School Science Lessons
2024-04-26
Pressure, liquid pressure, vapour pressure, weight
(UNPh12
Contents
12.1.01 Pressure
12.1.06 Altimetry, height and altitude
Experiments
12.1.0 Liquid pressure, fluid pressure, hydrostatics
12.2.0 Beverage can, drink-can
12.3.0 Weight and pressure
12.4.0 Gas, Molecular weight, Density, (Table 1)
12.5.1 Saturation Vapour Pressure, (SVP),
12.6.0 Sound
12.1.0 Liquid pressure, fluid pressure, hydrostatics
Experiments
12.1.1 Balancing water columns, Pascal's vases
9.240 Blood pressure
12.1.2 Dropping plate
12.1.3 Hydraulic lift
12.1.4 Hydraulic ram, water ram
12.1.5 Inverted test-tubes, test-tube rising automatically, upwards falling test-tube, pushed up test-tube
12.1.6 Liquid pressure depends on the density of the liquid
12.1.7 Manometer, water manometer, pressure gauge
12.1.8 Measure pressure with an U-tube
12.1.9 Liquid pressure and height / density
12.1.10 Pascal's vases
12.1.11 Pressure applied to a sealed fluid is transmitted equally through the fluid, Pascal's law, Pascal's principle
12.1.12 Pressure increases with depth, closed funnel at different depths in water
12.1.13 Water pressure is the same in all directions, Pascal's fountain
12.1.14 Raise heavy weights with water pressure
12.1.15 Water pressure changes with depth
12.1.16 Water pressure does not depend on the size of the container
12.1.17 Water wheel
12.1.18 Water does not compress
12.2.0 Beverage can, drink-can
12.2.1 Beverage can composition
12.2.2 Beverage can opening
12.2.3 Beverage can shaking or tapping
12.2.4 Beverage can pressure
12.3.0, Weight and pressure
Experiments
12.3.1 Cut ice with pressure
12.5.2 Pressure safety
12.3.2 Stand on a bed of nails
12.3.3 Weigh car with a tyre gauge, Bourdon gauge
12.3.4 Weight and pressure
12.3.5 Weight of a floating body
12.3.6 Weight on a beach ball
12.3.7 Atmospheric pressure water spray
12.6.0 Sound
12.4.1 Loudness, threshold of hearing, audible limits
12.4.2 Sound pressure level
12.4.3 Threshold of hearing
12.4.4 Decibels dB, Sound pressure units (Pa)
12.4.5 Musical scale, tones, sharps and flats, equal temperament scale
12.4.6 Musical instruments
12.4.7 Wine glass resonance
12.4.8 Wine glass breaks with voice resonance
12.4.9 Stationary waves in Chinese temple "fish wash" dishes
12.1.01 Pressure
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
29.92 " Hg = 1.0 atm = 101.325 KPa = 1013, 25 mb
Atmosphere, atmospheric pressure (Greek atmos vapour - sphaira. globe, ball)
1 torr = 1 millimetre of mercury (1 mmHg) = 133.322 Pa
1 pound-force per square inch = 1 lbf / in2 (psi) = 6894.76 Pa
For Standard Temperature and Pressure, STP, 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 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.
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).
Static pressure, P = height × density × g, pressure and depth.
Pressure occurs in all directions
,
Water pressure: "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.
Pressure depends upon the density of the liquid
12.1.1 Balancing water columns, Pascal's vases
See diagram 12.2.5: Pascal's vases
1. Drill a hole in or remove the bottoms from several plastic bottles of different shapes, but of about the same height.
Fit the bottles with stoppers or corks carrying glass tubes as shown in the diagram.
Connect the bottles together as shown.
Pour coloured water into the bottles until they are nearly full.
This experiment shows that in a given liquid, pressure is independent of the size or shape of the vessel and depends only on the depth.
12.1.2 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.
12.1.3 Hydraulic lift
Water pressure can raise freight and passenger lifts.
1. Connect a rubber hose to a motor car hand pump and bind the connections with wire and adhesive tape.
Connect the other end of the hose to a water tap.
Fix a weight to the handle of the pump.
Turn on the water tap and see the water pressure lift the weight.
12.1.4 Hydraulic ram, water ram
See diagram 4.198: Hydraulic ram
Hydraulic rams are sometimes used to raise water from a low level
to a higher level.
A flowing stream of water operates them.
1. Make a model hydraulic ram
Remove the bottom of a plastic drink bottle.
Fit the bottle with a one-hole rubber stopper carrying a short length of glass tubing.
Connect the glass tubing to a glass or metal T-tube that has a piece of rubber tubing on one end and a jet tube connected to it with a rubber tube.
Fill the bottle with water and pinch the tube at the end.
Let the water run from the end of the tube.
Stop the flow suddenly by quickly pinching the tube, and note the height to which the water squirts from the jet tube.
