Fire Suction

Physics allows us to manipulate the world around us in fun and creative ways.  The fire suction experiment is a good example.  We want to grab a penny from a plate that is covered in water, but we don’t want to touch the water.  At our disposal we have an empty soda can, some matches, and a glass.  What do we do? Well, we light the matches and put them inside of the coke can (cut in half) and cover the water and can with the glass as seen in the video.  As if by magic the water gets sucked up into the glass leaving the penny exposed to the air!  Walah!

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So what was actually happening?  We are taking advantage of a principle called the Ideal Gas Law: PV=nRT.  Since the volume of the glass won’t change, the important part for this experiment is that the pressure exerted by a gas is proportional to its temperature.  So as the gas gets hotter it exerts more pressure.  The glass separates our system so that there are two different sources of air pressing on the water covering the penny: the air inside the glass and the air outside the glass. The match heats up the air on the inside.

As the temperature of the air inside the glass drops, the pressure it exerts on the water also decreases.  That decrease in pressure causes the water outside the glass to get pushed into the glass by the air outside the glass.  Eventually the system balances with all of the water sucked into the glass.  That’s pretty cool!

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