UNPh12

School Science Lessons
Pressure in fluids, Pascal's principle, atmospheric pressure, siphons, barometers, pumps, syringes
2009-10-11
Please send comments to: J.Elfick@uq.edu.au
See: Interesting websites

Table of contents
12.1.0 Statics of fluids, static pressure, the pascal (Pa)
12.2.0 Pressure, liquid pressure
12.3.0 Pressure, atmospheric pressure
12.4.0 Siphons, pumps
12.6.0 Barometers, measure atmospheric pressure
12.7.0 Pumps, syringes

20.0.0 Gas laws relate the pressure, temperature and volume of gas
20.0.6 Standard temperature and pressure, STP, density of gases

12.1.0 Statics of fluids, static pressure, the pascal (Pa)
See: Table of saturated vapour pressure over water, Psvp
1.38 Air games (Primary)
1.39 Air in bags (Primary)
3.37 Air takes up space (Primary)
12.1.01 Pressure definitions
12.1.02 Standard temperature and pressure, s.t.p.
12.1.03 Pressure, P, stress
12.1.04 Atmospheric pressure
12.1.05 Conversions between units of atmospheric pressure
12.1.06 Altimetry, height and altitude
12.1.1 Weight and pressure
12.1.2 Weigh car with a tyre gauge, Bourdon gauge
12.1.3 Cut ice with pressure

12.2.0 Pressure, liquid pressure
4.192 Water pressure does not depend on the size of the container
5.18 Feel our pulse (Primary)
5.21 Water finds its own level (Primary)
6.35 Burn candle over water, candle burning in inverted jar over water (Primary)
12.2.1 Make a simple water manometer, make a pressure gauge
12.2.2 Pressure depends upon the density of the liquid
12.2.3 Pressure is independent of size and shape of the container
12.2.4 Pressure increases with depth, closed funnel at different depths in water
12.2.5 Balanced water columns, Pascal's vases
12.2.5.01 Spirit level, level tube apparatus
12.2.6 Pressure is the same in all directions, Pascal's fountain
12.2.7 Pressure applied to a sealed fluid is transmitted equally through the fluid, Pascal's law, Pascal's principle
12.2.8 Inverted test-tubes, test-tube rising automatically, upwards falling test-tube, pushed up test-tube
12.2.9 Dropping plate
12.2.10 Pascal's diaphragms
12.2.15 Weight on a beach ball
12.2.16 Compression of liquids / gases
12.5.3 Hydraulic ram, water ram, water hammer
12.6.4 Card on inverted glass
12.6.5 Pressure due to height
12.6.6 Effect of pressure on the boiling point of pure water
20.0.6 Standard temperature and pressure, STP, Standard Atmosphere

12.3.0 Pressure, atmospheric pressure
1.42 Drinking straw game (Primary)
4.153 Three holes can, 3-hole can, a vase with three holes, spouting cylinder
4.229 Mercury barometer, barometric pressure, atmospheric pressure
4.229.1 Mountain sickness and hyperventilation
4.230 Aneroid barometer
4.238 Volume and pressure of air, Boyle's Law
4.238.1 Scuba diving and Boyle's law
4.240 Make a model of the lungs
4.241 Oxidation and air pressure, steel wool over water
4.242 Air streams, Bernoulli theorem
4.243 Cold air is heavier than warm air, inverted paper bag balance
5.26 Air pressure (Primary)
6.35 Burn candle over water (Primary)
12.1.04 Atmospheric pressure
12.1.05 Measure atmospheric pressure
12.3.01 Blood pressure
12.3.1 Finding air
12.3.2.1 Air takes up space, transfer air under water
12.3.2.2 Cork under drinking glass, paper does not get wet
12.3.2.3 Funnel in the neck of bottle
12.3.2.4 Bag of air into and out of a jar
12.3.2.5 Tapping a box
12.3.3 Air has mass, air has weight
12.3.3.1 Carbon dioxide has mass
12.3.5 Air exerts pressure in all directions
12.3.5.1 Drinking straw, finger on drinking straw, glass tube
12.3.5.2 Inverted drinking glass
12.3.5.3 Plumber's force cup, suction cup, suction disc
12.3.5.4 Plumber's force cups as Magdeburg hemispheres
12.3.5.5 Wet suction with a Petri dish
12.3.5.6 Vacuum cleaner
12.3.6 Push a drinking straw through a potato
12.3.7 Water reservoir for chicken drinker
12.3.8 Water rises in a downwards floating beaker, pressure under an inverted beaker
12.3.9 Lift water with air pressure
12.3.10 Automatic drinking glass
12.3.11 Holes in a drink-can, finger-regulated watering can, jar at different angles
12.3.12 Inverted dish sticks to a smooth board
12.3.13 Air pressure crushes a can, collapsing can
12.3.14 Balanced balloons
12.3.15 Heavy newspaper, air has mass, buoyancy in air
12.3.17 Inflate balloon with low pressure and high pressure
12.3.19 Density of hot air and cold air
12.3.20 Density of air with a balloon
12.3.21 Equidensity bubbles
12.3.22 Freon and air
12.3.23 Lifting power of balloons
12.3.24 Pouring gases
12.3.25 Weight of air
12.3.26 Cork sticks to the bottom of a beaker
12.3.27 Egg in a bottle, balloon in a flask, tie a knot in a bone
12.3.28 Balloon with cup "ears"
12.3.29 "Cupping"
12.3.30 Bottles stick together
12.4.5 Flask fountain
12.7.0 Atmospheric pressure, Torricelli
37.3 Make a deflection anemometer

12.4.0 Siphons, pumps
2.12 Siphon and water spray (Primary)
12.4.1.0 Simple siphon
12.4.1.1 Siphon fountain
12.4.1.2 Siphon replaces water for fish
12.4.1.3 U-tube siphon
12.4.1.4 Siphon in a bell jar
12.4.1.5 Siphon mechanism apparatus
12.4.1.6 Measure pressure in a siphon
12.4.1.7 Mechanical siphon
12.4.1.8 Self-starting siphon
12.4.1.9 Mariotte flask and siphon
12.4.1.10 Turnover siphon