Let the water flow and stop alternately, and you have a working model of the hydraulic ram.
12.1.5 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.
12.1.6 Liquid pressure depends on the density of the liquid
See diagram 12.2.4: Manometer
2. Repeat the experiment with a container of methylated spirit, pure water and salt water.
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.
12.1.7 Manometer, water manometer, pressure gauge
See: Manometer (Commercial).
See diagram 12.2.4: Manometer 1
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.
Experiments
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.
12.1.8 Measure pressure with a U-tube
See diagram 4.189: Measure pressure in liquids with a U-tube
1. 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.
12.1.9 Liquid pressure and height / density
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 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. Repeat the experiment with a container of methylated spirit, pure water and salt water.
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, the more dense salt water exerts more pressure than pure water.
12.1.10 Pascal's vases
See: Pascal's Laws Pascal's Vases, (Commercial).
Pascal's vases is a communicating vessel for observing the liquid levels in the vases.
Pressure is independent of size and shape of the container.
See diagram 12.2.5: Pascal's vases
See diagram 12.2.5a: Communicating vessel for liquid level
See diagram 12.2.4: Manometer
1. Pascal's vases connects 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. Communicating vessel for liquid level shows the equilibrium of the same liquid in vessels of different cross section.
The apparatus contains five connecting vessels with different shapes.
3. 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
4. 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.
5. Cut the bottoms from different shaped plastic bottles.
Fit one bottle with a one-hole stopper and the others with a two-holes stoppers.
Connect the first bottle with a two hole stopper to a tap or reservoir and to 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."
6. A manometer consists of plastic tubing attached to one arm of a U-tube.
Prepare different size containers containing same liquid, e.g. water.
Stretch a piece of thin rubber 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.
7. Use a wide mouth beaker filled water.
Use three glass tubes open at both ends, with various shapes: straight tube is straight, tube is bent 90o, tube 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.
8. Pascal's apparatus shows that the pressure of a liquid varies with the depth and does not depend on the shape of the container.
It is a stand holding two glass vessels.
Fill the first vessel with dyed water and note how the pointer shows the position at which the pressure of the water overcomes the flanged seal.
Repeat the experiment with a second vessel to show that the thrust on the base depends on the area of the base and the depth of the liquid, but not on the shape of the container.
12.1.11 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.
12.1.12 Pressure increases with depth, closed funnel at different depths in water
Hydrostatic pressure increases with the height of a water column, (the depth from the surface).
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 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.
12.1.13 Water 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.
See diagram 4.193: Spurting tennis ball
6. 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.
12.1.14 Raise heavy weights with water pressure
See diagram 4.195: Raise heavy weights with water pressure
1. Use a rubber hot water bottle.
Put a one-hole stopper carrying a short glass tube tightly in the neck.
Punch a hole in the bottom of a plastic container and make it large enough to take a one-hole stopper.
Put a short piece of glass tubing through the stopper.
Connect the water bottle and the container with 1.25 m of rubber tubing.
Wind wire or adhesive tape firmly around the connection at the bottle.
Fill the bottle, tube and can with water.
Place the bottle on the floor and put a wooden board, books or other heavy objects on it.
Raise the plastic container above the level of the floor.
The books rise.
Note how heavy a weight you can lift by raising the plastic container.
12.1.15 Water pressure changes with depth
See diagram 12.2.4: Manometer
1. Cut one end off a tall plastic drink bottle to make a tall container.
Place the funnel of the manometer at different depths to measure the pressure.
12.1.16 Water pressure does not depend on the size of the container
See diagram 12.2.4: Manometer
Jet aircraft are usually refuelled by pumping fuel up through a hole in the bottom of the fuel tank.
The pump has to overcome only the weight of the column of liquid fuel above the hole, and with the same diameter as the hole, not the weight of the whole fuel contents of the fuel tank.
1. Use a large container of water and a small container of water.
Hold the mouth of the filter funnel at the same depths in the containers.
The corresponding differences in height of liquid in the U-tube of
the manometer are the same.
2. Repeat the experiment using a large container of water and a small container of water.
Hold the mouth of the filter funnel at the same depths as before.
The corresponding differences in height of the coloured water in the U-tube are the same.
12.1.17 Water wheel
5.8 Water wheel (Primary)
See diagram 4.199: Overshot water wheel
See diagram 12.278: Water wheel
1. Use a cylindrical cork and push in one the cutting edges of safety razor blades to make paddles.
Insert two needles in the centre of each circular face to act as an axle.
Grasp the needles in each hand to hold the water wheel in a stream of water from a tap.
See undershot and overshot water wheels.
12.1.18 Water does not compress
Some teachers have tried the following experiment several times and cannot get the desired effect.
Water just squirts out!
1. Fit a bottle with a one-hole stopper with a long glass tube passing through it.
Fill the bottle with water.
