Arcs in a microwave oven

[Guineafowl]

Some fireworks in our microwave from peeling paint got me thinking...

If you place a fork inside, arcs will form between the tines. Why? Presumably the EM field is inducing different potentials on each tine, and the pointiness concentrates the field enough to exceed the breakdown voltage there.

But the tines are shorted together at the base, so how does a potential difference build up between them?

 

[.Scott]

The fork acts as an antenna and the potential builds up through the metal.
The voltage difference is greatest at the ends of the tines where there can be maxima in the standing wave.

 

[Baluncore]

The gap between the tines of the fork form an open circuit slot antenna. A circulating current may only flow around the slot if there is dielectric breakdown between adjacent points of the fork.

 

[Guineafowl]

The gap between the tines of the fork form an open circuit slot antenna. A circulating current may only flow around the slot if there is dielectric breakdown between adjacent points of the fork.

Yes - this is what I can’t quite visualise: surely the metal, conductive fork has essentially zero dielectric breakdown voltage. Or does the EM field somehow maintain the various potential gradients, making the only way out to be the gap between the tines?

 

[.Scott]

Yes - this is what I can’t quite visualise: surely the metal, conductive fork has essentially zero dielectric breakdown voltage. Or does the EM field somehow maintain the various potential gradients, making the only way out to be the gap between the tines?

The effect you are seeing does not depend on any resistance in the metal.

Think of a physical pulse following along a taunt cable. When the pulse reaches the end of the cable, it is reflected - but it will tug hard on the connection to the wall. The same kind of thing is happening within the metal of the fork. The microwaves are causing waves to travel back and forth through the metal. But you will get peak voltage when the wave reaches a dead end.

 

[Averagesupernova]

It took me a long time to wrap my head around the fact that a short conductor such as an antenna can have a large voltage from one end to the other. RF does not behave as does DC.

 

[Baluncore]

Or does the EM field somehow maintain the various potential gradients, making the only way out to be the gap between the tines?

The complement of a metal dipole is a slot in a metal sheet. Currents are induced to circulate around the holes in the sheet.
http://electriciantraining.tpub.com...-Antenna-And-Its-Complementary-Dipole-163.htm

There are different modes available for resonance in a fork. Are the arcs clearly between the tines or do they radiate from the tines to something else?

2.45GHz has a wavelength of 120 mm. A half wavelength is 60 mm which is about 2½ inches. So I would expect inter-tine currents to be currents circulating around the gaps and between tines, rather than the electric dipole resonance of the entire fork which would induce similar voltages in the parallel tines. There may be an initial voltage mode that initiates the ionisation of air needed to support the current circulation mode. That would be the equivalent of the spark gap T/R switch in an early RADAR set.

 

[.Scott]

There are different modes available for resonance in a fork. Are the arcs clearly between the tines or do they radiate from the tines to something else?

I haven't done this with a fork. But I have done it with a tiny triangle of aluminum foil left on the rim of a 1-teaspoon butter container. That little tab arced even though it was only a few mm on a side.
So even though I mentioned "standing waves" before, the effect you see in a microwave does not depend on standing waves. The metal is simply absorbing the microwaves and reradiating the energy as an electric current.

 

[Guineafowl]

<Moderator's note: Quotation of deleted post edited out.>

Interesting - but metal in a microwave gets hot and arcs, so it must be absorbing some energy, no? And surely a microwave antenna is made of metal?

 

[Baluncore]

So even though I mentioned "standing waves" before, the effect you see in a microwave does not depend on standing waves.

When microwave energy is introduced into a closed conductive metal box, there will be internal reflections. Standing waves will be formed within the oven.

A small conductive object in that space will act as an EM scatterer. Efficient reflection or scattering of EM fields requires that RF currents flow on the surface. The eddy currents that flow on the surface of a metal conductor effectively “induction heat” the metal by W = I²R.

There will also be a voltage developed across a conductive object, determined by surface impedance and size. Microwave oven design is optimised as a "dielectric heater" to heat food, therefore it has high electric fields. A conductor in that field will develop high RF voltage steps at the edges that will ionise the air and produce a glow, or an arc to a nearby object. Thin foils have a very tight radius of curvature at the edge so that is where the discharge occurs.

The metal is simply absorbing the microwaves and reradiating the energy as an electric current.

Energy can travel through space as an EM wave, or as a stream of ionised material. A current cannot be radiated except as a stream of electrons or ions.

Interesting - but metal in a microwave gets hot and arcs, so it must be absorbing some energy, no? And surely a microwave antenna is made of metal?

Induced eddy currents heat metal placed in an EM field or microwave oven.

A microwave receive antenna could be said to convert the E and M fields propagating in free space, into a V and I wave guided along a transmission line. The conductive metal is needed to lead the energy along a protected path to the front-end electronics.

 

[.Scott]

<Moderator's note: Quotation of deleted post removed..>

When I used the word "absorb", I mean as an antenna. It is taking in the energy. Receiving. Conducting.
As for the standing waves, since the effect can be seen with tiny pieces of metal that are substantially less than a quarter wavelength, it is clear that "harmonics" or standing waves are not required.

