About this blog

Physics can be difficult to learn, but this blog aims to help you get into physics by connecting your GCSE physics lessons with things you see in the world around you.

Tuesday, 6 September 2011

Under Pressure

Pressure is an accepted part of today’s society, and in some ways it makes us want to excel and achieve our goals. But it can also make us feel terrible. Thankfully there are some good pressures around, very important indeed for keeping us in our place on earth, for keeping our eyes inside our heads, our lungs breathing and more!

Yes, I’m talking about pressure in the fluids around us. Let’s start with water pressure because it’s easy to see. Anyone who’s been swimming in a pool with a nice deep end knows that if you dive down to the bottom it feels a bit odd. Your head might even be able to feel the pressure increase. A simple experiment will show you why.

Take a 2 litre drinking bottle (fizzy drinks should be replaced with water as they have their own pressure too, and we don’t want an explosion!) outside, perhaps with a friend or two to help hold it and make holes. Very carefully use a couple of pairs of compasses, or something else that can make a small hole quickly, and punch a few holes in the bottle at varying levels.

You’ll find (apart from the fact that you might now be a bit soggy) that the lower holes have faster flowing jets that travel further than holes higher up. This is because of the water pressure. The pressure exists because of the weight of the water pushing down, so the lower you go, the more weight there is on top of that section, making the pressure higher. This is also one of the reasons why animals at the bottom of the sea look so weird, as they have to withstand some astonishingly high pressures.

And similarly, we are designed to live on the earth’s surface, where the problem isn’t the water pressure but the air pressure. In just the same way as the weight of the water makes the pressure, the weight of the atmosphere creates air pressure, called atmospheric pressure. I know what you’re thinking, air doesn’t weigh anything. And to you it probably doesn’t, since you have never encountered a large enough piece of air!

Imagine that the air molecules are pieces of popcorn. They are light and you can hold a few in your hand and hardly feel it. But fill some bin bags and you’ll be feeling the difference already. And it’s the same with air molecules, whose weight can be measured on the nanoscale. Gather enough together and they become heavy, just like in our atmosphere.

So spare a thought for some of the tiny creatures who walk the land, for them it may seem like the air weighs plenty, and that they are walking through treacle. It seems none of us can escape the pressure!

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