Class practical of air

Observing that air springs back when the pressure on it is released.

Apparatus and Materials

For each student group

• Plastic syringe
• Bicycle pump

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

1. Close the outlet of the syringe with a finger, and compress the air in the syringe. Notice its springiness as the force on it is increased or decreased.
2. Repeat with a bicycle pump.

Teaching Notes

• Take care: syringes can easily be filled with water! Whilst this will demonstrate that liquids are less compressible than gases and any air bubble remaining will shrink in size it might not be what the unwary teacher had in mind!
• Gases are squashy. Compressing a plastic syringe full of air allows you to feel its springiness as the piston bounces back as soon as the force is released.
• Questions to ask:
• How could you tell whether it is the air that makes it difficult to press the piston down, (the air that they can feel pressing back) or the piston rubbing the side of the barrel?
• What makes the air press on things?
• Gases can exert a considerable pressure on anything that holds them. How do they exert that pressure? What is pressure?
• Simple pressure-related activities:
• Shut your mouth and puff out your cheeks and feel your cheeks with your fingers.
• Feel the lightly compressed air in your chest driving out through your mouth and nose when you breathe out.
• Hear carbon dioxide bursting out from a bottle of lemonade.
• Feel gas escaping from a cylinder and listen to it.

The experiment was safety-checked in July 2007

Demonstration

Discussing the effect of area on pressure.

Apparatus and Materials

• Evesham pressure apparatus (see technical note 1)
• U-tube, 1.5 m tall, mounted on a board (see technical note 2)
• Slotted bases, 2
• Masses, 0.5 kg , 5
• Food colouring

Health & Safety and Technical Notes

Read our standard health & safety guidance

The Evesham pressure apparatus has two movable platforms – the smaller is 10 cm x 10 cm, the larger 20 cm x 20 cm (see diagram below). There are outlets at either end of the apparatus. When the air pressure inside the apparatus is increased, the platforms rise. Weights placed on top provide a pressure to balance the air pressure. Knowing the weights, and the areas of the platforms, you can calculate the pressure.

The 1.5 m manometer is filled with water coloured with a few drops of food colouring to make it clearly visible to the class. It is set up vertically using two slotted bases to hold it.

Procedure

1. Connect one of the outlets from the apparatus to the manometer with a 1 m length of rubber tubing.
2. Connect the other outlet to another 1 m length of tubing into which you can blow. Use a disposable mouthpiece.
3. Put one 0.5 kg mass on each platform.
4. Blow into the tube. The larger platform will rise. Note the difference in levels in the manometer at which the platform starts to rise. Warn students not to suck.
5. Repeat with two 0.5 kg masses on the larger platform, and still only one on the smaller platform. Again, the larger platform rises first. The manometer difference is again noted when the platform starts to rise. About twice the difference is obtained.
6. Repeat using three 0.5 kg masses on the larger platform, arranged as symmetrically as possible. Again, the larger platform rises first.
7. Finally put four 0.5 kg masses on the large platform with still the one 0.5 kg mass on the smaller platform. Both now rise together, showing that the pressure on each is the same.

Teaching Notes

• This is not a perfect demonstration but it helps to connect the idea of pressure as force/area with the behaviour of a working gauge.
• It is difficult to produce a reliable and accurate piece of equipment to teach the concept of pressure.
• You can estimate the pressures in a U-tube as follows. Attach the U-tube to a squashable reservoir of water (e.g. a polythene bag or a balloon filled with water). Place a square block on top of the reservoir with a load on top of the block. Since both force (load) and area (of the block) are known, you can calculate the pressure in the water (force/area). In practice, tensions in the envelope apply extra forces and make the observed pressure quite different from the simple one expected. However, it may lend qualitative support to the idea that a manometer (vertical tube of liquid) is measuring something like force/area.

The experiment was safety-checked in July 2007

Class practical

Feeling forces with two nylon syringes of different sizes connected together.

Apparatus and Materials

• Syringe, nylon, large
• Syringe, nylon, small
• Plastic tubing to connect the syringes

Health & Safety and Technical Notes

Read our standard health & safety guidance

If the syringes need to be disconnected, the wrong method is to pull on the plastic tubing. This merely tightens the tubing around the end of the syringe and breakage is likely. The correct method is to push the tubing towards the syringe and it will come apart easily.

