MadSci Network: Physics

Re: When trying the 'Plasma Light bulb' exp, I had ODD results...

Date: Fri Apr 13 11:01:41 2001
Posted By: Edward Peterson, Staff, Chemical Engineering, S&B Engineers and Constructors
Area of science: Physics
ID: 985705406.Ph

You have the clue to the actual result with your question.  What acually 
happened is due to the following set of occurences.

The filiment,that usually causes the filaments of plasma was destroyed by 
the microwave plama het very quickly.  The filament support broke and fell 
as as a small chunk onto the glass of the bulb.  Air had replaced the 
vacuum during this experiment,eliminating the vacuum.

A vacuum interacts well with microwave energy, allowing a plasma to form.  
With the vacuum gone, the microwave energy looks for the next best target. 
The nex best target was a clump of glass or ceramic or both.  The ceramic 
is the material that holds the filament in place at the bottom of the bulb.

With the vacuum gone, the microwave finds a clump of material that has a 
low thermal conductivity (ability to lose heat by conduction) and starts 
putting its energy into it.  The energy goes in fast enough to  cause the 
temperature to rise to the glass's melting point. This is hard to do, 
because small objects lose heat quickly. Molten glass is a much more 
effective absorber of microwave energy, so when even a small amount of 
glass has melted, it melts much more quickly.  That's when the light show 

How can one reproduce this?

With a lightbulb, this would be difficult.  I piled some broken window 
glass on some clean, white sand inside a PYREX beaker.  The glass shards 
heated up after some time and eventually produced the same results you 
described.  I have done this in a laboratory environment, so DO NOT DO 
THIS AT HOME.  A fire could result with the high temperatures you will be 
reaching.  Molten glass may be the hottest substance you ever encounter in 
your life, so do not touch the glass, even with oven mits. I stopped the 
experiment within seconds of seeing the glass start to melt. I do not 
recommend that you do this experiment.  I simply point out that I have 
seen the same effect you describe.

Dr. Ed Peterson.

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