4. Density and pressure

It turns out that 1cm of H2O has a mass of 1g. How does that convert into Kg/m3?

How many cm3 are in 1m3 ?

Why? Cos that is how many grams are in 1m3, you figure it out! So how many Kg/m3?

The specific gravity, swater = 1, obviously!

Definitions and units.

in physics because we deal with life size objects which are best measured in kg and meters we use the units kg/m3

in chemistry the measurements of reactions are usually (in a lab) smaller so the prefferred units are g/cm3


This is excellent introduction to pressure

A student holding a school bag with the strap and then with a string attached. What was the difference

Why does a tractor have large wheels

What would happen if you cycled a racing bike across a muddy field?

Why do mountain bikes have wider wheels and tyres?

What other construction machinery do they use?

clue - whats a young butterfly ??

Calculate the pressure exerted by your science book on the bench. To do this you will need to use the equation for pressure and for weight and calculate the area of the cover of the book

Pressure in liquids and gases.

In this case the force is caused by the weight of the mass above the point at which we are concerned about the Pressure.

A manometer is a small device that can be used to measure pressure,

the following link is a great explanation on how to use them

Boyle’s law.

Archimedes’ principle.

Arch..... Apparent wgt loss (upthrust) = wgt of displaced water

Float.....Apparent wgt loss (up thrust) = wgt of displaced water = wgt of item

more vids here

Law of flotation.

Demonstration of atmospheric pressure,

e.g. collapsing-can experiment.

Appropriate calculations.

Demonstration only.

  1. Take a soft drinks can
  2. Pour a little water in the bottom of the can.
  3. Heat it up on a bunsen or a hotplate
  4. When a constant cloud of steam is coming out
  5. Using a beaker tongs, held in back hand grip,
  6. quickly transfer the can to a basin of cold water
  7. observe

Calculations not required.

Atmospheric pressure and weather.

1 athmosphere (or the standard athmospheric pressure is 101,325 Pa

1 psi = 6.894×103 Pa

Calculate how much force this is pushing down on your physics book with.

the Irish weather forecast service do the usual old predictions and weather warnings, but in the following link you might be able to see how they predict the weather using the pressure,

btw H = high pressure, and L = low pressure

Weather forcasting and lots more about the sea

Winds blow away from a high pressure zone

barometers ?

A barometer is an instrument for measuring atmospheric pressure, thus in the 19th century they were used to predict the local weather. An increase in athmospheric pressure meant that the weather was improving and vica versa.

The “bends” in diving, etc.

The Bends


The Ideal Gas Law by Walter-Fendt

The single particle 1d gas & pressure from Fowler

Particles at diff heat!

Boyle’s law.

Boyles Law states when the temperature is held constant the Volume of a Gas is Inversely Proportional to the Pressure on it.

Big Pressure -> Small Volume

Small Pressure -> Big Volume

at a constant temperature

When carrying out an experiment to verify Boyles law, You change the Pressure and observe the change in Volume.

In order to represent this on a graph, one of the values should be put as the reciprocal like 1/Pressure or 1/Volume. Only one should be inverted.

Make a table out with P V & the chosen reciprocal. Plot your graph carefully, the inverted variable might be difficult to manage, follow the graphing rules.


A siphon (also spelled syphon) is a continuous tube that allows liquid to drain from a reservoir through an intermediate point that is higher, or lower, than the reservoir, the flow being driven only by the difference in hydrostatic pressure without any need for pumping. It is necessary that the final end of the tube be lower than the liquid surface in the reservoir.


Liquids can rise over the crest of a siphon because gravity pulls on the greater weight of the liquid in the longer outlet leg, allowing the liquid to flow to a lower potential energy state. Siphons can be most easily understood using the conservation of energy. If given the opportunity, liquid will flow downward with gravity to attain a lower energy state. In a siphon, the liquid first rises over a barrier, temporarily increasing its potential energy, so that it can then flow down to a level lower than its starting point, experiencing a net decrease in energy.

