School Science Lessons
12. Atmospheric pressure, liquid pressure, pumps, syringes, siphons, weight and pressure
2012-05-04b SP
Please send comments to: J.Elfick@uq.edu.au
\
Table of contents
12.0.0 Pressure
12.3.0 Atmospheric pressure, air pressure
12.2.0 Liquid pressure, fluid pressure, hydrostatics
12.1.01 Pressure definitions
11.4.0 Pressure in fluids, Buoyancy, flotation, Archimedes' principle, density of fluids
20.3.0 Pressure law, Constant volume, p / T = constant
12.7.0 Pumps, syringes
12.4.0 Siphons
12.5.0 Weight and pressure, pressure definition
12.3.0 Atmospheric pressure, air pressure
12.3.0 Atmospheric pressure, air pressure
4.220 Atmospheric pressure (experiments) UNESCO
6.18 Atmospheric pressure (Primary)
12.1.05 Atmospheric pressure, Conversions between units of atmospheric pressure
12.1.04 Atmospheric pressure water spray
13.1.0 Atmospheric pressure, barometer, Torricelli
37.11 Air exerts pressure in all directions
1.38 Air games (Primary)
12.3.3 Air has mass, air has weight
1.39 Air in bags (Primary)
5.26 Air pressure in all directions (Primary)
3.37 Air takes up space (Primary)
12.3.2.1 Air takes up space, transfer air under water
4.230 Aneroid barometer
12.3.10 Automatic drinking glass
12.3.2.4 Bag of air into and out of a jar
12.3.14 Balanced balloons
12.3.27.1 Balloon in a flask
12.3.28 Balloon with cup "ears"
13.7.0 Barometers
12.3.30 Bottles stick together
6.35 Burn candle over water (Primary)
12.3.3.1 Carbon dioxide has mass
4.243 Cold air is heavier than warm air, inverted paper bag balance
3.32.1 Composition of the atmosphere and greenhouse gases
12.3.26 Cork sticks to the bottom of a beaker
12.3.13 Crushed can, crush a tin can with atmospheric pressure, collapsing can
12.3.13.1 Crushed plastic drink bottle
12.3.29 "Cupping"
12.3.5.1 Drinking straw, finger on drinking straw, glass tube
1.42 Drinking straw game (Primary)
12.3.20 Density of air with a balloon
12.3.19 Density of hot air and cold air
12.3.27 Egg in a bottle
12.3.21 Equidensity bubbles
12.3.1 Find air in liquids
12.4.6 Flask fountain
12.4.4 Force pump
12.3.22 Freon and air
12.3.2.3 Funnel in the neck of a bottle
12.2.11 Hero's Fountain
12.3.17 Inflate balloon with low pressure and high pressure bottle
37.39.2 Inversion layers
12.3.5.2 Inverted drinking glass
37.39.1 Layers of the atmosphere, lapse rate
12.3.9 Lift water with air pressure
12.3.23 Lifting power of balloons
4.240 Model lungs
12.3.5.7 Magdeburg hemispheres, vacuum pumps
4.241 Oxidation and air pressure, steel wool over water
12.3.5.3 Plumber's force cups, suction cup, suction disc
12.3.5.4 Plumber's force cups
12.3.24 Pouring gases
24.2.4 Pressure and boiling point of water
3.8 Pressure of the atmosphere affects the boiling point
12.1.01 Pressure definitions
37.4.0 Pressure tube anemometer, wind speed indicator
12.3.6 Push a drinking straw through a potato
12.3.2.2 Pushed down drinking glass
20.0.6 Standard temperature and pressure, STP, Standard Atmosphere
12.3.2.5 Tapping a box
4.153 Three holes can, 3-hole can, vase with three holes, spouting cylinder
12.3.5.6 Vacuum cleaner
4.238 Volume and pressure of air, Boyle's Law
12.3.7 Water reservoir for chicken drinker
12.3.8 Water rises in a downwards floating beaker, pressure under an inverted beaker
12.3.25 Weight of air
12.3.5.5 Wet suction with a Petri dish
37.3.0 Wind speed indicator, Deflection anemometer
12.2.0 Liquid pressure, fluid pressure, hydrostatics
12.2.0 Liquid pressure
4.190 Liquid pressure experiments, UNESCO
12.3.01 Blood pressure
12.2.9 Dropping plate
5.18 Feel your pulse (Primary)
12.2.8 Inverted test-tubes, test-tube rising automatically, upwards falling test-tube, pushed up test-tube
12.2.1 Manometer, water manometer, pressure gauge
12.2.10 Pascal's diaphragms
12.2.5 Pascal's vases, balanced water columns
12.2.7 Pressure applied to a sealed fluid is transmitted equally through the fluid, Pascal's law,
Pascal's principle
12.2.2 Pressure depends upon the density of the liquid
12.2.4 Pressure increases with depth, closed funnel at different depths in water
12.2.3 Pressure is independent of size and shape of the container
12.2.6 Pressure is the same in all directions, Pascal's fountain