Procedure

1. Fill the syringes and connecting tube with water.

2. One student holds one syringe, while another student holds the other. They try pushing water to and fro.
3. Change over so that the one who had the larger syringe now has the smaller one.
4. Hold both syringes so that you can feel how the forces differ.

Teaching Notes

• The syringes can be air filled or water filled.
• One student should try pushing the in-going piston while another feels the out-going piston pushing out.
• The two syringes have different areas of cross section but students will not know about the difference of force between the two until they take both syringes into their own hands and feel what happens. Bigger forces acting over bigger areas create the same pressure as a small force on a small area. This is the principle of the hydraulic press.
• Simple numerical examples are now needed to reinforce the discussions that pressure is equal to force/area. For example:
• A mass of 2 kg in the earth's gravitational field of 10 N/kg will have a force on it of 20 N. It will create a pressure on an area of 4 m ² of 5 N/m ²
• - if the pressure is 40 N/m ² then the force on 2 m ² would be 80 N.

The experiment was safety-checked in July 2007

Demonstration

Using U-tube manometers to measure the gas supply pressure.

Apparatus and Materials

• U-tube manometer, 0.5 m tall
• Slotted base
• Methyl orange
• Length of Bunsen tubing
• 0.5 metre rule
• Trap (to prevent the water getting into the gas supply)

Health & Safety and Technical Notes

Ensure the lab is well ventilated.

A thin film of oil on top of the mercury will prevent the vapour escaping.

Read our standard health & safety guidance

The 0.5 m manometer is filled with water coloured with a few drops of methyl orange to make it clearly visible. It is set up vertically using the slotted base to hold it.

You should place a trap between the gas tap and manometer to prevent water from getting into the gas supply.

Procedure

1. Connect the manometer to the gas supply by Bunsen tubing, via the trap.
2. Turn the gas tap on, slowly at first, and keep it on. Measure the difference in levels in the manometer arms.

Teaching Notes

• Before the gas tap is connected, the levels of the water in both arms of the manometer should be the same, showing that atmospheric pressure pushes down equally on both columns of water. When the gas tap is connected to one arm and the gas is turned on, one arm has a downward pressure due to the atmosphere and the gas and the other arm has a downward pressure due to the atmosphere only. The difference in water levels is caused by the difference in pressures on the water in each arm.
• If both arms are connected to the gas supply, then the level of water in each tube is once again the same and so the pressure difference is zero.
• The manometer tube can be filled with oil or mercury to show that the gas still produces a difference in the level of the liquid but the value is greater in the case of oil and less in the case of mercury. "Has the gas pressure changed? No! The gas company is unaware of the instrument which is connected". However, pressure is measured as force/area and the weight of extra liquid supported by the gas pressure is equal to the volume of extra liquid (related to height of liquid) x density of the liquid.

The experiment was safety-checked in July 2007

Class practical

Apparatus and Materials

For each student group

• Mounted U-tube, 3 m tall (see Technical note 1)
• Mouthpiece tubes, disposable, 2 (see Technical note 2)
• Food colouring
• Metre rule
• Trap (to prevent the water being 'sucked' into the mouth)

Health & Safety and Technical Notes

Read our standard health & safety guidance

The large U-tube manometer is made of clean transparent plastic tubing, secured to a board. It should preferably be attached to the wall. You may find it easier to avoid air bubbles in the manometer if the tube is filled before fixing it to the board. Make a preliminary estimate of the length of water column required. Immerse one end of the tubing in a beaker of coloured water and suck at the other end until there is the required length and finally fix the tube to the board.

For hygiene, disposable mouthpieces are desirable, of the sort used for medical purposes.

Place a trap between the gas tap and manometer to prevent water from getting into the gas supply.

Procedure

1. Half fill the U-tube with water together with some food colouring as a dye (it makes the water clearly visible and is non-staining).
2. Measure your lung pressure by blowing into the manometer and measuring the difference in water levels between the two arms.
3. Insert a small trap. Repeat by sucking.

