Atmospheric pressure, Air pressure, Barometers. School Science Lessons
2024-09-17
Atmospheric pressure, balloons, barometers
(UNPh12a)
Contents
12.1.0 Atmospheric pressure, air pressure
1.0 Air (Primary)
12.1.1 Atmospheric pressure, air pressure
12.1.2 Conversions between units
Experiments
12.1.3 Air pressure in all directions
12.1.4 Atmospheric pressure water spray
12.1.5 Air has mass, air has weight, weigh a balloon
12.1.6 Air has mass, break ruler under newspaper
12.1.7 Air takes up space, transfer air under water
12.1.8 Automatic drinking glass
12.1.9 Bag of air into and out of a jar
12.1.10 Balanced balloons
12.1.11 Balloon pushed into a flask
12.1.12 Bottles stick together
12.1.13 Carbon dioxide has mass
12.1.45 Card on inverted glass
12.1.14 Cork sticks to the bottom of a beaker
12.1.15 Crush drink can with atmospheric pressure, collapsible can
12.1.16 Crushed plastic drink bottle
12.1.17 "Cupping", (partial vacuum)
12.1.18 Drinking straw, finger on drinking straw, glass tube
12.1.19 Egg in a bottle
12.1.20 Expanded balloon with cup "ears"
12.1.21 Funnel in the neck of a bottle
12.3.15 Heavy newspaper, air has mass, buoyancy in air
12.1.22 Hero's Fountain
12.3.11 Holes in a beverage can
12.1.23 Inflate balloon with low pressure and high pressure bottle
12.3.12 Inverted dish sticks to a smooth board
12.1.24 Inverted drinking glass
12.1.25 Lift water with air pressure
12.1.26 Lifting power of balloons containing different gases
12.1.27 Magdeburg hemispheres, vacuum pumps
12.1.28 Measure atmospheric pressure with rubber suction cup
12.1.29 Oxidation and air pressure, steel wool over water
12.1.30 Plumber's force cups, suction cup, suction disc
12.1.31 Pouring gases
12.1.32 Potato puncture, push drinking straw through potato
12.1.33 Pushed down drinking glass
12.1.34 Shrinking balloons
12.1.35 Standard atmosphere
12.1.36 Standard temperature and pressure, STP, density of gases
12.1.37 Syringe lift pump
12.1.38 Syringes for investigating air pressure
12.1.39 Tapping a box
12.1.41 Vacuum cleaner
12.1.42 Water rises in a downwards floating beaker, pressure under inverted beaker
12.1.43 Weight of air
12.1.44 Wet suction with a Petri dish
12.1.1 Atmospheric pressure, air pressure
See: Barometers, (Commercial).
Atmospheric pressure is exerted by the weight of the air above any place of the surface of the earth.
At sea level atmospheric pressure will support a column of mercury 760 cm high, in SI units 101.325 kilopascal, called one standard atmosphere, but less with an increase in altitude.
The mercury is at temperature 0oC and the value of g is that at sea level, latitude 45o.
Variations in the atmospheric pressure is measure by a barometer.
The standard atmosphere is a hypothetical atmosphere having the approximate average state of the real atmosphere in which the pressure and temperature is defined at all heights.
This internationally agreed standard atmosphere is used for assessing the performance of altimeters, aeroplanes and other devices that change their position of height in the atmosphere.
Sea level is the hypothetical level of the surface of the sea, the ordnance datum, and the barometric standard.
When bubbles form in a liquid, the vapour pressure inside the bubble is slightly greater than the atmospheric pressure above the liquid.
The hole in the barrels of ball point pens is to allow equal pressure inside the pen with atmospheric pressure to help prevent ink leakage.
12.1.2 Conversions between units of atmospheric pressure
1 atmosphere (atm) = (1.01325 bar) = (1 01325 mb) (millibar) = (1.01325 105 Pa) = (1.01325 × 105 N / m2) = (101 325 N m-2)
(Pa) = 1.01325 × 105 N m-2 = (101.325 kilopascal) (kPa) = (1.013 × 106 dyne / cm2) = (760 mm Hg) = (760 torr) = (~760 mm Hg) = (14.7 lb / in2) (14.7 pounds force per square inch) (2 116 pounds force per square foot).
12.1.3 Air pressure in all directions
See diagram 37.117: Sucking water up.
See diagram 4.227.1: Finger on drinking straw.
See diagram 4.227.2: Inverted glass: A Glass jar, B Water, C Paper, D Air pressure.
See diagram 4.227.3: Covered jar at different angles: A Air pressure, B Cardboard, C water.
See diagram 4.227.4: Plumber's force cups: A Two plumber's force cups, B Wooden handle, C Rubber.
1. Finger on drinking straw
Hold a finger over the end of a piece of straight glass tube or drinking straw and lower the tube into a container of coloured water.
Water does not replace the air in the tube.
Remove the finger and water enters the tube.
Replace the finger on the top of the tube and then lift the tube from the container.
The water remains in the tube, because the effect of the air pressure up the tube is greater than the weight of the water.
Remove your finger and the water falls out of the tube.
2. Inverted glass
Fill a drinking glass with water.
Cover the glass with a flat piece of glass or cardboard so that no air exists between the cover and the water.
Turn the glass and cover upside down.
The cover remains in place, because the pressure of the air pushing up is greater than the pressure of the water pushing down.
3. Holes in a beverage can
Make a small hole in the top of a beverage can.
It is very difficult to suck the drink through the hole or to pour the drink into a glass.
Make a second hole in the beverage can.
Now it is easy to suck the drink through the hole or to pour the drink into a glass.
Sucking reduces pressure at one hole in the can so the air pressure acting through the second hole forces drink into your mouth or lets you pour the drink into a glass.
4. Push a drinking straw through a hole in a beverage can
Make a hole with a nail near the bottom of an open metal can.
Fill the can with water.
Hold the palm of the hand tightly over the top and water stops running from the hole.
Remove the hand and water runs from the hole.
5. Sucking water up
Fit a flask with a two-holes stopper.
Fit the stopper with a straight glass tube and a bent glass tube through the holes.
Pour water into the flask and fix the stopper in the flask tightly.
You can suck water up the straight tube.
However, if you close the end of the bent tube with your finger, you cannot suck up water through the straight tube.
Remove the finger and sucking is easy again.
Instead of sucking, blow through the straight glass tube.
Increased atmospheric pressure on the surface of the water causes water to rise up the bent glass tube.
6. Suction disc
A rubber suction disc stays on a smooth window, because no air exists between the disc and the window.
The pressure of the atmosphere on the rubber disc keeps it pressed to the window.
7. Plumber's force cups
7.1 Press a plumber's force cup against a flat surface, e.g. the top of a stool, and lift the stool.
The force cup works better if the rubber is wet.
The force cup works, because almost no air remains between the object and the force cup.
However, the air in the atmosphere is pressing down on the rubber with atmospheric pressure.
7.2 Wet the rims of two plumber's force cups.
Press the rubber cups tightly together.
Try to pull them apart.
This experiment is similar to the historical demonstration of air pressure called the Magdeburg hemispheres.
8. Vacuum cleaner
Vacuum (Commercial).
A vacuum cleaner pumps some air away from over the dirty carpet creating a "partial vacuum".
Air rushes in to replace the air pumped out and when it is pumped away it takes with it the dirt from the carpet.