12.6.0 Barometers, measure atmospheric pressure
4.229 Mercury barometer, barometric pressure
4.230 Aneroid barometer
12.6.1 Atmospheric pressure causes liquids to rise in a sipping straw
12.6.2 Measure atmospheric pressure with a bicycle pump
12.6.3 Measure atmospheric pressure with a rubber suction cup
12.6.4 Card on inverted glass
12.6.5 Pressure due to height
12.6.6 Effect of pressure on the boiling point of pure water
23.2.5 Torricelli tube

12.7.0 Pumps, syringes
4.223 Plastic syringes and air pressure
12.4.3 Syringe lift pump
12.4.4 Simple test-tube force pump
12.6.2 Measure atmospheric pressure with a bicycle pump

12.1.01 Pressure definitions
The pascal (Pa) is the SI unit of pressure, (Blaise Pascal 1623 - 1662).
1 pascal (Pa) = 1 newton per square metre = 1 N/m²1 hectopascal (hPa) = 1 millibar (mb) = 100 Pa
1 bar (bar) = 10⁶dyn/cm² = 100 000 Pa
1 millibar = 100 Pa
1 atmosphere (atm) = 760 mmHg = 101,325 Pa
1 torr = 1 millimetres of mercury (1mmHg) = 133.322 Pa
1 pound-force per square inch = 1 lbf/in² (psi) = 6 894.76 Pa

12.1.02 Standard temperature and pressure, s.t.p.
Temperature 273.15 K or 0^oC
Pressure 101 325 Pa or 760.0 mmHg

12.1.03 Pressure, P, stress
Pressure, or stress, is the force per unit area on a surface, P = F / A and has the SI units newton / metre², Nm^-2or pascal, Pa. 1 Nm^-2= 1 Pa. The value of pressure changes by changing the area of the subject or by changing the force acting on the surface of the subject.
In meteorology other units are often used, 1 bar = 10⁵Pa, equivalent to 750.062 millimetres of mercury (mm Hg) at 0^oC, and gravitational acceleration of 9.80 665 m s^-2, a standard gravity. In meteorology, weather forecasting, the commonly used unit of atmospheric pressure is the millibar, (mb) 10^-3 bar, now called the hectopascal, (hPa) So 1 millibar (mb) = 1 hectopascal (hPa) = 100 pascals (Pa). However, some people reject the term hectopascal as just being a way to keep using millibars.

12.1.04 Atmospheric pressure
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 0^oC and the value of g is that at sea level, latitude 45^o. 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.

12.1.05 Conversions between units of atmospheric pressure
1 atmosphere (atm) = (1.01325 bar) = (1 01325 mb) (millibar) = (1.01325 10⁵ Pa) = (1.01325 x 10⁵ N / m²) = (101 325 N m^-2) (Pa) = 1.01325 X 105 N m^-2 = (101.325 kilopascal) (kPa) = (1.013 x 10⁶dyne / 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).

12.1.06 Altimetry, height and altitude
An altimetry setting is a description in millibars of the atmospheric pressure at a particular level at a particular time, i.e. the vertical position of an aircraft. If before takeoff, a pilot sets the atmospheric pressure at that particular aerodrome level (QFE), e.g. 1 008 millibars, on a sensitive altimeter in the cockpit, that altimeter will indicate its height above or below that reference level. However, if the altimeter is set to the atmospheric pressure corresponding to mean sea level at a particular place at a particular time (QNH), the altimeter will show altitude, the vertical distance above mean sea level. Mean sea level refers to the average mid-level between high and low tide at a particular place. Standard pressure refers to a constant pressure of 1 013.2 millibars assumed to be the average value of mean sea level pressures throughout the world. An altimeter set to 1 013.2 millibars reads pressure height that is a common reference called flight level zero.

12.1.1 Weight and pressure
See diagram 12.1.1: Difference between pressure and weight
1. Observe the change in pressure due to a change in area. Put a heavy house brick on mud, stand on biggest area, stand on the long side, second biggest area, stand on the short side, smallest area. Observe that it sinks deepest when the area of the brick touching the mud is least. The weight of the brick remains the same but the pressure the brick exerts on the mud changes as the area of contact changes. Other examples include 1. Walking on sand wearing wide shoes is easier than wearing narrow shoes. 2. Needing more force to cut with a blunt knife than a sharp knife because the surface of the cutting edge of the blunt knife is greater than the sharp knife and so the pressure exerted is smaller with the same force.
2. Use a block of wood with two different dimensions, e.g. 10 cm X 15 cm. Put the block on Plasticine (modelling clay) or mud, with the larger face down. Repeat the experiment with the smaller face down. Record the different depths the block sinks. Add a weight to the block to make it sink deeper. When the smaller face is down, the block sinks deeper than when the larger face is down. Pressure = force/area. The force down, i.e. the weight, is the same but the area of the smaller face is less than the area of the larger face. When the block has the smaller face down, it exerts more pressure.
3. Cut with a sharp knife and a blunt knife, or dig with a sharp spade and a blunt spade. You can cut deeper with the sharp knife because the surface area of the knife edge is less and applies more pressure. Pressure = force / area, so the greater the area, the less the pressure.
4. Stand on mud wearing flat shoes and high heel shoes. You sink deeper wearing high heel shoes because the surface area is less. Formerly, ladies could not wear high heel shoes in aircraft because the pointed heel might make holes in the aluminium floor.

