Register to reply

Light bulb in a microwave

by Will
Tags: bulb, light, microwave
Share this thread:
Will
#1
Sep27-03, 09:34 PM
P: n/a
A friend of mine showed me a neat trick the other day. He put a light bulb in a cup of water, put it in a microwave and turned it on. The bulb blinked on and off. So whats going on here? Is the water transporting the current via oxidation/reduction? Because I thought the microwave energy was only enough to change the vibration of the water molecules, not enough to ionize them.
My physics prof. did not believe me when I told him this was an incandescant bulb with a filament. He did a demonstration in class with a florescent bulb held near a Van Der Graff generator, and it lit up. So what is the difference between an florescent and incandescant light bulb?
Phys.Org News Partner Physics news on Phys.org
A new, tunable device for spintronics
Watching the structure of glass under pressure
New imaging technique shows how cocaine shuts down blood flow in mouse brains
Claude Bile
#2
Sep29-03, 01:29 AM
Sci Advisor
P: 1,477
A flourescent bulb emits light through flourescence (!), that is, atoms are excited by an electrical discharge and then emit photons as they relax back to their ground state.

Incandescent bulbs get really hot (Due to the high resistance of a thin filament) and emit as a blackbody and are spectrally much broader than flourescent bulbs (which basically means they are less efficient).

The flourescent bulb lit up because it was supplied with electricity from the Van Der Graaf generator. I'm not sure how a microwave field could induce current in a bulb.

Claude.
Duncan
#3
Sep30-03, 10:00 AM
P: 13
Hello,

The incandescent bulb is likely to be lit due to current in the filament set up by the microwaves directly. Microwaves waves applied to metals will in general create electric currents. To see this you could place some aluminuim foil in a microwave and see the sparks. Though I would not reccomend it. As the filament of a bulb has such a high resistance it glows from the heat generated. The bulb was placed in water to protect it from the full effect of the microwaves otherwise it would melt the filament or maybe blow up the bulb.
As a matter of interest I found an article that claims to allow you to measure the speed of light with a microwave. So impress you friend back!

http://physics.about.com/cs/opticsex...s/a/290903.htm

Duncan

chroot
#4
Sep30-03, 01:14 PM
Emeritus
Sci Advisor
PF Gold
chroot's Avatar
P: 10,427
Light bulb in a microwave

Duncan,

Microwaves applied to metals do not create currents. Furthermore, your assertion that the arcing due to foil is caused by such currents is also wrong. The arcing is due to the strong electric field near pointed bits of metal and the high voltages used by the magnetron to generate microwaves. The arcing has nothing to do with the microwaves themselves.

- Warren
Claude Bile
#5
Sep30-03, 06:54 PM
Sci Advisor
P: 1,477
Of course, that is probably also why the bulb lights up.

Claude.
Creator
#6
Sep30-03, 06:59 PM
P: 555
Very interesting,Will; interesting enough that I tried it myself.
And yes it works..
So in the interest of settling this rather theoretical debate, lets go experimental...
I tried it with a burned out bulb, with the same effect- off and on glow; sometimes greenish.
I thought at first it appears as though the glow may be due to microwaves interacting with the frost on the glass, or even some gas in the bulb. However it is still also possible that the filament is the source. ???

So why don't you keep trying : Ist with a clear (non-frosted bulb).
If that doesn't work just tear a bulb apart and start nuking each part individually.
Or do you just want someone to give you the correct answer? Wouldn't you rather be a part of the exciting world of destroying things in the name of science?
Why should everyone else have all the fun when you can experience nuclear destruction first hand right in your very own kitchen!!
Let me know how it turns out....

Creator
BoulderHead
#7
Sep30-03, 07:14 PM
P: n/a
Here's a couple of links;

http://apache.airnet.com.au/~fastinf.../tungsten.html
annoying audio.


http://home.earthlink.net/~marutgers...microwave.html
has movies of the events you can view.

[edit]
Say, there is a video at the first site too. Well, I'm off to view them...
Mr. Robin Parsons
#8
Sep30-03, 07:18 PM
P: 1,560
Might be nice if you remind this person, Will, of the safety hazards, and the idea of protective gear when doing things like breaking lightbulbs, don't want anyone losing an eye, or anything else, for their inquisitiveness.

P.S. Neat site Boulderhead, note the safety warning.......
chroot
#9
Sep30-03, 07:19 PM
Emeritus
Sci Advisor
PF Gold
chroot's Avatar
P: 10,427
The off and on glow would be due to the duty cycle of the magnetron -- the microwave doesn't nuke food constantly, but in pulses.