The more air that you remove the greater the force from the atmospheric pressure to replace it.
In a laboratory, it is impossible to pump all the air out of an enclosed space to create a perfect vacuum.
"Nature abhors a vacuum" (Spinosa, 1677, in Ethics).
Players of golf and baseball may complain about "heavy air" during damp weather, but the weight of a volume of air is actually less during bad weather caused by low pressure.
12.1.4 Atmospheric pressure water spray
See diagram 12.1.04: Atmospheric pressure water spray.
Boil some water in a round bottom flask fitted with a one hole stopper and glass tube.
After a steady stream of seams comes out of the glass tube, quickly invert the flask so that the end of the glass tube is under water.
Water rises up the glass tube and sprays into the round bottom flask.
The loss of steam from the flask created a partial vacuum.
Atmospheric pressure acting on the surface of water in the container forced water up into the flask.
12.1.5 Air has mass, air has weight, weigh a balloon
See diagram 12.3.3: Weigh a balloon.
1. Inflate a balloon with a bicycle pump.
Put the balloon on a platform balance and note its mass.
Remove the balloon, but leave the weights on the other platform.
Deflate the balloon and replace it on the balance pan.
Disregarding the effect of buoyancy, the balloon has less mass.
2. Inflate a balloon and put it on a sensitive balance.
Record the weight, e.g. 2.1 g.
Deflate the balloon and weigh again, e.g. 1.9 g.
The weight is less than before, because air exerts the weight force.
The first weight is less than the true weight, because a balloon has a large volume in relation to its mass.
So you have noticed a significant upward force due to buoyancy.
You live in a "sea of air", but you can usually disregard the effect of buoyancy when you weigh things, because the volume of what you are weighing is small in comparison to its mass.
The pressure inside the balloon is greater than atmospheric pressure, because the elastic rubber compressing the air inside the balloon.
So the density of the air inside the balloon is greater than the density of the air it has displaced.
So the air in the balloon has a greater mass than the volume of air that was displaced.
The apparent mass of the balloon is reduced by the mass of the volume of air the balloon has displaced.
3. Inflate a balloon or basketball or volleyball or soccer ball and put it on a sensitive balance reading to at least the nearest 0.1 g.
Record the weight.
Deflate, but do not collapse the balloon or ball and weigh again.
Collapse the balloon or ball as much as possible and weigh again.
The weight is less than before, because air exerts the weight force.
The first weight is less than the true weight, because a balloon has a large volume relative to its mass, so there has been a significant upward force caused by buoyancy.
You live in a "sea of air", but you can usually disregard the effect of buoyancy when you weigh things, because the volume of what you are weighing is small in comparison to its mass.
4. Before beginning, adjust the zero of a sensitive balance.
Weigh an empty plastic beaker, an empty rubber balloon and a paper clip.
Note the precise position of the pointer.
Inflate the rubber balloon to the maximum extent.
Fasten it with the paper clip weighed with it.
Place the inflated balloon on the balance and note the position of the pointer.
Pump up the balloon first.
Note the precise position of the pointer.
Prick the balloon with a needle.
The position of the pointer after all gases are gone out of the balloon.
The balance shows a higher mass.
In this experiment you measure only a fraction of the total weight of the air in the balloon.
This is because the effect of buoyancy, since the mass of the air inside the balloon is greater than the mass of the air displaced by the expanded balloon.
However, the experiment does show that air has weight.
5. Place a 1 m flat stick on a table so nearly half the length hangs over the edge of a table.
Lay a sheet of newspaper over the end of the flat stick on the table and smooth it down.
Give the other end of the flat stick a sharp blow.
The flat stick breaks over the edge of the table.
The stick breaks, because the air pressure on the large sheet of paper exerts a force down on the paper.
6. Let the air out of a basketball.
Weigh the soft basket ball.
Pump 10 strokes of air into the ball and weigh again.
Pump 10 more strokes of air into the ball and weigh again.
7. In China, people used to buy coal gas in bags from the gas company.
During a cold winter they used to rock the bag with their foot to check how much gas was still in the bag.
12.1.6 Air has mass, break ruler under newspaper
1. Inflate a balloon and put it on a sensitive balance.
Record the weight.
Deflate the balloon and weigh again.
The weight is less than before, because air exerts the weight force.
The first weight is less than the true weight, because a balloon has a large volume relative to its mass, so there has been a significant upward force caused by buoyancy.
You live in a "sea of air", but you can usually disregard the effect of buoyancy when you weigh things, because the volume of what you are weighing is small in comparison to its mass.
2. Place a flat stick about a metre long so that nearly half of it hangs over the edge of a table or desk.
Lay a full sheet of newspaper over the end of the stick on the table and smooth it down carefully.
Give the other end of the stick a sharp blow with your hand or a wooden mallet.
The stick will break over the edge of the table.
The stick breaks, because the inner end has been held down by air pressure on the large sheet of paper.
Stand to one side when hitting the stick.
3. Place a thin flat stick on a table so nearly half the length hangs over the edge of a table.
Lay a sheet of A4 printer paper over the ruler.
Hit the end of the flat stick, like a karate chop.
The paper and stick rise up and fall down.
Place a thin flat stick on a table so nearly half the length hangs over the edge of a table.
Lay a sheet of newspaper over the end of the flat stick on the table, fold facing down, and smooth it down.
Hit the end of the flat stick, like a karate chop.
The flat stick breaks over the edge of the table.
The stick breaks, because the air pressure on the large sheet of paper exerts a force down on the paper.
The force on the ruler = atmospheric pressure X area of the newspaper.
4. Place a full double page sheet from a broadsheet newspaper over a ruler protruding out over the edge of the desk.
Hit the end of the ruler sharply.
The ruler breaks and page stays in place.
The air pressure pressing down on the newspaper holds the newspaper down despite the force of the ruler pushing it up.
5. To remove atmospheric pressure from the experiment, use two more rulers at each side of the protruding ruler.
Now the air pressure is the same above and below the newspaper.
Hit the end of the central ruler sharply.
The ruler breaks and page stays in place.
The weight of the newspaper broke the ruler.
Place a small weight with the same weight as the newspaper on the edge of the ruler over the table.
Hit the end of the ruler sharply.
The ruler breaks again.
12.1.7 Air takes up space, transfer air under water
Aquarium, (Commercial).
See diagram 12.303.3: Transfer air under water.
See diagram 4.225.1: Air takes up space.
1. Hold two jars under water one contains air and the other is full of water.
Tilt the jar full of air up to transfer bubbles of air into the second jar.
2. Use an aquarium nearly full of water.
Lower a drinking cup mouth down into the bucket.
Use your other hand to lower another cup mouth down into the bucket.
Tilt its mouth upwards to let it fill with water.
Hold the second cup mouth downwards above the first one.
Tilt the first cup to let the air escape slowly to fill the second one.
3. Almost fill a fish tank with water.
Lower a drinking glass, mouth downward, into the fish tank.
With your other hand lower another glass into the fish tank.
Let this second glass fill with water by tilting its mouth upwards.
Now hold this glass above the first one mouth downwards.
Carefully tilt the first glass to let the air escape slowly.
Fill the second glass with air from the first glass to transfer the air under water.
Air replaces some water in the second glass.
4. Place the funnel in the neck of the bottle.
Seal the space between the funnel and the neck of the bottle with heavy grease or Plasticine (modelling clay).
Pour water slowly into the funnel.
