Physics Photo of the Week

February 12, 2010

Fire Syringe - Katie Learned and Meron Amare each contributed to the discussion.  The Physics class took the photos.

What you see in this picture is a bit of cotton bursting into flame at the bottom of a device called a fire syringe. The syringe itself is airtight once the lid is screwed on, trapping a certain amount of air inside its cylindrical walls. In the lid is a plunger, which can be used to compress the trapped air inside. A bit of cotton was placed in prior to sealing the syringe. When the plunger was depressed, the sudden increase in pressure resulted in a sharp spike in the internal energy of the compressed gas in the syringe—enough energy to cause the cotton to burn up in an instant.

The compression in this demonstration is an adiabatic process—a process where no heat is transferred.  It is thermally insulated.  The work pressing the plunger is converted into internal energy of the air trapped inside.  Internal energy is directly related to the temperature of the system.  Therefore when we apply work on the piston, which then changes to internal energy, we are raising the temperature so high that it burns a small mass of a flammable substance such as cotton.

The video clip shown at right consists of 12 images played slowly at 2 frames per second.  This slows the video a factor of 15 from the original 30 frames per second.  The 12 frames covers a time of 0.4 second.  In the first two frames the wad of cotton is visible in the bottom of the tube before it is ignited by the adiabatic compression of the air. 

This device is manufactured specially for this demonstration.  It is made of very thick walls to prevent breakage.  Previous versions of this have been made of glass and have suffered damage due to the large forces needed to press the plunger.



Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to dcollins@warren-wilson.edu. 

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.


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