Insert the stopper tightly until the water rises in the medicine dropper.
Grasp the bottle in your hands and squeeze as hard as you can.
Water rises in the tube, because you cannot compress water.
(Bulk modulus for water = 2.1 GPa, so water is not completely incompressible!).
12.1.06 Altimetry, height and altitude
See: Altimeters (Commercial).
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.
12.2.1 Beverage can composition
Beverage is a prepared drink, but not water, e.g. tea, soft drink, alcoholic drink
Beverage can, aluminium can, beer can, "can", drink can, drinking can,
soda pop can, soft drink can, tin-plated steel can
1. Beverage cans are cylindrical containers made of aluminium or tin-plated steel in which food and drink can be stored and hermetically sealed.
2. The sizes and composition of the walls of the can may vary as follows:
2.1 USA and Canada, "aluminium cans", 355 mL, 92-97% Al, <5.5% Mg, <1.5%, Mn, <0.15 % Co
The empty beverage can weight is 15 g.
2.2 Australia, 375 mL
2.3 Europe, China, India, 330 mL
3. Aluminium is highly suitable as a packaging material for beverages, but have a protective epoxy resin polymer coating applied on the inside to prolong storage life so acids and salts in certain foods or beverages do not contact the metal.
4. Aluminium does not corrode easily,
5. Aluminium has density 2.70 g / cm3, compared with steel
7.86 g / cm3, so it is very light and cuts down on transport costs.
6. Aluminium transfers heat 2.4 times faster than iron, and very thin sheets can be produced, so heat is lost and gained by aluminium very quickly, giving it ideal qualities for cooking and as a cold drink container
7. Aluminium can be rolled into extremely thin foil, and can be cast and joined, yet still retain much of its strength, so less of it is needed as a light packaging material.
8. Aluminium has melting point of 660oC, compared with 154oC for iron, so less energy is required for processing and recycling it.
9. The lids of beverage cans may be a different composition than the cup portion of the can.
12.2.2 Beverage can opening
Beverage can opening, bayonets, can piercers, pull tabs (ring pulls), push tabs, stay-tabs (pop tabs)
1. Knife or bayonets were first used to open hermetically-sealed cans at home and during military service.
2. Can piercers, church keys, used a lever action to dig a triangular hole in the top of the beverage can with the cut metal being pushed inside the can.
3. "Pull tabs", ring pulls, pulled of a section of the top of the beverage can, so that the ring and cut metal piece became detached.
However, the separated pull tabs became an environmental hazard, and the opening in the top of the beverage can had sharp edges, so adolescents could use them as dangerous rings on small fingers.
Some old people did not have the strength or dexterity to use pull tabs.
"Push tabs" detached an opening in the top of the can by pressing down, but people could cut their fingers on the cut metal.
5. "Stay-tabs", stay-on tabs, pop-tabs, are favoured nowadays, because they are easy to use, and cannot be detached to cause pollution or injury.
12.2.3 Beverage can shaking or tapping
To avoid an explosive emission of beverage and gas from a shaken soda pop can, some people lightly tap the side of the upright can before pulling on the pull tab.
This practice may be regarded as an urban myth.
Tapping may release some bubbles from adhering to the walls of the soda pop can to collect at the top of the beverage just inside the ring pull.
So when the drink-can is opened, these bubbles do not push through the beverage, but push out some beverage explosively.
The time taken for the tapping gives some time for the carbon dioxide to dissolve back into the beverage, compared with a soda pop can being opened immediately after the shaking.
The beverage is emitted less forcefully if the soda pop can is opened very slowly, or put back in the refrigerator to increase the absorption of carbon dioxide.
12.2.4 Beverage can pressure
Beverage cans, e.g. "7 UP", Coca Cola Classic"
Carbonated water pressure in "7 UP" = approximately 207 kpa
"Coca Cola Classic" contains 3.7 vols of CO2 at 24oC = 380 kpa
A soda can is about 17 psi above atmospheric pressure 17 psi = 117.21087 kpa | 1 psi = 6.89475 kpa
Pressure in a 12 oz. soda can:
If the soda was carbonated to 3.0 volumes of CO2, in refrigerator at -4.4 degrees C.
117 kPa | (4oC, when canned) | 248 kPa | (21oC, at room temperature) | (1 atm = 101.3 kPa = 14.7 psi).
12.3.1 Cut ice with pressure
See: Ice Model (Commercial).
See diagram 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.
Apply pressure and observe how the wire makes its way slowly through 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 into 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.
12.3.2 Stand on a bed of nails!
Demonstrate the essential difference between pressure and force by distributing the lecturers weight over a bed of nails.
Show the difference between pressure and force by distributing the weight over a bed of nails.
Distribution of weight over a large enough area reduces the pressure below the piercing breakpoint of skin.
Construct a bed of 900 nails evenly spaced on a 1 cm X 1 cm grid.