The water stops running, because the air takes up space.
Repeat the experiment and pour in water until it comes nearly to the top of the funnel.
Use a nail to punch a hole through the seal.
All the water drops into the bottle.
The water replaces the air that comes out through the punched hole.
5. Pour water into a large glass container until it is half full.
Float a cork on the water and lower a drinking glass, mouth downward, over the cork.
Repeat the experiment with a piece of paper wedged tightly into the bottom of the glass.
The paper does not get wet.
12.1.8 Automatic drinking glass
See diagram 12.3.10: Water rises in glass.
Put a shallow pan with a little water on a table.
Light a piece of paper and immediately put it into the drinking glass.
Quickly invert the glass and place it in the pan.
Water in the pan is pushed up into the glass.
12.1.9 Bag of air into and out of a jar
1. Use a plastic bag with circumference slightly more than the circumference of a wide mouth glass jar.
Shake the bag until it is full of air.
Use adhesive tape to connect the mouth of the bag to the mouth of the jar in an air-tight connection.
Try to push the bag into the jar with your closed fist.
You cannot push the bag into the jar against the pressure of air in the bag and jar.
2. Undo some of the adhesive tape so that you can push the bag firmly into the jar.
Push the bag firmly against the inside of the jar.
Replace the adhesive tape to make an air-tight connection again.
Attach a oop of adhesive tape to the bottom of the bag inside the jar.
Try to pull up the bag by pulling on the adhesive tape.
You cannot pull the bag out, because the atmospheric pressure inside the bag is greater than the pressure of the air between the bag and the inside of the jar.
12.1.10 Balanced balloons
Inflate two identical balloons until they are the same size and tie the inlets securely with string.
Make a loop at the end of each string.
Attach another string to the centre of a metre rule.
Use the loops to attach one balloon to each end of the metre rule.
Hold up the string attached to the centre metre rule and the move the loops until the balloons are balanced and the metre rule is horizontal.
Carefully cut the string tied to one of the balloons.
The balloons are no longer balanced.
12.1.11 Balloon pushed into a flask
Boil water in a flask for 5 minutes then attach the mouth of a deflated balloon to the mouth of the flask.
Hold the flask in the sink and wash it with water from the cold tap.
Atmospheric pressure pushes the balloon into the flask.
12.1.12 Bottles stick together
1. Put a piece of wet filter paper or absorbent paper over the mouth of a bottle.
Light a twisted piece of paper and drop it into an identical bottle.
Immediately invert the bottle with the wet filter paper over it.
Press down on the top bottle so that the wet filter paper forms a seal between the mouths of the two bottles and the flame goes out.
You can now lift both bottles by grabbing only the top bottle.
Similarly you can invert the two bottles and lift up both bottles by holding the other bottle on top.
2. If you try to repeat the experiment with dry filter paper between the bottles it does not work, because air can get in through the dry paper.
12.1.13 Carbon dioxide has mass
Tie string A to the centre of a uniform rod and tie strings B and C to each end of the rod.
Attach strings B and C to the ends of the ring pull of cola beverage cans B and C.
Pull up on string A so that the rod and supported beverage cans are no longer touching the table and adjust the position of strings A and B along the rod so that the rod bearing the two beverage cans is horizontal.
Keep hold of string A and lower the rod so that the beverage cans B and C are resting on the table.
Carefully pull on the ring-pull of beverage can B that carbon dioxide escapes, but the cola is not lost by too much fizzing or splashing.
Pull up on string A again until the beverage cans B and C are suspended, i.e. no longer resting on the table.
Note that the rod is no longer horizontal, because beverage can B has lost carbon dioxide.
The rod is now sloping with beverage can C suspended lower than beverage can B.
12.1.14 Cork sticks to the bottom of a beaker
Hollow out a suction cup in the bottom of a cork so it will stay stuck at the bottom of a beaker as you add water.
12.1.15 Crush drink can with atmospheric pressure, collapsible can
Crushed can, collapsing beverage can with atmospheric pressure, collapsible can
Collapsible can, (Commercial).
1. Use a flat-sided screw top tin can with a tightly fitting cap that has been thoroughly rinsed then left open to dry.
Put a few centimetres of water in the tin can.
With the cap off, heat the water until it boils and steam comes out.
Stop heating and immediately hold the tin can with a dry cloth to screw on the cap very tightly using insulated gloves.
You may use petroleum jelly to get a tight seal.
Allow the tin can to cool by putting it under a cold water tap or covering with a wet towel.
The sides of the can will slowly collapse inwards.
When the water boils, the steam drives the air from the tin.
When cool, the steam condenses to form water again, causing much lower pressure inside the tin than outside it.
The tin can collapses, because the external pressure is greater than the internal pressure.
So there are more air molecules pushing the sides of the can in than there are molecules inside the can pushing the sides out.
A rectangular tin can is better for the experiment than a round tin can because flat surfaces are more easily affected by changes in pressure than curved surfaces.
Repeat the experiment by removing air from a thin wall can with a vacuum cleaner.
2. Fill a beverage can with hot steam.
Place the beverage can open side down on ice.
The beverage can quickly collapses as the steam condenses.
3. Boil water in a can and cap.
As the vapour pressure is reduced by cooling the can collapses.
Pump out a beverage can slightly, put it in a vacuum chamber and blow it back up again.
4. Boil water in an opened soft drink can until it is full of steam.
Use a pair of tongs to quickly invert the can to hit against the bottom of a bucket of water.
The steam in the can condenses, the internal pressure falls, and the can collapses.
The can is crushed by the atmospheric pressure and water pressure on the outside walls of the can.
If there is no seal between the open end of the can and the bottom of the bucket, the partial vacuum in the can will draw in water and the can will not collapse.
12.1.16 Crushed plastic drink bottle
Use a 2 -litre plastic drink bottle with a screw on plastic cap.
Half fill the drink bottle with boiling water.
Be careful! Quickly rotate the drink bottle to swish the water around, then pour out the water and screw on the plastic cap.
Hold the drink bottle under a tap to pour water over it.
The sides of the drink bottle collapse inwards crushed by the difference between atmospheric pressure and the pressure inside the drink bottle.
When the drink bottle was half filled with hot water, the air in the other half was warmed by it.
When the cap was screwed on and the drink bottle cooled, the warm air inside the drink bottle cooled and contracted to decrease the pressure inside the drink bottle.
12.1.17 "Cupping"
Vacuum (Commercial).
A now discarded and probably useless medical treatment was called "cupping".
Wide mouth cups were heated internally with a candle then placed mouth downwards on the patients skin.
The heated air in the cups cooled causing a partial vacuum leaving a red area of skin where the capillaries had expanded.
This procedure was supposed to draw out the bad "humours", which in those days were thought to cause disease.
The procedure is still practised in some countries in Asia.
Do not ask students to do this, but teachers who are brave or foolhardy enough can drop a piece of burning paper into a small beaker or drinking glass, then press the mouth of the beaker against the palm of the hand or forehead so that the flame goes out!.
The beaker will stick to the skin and leave a red mark.
12.1.18 Drinking straw, finger on drinking straw, glass tube
See diagram 12.305.2: Air supports water in a glass tube.
1. Hold a finger over the end of a piece of straight glass tube or drinking straw and lower the tube into a container of coloured water.
Water does not replace the air in the tube.
Remove the finger and water enters the tube.