12.1.2 Weigh a car with a tyre gauge, Bourdon gauge
See diagram 12.1.2: Tyre mark
By measuring P and A, you can calculate the approximate weight of the car. Read the tyre gauge. It is the internal pressure of the tyre, P₁. (P = P₁ - P_0),where P = the pressure by which the internal pressure in the tyre exceeds the outside pressure, and P_o= atmosphere pressure. Measure each pressure P in the four tyres of a car. Measure each area of the tyres in contact with the ground. You could drive the car on to graph paper.
Calculate the weight of the car: F = PA, (F = P₁- P₀ × A).
An example of experiment using a Mitsubishi Lancer Wagon:
Atmospheric pressure = (1 020 hPa 1.02× 10⁵Pa), Tyre pressure = (210 kPa 2.1 × 10⁵ Pa) in each tyre, as recommended in the Operator's Manual. Area of each tyre touching the ground = (14 cm × 24 cm).
Total areas of tyres touching the ground = (0.14 × 0.24 × 4 = 0.1 344 m²).
Weight of car = (P₁- P₀× A) = (2.1 - 1.02 ×10⁵× 0.14 × 0.24 × 4) = (0.1452 ×10⁵newton) = (1 482 kg).
However, the Operator's Manual states that the weight of the Mitsubishi Lancer wagon is 1 080 kg, so the measured area of the tyres touching the ground was too high. Only the raised tyre tread was touching the ground.
The area of tyre touching the ground should have been = (1 080 × 9.8 / 1.08 × 10⁵= 0.098 m²)
Part of the error could also be that the manufacturer gives the dry weight, i.e. no fuel. A full tank could weigh up to 60 kg. Also, other things may be in the car that could add to this mass.

12.1.3 Cut ice with pressure
1. Put an ice cube on top of an open empty plastic bottle. Shape a piece of wire so that it has a hook at each end and is just broader than the ice cube. Hang equal weights on the two hooks so that the wire presses down on the top surface of the ice. Put the apparatus in a refrigerator. The wire will sink down though the ice cube cutting it in two.
2. Use 1 m of thin strong steel wire, e.g. piano wire. Attach each end to a broom handle. Loop the wire round a block of ice it and pull tightly so that the wire exerts pressure on the ice. The pressure causes the ice to melt where the wire touches the ice. Release the pressure and the ice becomes solid again.
3. Force a knife ice. Be careful! The ice melts at the edge of the blade. So an ice skater skates on a thin layer of water!
4. Force two cubes of ice together. The ice melts where they meet. Release the pressure and the melted ice freezes again.

12.2.0 Pressure, liquid pressure
Pressure in liquids, measuring pressure, liquid pressure, statics of fluids, Static pressure, P = height X density X g, pressure and depth, pressure in all directions, water finds its own level, water supply, Measuring Pressure, pascal, Pressure gauge, manometer, U-tube, liquids exert pressure
The pressure produced by liquid at any point within the liquid P = dgh, where h is the distance from the point to the liquid surface, d is density of the liquid, g is gravitational acceleration. The absolute pressure at a depth from the surface open to the atmosphere is P_o+ dgh, where P_ois the atmospheric pressure.

12.2.1 Make a simple water manometer, make a pressure gauge
See diagram 12.2.4: Manometer
1. Fill about half depth of an U-tube with water. Attach plastic tubing over one arm of the U-tube. Gently blow into the plastic tubing. Observe heights of the water in the arms of the U-tube. The greater the pressure of the air from your mouth, the greater the difference in the heights of the water in the arms of the U-tube. Do not blow too violently in case the water goes out of the tube.
2. Half fill a U-tube with coloured water. Stretch a piece of thin rubber loosely over the mouth of a filter funnel and tie it securely. Attach the stem of the filter funnel to one arm of the U-tube with rubber tubing. Hold the mouth of the filter funnel at different depths in a container of water. Record the depths of the mouth of the funnel in the water corresponding to the differences in heights of the coloured water in the U-tube. This U-tube is being used as a pressure gauge.

12.2.2 Pressure depends upon the density of the liquid
See diagram 12.2.4: Manometer
1. Prepare different liquids, e.g. pure water and saltwater, or pure water and spirits in identical containers. Use a manometer with plastic tubing attached to one arm of the U-tube. Stretch a piece of thin rubber from a balloon over the mouth of a filter funnel and tie it in place. Attach the open end of the plastic tubing to the stem of the funnel. Hold the mouth of the funnel at the same depth in containers containing different liquids. Observe the differences in the height of the water in the two arms of the U-tube. When the mouth of the funnel is at the same depth in each liquid, the pressure is greater in the more dense liquid.
2. Use a beaker of methylated spirit, pure water and saltwater. Hold the mouth of the filter funnel at the same depths in the three liquids and note the corresponding differences in the heights of the coloured water in the U-tube. At the same depth, the less dense methylated spirit exerts less pressure than pure water and the more dense salt water exerts more pressure than pure water.

12.2.3 Pressure is independent of size and shape of the container
See diagram 12.2.4: Manometer
1. Prepare different size containers containing same liquid, e.g. water. Construct a manometer with plastic tubing attached to one arm of the U-tube. Stretch a piece of thin rubber from a balloon over the mouth of a filter funnel and tie it in place. Attach the open end of the plastic tubing to the stem of the funnel. Hold the mouth of the funnel at the same depth in different size containers containing the same liquids. Observe the height of the water in the two arms of the U-tube. At the same depths the pressures are the same. Liquid pressure does not depend on the size of the container.
2. Use a large beaker of water and a small beaker of water. Hold the mouth of the filter funnel at the same depth in each beaker. The corresponding differences in height of the coloured water in the U-tube are the same.
3. Use a wide mouth beaker filled water. Use three glass tubes open at both ends and bottom ends with various shapes including, one tube is straight, one tube is bent 90^o, and one tube is bent 180^o. Insert the tubes into water such that the mouths are the same depth below the surface. Fill the tubes with coloured kerosene, or with another coloured fluid less dense than water until all of the water is just pushed out of the tubes leaving only coloured fluid in the tubes. Kerosene does not run out of the ends and the height of kerosene in each tube is the same.