Place two chairs either side of the bed of nails and initially support the weight using these chairs.
Lightly place the bare feet on the bed of nails then slowly release the grip from the chairs to transfer the whole weight slowly onto the nails.
Initially the force required to burst a single balloon may be found.
This is compared to the case where the lecturers weight is distributed over about 300 nails.
12.3.3 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.
12.3.4 Weight and pressure
See: Weight, (Commercial).
See diagram 12.1.1: Weight and pressure
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.
2. 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.
3. Walking on sand wearing wide shoes is easier than wearing narrow shoes.
Snowshoes are worn to decrease the pressure on the snow.
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.
4. More force is needed 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.
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. A hypodermic needle penetrates flesh easily, because the end of the needle is very sharp.
A small push on the plunger produces a huge pressure on the flesh.
Also, the small area of the edge of the hypodermic needle restricts damage to the flesh.
6. Fishermen who practice the dangerous pastime of fishing from rocks near the sea in Sydney, Australia, always lift one leg when a wave breaks over the rock to increase the pressure of one foot on the rock.
Despite this caution, many get swept off the rocks and drown, so now some local governments requires fishermen to wear life vests.
12.3.5 Weight of a floating body
1. Fill an overflow can with water and let it run out until the surface is level with the spout.
Select a piece of wood that floats half or more submerged in the overflow can.
Weigh the piece of wood with a spring balance.
Weigh the catch bucket.
Put the catch bucket under the spout.
Put the wood block in the overflow can and note the balance reading.
Find the weight of the displaced water by subtracting the weight of the catch bucket from the total weight of catch bucket and water.
The weight of the water displaced is equal to the weight of the object.
12.3.6 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
12.3.7 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.
12.4.1 Loudness, threshold of hearing, audible limits
Loudness measures the human perception of sound.
A sound wave of high intensity is perceived as louder than a sound wave of lower intensity, but the sensation of sound is proportional to the logarithm of the sound intensity for most individuals.
Loudness level is defined by a scale corresponding to the sensation of loudness.
The zero on this scale = the sound wave intensity, Io = 1.00 × 10-12 W / m2, corresponding to the weakest audible sound.
The loudness level, beta = 10 log (I / Io).
The psychological reaction to intensity is loudness.
If the intensity of a sound is increased, the sound is perceived as louder.
Loudness, however, is strongly dependent on frequency.
If the physical intensity of a sound is kept constant and the frequency is changed, the resulting psychological loudness varies significantly.
Loudness increases as the size (or amplitude) of the sound vibrations increase.
12.4.2 Sound pressure level
Sound pressure level (SPL) is the pressure level of a sound, measured in decibels (dB).
Loudness (or sound pressure) is measured in decibels (dB).
30 dB has ten times more intensity than 20 dB.
40 dB has tens times more intensity than 30 dB.
In other words 40 db has 100 times more intensity than 20 dB.
Sounds consistently greater than 80 dB will damage your heating, however, home fire alarms may emit beeps of 85 dB when being tested and when the alarm sounds.
Sounds greater than 140 dB may burst your eardrums.
As sound travels from its source the amplitude decreases so the loudness decreases.
12.4.3 Threshold of hearing
The threshold of hearing, hearing threshold, lists the lowest sound level that can be detected by ear and the highest sound level that can be tolerated by humans.
The audible limits of humans is approximately frequency range 20 Hz to 20, 000 Hz
Some sound levels in decibels (dB):
Threshold of hearing 0 dB, is the reference level, but the average level is about 4 decibels, i.e. 2.5 X 10-12 watts/ m2.
Rustle of leaves 10 dB,
Whisper (at 1 m) 20 dB,
City street, no traffic 30 dB,
Office, classroom 50 dB,
Normal conversation (at 1 m) 60 dB.
Current prescribed decibel limit for licensed premises in State of Queensland:
75 dB, Jackhammer (at 1 m) 90 dB,
Rock band 110 dB,
Threshold of pain 120 dB,
Jet engine (at 50 m) 130 dB,
Saturn rocket (at 50 m) 200 dB.
The decibel (dB) has no dimensions.
Decibel, dB is the logarithmic unit used for human audibility measurements ranging from 1, just audible, to 120, just causing pain.
The linear scale ranges from 1 to 1012 change in sound pressure.
A doubling of sound pressure corresponds to 6 dB.
A doubling of sound loudness corresponds to a tenfold increase in sound pressure, 20 dB.
A different decibel scale is used for measuring the output of audio amplifiers in terms of intensity.
The normal ear can distinguish intensities down to about 1 dB.
