Sunday, March 05, 2006

A Candle in A Jar

Seeing the Weight Of Air

Did you know that air has mass? Because the weight of the atmosphere has been pressing down on you all your life and your body is adapted to it, you don't even notice it! But try traveling to a place at high altitude and you'll quickly notice it's lack.

Even if you are intellectually aware of the mass of air, there are a few experiments like this one (and the "air weighing balance" and "Bubble Inertia") that allow you to directly observe the air's mass.

Things you will need:

1) A candle

2) A jar. The jar needs to be tall enough that top of the candle will be well below its lip when the candle stands in the jar. You won't need a lid for the jar, so don't worry if you don't have one.

3) Plasticine to help the candle remain standing when the jar moves. I found that play-doh had trouble sticking to the glass jar and the candle, making the experiment difficult to set up!

4) Matches or a lighter. Like all experiments using fire, if you're not allowed to play with fire by yourself, ask an adult to help you with this experiment.

What to do:

1) Use the plasticine to stand the candle inside the jar. Make sure it's held securely so that it stays standing when you shake the jar gently from side to side.

2) Light the candle. Either use another candle to reach in and light it or turn the jar upside down.

3) Now, before you go any further, try to guess what the flame will do when you move the jar from side to side.

4) Now that you have your guess, try it out. Move your jar slowly from side to side. Pay close attention to the flame. Which way does the flame move when the jar starts moving? When you stop the jar? If you are not sure why this is interesting, think about which way you move when you are in a car that starts or stops.

What happens?

When you ride in a car that is accelerating, you feel a "force" that pushes you in the opposite direction. When the car speeds up, you're pushed backwards into your seat. When the car turns left, you're pushed to the right. When the car brakes, you're pushed forwards.

These "forces" are not true forces - nothing is actually pushing you forward when the car brakes suddenly. Instead, these "pseudoforces" are due to your inertia and its tendency to travel in straight lines at constant speeds (Newton's First Law).

However, the flame moves differently. When the candle accelerates to the left, rather than trailing to the right the candle flame points to the left! Then, when the jar is moving at a constant velocity the flame points upwards. Finally, as the jar decelerates back to stationary, the flame trails to the right!

We know that it's not just a breeze causing the unusual motion - the walls of the jar protect the flame from the wind. When everything is stationary, the flame points upwards. This is because the flame heats the air around it. Hot air is lighter than cold air, so the cold air sinks to the bottom of the jar, displacing the warm air. This causes the warm air to rise straight up, so the flame also points upwards.

When the jar is accelerating, the inertia of the air causes it to be pushed to the back of the jar (just like the way you are pressed back into your seat in an accelerating car). Because the cold air is heavier than the light air, it accelerates slower and is pressed back harder against the back of the jar, pushing the hot air slightly forward as well as upwards. Thus, the candle flame points forwards. Similarly, when the jar comes to a stop, the cold air goes to the front, pushing the warm air (and the flame) to the back.

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