “torr,” after Torricelli. To construct a mercury barometer, fill a tube with a liquid. Invert then tube in a dish of liquid holding your thumb over the top of the tube until the the tube is immersed in the bowl of liquid, the atmospheric pressure will keep the liquid in the tube from emptying such that the weight of the liquid in the tube equalize with the atmospheric pressures. (Do not do this with mercury because of its toxicity) Mercury was used because it is a very heavy liquid, so the tube could be relatively short. The tube in a mercury barometer still has to be over a meter long. Students may want to try building a barometer using colored water. How high would the tube need to be? Merucy is about 11 times more dense than water. What if they used milk or some other liquid, would the height be the same?

As atmospheric pressure increases… The mercury in the tube rises.

As atmospheric pressure decreases… The mercury in the tube falls.

Another Instrument used to measure pressure… Manometer

Manometers Cont.