12.2.4 Pressure increases with depth, closed funnel at different depths in water
See diagram 12.2.4: Manometer
1. Prepare a container with liquid, e.g. water. Construct a manometer with plastic tubing attached to one arm of the U-tube. Stretch a piece of thin rubber from a balloon over the mouth of a filter funnel and tie it in place. Attach the open end of the plastic tubing to the stem of the funnel. Hold the mouth of the funnel at different depths in a container of water. Observe the differences in height of the water in the two arms of the U-tube. The greater the depth the greater the difference in the heights of the water in the manometer. The greater the depth, the greater the pressure.
2. Cut one end off a tall plastic drink bottle to make a tall container. Use the pressure manometer to place the funnel at different depths to measure the pressure.
3. Weigh a water column. Suspend a tube from a spring scale in a beaker of water and suck water up into the tube Why does the scale reading increase? Suspend a tube open at the bottom from a spring scale in a beaker of water and partially evacuate the air from the tube.

12.2.5 Balancing water columns, Pascal's vases
See diagram 12.2.5: Pascal's vases
1. Connect 6 tubes of various shapes to a common water reservoir.
2. Fit plastic bottles of different shapes. Pour water into the bottles. Observe the level of the water at balancing in the bottles. The height of the water in each bottle is the same. Liquid pressure is independent of the size or shape of the container.
3. Cut the bottoms from different shape plastic bottles. Fit one bottle with a one-hole stopper and the other with a two-holes stopper. Connect the first bottle with a 2-hole stopper to a tap or reservoir and the second bottle. Connect all the bottles with glass tubing and rubber connectors. The last bottle has the one-hole stopper. Invert the attached bottles. Turn on the tap so that water flows into the bottles. The level of water is the same in the differently shaped bottles. Pressure in a liquid is independent of the size or shape of the vessel and depends only on the depth. Some people say "Water finds its own level."

12.2.6 Pressure is the same in all directions, Pascal's fountain
See diagram 12.2.6: Pascal's fountain
1. Pressure is independent of direction. Lower into water a thistle tube covered with a diaphragm or rubber membrane and connected to a manometer and oriented in different directions. Join 3 thistle tubes filled with coloured alcohol and capped with rubber membranes. Twist the ends bent in various directions, or twist one tube to show the same phenomenon. (Pressure dependent on depth fallacy. The manometer used in the demonstration is calibrated based on the law under investigation.)
2. Pressure at equal depths in a fluid is the same in all directions. Observe the pressure at a depth of the static fluid is the same in all directions Punch four holes in a plastic container at the same height from the base. Plug the holes and fill the container with water. Take out the plugs. Compare the distances the water travels when it shoots out through the holes to hit the ground. The distances before the water hits the ground are the same if the holes were the same height from the base.
3. Water pressure is the same in all directions. Punch holes around the base of a tall metal can with a nail. Cover the holes with a strip of tape. Fill the can with water and hold it over a sink. Strip off the tape. The distance the streams shoot out from the holes is the same in all directions.
4. To make Pascal's fountain, use a piston to apply pressure to a round glass flask with small holes drilled at various points. Water squirts out equally in all directions when forced out of a sphere by a tube fitted with a piston.
5. Cut six pieces of glass tubing 3 cm long. Cut small holes in a tennis ball just big enough to insert the glass tubes. Place the holes at top and bottom, right and left, nearest you and farthest from you. Put the tennis ball and glass tubes in a bucket of water. Squeeze out all the air so the tennis ball is full of water. Take out the tennis ball and hold it in your hand with your fingers around it but not over the glass tubes. Squeeze the tennis ball. The same amount of water squirts out through the glass tubes in all directions.

12.2.7 Pressure applied to a sealed fluid is transmitted equally through the fluid, Pascal's law, Pascal's principle
See diagram 12.2.7: Squeeze bags, squeeze tennis ball
1. Squeeze a flask capped with a stopper and small bore tube.
2. Squeeze a sealed plastic bag. Observe the direction and values of applied pressure transferred by a sealed liquid Use a durable plastic bag and punch holes at its bottom with needle. Half fill the bag with water. [Doing this by opening the bag in a bucket of water may be easier and less messy.] Remove all the air from the bag. Invert the bag. Squeeze the bag tightly. The speed of water streaming out of the holes in various directions is equal.
3. Squeeze a tennis ball. Insert four short tubes into a tennis ball in different directions. Plug three of the tubes and fill the ball with water from the last tube. Hold the ball in your hand and then squeeze tightly. The same amount of water shoots out of the tubes in the four directions [This may be difficult to observe]. 4. Squeeze a plastic bag attached to a water tap. Use a durable rubber or plastic bag and punch holes at its bottom with needle. Attach the bag over a water tap and tie the mouth tight with string. Turn on the water. Water squirts out of the holes in different directions. Sealed liquid will transfer pressure on it in all directions and the change in pressure is equal in all directions. Note that the holes at different heights will show different pressures since dgh, density × gravity × height, is also a factor.

12.2.8 Inverted test-tubes, test-tube rising automatically, upwards falling test-tube, pushed up test-tube
See diagram 12.2.8: Test-tube moves up
1. Select two test-tubes so that one just fits into the other. Half fill the larger test-tube with water. Put the smaller test-tube in the larger test-tube to float on the water. Push down on the small test-tube so that water overflows. from the larger test-tube and no air remains between the two test-tubes. Invert the test-tubes over the sink by holding only the larger test-tube. Give the smaller test-tube a slight push up. The smaller test-tube moves up as water falls down into the sink because the atmospheric pressure pushes it up.
2. Use two test-tube with nearly the same diameters. Fill the larger test-tube with water. Insert the end of the smaller test-tube deeply into the larger test-tube. Hold the test-tubes separately with your hands and turn them mouth downwards. Stop holding the smaller test-tube. The smaller test-tube rises in the larger test-tube with water dropping out. Repeat the experiment gradually increasing the depth of the smaller tube in the larger test-tube until at height h0 the thinner tube neither goes up or down when the larger tube is inverted. Measure the weight and the inner diameter of the thinner tube.

