 |
Kitchen Science ExperimentsThe Naked Scientists: Science Radio & Science Podcasts |
|
||||
Listen to Experiment Part 1 Part 2  ...or download as MP3 [1] [2] Next Experiment: Margarine Planets  If you like Kitchen Science then you'll love Crisp Packet Fireworks, our new book of home experiments, complete with illustrations and explanations. Find out more |
 Kitchen Science RSS Feed |
Build your own airbrush - Applying Bernoulli
Find out how to build an airbrush to produce beautifully smooth paint finishes, and what it has to do with a car engine.
What you need
| A straw |
|
| |
| Scissors |
What to Do
Cut the straw into two halves.
Hold one of them in your mouth.
Put the other half into the water and hold or tape it so that it sits vertically with about 1-2cm emerging from the surface.
Use the straw in your mouth to blow across the top of the other straw quite hard.
If nothing happens try lowering the straw in the water.
What may Happen
You should find that when you blow hard enough you produce a spray of tiny water droplets, just like an airbrush.
If you put some colour or paint into the water you can use it to paint with (thought this could get very messy very quickly so make sure you do it outside.
What is going on?
Rapidly moving air will be at a lower pressure than when it is stopped, this is called Bernoulli's principle. So when you blow across the top of the straw, the rapidly moving air is at a lower pressure than the air above the water in your glass.
|
|
One straw is set up to blow over the top of the other one. | The fast moving air is at a lower pressure than the air above the water, so the water is pushed up the straw. |
If you blow hard enough the air above the glass will push the water all the way up the straw and out of the top where the rapidly moving air breaks the water up into tiny droplets which are good for painting with.
|
Eventually the water gets to the top of the tube and is broken up into small drops by the fast moving air. |
Commercial airbrushes work exactly the same way, they just use higher pressures and better designed nozzles and produce smaller droplets.
It is also how fuel is mixed with air in an engine's carburettor air moves across a nozzle and fuel is sucked up and forms a fine flammable mist which is then burnt in the engine.
Why is fast moving air at a lower pressure?
Air being a gas can only be accelerated by a change in pressure. If the air is slowing down then it must be going from an area of low pressure to a higher pressure, so the rapidly moving air must be at a lower pressure than the slower moving air.
I don't get it. If you spray a stream of air from a high pressure tank it slows down, but it's moving from high pressure to the lower pressure atmosphere?
The contents of this site are © The Naked Scientists® 2000-2010. The Naked Scientists® and Naked Science® are registered trademarks.