12.4.4 Decibels dB, Sound pressure units (Pa)
0 dB: 2 × 10-5 Pa,
10 dB: just audible, the sound of falling leaves,
20 dB: empty broadcasting studio 2 × 10-4 Pa,
30 dB: soft whisper at 5 m,
35 dB: quiet library,
40 dB: bedroom, no conversation 2 × 10-3 Pa,
50 dB: very quiet, 55 dB: light traffic at 15 m,
60 dB: air conditioning at 6 m.2 × 10-2 Pa,
65 dB: normal conversation,
70 dB: light freeway traffic,
80 dB: annoying sound level 2 × 10-1 Pa,
85 dB: pneumatic drill at 15 m,
90 dB: heavy truck at 15 m,
95 dB: very annoying,
100 dB: 105 dB: jet plane take-off at 600 m,
110 dB: riveting gun close by,
115 dB: maximum vocal voice without amplification,
117 dB: discotheque at full blast,
120 dB: jet take-off at 60 m 2 × 10 Pa,
30 dB: limit of amplified speech,
135 dB: painfully loud,
140 dB: on aircraft carrier deck 2 × 102 Pa.
12.4.5 Musical scale, tones, sharps and flats, equal temperament scale
See diagram 26.3.2.3: Staff notation of the major diatonic scale
Often people use the word "musical sound" for something they want to hear, and "noise" for what they do not want to hear.
A tuning fork emits an almost pure note of one frequency.
Musical sound is made up of superposition of a set of fundamental and harmonics with different frequencies and amplitudes according to certain law.
For example, consider two sounds, one a mixture of harmonics (frequencies related by integer ratios) and the other a mixture of frequencies with no integer relationship among them.
The first sound will result in an identifiable pitch, that of the fundamental frequency, and is called a musical sound.
The second sound, i.e. noise, will have a much different quality, so different that it may not even have an identifiable pitch.
Thus the difference between music and noise is a gross example of quality.
The sound, transitory and declined quickly is an explosion.
A series of musical notes is called a scale.
The pitch interval for most musical scales is the octave.
In European music the octave is divided into seven unequal parts called the major diatonic scale.
The lowest note is called the keynote.
The eight notes are shown by letters.
Pitch intervals are called major tone, minor tone and limma.
The note called middle C, 264 hertz, is in the middle of the piano keyboard.
The Chinese music scale is divided into fewer parts.
Tuning forks used in science do not match the musical frequencies, e.g. A 440 Hz is a musical note, A 426.7 Hz is the scientific note.
The scientific tuning scale is arranged around middle C, 256 Hz, but (C is 261.6 Hz on the musical scale.
The C notes of the scientific scale are all multiple of 2, so the first C above middle C is 512 Hz
Table 26.3.2.3
| Note |
"middle" C |
D |
E |
F |
G |
A |
B |
C |
| Relative frequency |
24 |
27 |
30 |
32 |
36 |
40 |
45 |
48 |
| Actual frequency, Hz |
264 |
297 |
330 |
352 |
396 |
440
pitch standard |
495 |
528 |
| Pitch intervals |
C: D 27/24 = 1.125 |
D: E 30/27 = 1.111 |
E: F 32/30
= 1.066 |
F: G 36/32 = 1.125 |
G: A 40/36 = 1.111 |
A: B 45/40
=
1.125 |
B: C 48/45
= 1.066 |
- |
| Tone |
9 to 8 Major tone |
10 to 9 Minor tone |
16 to 15 Limma |
9 to 8 Major tone |
10 to 9 Minor tone |
9 to 8 Major tone |
16 to 15 Limma |
- |
The difference between a major tone and minor tone, called a comma, = (9 / 8) / (10 / 9) = (81 / 80).
The difference between a minor tone and a limma, called a diesis, = (10 / 9) / (16 / 15) = (25 / 24).
Additional notes to the major diatonic scale, called sharps and flats, raise or lower a note by a diesis.
Frequency of A sharp, A#, = 440 × (25 / 24) = 458.3 Hz.
Frequency of A flat, Ab, = 440 × (24 / 25) = 422.4 Hz.
Frequency of G# = 396 × (25 / 24) = 412.5 Hz.
Frequency of Gb = 396 × (24 / 25) = 280.2 Hz.
The scale of 22 notes containing all the sharps and flats is called the chromatic scale.
In an equal temperament scale the interval between aa note and its octave is divided into equal pitch intervals, e.g. twelve tone scale divides by 12 to give interval = x.
Pitch interval of octave = 2, so x12= 2, 12 log x = log 2, x = 1.059 (similar to a limma, 16 / 15).
This equally tempered scale based on A = 440 Hz pitch standard.
Note cases where similar note become mistuned to become the same frequency, e.g. G# and Ab.
Table 26.3.2.3.1
| Note |
C |
C# Db |
D |
D# Eb |
E |
F |
F# Gb |
G |
G# Ab |
A |
A# Bb |
B |
C |
| Frequency |
261 |
277 |
294 |
311 |
330 |
349 |
370 |
392 |
415 |
440 |
466 |
494 |
522 |
Experiments
1. Use tuning forks in different frequencies to create chords.
Specific ratios between notes are in harmony.