12.2.9 Dropping plate
Water pressure holds a glass plate against the bottom of a glass tube inserted into a beaker of water until the pressure is equalized by another fluid poured into the tube. Pour water into the tube until the plate drops off.

12.2.10 Pascal's diaphragms
A closed container has several protruding tubes capped with rubber diaphragms. Push on one and the others go out.

12.2.15 Weight on a beach ball
Place a 25 kg weight on a circular wood disc on a beach ball and blow up the beach ball. Lift a 12 kg weight with your lungs by blowing it up on a beach ball.

12.2.16 Compression of liquids / gases
Pound in a nail with a bottle completely filled with boiled water.

12.3.01 Blood pressure
Systolic blood pressure refers to the pressure when the heart pumps blood into the circulation. Diastolic blood pressure is when the heart relaxes and takes in blood. Human blood pressure is measured in millimetres of mercury, mmHg. The normal blood pressure for adults over 18 years is less than 130 mmHg systolic and 85 mmHg diastolic. The normal blood pressure for adults over 65 years is less than 140 mmHg systolic and 90 mmHg diastolic. Persons with blood pressure higher than these levels suffer from hypertension and may be more likely to have strokes, heart attacks and kidney failure. Hypertension can be treated without medication by following a nutritious low-fat diet, restricting sodium chloride in food, limiting alcohol consumption, avoiding stress, and regular exercise to make you "out of breath". Normal range of values of blood pressure of resting persons, in mm Hg: 3 months 90/50, 10 years 90-125/60, adult 95/60 to 140/90.

12.3.1 Finding air
1. Put a bottle under water: Lower a bottle mouth down into a container of water. Slowly tip the mouth of the bottle towards the surface of the water. Bubbles of air rise to the surface. The bottle was not empty.
2. Place a lump of soil in a container of water. Bubbles of air rise from the soil.
3. Place a house brick in a container of water. Bubbles of air rise from the brick.
4. Observe soda water: As temperature increases, density of water decreases, thus reducing the pressure in the water allows air bubbles to form. For example, opening the top of a carbonated drink allows CO₂bubbles to form. Fill a dry and clean beaker with water. Observe any bubbles. Put the beaker on a tripod and heat it with a spirit lamp. Observe it again. Some new bubbles of air will appear on the side of the beaker
5. Observe a carton full of fruit juice. Insert a drinking straw into the little hole covered with silver paper drink the fruit juice. Hold the empty carton with the suck hole directed at your ear. Pinch the box suddenly. Feel a stream of air spurting from the box and hear the sound caused by air. Jump on the empty carton. Hear a loud "pop" cause by the compressed air escaping from the carton.
6. Push a narrow neck bottle mouth down into a jar of water: Slowly tip the mouth of the bottle towards the surface of the water. Observe bubbles of air rising.
7. Fill a glass with water: Let it stand in a warm place for several hours. Observe bubbles of air rising.

12.3.2.1 Air takes up space, transfer air under water
See diagram 12.3.2.1: Transfer air under water
1. 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.
2. 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.
3. 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.

12.3.2.2 Cork under drinking glass, paper does not get wet
See diagram 12.3.2.2: Push down air
1. Pour water into a bucket. Float a cork on the water. Lower a drinking glass, with a piece of paper sticking on the bottom of it, mouth downward, over the cork. The cork drops down with the water under the glass and does not go into the glass. Take the glass out of water. The paper at 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.

12.3.2.3 Funnel in the neck of bottle
See diagram 12.3.2.3: 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 2-hole stopper. All the water drops into the bottle. The water replaces the air that comes out through the second hole in the stopper.

12.3.2.4 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 loop 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.

12.3.2.5 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.

12.3.3 Air has mass, air has weight
See diagram 12.3.3: Weigh a balloon
1. 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.
2. 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 wigh 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 because of 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.
3. 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.
4. 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.
5. 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.

12.3.3.1 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 drink-cans B and C. Pull up on string A so that the rod and supported drink-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 drink-cans is horizontal. Keep hold of string A and lower the rod so that drink-cans B and C are resting on the table. Carefully pull on the ring-pull of drink-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 drink-cans B and C are suspended, i.e. no longer resting on the table. Note that the rod is no longer horizontal because drink-can B has lost carbon dioxide. The rod is now sloping with drink-can C suspended lower than drink-can B.

12.3.5.1 Drinking straw, finger on drinking straw, glass tube
See diagram 12.3.5.1: Finger on drinking straw
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. Have a soda straw contest to measure how far a person can suck water up through a very long tube.

12.3.5.2 Inverted drinking glass
See diagram 12.3.5.2: 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 and let the cardboard free. You can find the cardboard does not fall.
3. Put the inverted glass on a smooth top of a table. Draw off the cardboard on to the top of the table. You can find water does not stream out of the glass. [The cardboard must be quite thin. If you allow air to enter the glass while the cardboard is removed the water will fall 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. 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.

12.3.5.3 Plumber's force cup, 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. Lift a cube of aluminium with a glass handler's suction cup.
5. 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.

12.3.5.4 Plumber's force cups as Magdeburg hemispheres
See diagram 12.3.5.4: 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.
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. A famous experiment at Magdeburg, Germany in 1654, 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. You encounter this kind of event in daily life sometimes, e.g. boil water or soup in a pan with a well fitting top then cool the pan. Try to open the cover only to find the cover to be sticking to the pan.
3. Evacuate a pair of Magdeburg hemispheres are evacuated with a pump. Pump out flat plates separated by an O-ring and hang weights. Separate the hemispheres by placing in a bell jar and evacuating it.

