Fluid pressure

Compares pressures
Compares pressures

This is a manometer. It’s just a plastic U-tube half-filled with coloured water. Attach one end to the gas tap. The gas pressure pushes down on one side, pushing the other side up. We can say that the excess pressure over and above atmospheric pressure is supporting a column of water 4.9cm or 0.049m high. We can then use

P = hdg

to work out this pressure in Pascals .

Let h = 4.9cm or 0.049m. Look at the picture, people….

The density of water is 1000kg/m³ and g is about 10m/s²

so, excess pressure, P, over and above atmospheric pressure

P = 0.049 x 1000 x 10 =490Pa

Not difficult, is it?                 Thanks, Sheikha…

The weight of air above us is quite another matter, however. Let the air above us be just over 8km high and of uniform density of 1.2kg/m³. It isn’t, of course, the further up we go, the less dense the air, but the atmosphere extends to, well, space at a height of about 100km, so if it was of uniform density, it’d be about five miles or 8km high

Using P = hdg:

P = 8,000 x 1.2 x 10 = 96,000Pa – more or less 100kPa. Huge. What does this incredible pressure do to us? Well, nothing since the air in our lungs is also at this pressure, so the pressures outside and inside balance out. But, what if they didn’t? Our lungs would collapse as if an elephant had trodden on our chest! We can show what happens in the lab. Have a look at this..


The can is like a petrol can (but empty, of course!). Put a little bit of water in the bottom, open the lid and heat it with a bunsen burner. The water boils, steam driving all the air out of the can. Screw the top on quickly to prevent the air rushing back in. The pressure inside is small – it’s a partial vacuum in there. But, the air outside is still at 100kPa – look what happens to the can…


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