20.3.0 Pressure law, p / T = constant
4.244 Scuba diving and Boyle's law
16.2.2.5 Spirit level (level tube) accelerometer
20.0.6 Standard temperature and pressure, STP, Standard Atmosphere
SVP: Table of saturated vapour pressure over water, Psvp
5.21 Water finds its own level (Primary)
12.2.1 Manometer, water manometer, pressure gauge
4.192 Water pressure does not depend on the size of the container
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 / m2
1 dyne cm2 = 10-1 pascal (Pa)
1 bar = 105 pascal (Pa)
1 hectopascal (hPa) = 1 millibar (mb) = 100 Pa
1 millibar = 100 pascal (Pa)
1 bar (bar) = 106 dyne / cm2 = 100 000 Pa
1 bar = 750.07 mm Hg
1 atmosphere (atm) = 760 mmHg = 101,325 Pa
Atmosphere, atmospheric pressure (Greek: atmos, vapour - sphaira, globe, ball)
1 torr = 1 millimetres of mercury (1 mmHg) = 133.322 Pa
1 pound-force per square inch = 1 lbf / in2 (psi) = 6 894.76 Pa
For Standard temperature and pressure, s.t.p., the temperature = 273.15 K or 0oC and the
pressure - 101 325 Pa or 760.0 mmHg.
Pressure, or stress, is the force per unit area on a surface, P = F / A and has the SI units
newton / metre2, Nm-2 or 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 = 105 Pa, equivalent to 750.062 millimetres of mercury
(mm Hg) at 0oC, 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.
12.5.0 Weight and pressure
12.1.1 Weight and pressure
12.1.06 Altimetry, height and altitude
12.1.3 Cut ice with pressure
21.0.1.1 Stabbed rice experiment (pressure)
20.0.6 Standard temperature and pressure, STP, density of gases
8.1.0 Mass and weight, weighing devices (balances)
12.1.01 Pressure definitions
3.2.0.0 Weight, standards of weight
36.108.02 Weight of an object and g
12.2.15 Weight on a beach ball
12.1.2 Weigh car with a tyre gauge, Bourdon gauge
12.7.0 Pumps, syringes
12.4.4 Force pump
12.4.2 Lift pump
13.1.2 Measure atmospheric pressure with a bicycle pump
4.223 Plastic syringes and air pressure, Boyle's law
12.4.5 Simple test-tube force pump
12.4.3 Syringe lift pump
12.1.04 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.
12.1.05 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).
12.1.06 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 the following:
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.
3. Use a block of wood with two different dimensions, e.g. 10 cm × 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.
4. 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.
5. 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 - P0), where P = the pressure by which the internal pressure in
the tyre exceeds the outside pressure, and Po = 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 × 105 Pa), 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 × 105 newton) =
(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
through 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 × density × g, pressure and depth, pressure in all directions, "water finds its own level"
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 Po + dgh, where Po is the atmospheric pressure.
12.2.1 Manometer, water manometer, pressure gauge
See diagram 12.2.4: Manometer 1 | See diagram 12.268: Manometer 2
A manometer measures the pressure of a gas or liquid. It consists of an U-tube containing a liquid,
e.g. water, oil, or mercury, with one limb of the U-tube connected to a enclosure containing the gas or
liquid, with the other limb open to the atmosphere. The open end may contain a float as part of a
recording mechanism.
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 90o, and one tube is bent 180o. 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 Pascal's vases, balanced water columns
See diagram 12.2.5: Pascal's vases
1. Connect 6 tubes of various shapes to a common water reservoir. Water levels in containers with
different shapes but attached to a common reservoir reach the same height regardless of the shape of the
container.
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 shaped plastic bottles. Fit one bottle with a one-hole stopper and the
other with a two-holes stopper. Connect the first bottle with a two-holes 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.