Teaching Notes

• Students may discover for themselves when using short (0.5 m) manometer tubes that if they blow into them then the water can be blown out. That is the time to introduce them to a 3 m tube so that they can measure their lung pressure as the difference in levels between the water in the two arms.
• A statistical distribution of the class results can be produced and explanations for the variation sought. Perhaps you can tell who smokes, or who has asthma.
• In their efforts to get the water up and out of the tube some may discover resonance while bouncing the water over. This can be messy but it is a delight too.
• Some may try to see how far below atmospheric pressure they can go by sucking the water. Ensure that there is no danger of the water being swallowed by using a trap.

The experiment was safety-checked in July 2007

Demonstration

Does the area of a manometer tube matter when measuring pressure?

Apparatus and Materials

• Mounted U-tube manometer, 1.5 m tall
• Manometer with unequal limbs, 0.75 m tall
• Aspirator (10 litres)
• Food colouring

Health & Safety and Technical Notes

Read our standard health & safety guidance

Set up the 1.5 m manometer vertically using the slotted base to hold it. One arm should be removable, so that it can be replaced with a tube of the same length but greater diameter. If this is not possible, then join together two unequal arms.

Fill it with water, coloured with a few drops of food colouring to make it clearly visible.

The gas pressure is generally about 25 cm of water. When filling the tubes there must be sufficient water to allow for this change in level.

Procedure

1. Connect the local gas supply to the manometer.

2. Replace one arm of the manometer with a tube of greater diameter and repeat.

3. Connect the gas supply to the aspirator. It should be connected to each end of the aspirator in turn, as indicated in the diagrams below, to show that the difference in level is still the same.

Teaching Notes

• These three experiments should be set up side by side.
• Ask: "would it make any difference to the manometer reading for a given pressure if the cross-section area of the liquid in the tube were changed on one side?" Surprisingly for young students, the level difference is always the same.
• The manometers with unequal arms should be connected up to the gas supply first from one side and then from the other to see if it matters which side is the input.
• For those who can manage a mathematical argument about the pressure being equal to the extra weight of water supported/area then they will see that the height of the extra column of water is independent of the cross sectional area. For some the pressure equation p = hρg can be explored.
• For some particular pressure, explore the equation p = hρg.
• h = difference in height of columns
• ρ = density of liquid in the columns
• g = gravitational field strength = 10 N/kg

The experiment was safety-checked in July 2007

Class practicals

An experiment to introduce the Bourdon gauge.

Apparatus and Materials

For each student group

• Bourdon gauge
• Paper Bourdon gauge also known as party blow outs
• Foot pump

Health & Safety and Technical Notes

Read our standard health & safety guidance

The Bourdon gauge reads absolute pressure from 0-150 kPa (in Imperial units, 0-20 pounds per square inch).

You can buy party blow-outs that are silent!

Procedure

1. Pass paper Bourdon gauges round the class and ask how they work. (The pressure inside makes the flattened tube uncoil.} Students should have one each and not share.
2. Fit the Bourdon gauge with a disposable mouthpiece and use it to measure excess lung pressure by blowing into a rubber tube connected to it, fitted with a disposable mouthpiece.
3. Use the Bourdon gauge to measure the pressure obtained when the foot pump is used gently.

Teaching Notes

• This is an introduction to a direct reading pressure gauge. It should be used to measure any convenient pressures such as excess lung pressure and excess pressure produced when a car foot pump is attached to it and used gently. The toy Bourdon gauge helps to show how increased pressure straightens out the tube whose end is connected to a pointer.
• The Bourdon gauge does not register zero when it is free standing but rather the zero is calibrated to register the current air pressure of about 10 ⁵ N/m ². This is the same as the pressure created by 1 kg supported on 1 cm ² . (Old Imperial units are still often found on tyre pumping machines, recording a pressure of 14 to 15 pounds per square inch (psi) for atmospheric pressure.) Balancing a kilogram on the end of a finger will give a feel for the extra pressure on the finger. Extra because there is already the atmospheric pressure pressing on the finger in all directions.
• A Bourdon gauge with a glass back is ideal, as this reveals the curved tube connected to the inlet, and the mechanism by which its unbending moves the needle across the dial.

The experiment was safety-checked in July 2007