12.3.5.5 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 cm²of 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.

12.3.5.6 Vacuum cleaner
A vacuum cleaner pumps some of the air away from over the dirty carpet. A "partial vacuum" is created. Air rushes in to replace the air pumped out and when it is pumped away it takes with it the dirt from the carpet. 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. "Nature abhors a vacuum" (Spinosa, 1677, in Ethics).

12.3.6 Push a drinking straw through a potato
See diagram 12.3.6: Push straw through potato
1. Try to push a drinking straw through a potato and find that the straw is too weak and collapses. 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 the straw enough strength to prevent its bending.
2. Put the index finger over one end of a drinking straw. Hold a potato in the other hand. Push the straw quickly 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 the drinking straw enough strength to prevent its bending.
3. Hold a potato and press 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. 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!

12.3.7 Water reservoir for chicken drinker
See diagram 12.3.7: Make a chicken drinker
Use a flat dish that chickens can drink from. Fill the dish with water. Use a large bottle filled with water. Close the bottle tightly. Hang the bottle with a thread, mouth down and immerse under the water level in the dish. Then remove the cover of the bottle under the water. The water level in the bottle does not drop because the atmospheric pressure on the water surface of the dish holds up the water. If chickens drink some water and the water level in the dish falls below the mouth of the bottle, water in the bottle can drop automatically and the water level in the dish goes up again. When water level in the dish rises to the mouth of the bottle, water in the dish stops falling automatically again.

12.3.8 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.

12.3.9 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 into the test-tube.
2. Fit a test-tube with a one-hole cork and 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.

12.3.10 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.

12.3.11 Holes in a drink-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 drink-can. Make a hole with a nail near the bottom of an empty drink-can. Block the hole with your finger and fill the drink-can with water. Invert the drink-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 drink-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 drink-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 drink-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 5 to 10 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.

12.3.13 Air pressure crushes a can, collapsing can
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 using 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.
2. Repeat the experiment by removing air from a thin wall can with a vacuum cleaner.
3. Boil water in a can and cap. As the vapour pressure is reduced by cooling the can collapses. Pump out a can slightly put it in a vacuum chamber and blow it back up.

12.3.14 Balanced balloons
Use 2 identical balloons, a metre rule, string. Inflate both balloons until they are about the same size and tie securely with string. Make a loop at the end of each string. A attach string to the centre of the metre rule. The attach one balloon at each end with the loops. Hold up the string attached to the metre rule the move the loops until the balloons are balance and the metre rule is horizontal. Carefully cut the string tied to one of the balloons. The balloons are no longer balanced.

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.
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 X 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 layed 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 x 80 cm newspaper is approximately 60 x 80 x 1 kg = 4, 800 kg, is close to the weight of two large cars. So lifting the newspaper with the thin stick was impossible!

12.3.17 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 2-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.

12.3.19 Density of hot air and cold air
Heat one of two cans hanging from a balance.

12.3.20 Density of air with a balloon
Temperature and mass of dry air, kg / m³, at s.t.p.0^oC 1 292 kg / m³
25^oC 1 184 kg / m³ (Maximum water content = 0.023 kg / m³)
Use carbonated water to fill a balloon for use in measuring the density of air.

12.3.21 Equidensity bubbles
Blow a soap bubble with air and then gas to give a bubble of the same density as the surrounding air.

12.3.22 Freon and air
Fill a pan with Freon and float a balloon on it to show the difference in density with air.

12.3.23 Lifting power of balloons
Fill balloons to the same diameter with different gases and show difference in lifting power.

12.3.24 Pouring gases
Pour carbon dioxide into one of two beakers on a platform balance.

12.3.25 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.

12.3.26 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.

12.3.27 Egg in a bottle, balloon in a flask, tie a knot in a bone
1. Cover a fresh egg with vinegar or dilute acid hydrochloric acid. Change the solutions each day for 2 days. The dilute acid dissolves most of the egg shell or bone composed mainly of calcium salts. Boil water in a flask. 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 soft egg will squeeze down the neck of the flask due to atmospheric pressure. The egg is NOT sucked into the bottle!
2. Peel the shell off a hard-boiled egg then put the peeled egg narower end down end in the mouth of bottle with a fairly wide mouth, e.g. a milk bottle. Light a small piece of paper, 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. Or you can drop the burning paper into the bottle, wait until the paper 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.
3. 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. Another way to get the egg out is to 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 hugh 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!
5. 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.
6. Cover a fresh egg with vinegar or dilute acid hydrochloric acid. Change the solutions each day for 2 days. You can now tie a knot in the bone!

12.3.28 Balloon with cup "ears"
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.

12.3.29 "Cupping"
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.

12.3.30 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.

12.4.0 Siphons, pumps
Siphons, lift pump, force pump, siphons fountain, syringe, bicycle pump, vacuum pump, Boyle's law
See diagram 12.4.1.0: Difference in heights of water columns | See diagram 12.4.05: Two siphon systems and as syringe
Siphons are tubes used to move liquids from a higher container to a lower container, e.g. you can siphon gasoline out of a gasoline tank in a motor car. Pumps are machines used to move liquids or gases.

1. You can define the siphon, as a pipe system consisting of two legs as an inverted J, used to carry a liquid from one vessel to another vessel at a lower level, over an intermediate level higher than both levels. When both legs of the siphon are full, the hydrostatic force due to gravity is larger on the longer leg, thus causing the liquid to move up the shorter leg, over the bend, and down the longer leg. Start the siphoning process by 1. filling a siphon with liquid before placing it into its operating location or 2. by applying suction at the lower end after the tube is in position. Once started, the flow will continue until the liquid level in both vessels is equal, or until the level in the higher vessel falls below the inlet of the tube when air is sucked in and the siphoning action stops."

