1. Submitted To,
Smt Linimol KS
(lecture and my teacher in physical
science)
SNTC Sreekandewaram,poochakkal p.o.,
Cherthala - 688526
Submitted By,
Syamaprasad.S
physical science
Rg.no:1821438302
2. • Atmosphere is kept by the planet’s gravity
– Low mass (small) planets= low gravity
=almost no atm.
– High mass (large) planets = high gravity
= thick atm.
• Gravity and pressure
– Air pressure depends on how much gas there
is i.e. The atmospheric thickness.
3. Gravity and Atmospheric Pressure
• The stronger the gravity, the more gas is held by the
planet and also increases the weight of atmosphere on
unit area
6. Gravity pulls the air moleculesGravity pulls the air molecules
toward the earth, giving themtoward the earth, giving them
weight. The weight of the airweight. The weight of the air
molecules all around us ismolecules all around us is
called the air pressure.called the air pressure.
8. Atmospheric Pressure
Gas pressureGas pressure
depends on bothdepends on both
density anddensity and
temperature.temperature.
Adding airAdding air
moleculesmolecules
increases theincreases the
pressure in apressure in a
balloon.balloon.
Heating the airHeating the air
also increasesalso increases
the pressure.the pressure.
9. Air pressure isAir pressure is
equal in allequal in all
directions.directions.
Pressure = force per unit areaPressure = force per unit area
12. In 1643, Evangelista TorricelliIn 1643, Evangelista Torricelli
invented the barometerinvented the barometer
13. Torricelli’s barometerTorricelli’s barometer
used a glass columnused a glass column
suspended in a bowl ofsuspended in a bowl of
mercury. The pressuremercury. The pressure
of the air moleculesof the air molecules
pushed the mercury uppushed the mercury up
into the glass tube.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.
15. The Mercury BarometerThe Mercury Barometer
Good:Good: Bad:Bad:
•Simple to constructSimple to construct
•Highly accurateHighly accurate
•Glass tube is fragileGlass tube is fragile
•Mercury isMercury is veryvery toxictoxic!!
17. Most aneroidMost aneroid
barometers have abarometers have a
needle which can beneedle which can be
set to remember theset to remember the
previous reading.previous reading.
18. An aneroid barometerAn aneroid barometer
uses a cell which hasuses a cell which has
had most of the airhad most of the air
removed.removed.
As the air pressureAs the air pressure
around the cellaround the cell
increases, it pressesincreases, it presses
on the cell, whichon the cell, which
causes the needle tocauses the needle to
move.move.
Television weather forecasters usually give barometricTelevision weather forecasters usually give barometric
pressure in inches of mercury. However, meteorologistspressure in inches of mercury. However, meteorologists
measure atmospheric pressure inmeasure atmospheric pressure in millibars.millibars.
MILLIBARS
19.
20.
21.
22. Changing PressureChanging Pressure
AA falling barometerfalling barometer = decreasing air pressure.= decreasing air pressure.
This usually means:This usually means:
Falling barometer readings usually
indicate the approach of an area of
low pressure. Low pressure readings
are usually associated with storm
systems. Tornadoes and hurricanes
can produce very low barometric
readings.
Editor's Notes
Your weight is the result of gravity pulling your mass down on the bathroom scales. Note that weight has units of a force, such as pounds.
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.
Because air is a fluid, force applied in one direction is distributed equally in all directions. Thus the downward pull of gravity on air molecules produces air pressure in all directions.
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.”
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?
Although mercury has been used for hundreds of years, its toxic effects have only been fully realized in the last few decades. Students should NEVER handle mercury or broken mercury thermometers or barometers. Mercury should also never be thrown in the trash or washed down the drain, since it moves easily up the food chain from fish to humans. A local health department or environmental professional can assist with disposal of old or broken mercury instruments. GLOBE instrument specifications call for organic-fluid (non-mercury) or digital thermometers, except for the analog min/max thermometer, which is mounted in a shelter and is not handled by the students.
Knowing how the air pressure is changing is as important as knowing the actual barometric reading. The set needle allows students to compare the current reading to the previous one. If the current reading is less than the previous one, the barometric pressure is falling. If the current reading is more than the previous one, the pressure is rising. If it is the same as the previous reading, the pressure is said to be steady. Weather forecasters often use the phrases “falling barometer,” “rising barometer,” or “steady barometer” as a way of referring to the change in atmospheric pressure.
The word “aneroid” means “no air,” and refers to the partial vacuum inside the cell. The aneroid cell is shaped like a bellows, so that it can flex as air pressure changes. Increasing air pressure compresses the cell, causing the needle to register a change. Decreasing pressure allows the cell to expand, causing the needle to move in the opposite direction.
The use of inches of mercury is a hold-over from the days of mercury barometers. It refers to the actual height of mercury in the glass tube. Millibars are metric system units, and as such are readily understood by scientists around the world.
As we have noted earlier, higher elevations have fewer air molecules pressing downward, and so atmospheric pressure is lower. This means a barometer will read lower as it is carried to a higher elevation. Airplanes use a special type of barometer, called an altimeter, to measure altitude.
Rising barometer readings indicate that a high pressure system is approaching. Higher atmospheric pressure is usually associated with fair weather and clearing skies.
Falling barometer readings usually indicate the approach of an area of low pressure. Low pressure readings are usually associated with storm systems. Tornadoes and hurricanes can produce very low barometric readings.
