Fun with Air Pressure

Fun with Air Pressure

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Today we’re going to learn about the effects of air pressure.  I’m going to show you how to get an egg into and out of a bottle using nothing but a change in air pressure inside the bottle.

Preparation – what you will need

1. A bottle with a large opening – like an empty Tomato Sauce bottle

2. Strips of paper

3. A lighter or matches (remember to get mom or dad to watch you)

4. 1 Hard Boiled egg (peeled)

Make sure the bottle opening is just smaller than the egg.  The egg will need to rest on the opening of the bottle – not fall through it.  But make sure that it’s not too small of an opening either else the egg won’t be able to squish through

Method:

Kids – be sure to do this with adult supervision.  It does involve fire, and cooking!

  1. Boil the egg for about 8 minutes to make sure it’s nicely hard boiled.
  2. Peel the egg and get it ready by the bottle.
  3. Make sure the paper strips can fit through the opening of the bottle easily.
  4. Light the paper strips, put them into the bottle and quickly rest the egg on the opening of the bottle.
  5. Sit back and watch what happens

If everything worked according to plan, as soon as the paper stops burning, the egg will be sucked into the bottle.  Pretty cool huh?

To get the egg back out again:

  1. Tilt the bottle over slightly and get as much of the paper that’s remaining inside the bottle out.
  2. Then, with the bottle held in the air as if you were going to drink from it, place your lips over the edges of the bottle and blow into it as much as you can.
  3. Make sure the egg drops to the bottom of the bottle again and then stop blowing.
  4. Get ready to catch the egg and quickly take your mouth away from the opening.

Again, if everything worked nicely, the egg should pop out of the bottle.
But why did it do that??

Three things you need to remember:

  1. When you heat air up it expands and takes up more space than it did when it was colder.
  2. When the air cools down again, it contracts again.
  3. Air doesn’t like having different pressures in different locations.  What that means is that when you cool the air down it likes to take up the same amount of space that it did before you heated it up.

That’s what happens when you put the paper strips into the bottle and close the top of the bottle with the egg.  If you watch closely you’ll see the egg wiggle around a bit on the opening of the bottle.  What’s happening there is that when the air inside the bottle gets heated up and expands, some of it actually pushes past the egg.  And then when the available oxygen inside the bottle is used up and the flame goes out, the air cools down again.  And when it cools down it creates what we scientists call a “partial vacuum”.  What that means is that there is too much space for the amount of air that remains inside the bottle (remember, some of it escaped when it was heated up).  Now, normally if you didn’t place the egg over the opening, air would come rushing in to take up the space, but since it can’t do that, it sucks the egg in.  Once the egg is inside the bottle and the opening is freed up again, air rushes in to fill up the remaining space.

When you’re getting the egg out of the bottle, you’re doing the opposite.  By blowing hard into the bottle, you’re forcing more air into the bottle than can actually fit into the bottle under normal circumstances.  And then when you take the source of pressure away (i.e. your mouth) the air needs to escape so that the pressure of the air inside the bottle is the same as the pressure of the air outside the bottle.  But since the egg is in the way, it pushes the egg out first.

Neat huh?

An everyday example of how air pressure affects us:

Have you ever wondered what causes the wind to blow?  It’s not the flapping of tree branches that causes the wind.  What actually causes wind is differing air pressure.  When the sun heats the Earth up, not everywhere on the planet gets to the same temperature.  And as we’ve seen in this experiment, air doesn’t like to be at different pressures.  So wherever there is a difference in temperature between two places, the air will flow from where it is at a higher pressure (i.e. where there is more air in a certain spot) to where it is at a lower pressure (i.e. where there is less air in a certain spot).  Or to put it more simply, the air will blow from where it is colder to where it is hotter; Because where the air is hotter, it expands and takes up more space meaning there is less air in the same size space where it is hot than where it is cold.  And like I said, air doesn’t like that – Air likes to have the same amount of air in the same space at all times.

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