2. There is more than one explanation of how the siphon works. Explanation 1: The forces of cohesion, surface tension between water molecules, allows the water in the short side of the siphon to be pulled up the tube by the greater weight of the water in the long side. The siphon acts in much the same way as the longer end of a chain hanging out of a bucket can pull the rest of the chain from the bucket. Explanation 2: Both surface tension and atmospheric pressure are required to make the siphon work. Gravity pulls down on the water in both sides of the siphon. However, because there is more water in the long side, the weight is greater in the long side than in the short side. If the columns of water are not allowed to separate, the water in the long side will pull the liquid up the short side. Atmospheric pressure keeps the water from separating. The water flowing down the long side reduces the pressure in the siphon and thus atmospheric pressure pushes water up into the short side of the siphon if it is greater than the pressure of the column of water in the short side of the siphon. If atmospheric pressure did not exist then there would be no reason for water to enter the short side of the tube. The following experiment, the siphon fountain, shows that the principle of operation for a siphon does not rely on surface tension alone.

3. After "Understanding the siphon" Kevin C. de Berg and Cedric E. Grieve, Australian Science Teachers Journal, 45 (4)
The explanatory principle based on comparing the weights of fluid in both legs of a siphon occurs in most textbooks analysis and in the work of the French scientist Pascal, 1663. The advantage of this principle is that it is relatively simple and applies to siphons as commonly employed. Its disadvantage is are that it applies only to siphons and not other fluid flow devices, e.g. syringes, and applies only where the siphon fluid is more dense than the external fluid. The following explanatory principle overcomes this disadvantages: The siphon in diagram 12.4.05 1. has a fluid disc of negligible thickness at X. You can consider the contending pressures either side, S and S1 of this disc at the moment the stopper is removed from X. The pressure on side S1 is atmospheric pressure, AP. To locate the pressure on side S of the disc, follow the pressure changes on the other side of the siphon starting at the surface of the fluid in the container where the pressure is atmospheric pressure, AP. At A the pressure is (Ap + 2). At B the pressure is (AP + 2 - 18). At X the pressure is (AP + 2- 18 + 6), i.e. (AP - 10). The pressure on side S1 exceeds the pressure on side S so the liquid will not siphon from the tube but move back into the longer leg and into the container.
In diagram 12.4.05 2. for this siphon, at X, the pressure on side S1 is again AP. On side S, the pressure (AP + 2 - 12 + 14), i.e. (AP + 4). The pressure on side S exceeds the pressure on side S1 and so fluid siphons from the beaker out of the longer leg of the siphon. In the syringe in diagram 12.4.05. The pressures on either side of a fluid disc of negligible thickness at the opening of the needle after the plunger has been moved back but before fluid has begun to flow. On side S the pressure is less than atmospheric pressure because a small sample of air at atmospheric pressure was expanded into larger volume. On side S1 the pressure is (AP + h). So the pressure on side S1 exceeds the pressure on side S and fluid moves into the syringe from high pressure to low pressure.

12.4.1.0 Simple siphon
See diagram 12.4.0: Siphons
Use two tall glass bottles and fill each about half full of water. Connect two 30 cm lengths of glass tube with a 30 cm rubber or plastic tubing. Fill the tube with coloured water and pinch it. Put a glass tube in each bottle of water. Siphon the water back and forth by varying the height of the bottles.

12.4.1.1 Siphon fountain
See diagram 12.4.1: Siphon fountain
1. Fit a flask with a 2-hole stopper. Through one hole put a jet tube that extends to about half way to the top of the flask and about 2 cm outside the stopper. Through the other hole push a short length of a glass tube so that it is just flush with the bottom of the stopper. Connect a 20 cm length of a rubber tube to the jet tube. Connect a 1 m length to the other glass tube. Put some water in the flask and insert the stopper. Put the short rubber tube in a container of coloured water on a table, let the longer rubber tube go into a pail on the floor and then invert the siphon. Coloured water spurts from the jet. You can make a double siphon fountain by making another flask unit similar to the first one and connecting them. While the water is maintained in the container, the fountain cannot be stopped.
2. Insert 2 pieces of glass tubing through a 2-hole stopper such that one extends farther than the other. Connect 30 cm of rubber tubing to each glass tube. Fill a large bottle with cold water. Pour 100 mL of water in a large jar and insert the 2-hole stopper with attached tubing. Invert the jar. The ends of the tubing in the bottle should be under water at all times. Observe what happened after inverting the jar. Observe the volume of the air pocket above the water in jar as the water poured in the empty bottle. Note that water was drawn up into the jar. The bottle filled with water had to stand higher than the other bottle. See how to make the fountain flow more or less. The water in the jar was needed to "prime" the siphoning action. After inverting the jar, the water ran down and into the empty bottle causing an increase in volume of the air pocket above the water in the jar and decreasing the pressure. The lower pressure caused the sucking up of the water from the bottle filled with water. The atmospheric pressure pushes water up into the larger jar. The greater the difference in height of the water levels in the two jars, the greater the flow of water. As soon as the water levels are the same height in both bottles the water flow stops as in a siphon.
3. Fit a glass container, or a flask made from a used electric bulb with a two-holes rubber stopper. Through one hole place a jet tube which will extend to about half way to the top of the flask and about 2 cm outside the stopper. Through the other hole push a short length of glass tube so that it is just flush with the bottom of the stopper. Connect a 20 cm length of rubber tube to the jet tube. Connect a 1 m length to the other glass tube. Place some water in the flask and insert the stopper. Put the short rubber tube in a container of water on a table, let the longer rubber tube go into a bucket on the floor and then invert the siphon. Add ink to the water to see the fountain better. Make a double siphon fountain by making another flask unit similar to the first one and connect them.