Replace the finger on the top of the tube and then lift the tube from the container.
The water remains in the tube, because the effect of the air pressure up the tube is greater than the weight of the water.
Remove your finger and the water falls out of the tube.
Tap the glass tube or straw against the side of the container so that one or two drops of water fall out leaving a small space between the surface of the water and the finger.
The air in this small space is at a much lower pressure than atmospheric pressure if the water stays in the tube.
Remove your finger and the water falls out of the tube due to its own weight and the atmospheric pressure above the falling water is equal to the atmospheric pressure below the falling water.
2. Put a glass tube or drinking straw under water so that it contains no air.
Press your finger against one end of the tube.
Take the tube out of the water.
Water remains in the tube.
Air supports water in a glass tube or drinking straw.
The external air pressure acts with uniform force in every direction in space.
In this experiment it applies pressure from below against the column of water in the glass tube.
The weight of the water creates a reduction in pressure between the water and the finger, since this pressure is lower than atmospheric pressure the water cannot flow out of the bottom of the tube.
When you remove the finger, the pressure above the water is the same as the pressure below the water and as a result the water flows out of the tube.
This process finds practical application such as the glass tube called a pipette to transfer known volumes of liquid.
3. In a drinking straw contest you can measure the maximum length of a vertical drinking straw, or linked drinking straws, used to suck water up.
If a vacuum exists above a non-volatile liquid then the maximum height it could be sucked up would be when the hydrostatic head of pressure = 1 atmosphere, i.e. 101, 325 pascals.
Pressure = hdg, i.e. height (m) × density of water (1000 kg per cubic metre) × g, acceleration due to gravity ( 9.81 metres per second per second) = 10.33 metres.
However, at a room temperature of 27oC (80.6oF) water has a vapour pressure of 3, 536 pascals.
It would begin to boil before a vacuum is reached.
So maximum vapour pressure you could apply by sucking = 101, 325 - 3, 536 = 97.789 pascals, and maximum height = 9.97 metres.
Some high school students can suck up to 2 metres vertical height while other students can double this height by first sucking, sealing the end of the drinking straw with the tongue, breathing out, then sucking again.
However, the tongue may get stuck at the end of the drinking straw and blood blisters may form.
12.1.19 Egg in a bottle
See diagram 12.3.27: Egg in a bottle.
1. Cover a fresh egg with vinegar or dilute acid hydrochloric acid.
Change the solutions each day for 2 to 7 days.
The dilute acid dissolves most of the egg shell or bone composed mainly of calcium carbonate.
Pick up the decalcified egg and drop it to show that it will bounce and not break.
Boil water in a flask.
The steam in the flask forces out some of the air.
Stop heating after boiling for 5 minutes then immediately put the egg with no shell in the opening of the flask.
Hold the flask in the sink and wash it with water from the cold tap.
The steam cools to form water drops that takes up less space and reduce the air pressure inside the flask.
The soft egg will squeeze down the neck of the flask, because the atmospheric pressure is greater than the pressure in the flask.
The egg is not "sucked" into the bottle!
2. Peel the shell off a hard-boiled egg then put the peeled egg narrower end down end in the mouth of bottle with a fairly wide mouth, e.g. a milk bottle.
Light a small piece of paper or a match, lift up the egg, drop the burning paper into the bottle and replace the egg.
The egg trembles as hot air leaves the bottle then the flame goes out and the egg is pushed into the bottle, because the atmospheric pressure is greater than the pressure of gases in the bottle.
3. Peel the shell off a hard-boiled egg then put the peeled egg narrower end down end in the mouth of bottle with a fairly wide mouth, e.g. a milk bottle.
Drop burning paper or a lighted match into the bottle, wait until it stops burning, then immediately place the egg on the opening.
The warmed air starts to cool when the burning stops and the egg seals the opening so the air pressure in the bottle decreases as the cooling continues.
Get the egg out by inverting the bottle so that the egg sits inside the mouth.
Then warm the bottle to make it expand and the egg is pushed out.
4. Blow upwards on the egg or use a jet of compressed air to create negative pressure below the egg so that the egg is pushed out by the pressure of gases in the bottle or blown upwards.
This is an application of the Bernoulli force.
Hold the bottle with its mouth near your mouth with its bottom slightly higher so that the egg is in the neck of the bottle, but not completely blocking the neck.
You must exhale a huge burst of breath to suddenly increase the inside air pressure.
The Bernoulli effect of the reduction of in air pressures at right angles to the air stream flow direction caused by the movement of air around the egg and the increased air pressure will push the egg out of the bottle.
Some people can catch the egg in their mouth then eat it!
12.1.20 Expanded balloon with cup "ears"
1. Use your breath to partly inflate a round balloon so that it is nearly spherical.
Wet the mouths of two identical plastic cups and hold them each side of the balloon, like "ears".
Inflate the balloon further while using your hands to keep hold of the cups and press slightly inwards.
Ask another person to tie a string around the mouth of the balloon and hold the end of the string.
Take your hand off the cups so that the balloon is held up only by the string.
The cups stick to the balloon.
When you inflated the balloon with the cups pressed in you increased the volume of air in the cups as the curvature of the balloon became less.
The pressure of air in the cups became less than atmospheric pressure.
Atmospheric pressure acting on the outside of the cups kept them pressed in on the balloon, like "ears".
Slowly release the string to let air out of the balloon.
At a certain decreased curvature of the balloon, the cups fall off.
Some teachers have reported no success with this experiment.
2. Inflate up a balloon to large orange size and pinch the inlet.
Wet the rims of two identical plastic containers, e.g. small jam containers.
Lay the containers on their sides each side of the balloon, with open ends touching the balloon.
Press the containers in on the balloon while a second person inflates the balloon more.
More inflation causes the curvature of the balloon in contact to decrease, and the air pressure inside the containers to decrease as the volume within increases.
Use this procedure to attach more plastic containers to the balloon.
Pick up the balloon with containers attached.
12.1.21 Funnel in the neck of bottle
See diagram 12.303.1: Funnel in neck of bottle.
1. Place the funnel in the neck of the bottle through a one hole stopper.
Seal the space between the funnel and the neck of the bottle with heavy grease or Plasticine (modelling clay).
Pour water slowly into the funnel.
The water stops running, because the air takes up space.
2. Repeat the experiment using a bottle with a two holes stopper.
All the water drops into the bottle.
The water replaces the air that comes out through the second hole in the stopper.
12.1.22 Hero's fountain
"Fountain Connection", connects two plastic soft drink bottles, Hero of Alexandria's water fountain (toy product)
See diagram 12.2.11: Hero's fountain.
Use two plastic drink bottles and half a bottle as a reservoir.
Half fill the bottles with water, attach the stoppers with the inserted glass tubes then connect the bottles and the reservoir.
Pour water into the reservoir until the glass tube from the reservoir to the lower bottle is full of water.
As water flows down this tube, because of the weight of water in the reservoir the air pressure in the lower bottle increases and this pressure is transmitted up the glass tube connecting the two bottles to the upper bottle.
So the increased air pressure becomes the same in both bottles.
The increased air pressure in the upper bottle forces water up and out of the outlet tube as a fountain.
Some of the water from the fountain can move down through the system again, but the height of the jet of water is constantly decreasing and the fountain can continue only until all of the water in the reservoir has run down into the bottom bottle.