Between G and C is a 3/2 ratio, called a fifth.
Between E and C is a 5/4 ratio, used in the C major chord.
Between C and A is a 6/5 ratio used in the A minor chord.
All octaves, e.g. middle C and the next C above middle C, are separated by doubled the frequency.
2. Tuning an instrument by holding the tuning fork to the body of the instrument while it is tuned by ear.
3. Tune a stringed instrument by striking the note and listening for beats as the sound from the instrument interferes with the sound from the tuning fork.
The beats occur less as the frequencies match.
12.4.6 Musical instruments
Woodwind instruments include the bassoon, clarinet, alto saxophone, cor anglais, oboe, flute, piccolo and alto and bass versions of some of the previous examples.
Woodwind instruments have a long column of air with lowest note played with all the tone holes closed, so the column is longest.
To shorten the column holes are opened, starting from the open end.
Air flow into the instrument is controlled by an air jet for flutes family and cane reeds for the other woodwind instruments
Aerophone (column of air vibrates to make the sound), e.g. flute, trumpet, pipe organ, Australian aboriginal didgeridoo.
Denser, harder timbers produce a better tone as the sound wave travels more easily through them.
As an aerophone is played the air in the resonating column becomes warmer and moister.
Humid air is less dense than dry air.
Warm air is also less dense than cool air.
With this double drop in the density of the air, the notes become sharper or slightly higher.
As the reed warms and moistens, more air is allowed to escape from the column and hence it becomes less dense and the notes become sharper or slightly higher.
2. Chordophone (strings are vibrated), e.g. violin, guitar, piano.
As the strings warm their tension cases and the notes become flat or slightly lower.
3. Idiophone (percussion instrument), e.g. cymbals, paired sticks, lagerphone.
4. Membranophone (a membrane is vibrated), e.g. drums.
Investigate how different notes are produced on the different instruments.
Look for a resonator or vibrating system for each instrument.
5. The sound vibration may be free or maintained.
Free vibration occurs when just one sound is made and then allowed to die out in time, e.g. plucking a guitar string or tapping a tuning fork once.
Maintained vibrations occur when the vibrations making the sound are continued, e.g. scraping a bow across the strings of a violin and playing a long note on a bugle.
6. Notes are created by altering the length and/or mass of the string, or by altering the length of the air column.
The energy comes from the hands, arms or breath of the player.
The resonator is the string, column of air or membrane.
7. Sounds are radiated by a variety of means.
Stringed instruments use the timber body of the instrument, some woodwind and brass instruments have bell shaped openings that radiate the sound, flutes and piccolos use their holes and hence
produce very little sound.
Each of these methods is called an impedance resonator.
Percussion instruments do not need impedance resonators since the object hit is large enough to move enough air to produce an audible sound.
12.4.7 Wine glass resonance
See diagram 26.3.3.1: Singing glass
When a wine glass vibrates the upper rim changes shape from circular to elliptical twice per cycle.
The frequency of sound is exactly the same if the wine glass is tapped or rubbed.
The frequency is inversely proportional to the internal radius squared.
The rubbed glass produces standing waves with one node near the point of contact of the finger.
A wine glass containing water does not behave like a closed pipe or a cylindrical glass tube over water.
For example if water is added to the wine glass the pitch is lower, because the water adds mass.
However, if an identical wine glass differs only in having thicker glass, the pitch is higher.
Experiments
1. Use a good quality wine glass.
Hold firmly the base of a wine glass to the table with one hand.
Wet a finger on the other hand then slowly wipe around the rim of the wine glass.
Gradually change the speed of rotation until a continuous ringing sound is heard.
Observe the vibration on the surface of the water.
Feel your finger gripping the rim of the glass as you rotate your finger.
Note what you feel when you reach the resonant frequency of the glass.
2. Use two similar thin walled glasses, e.g. wine glasses, on a table 2 cm apart.
Rub your clean finger around the rim of one glass until you hear a humming ("whining") sound.
The second glass will also start to vibrate and produce a sound.
To see the second glass vibrating, place a very thin wire across the rim of the second glass or put the same amount of water in both glasses and observe the surface of the water in each glass.
The second glass resonates with the first glass.
The pitch, note, produced by the two glasses are the same.
3. Clean your hands and place two clean wine glasses on a table.
Hold one wine glass tightly with a hand and make it touch the tabletop tightly.
Put a drop of vinegar on the index finger or thumb of another hand then rub the wine glass very slowly with the finger.
You can hear the sound from the wine glass.
Pour water into the wine glass then rub it again.
The pitch of the sound will change.
The finger is the vibrating source as it jolts over the surface of the glass due to friction.
If your finger is greasy it just slides over the glass and no sound is produced.
The wine glass is like a resonance box.
Rubbing causes the resonance.
Thus the pitch of the sound depends on the wine glass.