An occasional misunderstanding of siphons is that they rely on the tensile strength of the liquid to pull the liquid up and over the rise. While water has been found to have a great deal of tensile strength in some experiments (such as with the fascinating z-tube [5]), and some siphons may take advantage of such cohesion, common siphons can easily be demonstrated to need no liquid tensile strength at all to function. To demonstrate, the longer lower leg of a common siphon can be filled almost to the crest with liquid, leaving the top and the shorter upper leg containing only air at ambient pressure. When the liquid in the longer lower leg is allowed to fall, it will cause a partial vacuum at the top of the siphon, resulting in the liquid in the upper reservoir being pushed up into the partial vacuum by atmospheric pressure acting on the upper reservoir. The liquid will then typically sweep the air bubble down and out of the tube and continue to operate as a normal siphon. As there is no contact between the liquid on either side of the siphon at the beginning of this experiment, there can be no cohesion between the liquid molecules to pull the liquid over the rise. This demonstration may fail if the air bubble is so long that as it travels down the lower leg of the siphon it displaces so much liquid that the column of liquid on the longer lower leg of the siphon is no longer heavier than the column of liquid being pushed up the shorter leg of the siphon.

Once started, a siphon requires no additional energy to keep the liquid flowing up and out of the reservoir. The siphon will pull the liquid out of the reservoir until the level falls below the intake, allowing air or other surrounding gas to break the siphon, or until the outlet of the siphon equals the level of the reservoir, whichever comes first. Energy is conserved because the ultimate drain point is lower than the liquid level of the reservoir.

The maximum height of the crest is limited by atmospheric pressure, the density of the liquid, and its vapour pressure. When the pressure within the liquid drops to below the liquid's vapor pressure, tiny vapor bubbles can begin to form at the high point and the siphon effect will end. This effect depends on how efficiently the liquid can nucleate bubbles; in the absence of impurities or rough surfaces to act as easy nucleation sites for bubbles, siphons can temporarily exceed their standard maximum height during the extended time it takes bubbles to nucleate. For water at standard atmospheric pressure, the maximum siphon height is approximately 10 m (33 feet); for mercury it is 76 cm (30 inches).


A rough analogy to understand siphons is to imagine a long, frictionless train extending from a plain, up a hill and then down the hill into a valley below the plain. So long as the valley is below the plain, the part of the train on the valley side of the hill will be longer and heavier than the part on the plain side of the hill, so the portion of the train sliding into the valley can pull the rest of the train up the hill and into the valley. What is not obvious is what holds the train together when the train is a liquid in a tube. In this analogy, ambient atmospheric pressure and the intermolecular forces within the liquid hold the train together. If the train tries to crest a hill that is too high, the weight of the train will exert a force that exceeds the strength of the couplings between the train cars, causing them to break. This is equivalent to the pressure at the top of the siphon dropping below the liquid's vapor pressure, where the ambient atmospheric and intermolecular forces are no longer strong enough to keep the molecules in the liquid phase, and thus vapor bubbles will begin to form breaking the siphon. (Note that typical liquids have vapor pressures much lower than atmospheric pressure, and thus it often a good approximation to simply consider when the pressure in the liquid drops below zero.) The train analogy is demonstrated in a "siphon-chain model" [6] where a long chain on a pulley flows between two beakers

How to make a cartesian DIver

To Demonstrate Pressure is Inversely Proportional to Volume in a Gas

taken from "Physics on stage 3"

Boyles Law states when the temperature is held constant the Volume of a Gas is Inversely Proportional to the Pressure on it.

Big Pressure -> Small Volume

Small Pressure -> Big Volume

  1. Place a cylindrical marshmallow inside a large plastic syringe
  2. Insert the plunger to the half way mark.
  3. Place your thumb over the end of the nozzle.
  4. Push the plunger in, observe.
  5. Stop pushing the plunger, observe.
  6. Pull out the plunger and observe.

The Marshmallow is a sugary blob with lots of air bubbles in it. When the plunger is pushed in the gas is under pressure. This makes the volume (size) of the gas get smaller, including the bubbles of gas in the marshmallow, this makes the marshmallow get smaller.

The opposite effect can be seen using the wine stopper vacuum pump.

To Demonstrate Athmospheric Pressure

taken from "Physics on stage 3"

  1. Attach a string (about 1m) to a CD with a paper clip,
  2. Feed the string through a sheet of newspaper (preferably a broadsheet)
  3. Place the newspaper flat on the ground.
  4. Give the string a tug as to lift the CD

What happens?

Did you expect this?

Why do you think this happens.

Egg in bottle,

you can use a water ballon, taken from "Physics on stage 3"

  1. Fill a water balloon with enough water as so the balloon is bigger than the opening of the bottle
  2. Pour some boiling water in the bottle (make sure the bottle is not cold).
  3. Place the water balloon over the mouth of the bottle.
  4. Wait and observe.

What happened?

Why did this happen?

It happens because the air around us has a mass and therefore a weight, so it applys a force on everything below it. The athmosphere goes up about 5000m so all that air is lying on top of us!