To the end of the outlet tube, attach a plastic tube with, the same internal cross-section area to show that the water rises until its height above the surface of the water in the reservoir is equal to the height between the water levels in t
12.1.23 Inflate balloon with low pressure and high pressure
See diagram 12.3.17: Inflate balloon.
1. Insert two lengths of glass tubing, one straight and one with a right angle bend, into a two hole stopper.
Attach a small balloon to the lower end of the straight tube.
Fix the stopper with attached balloon into a glass jar.
Suck through the bent glass tube until the balloon is inflated.
The atmospheric air pressure acting through the straight tube is inflating the balloon.
Close the end of the bent tube with your finger.
The balloon remains inflated.
The pressure inside the balloon is atmospheric pressure.
The pressure inside the jar is lower than atmospheric pressure.
2. Remove your finger from the end of the bent tube.
The balloon deflates.
Blow through the straight tube.
The balloon inflates.
Put your finger over the end of the straight tube.
The balloon remains inflated.
The pressure inside the balloon is now higher than the atmospheric pressure.
The pressure inside the jar is at atmospheric pressure acting through the bent tube.
12.1.24 Inverted drinking glass
See diagram 12.305.1: Inverted drinking glass over table.
See diagram 12.3.5.2.1: Inverted drinking glass in the air.
1. Fill a drinking glass to the brim with water.
Cover the glass with something flat, e.g. piece of glass or cardboard or a playing card, so that no air remains between the cover and the water.
Invert the glass and cover.
The cover remains in place, because the pressure of the air pushing up is greater than the pressure of the water pushing down.
2. Repeat the experiment, but do not completely fill the glass.
Hold a piece of cardboard, e.g. a playing card, against the glass.
Invert the glass.
The cardboard does not fall.
3. Fill a drinking glass to the brim with water.
Cover the glass with waxed paper.
Inverted the glass on a smooth top of a table.
Pull away the piece of waxed paper.
Water does not stream out of the glass.
Move the glass slowly over the top of the table.
The water stays in the glass.
4. Repeat the experiment using soda water or fizzy lemonade.
The experiment does not work, because carbon dioxide gas comes out of solution and exerts pressure inside the glass.
5. Fill two identical drinking glasses to the brim with water.
Cover one glass with a piece of paper and invert it exactly over the other glass so that the rims are exactly in line.
Raise the inverted glass and the other glass does not move.
Replace the inverted glass exactly over the other glass and carefully remove the paper between them.
Lift the inverted glass while holding it at the bottom.
Both glasses rise if there is no air between the inverted glass and the other glass.
6. Repeat the experiment, but cover the glass with a piece of wet thin cotton cloth, cheesecloth.
Push the edges of the cloth against the outside of the glass.
Invert the glass holding around the cloth of the outside of the glass.
Some water flows through the cloth.
With your other hand, hold the glass by the bottom and let go with the other hand.
The wet cloth holds up the water in the glass.
The cloth does not fall, because of the forces of adhesion between the cloth, water and the outside of the glass.
Also the space above the water in the glass has air pressure less than air pressure.
Water has stopped flowing through the wet cloth, because of the forces of cohesion between water molecules in the holes in the cloth and the forces of adhesion between the water in the holes and the cloth.
7. Fill a drinking glass 3/4 with water and cover with a dry cotton cloth, e.g. a clean handkerchief.
Pick up the glass with one hand and use the other hand to pull the cloth edges under it.
Put the glass on the table and press down on the centre of the cloth until it just touches the surface of the water to form a concave shape.
Invert the glass.
Water does not passes through the cloth, which keeps its concave shape.
While holding the glass in the same inverted position use the other hand to pull up the cloth so that it is tight and straight across the mouth of the glass, like a drumhead.
Water does not pass through the cloth, but the upper surface of the water drops down in the glass to form a horizontal surface as air passes up through the cloth then through the water as small bubbles.
The water appears to be boiling.
When the cloth is moved down the water follows it down, because of its own weight leaving a vacuum between the water and the bottom of the inverted glass.
12.1.25 Lift water with air pressure
See diagram 12.3.9: Lift water with air pressure.
1. Fill a test-tube to 1 / 3 with water.
Insert a one hole stopper fitted with a straight glass tube.
Clamp the test-tube and heat over a spirit burner until the water boils.
When a large amount of water vapour has been lost, plunge the glass tube, mouth downward, into a container of coloured water.
Observe the water in the container pushed up by atmospheric pressure.
2. Fit a test-tube with a one hole cork stopper and straight glass tube.
Drive the air out of the test-tube by boiling water in it.
Invert it with the open end under the surface of a jar of water.
Atmospheric pressure will drive water up into the test-tube until it almost completely full.
12.1.26 Lifting power of balloons containing different gases
Fill balloons to the same diameter with different gases and show difference in lifting power.
12.1.27 Magdeburg hemispheres, vacuum pumps
"Magdeburg Cups", pair of Magdeburg hemispheres (toy product)
A famous experiment at Magdeburg, Germany in 1654, by Otto von Guericke (1602 - 1686) showed that two teams of horses could not separate a pair of large copper cups, Magdeburg Hemispheres, from which air had been removed by boiling water in them.
Otto von Guericke disproved the idea that "Nature abhors a vacuum" (Spinosa, 1677, in Ethics) to show that a vacuum creates no force by itself, but is just an absence of air and any other substance.
He invented the vacuum pump in 1654.
It has many applications, including vacuum tubes for medical and dental suction, milking machines, freeze drying and gas discharge tubes that contain gas at low pressure, used in electronics.
Experiments See diagram 12.3.5.7: Magdeburg hemispheres.
1. Use two identical drinking glasses.
Cut a square piece of newspaper with diameter 1 cm greater than the diameter of the open end of a glass.
Dip the paper in water to make is just damp and place it over the first glass.
Put a small candle in the bottom of the other second glass.
Light the candle in the second glass then invert the first glass with attached damp paper over it.
The rims of the two glasses must touch exactly.
The candle flame is extinguished when all the oxygen in the glass is converted to carbon dioxide.
Lift the top glass and the bottom glass also rises.
It may be difficult to separate the two glasses so you may have to twist the glasses to break the partial vacuum.
The previous explanation of the origin of the partial vacuum in this experiment was that the candle flame had "used up" or "consumed" the oxygen in the glass.
However, the product of the burning is carbon dioxide and water vapour.
A more likely explanation is that the candle flame had heated the air in the glass so that it had expanded and some air had left the glass.
When the flame was extinguished, because of lack of oxygen, the remaining air in the glass had cooled and contracted leaving a partial vacuum in both glasses.
Some of the air in the top glass had diffused through the damp newspaper to equalize the partial vacuum in the two glasses.
2. Apply a layer of grease to the rim of the hemispheres before pressing them together.
After evacuation close the valve and remove the sphere from the vacuum pump.
Students may attempt to pull the sphere apart.
After opening the valve, the hemispheres can easily be separated.
Separate the hemispheres by placing the apparatus in a bell jar and evacuating it with an electric vacuum pump.
The hemispheres fall apart.
3. Boil water or soup in a pan with a well fitting lid, then cool the pan.
Try to open pan, but the lid "sticks" to the pan.
12.1.28 Measure atmospheric pressure with a rubber suction cup
See diagram 4.232: Measure atmospheric pressure with a rubber suction cup.
1. Measure the area of the suction cup by pressing it down onto a sheet of glass above a sheet of graph paper and drawing its outline.