If you pour water into the wine glass, the mass increases and the pitch of the sound produced decreases.
Soldiers marching across a bridge in step can cause the bridge to vibrate violently if the frequency of their steps coincided with its natural frequency.
So when approaching a bridge the officer in charge should order "Break step!" so that the soldiers do not keep in step and cause a dangerous vibration.
4. Place a clean wine glass near a horn loudspeaker.
A light mirror acts as a detector of resonance.
Adjust the frequency of the signal generator through the 1000 to 1600 range until a circular Lissajous figure projected on the screen indicates resonance.
Increase the amplifier output until the wine glass explodes, because of the rim oscillations.
Tuning forks were formerly tuned by using Lissajous figures by shaving down excess length of fork to bring down to the correct frequency.
Lissajous figures are still used to analyse electromagnetic oscillations in LRC circuits.
Formerly, they were produced by tuning forks reflecting light pointed at two mirror-loaded forks vibrating at 90o.
Frequencies in ratio form Lissajous figures in different shapes.
5. Make a wine glass sing a pure tone by rubbing your degreased and wetted finger around the rim.
Vibrations are set up in the wall of the glass and resonance occurs in the air column.
Increase the volume of water inside the glass to change the frequency of the sound.
The pitch is lowered when you add water to the glass.
Compare resonance in a singing wine glass with resonance in a closed pipe.
Filling the glass with water would imply that the note goes up, not down.
However, the length of a wine glass is very short, so the frequency might be so high you just don't hear that note.
Observe whether the pitch proportional to the circumference, the diameter of the glass or the amount of liquid in the glass, the thickness of the glass, volume of water, height of water, percentage of water from the top or bottom of the glass, temperature.
Compare liquids of different density or viscosity, non-polar liquids, e.g. hexane with polar liquids, e.g. ethanol
Record the sound on a CRO and work out the frequency.
Try four variations.
The last variation has a solid column in the glass so there is less water, but the same water level.
12.4.8 Wine glass breaks with voice resonance
Hit an empty wine glass to make a "ping" sound.
The frequency of the "ping" is the natural (resonant) frequency for the wine glass to vibrate.
The louder the sound, the more violent the vibrations.
Put a short drinking straw in the wine glass held close to the mouth and sing the ping note.
Girls can usually sing the note easily, but boys may have to use falsetto.
When the drinking straw starts to vibrate sing the same note louder.
Getting the right note is difficult, so start lower singing "ee", then sing higher then lower until the straw vibrates then the wine glass vibrates violently and breaks.
The breakage works better if the sound is concentrated placing the glass behind a screen with a hole in it or by using electronic equipment to amplify the sound.
Usually more than 100 decibels are needed to break a wine glass.
The breakage is caused by the widening of tiny unseen cracks in the glass.
12.4.9 Stationary waves in Chinese temple "fish wash" dishes
The "fish wash" can still be seen in ancient Chinese temples.
It is a basin made of brass with two circular holders on each side of it, called "wash ears".
The name "fish wash" came from the picture of fish at the bottom of the "wash".
The fish wash can be used for an example of the phenomenon of resonance.
Fill the fish wash with water, wash your hands clean, then rub with two hands on the top of the "wash ears".
When your hands rub synchronous back and forth on the wash ears, the fish wash can buzz loudly and form sprays.
If you can rub continuously enough, the spray jump very high as if to spout though from the mouth of the fish.
As you rub the wash ears with your hands, the fish wash can produce vibration with the frequency of rubbing.
When the frequency of vibration caused by rubbing is equal to or near to the natural frequency of the vibrating object, the brass wall of the fish wash produces resonance, the amplitude increases rapidly.
However, due to the limitation of the bottom of fish wash, the vibration produced by it can be spread out.
Then the incident wave and reflected wave pile up each other at the wall of the fish wash to form a stationary wave.
The point of maximum amplitude is at an antinode, the point of minimum of amplitude is at a node.
It is easiest to produce a low resonance frequency by rubbing an object like a circular basin, i e. a vibration consisting of four antinodes and four nodes.
The place at maximum amplitude is on the wall of the wash can stimulate the surface of water immediately to form the spray.
As the four antinodes act in the meantime, there appears the spray splashing in all directions.
If you paint four fish at the places where the amplitudes are maximum on the wall of the fish wash, the spray comes as though from the mouths of the fish.