- Warren
Duncan
#10
Oct1-03, 08:22 AM
P: 13
Hello,

Chroot's reply to my post has greatly confused me. I can see how my assertion that currents in the filament caused it to glow may be wrong. But the statement from chroot that;

The arcing is due to the strong electric field near pointed bits of metal and the high voltages used by the magnetron to generate microwaves. The arcing has nothing to do with the microwaves themselves.
From what I can tell is the microwave is designed to trap micrwaves in its cavity to create a strong EM field. The EM field being the microwaves themselves. Metals with sharp edges disturb the electric field. This distubance either attracts or repels electrons at these edges. If there is another object of a diffrent potential difference (another edge of the foil) close by arcing will result. So isn't the metal directly influenced by the microwaves?


Microwaves, metal and arcing

Duncan
radagast
#11
Oct3-03, 08:57 AM
radagast's Avatar
P: 460
Originally posted by chroot
Duncan,

Microwaves applied to metals do not create currents. Furthermore, your assertion that the arcing due to foil is caused by such currents is also wrong. The arcing is due to the strong electric field near pointed bits of metal and the high voltages used by the magnetron to generate microwaves. The arcing has nothing to do with the microwaves themselves.

- Warren
Electromagnetic radiation in the microwave and radio frequencies induce current in conductors - otherwise antennas would not work. All electromagnetic radiation has a electric and magnetic field component.

If they do or do not generate a strong enough current to cause the above, I don't know. However, I have seen sparks coming off the lip of a china tea cup (in the microwave), where a thin layer of gold (one without breaks or pointed bits) formed a circle (circuit) at the lip of the cup.

And just to head off any potential objections, metal lustres on china and other pottery are actually metal. Metal oxides and salts are reduced either thru a reducing atmosphere in the firing or due to reducing agents in the lustre binding medium.
gm137
#12
Oct11-08, 03:23 AM
P: 2
I've performed this demonstration many times for my class: it's always a crowd pleaser.

As far as I'm aware, the bulb itself (e.g. the filiament) isn't lighting up: you couldn't induce a reliable modulated current in the changing environment of a microwave (where the object is constantly moving in and out of "hotspot" areas) anyway.

If you watch videos of this demonstration, you'll find that the bulb glows far brighter than it would normally, and undergoes a number of colour changes. I think that the microwave energy is promoting the argon atoms that surround the filament to an excited state, and then releasing that energy as light when they return to the ground state. I have experimented with an empty tube of argon, and also with a "blown" bulb (e.g. where the filiament has failed) and achieved a similar glow, so this seems to me to be a reasonable explanation.

The puspose of the water is to absorb some of the microwave energy: a bulb by itself will fracture and implode after about just 20 seconds of heating as the glass softens. In the following video, I microwave a bulb with a broken filiament and no water: the yellow glow at the end is characteristic of hot sillica. http://uk.youtube.com/watch?v=qUYJd4F2cuI
Redbelly98
#13
Oct11-08, 09:46 AM
Mentor
Redbelly98's Avatar
P: 12,069
Welcome to PF, gm137.

Here's another video with the lightbulb in water:



p.s. I'm not always a fan of reviving years-old threads, but I'm glad this one was. Thanks to YouTube, the doubters can be convinced.
Naty1
#14
Oct11-08, 11:24 AM
P: 5,632
Electromagnetic radiation in the microwave and radio frequencies induce current in conductors
Of course they do..the electromagnetic field carries the energy from the magnetron to the material being heated....Wikipedia explains the magnetron production of RF power for cooking at http://en.wikipedia.org/wiki/Magnetron
Bad Monkey
#15
Oct11-08, 11:56 PM
P: 30
This one shows a large bulb with no water.

What is going on inside the bulb itself can be more clearly seen.



Comments?
Redbelly98
#16
Oct12-08, 06:48 AM
Mentor
Redbelly98's Avatar
P: 12,069
Nice find.

There are warnings about placing metal objects in microwaves because of the arcing that will occur. I imagine the field is strong enough to remove electrons from the metal, which then ionize the surrounding air (or other gas), and presto! An electrical gas discharge, just like in gas discharge lamps. Except at a much higher frequency.

I'm guessing that the gas inside the bulb is more easily ionized than air, so that's where the discharge happens instead of, for example, in the air near the metal threaded part of the bulb.

I'm not sure what's causing the glow at the glass surface however.
gm137
#17
Oct13-08, 03:25 AM
P: 2
A good set of videos!

Re. the afterglow: that's the silica of the glass being heated to red heat: it's what will eventually cause the bulb to fail. Take a glass rod and put it in a gas flame, and you'll see the same effect. After an experiment where I left the microwave on for too long and shattered the bulb, I microwaved a small piece of the glass and achieved the same sort of glow.


Register to reply

Related Discussions
W light bulb Introductory Physics Homework 1
Light Bulb Introductory Physics Homework 3
Light Bulb in Ohm Law Introductory Physics Homework 4
Once upon a light bulb... Advanced Physics Homework 2
Light bulb General Engineering 6