Measure the diameter to calculate the area.
Record the area A.
Attach the hook of the spring balance to the neck of the sucker.
Use a spring balance to find the force required to pull the sucker away from a smooth surface, e.g. a sheet of glass.
Record the force, F.
Calculate the average atmospheric pressure.
When there is no air inside the sucker, by fully pressing the sucker on the smooth surface the force shown on the spring balance equals the force, because of atmospheric pressure acting on the sucker.
Pressure = force / area.
Repeat the experiment with different size suckers, then calculate the average atmospheric pressure.
The average atmospheric pressure at sea level is 1.033 kilogram per square centimetre, 14.7 pounds per square inch.
12.1.29 Oxidation and air pressure, steel wool over water
See diagram 12.318: Steel wool over water.
Some new types of steel wool, e.g. "Steelo", are manufactured from stainless steel, so do not rust!
1. Wash a small wad of steel wool in petrol to remove any grease.
Squeeze it out and then fluff it.
When it is dry, place the steel wool in a flask fitted with a one-hole stopper carrying a 40 cm length of glass tube.
Stand the flask and tube in a container of water with the end of the tube under water.
After a few hours, note that water is slowly drawn up into the glass tube.
As the oxygen in the air reacts with the iron to form rust (iron oxides, Fe2O3) the air pressure decreases in the flask.
Atmospheric pressure can push water up 1/5 the height of the glass tube.
2. Repeat the experiment with magnesium ribbon rubbed with sandpaper or white phosphorus.
12.1.30 Plumber's force cups, suction cup, suction disc
See diagram: 12.3.5.3: Plumber's force cup.
1. Press the force cup against a flat surface, e.g. the top of a stool.
Try to lift the stool with the force cup.
The force cup works better if it is wet.
Press the force cup against a flat surface such as the top of a stool.
The force cup works, because most of the air between the force cup and the stool surface was pushed out when the force cup was pushed onto the stool surface.
Thus the air pressure inside the force cup is less than outside and as a result atmospheric pressure pushes together the cup and stool.
The force cup works, because almost no air remains between the object and the force cup.
However, the air in the atmosphere is pressing down on the rubber with atmospheric pressure.
2. A rubber suction disc stays on a smooth window, because there is no air between the disc and the window.
It is kept there by the pressure of the atmosphere on the rubber.
You can use a force cup to clear a blockage in a drain.
3. If you don't mind ruining a good plumbers force cup, make a little hole in the rubber and it will not lift anything.
However, if you cover the hole with your wet finger it works as normal.
4. Place a square piece of rubber on a flat stool and lift the stool by pulling up on a handle attached to the rubber square.
5. Two plumber's force cups
See diagram 12.305.3: Two plumber's force cups.
1. Wet the rims of two plumber's force cups.
Press the rubber cups tightly together.
Try to pull them apart.
The force cup works, because there is no air between the object and the force cup, but the air in the atmosphere is pressing down on the outside of the rubber.
This experiment is similar to the historical demonstration of air pressure called the Magdeburg hemispheres.
12.1.27 Magdeburg hemispheres, vacuum pumps
2. Wet the rims of two plumber's force cups.
Press the rubber cups tightly together.
Try to pull them apart.
Pulling the force cups apart needs two strong students.
When the force cups are pushed together some air is pushed out of the cavity between the two cups leading to a lower pressure being produced in the cavity.
Since this pressure is less than the pressure outside the cavity, the cups are pushed together by atmospheric pressure.
12.1.31 Pouring gases
Pour carbon dioxide into one of two beakers on a platform balance.
12.1.32 Potato puncture, push drinking straw through a potato
See diagram 4.228: Push a drinking straw through a potato.
1. Put the index finger over one end of a drinking straw.
Hold a potato in the other hand.
Push the drinking straw quickly through the potato.
The air in the drinking straw is trapped between the index finger and the potato when the end of the drinking straw strikes the potato.
This compressed air gives the drinking straw enough strength to prevent its bending and keep it rigid.
2. Try to push a drinking straw through a potato and find that the straw is too weak and collapses.
The straw is not rigid enough to take the compression.
Now place your index finger over one end of a drinking straw, and push the straw quickly through the potato.
The straw now penetrates the potato successfully and, with practise, you can push the straw completely through the potato!
The air in the straw is trapped between the index finger and the potato when the end of the straw strikes the potato.
This compressed air gives rigidity to that end of the straw to prevent it from bending.
3. Hold a potato and press 4 cm from one end of a drinking straw on it so that it points to the centre of the potato.
Hit the other end of the straw very hard with the other hand.
It can go through the potato.
So you do not need to put your thumb over the end of the straw.
Drinking straws are long cylinders with thin walls so they are quite sharp.
To test the strength of a drinking straw use both hands to push the ends of a straw towards each other.
If you block the end of the drinking straw with your thumb when you push it into a potato, the increased air pressure may split the straw!
4. Use a gardening glove to hold a drinking straw in the middle pinched between the index finger and the thumb.
Stab the drinking straw into, and perhaps through, an uncooked potato.
Pressure = force / area.
The area of the edge of the drinking straw is small, so the pressure on the potato is great.
The smaller the area, the greater the exerted pressure.
12.1.33 Pushed down drinking glass
See diagram 12.303.2: Push down air.
1. Squash some cotton wool into the bottom of a drinking glass.
Pour water into plastic bowl to a depth greater than the length of the drinking glass.
Float a cork on the water.
Slowly lower the drinking glass, mouth downward, over the cork until the rim reaches the bottom of the plastic bowl.
The cork drops down with the level of water under the glass.
Take the glass out of water.
The cotton wool into bottom of the glass is still dry, because the air takes up the place between the bottom of glass and the mouth of glass.
2. Pour water into a large glass container until it is half full.
Float a cork on the water and lower a drinking glass, mouth downward, over the cork.
Repeat the experiment with a piece of paper wedged tightly into the bottom of the glass.
The paper does not get wet.
12.1.39 Tapping a box
Make a small hole in the wall of a small box or use a small drink carton with a hole for the drinking straw.
Put a lighted candle near the box so that the top of the flame is opposite the hole.
Tap on the box and see the flicker of the flame.
When you increase and decrease pressure on the box by tapping on it, air moves out and back into the box as shown by the movement in the flame.
12.1.41 Vacuum cleaner
Vacuum, (Commercial).
A vacuum cleaner uses an air pump to create a partial vacuum over a dirty carpet.
The vacuum cleaner is really a wind machine the forces a current of air with a propeller.
The partial vacuum occurs only when the inlet nozzle of the vacuum cleaner is pressed down on something.
Air rushes in to replace the air pumped out and when it is pumped away it takes with it the dirt from the carpet.
When the vacuum cleaner is used on a smooth surface., e.g. linoleum, the nozzle with the brush is used to raise the inlet above the smooth surface.
The smooth surface nozzle is used for carpets would just stick to the smooth linoleum and no current of air would form to carry dirt from the surface.
It is almost impossible to pump all the air out of an enclosed space to create a vacuum.
The more air removed the greater the force from the atmospheric pressure to replace it.
Experiment
Use a vacuum cleaner to create a partial vacuum over a soft marsh mallow.
It becomes puffed up when the air bubbles in the marshmallow expand within the elastic solids.
12.1.42 Water rises in a downwards floating beaker Pressure under an inverted beaker
See diagram 12.3.8: Water rises in beaker.
