Physics Photo of the Week
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.
Photo of the
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 firstname.lastname@example.org.
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.
see the Physics Photo
the Week Archive.
Observers are invited to submit
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