I meant there didn't need to be standing waves in the metal. Of course there could be some in the chamber. Nothing I've seen would argue one way or the other regarding the requirement of standing wave in the chamber.

I would also note that there are cases where arcing does not occur for some tens of seconds after the oven has been turned on (as with small aluminum fragments of CDs). So perhaps the metal is heating up and that is somehow contributing to the arcing.

 

[marcusl]

Scott, what bothers me is that your knowledge of E&M and of microwave engineering is rudimentary, to be generous, yet you make explanations and proclamations as though you are expert in the field. Asking questions is fine on PF, but disseminating misinformation is not.

 

[.Scott]

Scott, what bothers me is that your knowledge of E&M and of microwave engineering is rudimentary, to be generous, yet you make explanations and proclamations as though you are expert in the field. Asking questions is fine on PF, but disseminating misinformation is not.

You're right that I do not have any formal schooling in microwave engineering. But I have been working with radar for a few years now - and had worked a bit with it before that.
My language is not up to snuff, but that can be an advantage. I was the one on the team who was able to translate radar engineering speak into algorithm speak - an essential step in getting theory into an actual working device.

 

[marcusl]

Ok, so the metal doesn't heat up and the situation is altogether different than an antenna. An antenna delivers power from an EM wave into a resistive load. The load might get warm but the metal in the antenna does not. In fact, losses in microwave antennas and transmission lines are primarily due to dissipation in the dielectrics (look up loss tangent), not the metal.

 

[sophiecentaur]

Ok, so the metal doesn't heat up and the situation is altogether different than an antenna. An antenna delivers power from an EM wave into a resistive load. The load might get warm but the metal in the antenna does not. In fact, losses in microwave antennas and transmission lines are primarily due to dissipation in the dielectrics (look up loss tangent), not the metal.

Any metal structure can resonate at the appropriate frequency and that can involve very high levels of current at Voltage Nodes. These can cause heating after an interval of time. Otoh, the Voltage at an Antinode can occur more or less instantly and start an arc. I don't think there is a single simple answer to this one (any more than there is a simple answer to how any Antenna works).

 

[Dr_Zinj]

When microwave energy is introduced into a closed conductive metal box, there will be internal reflections. Standing waves will be formed within the oven..

Shall we really confuse him and mention making a microwave oven in the form of a truncated cone?

 

[Tom.G]

More irrelevant data:
My back-of-the-envelope calcs show the tines of the forks I use are 0.37 (3/8) of a wavelength, yielding 73% of the RF field voltage at the tip. The overall length is 1.5 wavelengths.

 

[Klystron]

My understanding is that a standing wave model adequately describes conditions within microwave (oven) cavities.
https://en.wikipedia.org/wiki/Standing_wave

Unwanted standing waves might cause problems during signal transmission; say in a a rectangular waveguide; but are effective in cavity resonators. Wikipedia contains decent descriptions of typical microwave tubes. Skip ahead to compare klystrons and magnetrons.
https://en.wikipedia.org/wiki/Resonator#Cavity_resonators

 

[sophiecentaur]

My understanding is that a standing wave model adequately describes conditions within microwave (oven) cavities.
https://en.wikipedia.org/wiki/Standing_wave

Up to a point. But when there is a significant load in there (a big enough lump of food) or a significant mismatch (a forbidden piece of metal) then the simple standing wave gets perturbed. This is particularly true when the added object is resonant, producing a dead short at a point in there.

 

[Klystron]

My understanding is that a standing wave model adequately describes conditions within microwave (oven) cavities. ...snip ...

Up to a point. But when there is a significant load in there (a big enough lump of food) or a significant mismatch (a forbidden piece of metal) then the simple standing wave gets perturbed. This is particularly true when the added object is resonant, producing a dead short at a point in there.

Yes, to be sure; the OP opened this thread with fireworks! In attempting to simplify the OP's voltage calculations using a standing wave model, I overlooked human factors in the 'experiment'.

Once (conductive) paint chips and silverware perturb the cavity, "all bets are off!", as the croupier shouts while spinning the wheel. Due to door interlocks -- meaning A/C is off thus no microwaves -- the paint chips and fork are already in the oven before the door is secured, power restored , and cavity resonance achieved.
[Please, folks on line, never exceed the operating parameters of your equipment or inhibit interlocks. --Norm.]

 

[Baluncore]

Once (conductive) paint chips and silverware perturb the cavity, "all bets are off!", as the croupier shouts while spinning the wheel.

It does not matter what dielectric material or conductive flakes are present in an oven, the standing wave pattern will obviously be more complex but there will still be one so long as there are reflections from the conductive walls. There is no requirement that standing waves in the real world be as simple as a textbook.

If the oven detected excessive reflected energy then it would probably reduce the duty cycle to regulate the magnetron temperature. A very clever oven might shut down with an error code or warning.

 

[sophiecentaur]

The SWR of an empty oven is only one factor in the volts that can exist within a structure that’s inserted. A large lossy lump of meat may reduce the VSWR by absorbing reflected waves and so could a fork with an arc that’s dissipating power.
I think the main risk when ‘playing’ with a microwave oven is probably physical damage to the case and glass and not to the magnetron itself.