1. Boil water in a large beaker.
Stop heating then put a small beaker, mouth down into the large beaker.
Note the level of water inside the small floating beaker.
Heat the large beaker again for a few minutes.
During this heating note the movement of the small beaker and movement of any bubbles.
Stop heating and note the level of the water inside the small beaker again.
The level of water rises in the small beaker.
Water vapour drives out air when the water is boiling.
As the small beaker cools, the water vapour condenses to water.
The level of water in the small beaker rises to replace the air displaced by the water vapour.
2. Put food colouring and boiling chips in 110 mL of water in a 400 mL beaker.
Heat until boiling.
Put an inverted 100 mL beaker or an inverted test-tube inside the 400 mL beaker.
Keep boiling, but do not let the 100 mL inverted beaker tip over.
Let the beakers cool to room temperature.
Observe the water level in the small beaker.
Note what is inside the inverted small beaker before the heating.
Note what happens to the water when it boils.
The small beaker keep bobbing up and dawn.
Note the bubbles.
The water level rises in the small beaker.
After the heating, you put a few drops of cold water on the small inverted beaker.
Boiling water changes from the liquid state into the gaseous or vapour state.
The water vapour formed under the inverted beaker replaces air.
The longer the water boils the more air is replaced by water vapour.
When the inverted beaker cools, the water vapour in it condenses to water reducing the pressure inside so water is pushed up inside the inverted beaker by the higher atmospheric pressure.
Pour cold water on the inverted beaker to increase cooling and the rising of the water.
12.1.43 Weight of air
Suspend an inflated tyre with a heavy duty spring and let the air out.
Place a large evacuated glass flask on a balance then let air in and note the increased weight.
Tape a one litre flask on a balance then pump out.
The loss of weight is about one gram.
Weight a glass sphere on a pan balance then evacuate it and weigh again.
12.1.44 Wet suction with a Petri dish
Fill a Petri dish with water.
Push it against a smooth surface, e.g. under side of a desk, but leave no air bubbles in it.
The Petri dish sticks to the surface.
By filling the dish with water, there is no air left and no air pressure working down on the dish.
The force down on the Petri dish is the weight of the Petri dish plus the water.
The force up on the dish against the surface is equal to the air pressure of about 1 kg per cm2 of dish surface area.
A dish with a 3 cm radius will have a force of about 27 kg holding it up minus the weight of the water and up the dish itself.
A dish sticking to the surface will stay there until water evaporates and air seeps into the dish.
The water acts as a seal to prevent the air to coming into the dish.
12.1.37 Syringe lift pump
[Some school systems do not allow the use of syringes in the classroom.]
See diagram 12.1.37: Syringe lift pump: A Glass case, B Rubber valve, C Cork piston, D Holes, D Intake tube.
1. Drill a hole through the centre of a cork, B, that makes a tight fit inside the glass tube of the syringe body, A.
Use a piece of hot wire to burn two small holes through the cork, C, on either side of the centre hole.
Pass a metal rod through the centre hole in the cork then expand the end after it has passed through the cork.
Cut a circular piece of flexible plastic, D, to the exact size of the cross-section area of the glass tube of the syringe body.
Cut a hole in the centre of the flexible plastic to allow the metal rod to pass through it.
Attach the inner edge of the plastic to the cork with glue.
The piston consists of the cork and metal rod.
The inlet valve is the piece of plastic.
The inlet is the nozzle of the syringe.
Push the piston down, then place the nozzle of the syringe under water.
Raise the piston.
During the upstroke the inlet valve should remain closed and water is drawn into the lower body of the syringe by reduced atmospheric pressure.
Lower the piston.
During the downstroke water moves up through the side holes while the inlet valve remains open.
Raise the piston.
During the upstroke the inlet valve should remain closed, the water above the piston is raised and water is drawn into the lower body of the syringe by reduced atmospheric pressure.
12.3.11 Holes in a beverage can
Holes in a beverage can, finger-regulated watering can, jar at different angles
See diagram 12.3.11: Finger regulation.
See diagram 12.3.11.1: Jar at different angles.
1. Use finger regulation to control water passing through a hole in a beverage can.
Make a hole with a nail near the bottom of an empty beverage can.
Block the hole with your finger and fill the beverage can with water.
Invert the beverage can while keeping your finger over the hole in the bottom.
The water does not run out through the ring-pull hole in the top of the beverage can.
Remove your finger and the water runs out.
2. Make a hole with a nail near the bottom of an open metal can.
Fill the can with water.
Hold the palm of the hand tightly over the top and water stops running from the hole.
Remove the hand and water runs from the hole.
3. Make a small hole in the top of a beverage can that has not been opened.
It is very difficult to suck the drink through the hole or to pour the drink into a glass.
Make a second hole in the beverage can.
Now it is easy to suck the drink through the hole or to pour the drink into a glass.
Sucking reduces pressure at one hole in the can so the air pressure acting through the second hole forces drink into your mouth or lets you pour the drink into a glass.
4. Fill a jar with water the place a piece of the glass over it so that no air remains under the piece of glass.
Lift the jar and turn it through different angles.
The water stays in the jar.
5. Fit a flask with a two holes stopper with a straight and a bent piece of glass tubing fitted through the holes.
Pour water into the flask and put the stopper in tightly.
You can suck water up the straight tube.
Close the end of the bent tube with your finger.
You cannot suck up water through the straight tube.
6. Make a finger-regulated watering can.
Melt one hole in the base of a plastic bottle with a hot iron wire.
Be careful! Do not burn yourself when making the holes with the hot wire!
Screw off the lid of the bottle and melt five to ten holes through the lid.
Block the hole at the bottle base with your index finger and fill the bottle with water then screw on the lid.
Invert the bottle./ Water streams out of the holes in the lid as your finger uncovers the hole in the base and water stops streaming as your finger blocks the hole.
It is convenient to use the simple apparatus to water flowers.
7. A rubber suction disc stays on a smooth window, because no air exists between the disc and the window.
The pressure of the atmosphere on the rubber disc keeps it pressed to the window.
8. A vacuum cleaner pumps some air away from over the dirty carpet creating a "partial vacuum".
Air rushes in to replace the air pumped out and when it is pumped away it takes with it the dirt from the carpet.
The more air that you remove the greater the force from the atmospheric pressure to replace it.
In a laboratory, it is impossible to pump all the air out of an enclosed space to create a perfect vacuum.
"Nature abhors a vacuum" (Spinosa, 1677, in Ethics).
9. The hole halfway down the outside shell of a "BIC" ball point pen is to equalize the pressure inside the pen.
These vents, or holes, in the pen barrels, basically help to prevent ink leakage.
Approximately 90 per cent of all pens are vented to prevent leakage.
Pens that do not have vented caps contain sealed ink systems and must be pressurized.
(from: Societe Bic / New Scientist)
12.3.12 Inverted dish sticks to a smooth board
See diagram 12.3.12: Dish sticks to board.
Use a shallow glass dish with a smooth rim or a Petri dish.
Fill the dish with water.
Place a heavy smooth board over the dish.
Push down on the board so that water overflows leaving no bubbles.
Pick up the board.
The dish full of water sticks to the board.
If the weight of the water and the dish is less than the force due to atmospheric pressure when the board is lifted, the weight of the water and the dish causes the pressure in the water filled cavity to be reduced.