12.4.0 Gas, Molecular weight, Density, (Table 1)
Gas
|
Formula
|
Molecular
weight |
Density, 20oC
NTP, g/L |
Density, 0oC
STP, g/L |
| Air |
- |
29 |
1.205 |
1.2929 |
| Ammonia |
NH3 |
17.031 |
0.717 |
0.771 |
| Argon |
Ar |
39.948 |
1.661 |
1.7837 |
| n-Butane |
C4H10 |
58.1 |
2.489 |
2.672 |
| Carbon dioxide |
CO2 |
44.01 |
1.842 |
1.977 |
| Carbon monoxide |
CO |
28.01 |
1.165 |
1.250 |
| Chlorine |
Cl2 |
70.906 |
2.994 |
3.214
|
| Ethane |
C2H6 |
30.07 |
1.264 |
1.356
|
| Ethene, ethylene |
C2H4 |
28.03 |
1.260 |
1.260 |
Ethyne, acetylene
|
C2H2 |
26 |
1.092 |
1.171 |
| Helium |
He |
4.02 |
0.1664 |
0.1785 |
| Hydrogen |
H2 |
2.016 |
0.0899
|
0.0899 |
| Hydrogen chloride |
HCl |
36.5 |
1.528 |
1.639
|
| Hydrogen sulfide |
H2S |
34.076 |
1.434 |
1.538
|
| Methane |
CH4 |
16.043 |
0.668 |
0.7165 |
| Natural gas |
- |
19.5 |
0.7 to 0.9
|
0.862
|
| Neon |
Ne |
20.179 |
0.8999
|
0.902 |
| Nitric oxide |
NO |
30.0 |
1.249 |
1.251
|
| Nitrogen |
N2 |
28.02 |
1.165 |
0.716
|
| Nitrogen dioxide |
NO2 |
46.006 |
- |
- |
| Nitrous oxide |
N2O |
44.013 |
.
|
1.977
|
|
| Oxygen |
O2 |
32 |
1.331 |
1.429 |
| Ozone |
O3 |
48.0 |
2.14
|
- |
| Propane |
C3H8 |
44.09 |
1.882 |
2.019
|
| Propene |
C3H6 |
42.1 |
1.748 |
- |
| Sulfur dioxide |
SO2 |
64.06 |
2.279 |
2.927 |
Water vapour (steam)
|
H2O |
18.016 |
0.804 |
- |
12.5.2 Pressure safety
1. A can of beans on a stove is potentially lethal, because the it can explode with force proportional to the size of the can.
Larger cans without a weakened pop open top may explode more violently than smaller cans
2. A 23 L water heater without functioning safety features may rupture, but not explode like a rocket.
A similarly disabled 110 L water heater may explode, sending it into the air.
A similarly disabled 200 L water heater may explode 150 m through the roof.
12.5.1 Saturation Vapour Pressure, (SVP)
NTP, Normal Temperature and Pressure is air at 20oC (293.15 K and 1 atm (101.325 kN / m2, 101.325 kPa, 30 inches Hg, 760 torr).
The NTP of dry air has a density of 1.204 kg / m3.
STP, Standard Temperature and Pressure is air at 0oC (273.15 K, and 1 atm (101.325 kN / m2, 101.325 kPa, 30 inches Hg, 760 torr).
The STP of dry air has a density of 1.275 kg / m3.
Standard sea level atmospheric pressure = 101 325 Pa
Air density, Kg / m3 = [1.2929 × 273.13 × (AP - ( SVP × RH ) / ( T + 273.13)] × 760
Where: T = oC, AP = Absolute Pressure, mm of Hg,
SVP = Saturation Vapour Pressure of air over water at temperature T,
RH = Relative Humidity (decimal), High air density readings indicate more oxygen in the air.
Saturation vapour pressure over water, (Table 2}
| oC |
kPa |
oC |
kPa |
oC |
kPa |
oC |
kPa |
oC |
kPa |
| 0 |
0.611 |
.
|
.
|
.
|
.
|
.
|
.
|
.
|
.
|
| 1 |
0.657 |
11
|
1.312 |
21
|
2.487 |
31
|
12.495 |
41
|
7.382 |
| 2 |
0.706 |
12
|
1.402 |
22
|
2.644 |
32
|
12.758 |
42
|
8.206 |
| 3 |
0.758 |
13
|
1.497 |
23
|
2.810 |
33
|
5.033 |
43
|
8.647 |
| 4 |
0.813 |
14
|
1.598 |
24
|
2.985
|
34
|
5.323 |
44
|
9.109 |
| 5 |
0.872 |
15
|
1.705 |
25
|
3.169
|
35
|
5.627 |
45
|
9.591 |
| 6 |
0.935 |
16
|
1.818 |
26
|
3.362 |
36
|
5.945 |
46
|
10.095 |
| 7 |
1.002 |
17
|
1.938 |
27
|
3.567 |
37
|
6.280 |
47
|
10.622 |
| 8 |
1.073 |
18
|
2.064 |
28
|
3.781 |
38
|
6.630 |
48
|
11.173 |
| 9 |
1.148 |
19
|
2.197 |
29
|
12.007 |
39
|
6.997 |
49
|
11.747 |
10
|
1.228 |
20
|
2.338 |
30
|
12.245 |
40
|
7.382 |
50
|
12.347 |