Re: Electronics inside the microwave.

From: Terry Given (my_name_at_ieee.org)
Date: 10/20/04 

Date: Wed, 20 Oct 2004 20:44:22 +1300

Rich Grise wrote:

> On Wed, 20 Oct 2004 06:24:06 +1300, Terry Given wrote:
>
>
>>Ian Stirling wrote:
>>
>>
>>>Terry Given <my_name@ieee.org> wrote:
>>>
>>>
>>>>Mac wrote:
>>>>
>>>>
>>>>
>>>>>On Fri, 15 Oct 2004 18:55:30 +0000, ChrisGibboGibson wrote:
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>Some amusing things to try in a microwave.
>>>>>>
>>>>>>Small flourescent tube, they light very brightly.
>>>>>>
>>>>>>Incandescent bulb, light very brightly, very briefly.
>>>>>>
>>>>>>Xmas tree lights.
>>>>>>
>>>>>>LEDs
>>>>>>
>>>>>>Blue bottles buzz like a demented thing.
>>>>>>
>>>>>>And finally, worms can travel much faster than I thought.
>>>>>>
>>>>>>Gibbo
>>>>>
>>>>>
>>>>>Also try compact disks. I recommend that you convince your friends to do
>>>>>it rather than risk doing any slight damage to your own microwave.
>>>>>
>>>>>One or two seconds is plenty of time to see the effect.
>>>
>>>
>>>>CDs are also fun when you have a dremel. but wear safety goggles.....
>>>
>>>
>>>I'd say "just say no", unless wearing full body armour and face protection.
>>>
>>>I tried this once, and ended up with shards of CD 2cm deep in the
>>>plasterboard.
>>>
>>
>>As did I. except it was thru 20mm Gib board (antipodean for plasterboard
>>:). I figured the rest of me (and the 5 guys watching) would heal....I
>>was wearing a face shield (safety goggles = double glazing = instant
>>headache :)
>>
>>If you Nukrowave the CD first, it self-destructs at much lower speeds.
>>
>
> Forgive my ignorance, but how do you blow up a CD with a Dremel?
>
> Thanks,
> Rich
>

Aha! odds on if you google it (i'm too lazy :) you'll find some nice
mpegs. If you have the dremel sander attachment (a 10mm dia or so
cylinder of rubber, about 10mm high around which a ring of sandpaper
sits), that just happens to fit nicely into the hole in the centre of a
CD (a slight interference fit). Crank it up... I suggest doing a little
bit at a time, and using your thumb to push the CD off the dremel. A
wall and a ramp allow you to make the cd spin in large loops - floor,
ramp, door, ceiling etc - the ramp shoots it backwards, but the rotation
makes it go forwards when it hits the ground. Each time it does the CD
gets damaged, and eventually flies to bits. I had a CD running at full
speed on my dremel without it bursting, but several did. Wear heavy
clothing, gloves and face protection. And ensure (not insure - grr)
no-one comes into the room while you do it. If I wasnt so lazy I'd
calculate the energy stored in the CD....oh OK, J=0.5M*(Ro^2+Ri^2)

Ro = 120mm, Ri = 15mm, t = 1.2mm so volume
V = 3.14*(120mm^2-15mm^2)*1.2mm = 53.44e-6 m^3. the CD is basically
polycarbonate, density Rho = 1.2g/cm^3 = 1200kg/m^3. sanity check:
density of aluminium = 2700kg/m^3, glass = 2600kg/m^3 so sounds OK.

M = V*Rho = 53.44e-6*1200 = 64g = 0.14lb (exactly what some of the DCD
shipping stuff I googled said)

J = 0.5*0.064*(120mm^2+15mm^2) = 468e-6 - agrees exactly with
http://hyperphysics.phy-astr.gsu.edu/hbase/ihoop.html#ihoop2

max speed = 35,000 rpm = 583.33Hz = 3665rad/s

Emax = 0.5*J*w^2 = 3kJ

*** with this at your peril. 3kJ is enough to cause a serious injury. I
have been in close proximity to a 17kJ bang (a large inverter shat
itself while I stood beside it) and that was scary. but nothing like the
1MJ flywheel that shat itself when Stan and I were testing it
(thankfully it had a 2" Al containment vessel, securey bolted to a very
heavy plate). At a spin test for the same product, we made it fail
catastrophically by overspeeding it with a uqill shaft - died at 7.2MJ,
inside a spin pit. and melted $6,000 worth of lead bricks. man did the
building shake.

you have been warned.

cheers
Terry

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