Since this pressure is less than atmospheric pressure, the dish is held against the board by atmospheric pressure.
12.3.15 Heavy newspaper, air has mass, buoyancy in air
1. Place a 1 m flat stick on a table so nearly half the length hangs over the edge of a table.
Lay a sheet of newspaper over the end of the flat stick on the table and smooth it down.
Give the other end of the flat stick a sharp blow.
The flat stick breaks over the edge of the table.
The stick breaks, because the air pressure on the large sheet of paper exerts a force down on the paper.
Role the newspaper of fold it many times and repeat the experiment.
The are of the newspaper is less and the ruler can raise it without breaking.
2. Put a flat thin stick or barbecue skewer on a table with a smooth top so that half of it hangs over the edge of a table.
Lay a large sheet of newspaper over the end of the stick on the table and smooth it down flat on the table with a clothes iron.
Hit the end of the stick with a sharp blow.
The stick breaks over the edge of the table.
Little air remains under the smoothed down newspaper paper, but full atmospheric pressure acts down on the newspaper.
The force down on the newspaper = atmospheric pressure × area of the newspaper.
3. Repeat the experiment with the same piece of newspaper paper folded several times.
When you hit the stick it does not break and the newspaper flies away.
The atmospheric pressure is equal on both pieces of the folded newspaper.
4. Put a stick on a table with a smooth surface and let it protrude 8 cm over the edge of the table.
Observe what happens when you hit the protruding end of the stick.
The stick flies up and you can catch the flying stick.
Put the stick back on the table as before, protruding 8 cm over the edge, and cover it with a newspaper laid flush with the edge of the table.
Smooth down the paper with your left hand, then strike the protruding end of the stick with your right hand, as a sudden sharp blow with the edge of the palm.
The stick breaks.
By smoothing the paper down, there was almost no air under it, but a column of air existed above the paper, pushing down on the paper with the atmospheric pressure, i.e. approximately 1 kg / cm2.
The total weight or force pushing down on a 60 × 80 cm newspaper is approximately 60 × 80 × 1 kg = 4, 800 kg.
So lifting the newspaper with the thin stick was impossible!
12.1.34 Shrinking balloons
Inflate three balloons fully.
Let about half the air out of one balloon.
Let out about one quarter of the air out from another balloon.
Measure the diameter of the three balloons.
Leave the balloons and the next day measure the diameters again.
Compare the diameters of the three balloons.
The full balloon shrinks faster than the half-filled balloon than the quarter filled balloon.
The rate of shrinkage depends on the pressure in the balloon.
12.1.45 Card on inverted glass
Put playing card onfull glass of water., then invert the glass.
Replace the glass by a 50 cm glass tube.
When half filled, it cannot be inverted.
12.1.35 Standard atmosphere
A standard atmosphere (International Standard Atmosphere, ISA), atm, is a hypothetical atmosphere used as a basis of comparing altimeters.
At mean sea level, 1 atm is a pressure of 101.325 Nm-2 (101, 325 pascals), equivalent to the pressure exerted by a column of mercury 760 cm high at 0oC.
The density of a gas is inversely proportional to the absolute temperature provided the pressure remain constant, ρ 2 / ρ 1 = T1 /T2, where ρ = density and T = absolute temperature.
The density of a gas is directly proportional to the pressure, if the temperature remain constant.
The standard atmosphere (USA, 1976 ) has mean conditions at sea level as follows:
Pressure 101325 Pa, Temperature 288.15 K (15oC), Density, ρ, 1.225 kg / m3, Standard gravity, g = 9.90665 m /s2, R = 8.31432 JK-1 mol-1,
and Composition: N2 (78.084%), O2 (20.9476%), Ar (0.934%),
CO2 (0.0314%), Ne (0.001818%), He (0.000524%), CH4 (0.0002%).
However, the international standard atmosphere (ISA) used in international weather data has conditions at sea level pressure 101325 Pa (1 atmosphere) at 15oC with lapse rate -6.5oC / km and a range of different conditions for specific layers of the atmosphere.
12.1.36 Standard temperature and pressure, STP, density of gases
Density of gases at STP (Table)
States of matter, at STP (Standard Temperature and Pressure) (0oC and 1 atmosphere pressure) are solid (s), or liquid (l), or gas (g), or aqueous solution, (dissolved in water), (aq).
STP refers to the standard conditions used in calculations of the effects of changing temperature and pressure.
IUPAC, set the standard of STP in 1982 as temperature 273.15 K (0oC, 32oF), absolute pressure 100, 000 Pa (1bar, 0.98692 atm)
Since 1982, STP has been defined as a temperature of 273.15 K (0 °C, 32 °F) and an absolute pressure of exactly 105 Pa (100 kPa, 1 bar).(Wikipedia) Formerly it was as follows and is still commonly used as the SATP, standard ambient temperature and pressure:
T = 0oC or 273.15 K, P = 760 mm Hg, 1 atm, or 101, 325 pascals, Pa (101.325 Nm-2).
The combined gas equations can be used to find the volume of a gas at STP, i.e. at 0oC and 760 mm Hg pressure.
At STP, one mole of gas occupies 22.4 litres, L.
12.1.38 Syringes for investigating air pressure
"Vacuum Container & Pump", reduced pressure increases balloon size, (toy product)
"Vacuum Stoppers", creates near vacuum in plastic syringes (toy product)
See diagram 12.4.3.2: Force pump.
[Some school systems do not allow the use of syringes in the classroom.]
1. With the tip sealed, use a syringe to compress air or to produce a partial vacuum.
Attach a small piece of plastic tubing to let you seal the tip with a pinch clamp or seal the syringe by pushing the tip into a wooden block drilled to the appropriate size.
With this base as a platform, use the syringe in a vertical position as a balance for measuring weight by air compression.
You can quantify all the following experiments, because syringes are already graduated.
2. Fill the syringe with a small amount of air and hang it inverted to serve as a "spring type" balance.
When the tip is sealed, use the syringe to compress the air or to produce a partial vacuum.
Attach a small piece of plastic tubing to allow you to seal the tip with pinch clamps.
Seal a syringe by pushing the tip into a wooden or plastic block that has been drilled to the proper size.
With such a base as a platform, use the syringe in a vertical position for applications such as serving as a balance for measuring weight by air ompression.
3. Fill the syringe with a small amount of air and hang it inverted to serve as a "spring type" balance.
4. Compress moist air within a syringe to cause water condensation and form "artificial rain".
5. Attach a piece of plastic tubing 20 or 30 cm long to make a simple syringe pump.
Put water in the tube to make an air thermometer or use 12 metres of tubing to make a water barometer.
6. Syringes and air pressure.
See diagram 12.301: Syringes and air pressure.
Couple two syringes with a piece of tubing to show pressure changes within closed systems.
You can easily quantify all these experiments, because syringes are already graduated.
7. To show compressibility of liquids, fill a syringe with water and then, having made sure no air is present, seal the outlet with the finger and try to move the plunger.
You cannot you move it.
Draw some air into the syringe.
Seal the outlet with the finger and try to move the plunger.
You can move the plunger, because air can be compressed.
8. Connect two syringes of different sizes and you can feel the pressure difference.
9. Compare water / air compression in a syringe.
You can compress a syringe filled with air with a large weight, but you cannot similarly compress a syringe filled with water.
10. Put water in the tube to make an air thermometer or use 12 m of tubing to make a water barometer.