12.4.1.2 Siphon replaces water for fish
Use a one metre length of 10 millimetre diameter rubber hose. Fill the hose with water then block both ends with fingers. Put one end into water in a tank for goldfish then move away the finger the end under water. Put another end into a bucket or basin on the floor below the end of the tube in the tank, then move away the finger from the end. Water streams out of the tank towards the bucket or basin.

12.4.1.3 U-tube siphon
Use two beakers half full of water with a connecting U-tube full of water. Lift one beaker then the other.

12.4.1.4 Siphon in a bell jar
Transfer water through an U-tube from a sealed flask to an open beaker when the assembly is placed in a bell jar and evacuated.

12.4.1.5 Siphon mechanism apparatus
This apparatus that shows atmospheric pressure, not cohesion, to be the basis for the siphon action.

12.4.1.6 Measure pressure in a siphon
Hook a manometer to the upper portion of a siphon.

12.4.1.7 Mechanical siphon
A mechanical model of a siphon consists of chain hung over a pulley to a lower level.

12.4.1.8 Self-starting siphon
Seal an inverted U-tube in the side of a beaker to make a self-starting siphon.

12.4.1.9 Mariotte flask and siphon
Use a gas siphon to transfer carbon dioxide from one beaker to another, intermittent siphon, Tantalus cup
Use a Mariotte flask to make a siphon with a constant flow because the height of an open tube inserted through the stopper of a jug with an outlet at the bottom regulates flow.

12.4.1.10 Turnover siphon
See diagram 4.4.1.10
1. Insert two pieces of glass tubing through a 2-hole stopper with a longer piece drawn out to form a jet. Connect the shorter glass tube with rubber tubing to a lower jar, A. Connect the longer glass tube with rubber tubing to a higher jar, B. Add water to an elevated jar, C. Insert the 2-hole stopper in C so that the jet end of the longer glass tube is above the water level. Fill jar B with water.
2. Invert jar C while keeping the ends of the rubber tubes in the jars. Water in jar C. starts to run down the rubber tube into jar A, increasing the volume of air in jar C. and decreasing the air pressure in jar C. Water in jar B. is pushed up the rubber tube by (atmospheric pressure - air pressure in jar C). The water shoots out through the jet end of the longer glass tube until the levels of water in jar B. and jar C. are the same.

12.4.3 Syringe lift pump
See diagram 12.4.3: Lift pump
[Some school systems do not allow the use of syringes in the classroom.]
See diagram 12.4.3.1: Syringe lift pump
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.

See diagram 12.4.3.2: Syringe lift pump
Wrap a string around one cork to make a tight fit inside the glass or metal tube. The other cork, with a piece of glass, bamboo or strong tubing, acts as an intake. Then drill a hole in the cork and push a piece of glass, bamboo or strong tubing through the hole ensuring a tight fit and push the assembly into the end of the glass or a metal tube used for the syringe body. This will be the intake. To make the piston drill a hole through the other cork to attach the metal rod. Use a piece of hot wire burn two more small holes through the cork on either side a parallel to the hole for the metal rod. Burn two holes through the piston with a hot wire and fit a thin piece of leather above them to act as a valve that closes on the up-stroke and yet allows liquid to pass through on the down stroke. To make the piston, drill a hole through the other cork to attach the metal rod. Then, with a piece of hot wire, burn two more small holes through the cork on either side, parallel to the hole for the metal rod. Put the glass, bamboo or strong tubing, acts as an intake, into water. Trim a piece of leather or rubber to the same size as the internal diameter of the syringe body. Cut a hole in the centre of the piece of leather or rubber to allow the metal rod to slide through it. Assemble the piston. Raise the piston, sucking water up under the piston into the syringe. To operate raise the piston up sucking water up under the piston into the syringe. Then push down the piston then the water raises above the piston of the syringe. Finally raise the piston up then the water streams out of the top of the syringe.

12.4.4 Simple test-tube force pump
See diagram 12.4.4: Test-tube force pump
1. Use a small and a large test-tube. Heat the bottom of each test-tube with an alcohol burner. When the bottoms are soft but not melting, punch a hole with a metal awl. Fit a ball bearing or small marble in each test-tube to act as a valve. Fit a cork with a bent glass tube at the end of the small tube. Put the small test-tube into the large tube. Wrap string around the inner test-tube so that it fits tightly in the outer one but can still slide up and down and secure the inner tube to act as a piston of a force pump. Put the bottom of the large tube into water in a beaker. Raise the piston then water is sucked up the large tube through the ball valve of the large tube. Push down the piston then the water raises the small tube through the ball valve of the small tube. Raise the piston then the water streams out of the small tube. Raise and push the piston continually then water streams out of the bent tube continually.
2. Use a thinner test-tube and a thicker test-tube. Heat the bottom of each of test-tube with a burner then punch a hole with a nail when the glass in the bottom is soft but not melting. When the test-tubes are cool, fit a ball bearing or marble to sit in the holes in the bottom to act as valves. Fit a cork with a bent glass tube passing through it into the end of the thinner test-tube. Wrap string around the thinner test-tube so that it fits tightly in the thicker test-tube but can still slide up and down. Put the thinner test-tube into the thicker test-tube. The thinner test-tube can act as a piston of a force pump. Put the bottom of the thicker test-tube into water. Raise the piston. During the upstroke water moves up through the ball valve of the thicker test-tube because of reduced atmospheric pressure. Lower the piston. During the downstroke water moves up through the ball valve in the thinner piston while the ball valve in the thicker piston remains closed. Continue to raise and lower the piston until water streams out of the bent tube.

12.4.5 Flask fountain
Boil some water in a flask and insert a one-hole stopper with a single tube then invert the apparatus with the lower end of the tube under water in a beaker. At first water moves slowly up the tube but as water touches the inside of the flask cooling quickens and water moves more quickly up the tube. Make the colour of the water change by putting dilute hydrochloric acid in the in the flask and bromothymol blue in the beaker.