12.2.11 Hero's Fountain
See diagram 12.2.11: Hero's fountain
Order online: Fountain Connection, Hero of Alexandria's water 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 the bottles.
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.
12.3.0 Atmospheric pressure, air pressure
Order online: Pressure Mat, air pressure, visualize its force
Order online: Pressure Pumper kit, clouds, refrigeration, air conditioning
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.
12.3.01 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 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 Find air in liquids
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. If
a glass of tap water is left for some time on the bench, bubbles form over the glass surface under the water
level. The air in the bubbles come from the air dissolved in the water reticulation system by the mains
pressure in the water pipes.
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 CO2
bubbles 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 12.303.3: 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 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.
12.3.2.3 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.
12.3.2.4 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
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.
12.3.2.5 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 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 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 the 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 12.305.2: 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. 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 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.
12.3.5.2 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 and let the cardboard free. 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.
12.3.5.3 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. 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
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 (see below).
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.
12.3.5.5 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
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.
12.3.5.6 Vacuum cleaner
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. 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.
12.3.5.7 Magdeburg hemispheres, vacuum pumps
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.
1. Use two identical drinking glasses. Cut a square piece of paper 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" 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, the remaining air in the glass had cooled and contracted leaving a partial
vacuum. Some of the air in the top glass had diffused through the damp paper to equalize the partial
vacuum in the two glasses.
2. You may 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. You could separate the hemispheres by placing in a bell jar and evacuating it with an electric vacuum
pump. The hemispheres would then fall apart.
12.3.6 Push a drinking straw through a potato
See diagram 12.306: 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!
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.
12.3.7 Water reservoir for chicken drinker
See diagram 50.6.3.3: Brooder 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 Crushed can, crush a tin can with atmospheric pressure, 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 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.
2. Repeat the experiment by removing air from a thin wall can with a vacuum cleaner.
3. Fill a drink-can with hot steam. Place the drink-can open side down on ice. The drink-can quickly
collapses as the steam condenses.
4. Boil water in a can and cap. As the vapour pressure is reduced by cooling the can collapses. Pump out
a drink-can slightly, put it in a vacuum chamber and blow it back up again.
12.3.13.1 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.
12.3.14 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. 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, 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 two-holes 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
See: Density of air
Heat one of two cans hanging from a balance.
12.3.20 Density of air with a balloon
See: Density of air
Temperature and mass of dry air, kg / m3, at s.t.p.
0oC 1 292 kg / m3
25oC 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
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 salts. Pick up
the decalcified egg and drop it to show that it will bounce and not beak. 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 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. 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 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!
12.3.27.1 Balloon in 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.
12.3.28 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.
12.4.2 Lift pump
See diagram 12.4.3: Lift pump
Downstroke 1: On downstroke valve A closes and valve B opens. Air passes from lower chamber to
upper chamber through valve B.
Upstroke 1: Reduced pressure in lower chamber causes valve B to close and valve A to open. Pressure
in inlet pipe is reduced so atmospheric pressure acting on water surface cause water to rise into lower
chamber.
Downstroke 2: Valve A is closes, valve B is open. Water passes from lower chamber to upper chamber.
Upstroke 2: Valve A is open and valve B is closed. Water is lifted up and passes out through outlet pipe
and more water enters through inlet pipe.
The atmosphere can support a column of mercury 76 cm vertical height, so h in the diagram can be no
more than 76 × 13.6 (relative density of density of mercury) = 1,030 cm.
12.4.3 Syringe 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 down stroke 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
2. 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 Force pump
See diagram 12.4.4: Force pump
Downstroke: Valve A is closed, valve B is forced open, air passes out of the chamber into the outlet pipe
Upstroke: Valve A is open, valve B is closed, . The pressure in the chamber is reduced and water rises
in the inlet pipe caused by atmospheric pressure on the water surface.
Downstroke: A is closed, water is forced through valve B into outlet pipe
Upstroke: Water in outlet pipe closes valve B. More water enters the chamber through valve A.
The air chamber causes the flow of water to continue during the upstroke. On the downstroke, water
enters the air chamber compressing the air inside it. On the upstroke, the air expands again to force some
air up the outlet pipe.
The atmosphere can support a column of mercury 76 cm vertical height, so h in the diagram can be no
more than 76 × 13.6 (relative density of density of mercury) = 1,030 cm.
12.4.5 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 down stroke 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.
12.4.6 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.