Discussion of pressure

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