# Pressure

measuring pressure, everyday applications of pressure

OP10 understand the relationship between pressure, force and area; perform simple calculations using this relationship

factors affecting pressure; pressure in fluids

1 Pascal = 1 Newton per meter squared

1P = 1Nm-2.

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

What do you think would make the most comfortable school bag?

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 ??

Solve the following problems

1. What Pressure would a boy of 600 N cause on a 1m2 slab
2. The same boy jumps onto a 0.5 m2 slab what pressure would he cause.
3. The same boy gets on a bike of 300N that has a contact area of 0.0001 m2 what is the pressure now?
4. The boy goes Ice skating on blades that are 0.1 m x 0.005 m each what pressure does he put on the ice.

Who puts more pressure on the ground, an elephant of Weight 32,000N with each foot having a 450 cm2 or a woman of weight 600N on high heels that have an are of 2.5cm2

OP11 investigate the relationship between pressure and depth for a liquid

units are still pascals

If we test how far water travels from different heights,

we see that the lower holes shoot the water further, this is because the amount of water above the hole is more massive.

Pressure increases as we go deeper in a fluid

P = pressure

p = density

g = gravity

h = height

Calculate the Pressure of the following

Density of water is 1000 kg/m3

Density of oil is 800 kg/m3

Gravity = 10 m/s2

Height = 30 m

Gravity = 10 m/s2

Height = 40 m

Take a tall vessel like a milk carton or a straight edged mineral bottle. and a ruler

Place 3 holes in it at different heights

At one of the heights you could also put a 2nd hole on the opposite side.

Cover each of the holes with Blu Tack or some similiar putty.

Fill the vessel to the top, Place the ruler under the hole.

Remove a piece of blu tack from one hole, measure how far the water gets shot.

Recover the hole and Refill the bottle, repeat for a different hole.

The Water goes further from the Lowest hole ..... because the Force increases as there is more water above (the weight of water above the hole is greater).

OP12 show that air has mass and occupies space

air has mass and occupies space

Mass of empty balloon = g

Mass of Air filled balloon = g

_______________________________

Mass of Air = g

Can a balloon lift a glass?

put balloon in a dry glass, blow it up, & lift carefully

OP13 understand that the atmosphere exerts pressure and that atmospheric pressure varies with height

atmospheric pressure and its relationship to weather

Met.ie

Syphon

A siphon 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.

### Theory

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).

### Analogy

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

OP14 examine weather charts to observe variations in atmospheric pressure and relate these to weather conditions

the Irish weather forecast service Met.ie do the usual 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

http://www.met.ie/forecasts/atlantic-charts.asp

Isobars are lines of equal atmospheric pressure drawn on a meteorological map.

High Pressure on a Weather map means there will be few clouds, warm in the day but cold at night

Low Pressure on a Weather map means cloudy, which means there is a possibility of rain. Cold days but warm nights.

When a Low or High pressure is over you, there will be no wind !

When the Isobar lines are close together, we get windy conditions (always from High Pressure to Low Pressure). When they are spread out the wind is less strong.

live weather charts

http://www.sat24.com

The following site has predictors on the weather forcasting,

http://www.wunderground.com/

local weather

the weather at dunsink observatory

http://weather.dias.ie/

check out the graphs, the past but definitly the live stuff ..... crazy good

## http://earth.nullschool.net/ A Global map of wind speed right now !!

some great links here too but more for younger children

http://www.met.ie/education/

### 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!

the disk hovercraft

Balloon

CD

glue

Use the glue to stick the cd to the spool

Inflate balloon

hold and attach over the spool

release and observe

food service tinfoil tray

bendy straw

scissors

paperclip,

blu-tak

Cut out diver shape from foil

Cut the straw about to about 1cm either side of the bend

Use the paper clip to hold both sides of the straw to the diver.

Add blutak to the divers feet but only so much as the diver will still float

put in 2l (or bigger) fizzy drinks bottle, filled with water

put on lid tightly

gently squeeze bottle & observe

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

Moving air,

see expt on blowing btw table tennis balls

table tennis ball in hair dryer stream/ water stream

aerofoil / spoiler

berrnolis

OP26 investigate the effect of pressure on the boiling point of water

We witnessed that as we lowered the pressure in the bottle, by removing the air, the water boiled. The temperature of the water was less than 100oC.

If the pressure is lowered the boiling point is lowered too. If the pressure is increased the boiling point increases too.

A pressure cooker increases the pressure, thus the temperature of the water, so the heat required to cook the food is delivered in a shorter time

live weather charts

http://www.sat24.com

The following site has predictors on the weather forcasting,

http://www.wunderground.com/

local weather

the weather at dunsink observatory

http://weather.dias.ie/

check out the graphs, the past but definitly the live stuff ..... crazy good