How can microwaves melt metal?

[Mk]

Can microwaves melt metal? As far as I know, it can't melt a spoon no matter how long you put it in a microwave oven. Is it possible to get microwaves to melt metal?

 

[FredGarvin]

If memory serves me correctly, microwaves excite molecules like fat molecules and the like.

From wiki:

A microwave oven works by passing microwave radiation, usually at a frequency of 2.45 GHz (a wavelength of 12.24 cm), through the food. Water, fat, and other substances in the food absorb energy from the microwaves in a process called dielectric heating. Many molecules (such as those of water) are electric dipoles, meaning that they have a positive charge at one end and a negative charge at the other, and therefore rotate as they try to align themselves with the alternating electric field induced by the microwaves. This molecular movement creates heat as the rotating molecules hit other molecules and put them into motion.

 

[Mk]

Yes, but there's no possibility of them melting some kind of metal?

 

[vanesch]

Well, microwaves induce currents in metals.

 

[Mk]

Well, microwaves induce currents in metals.

And the currents could for sure cause resistive Joule heating, which could heat and melt the metal?

 

[momentum_waves]

Gold foil on plates burns in a microwave.

 

[lpfr]

You can certainly heat metals in microwave ovens.
But the frequency is chosen to work better on polar dielectrics as water.
In metals lower frequencies work better because of the lesser skin effect. RF heating is or has been used in industry for years. Frequencies where near 100 kHz.
If you do this kind of experiences with your microwave oven, do not forget that the inner walls of the oven are metallic, and that they will heat. Beware which melt first.

 

[jessb]

http://www.popsci.com/popsci/how20/15499aa138b84010vgnvcm1000004eecbccdrcrd.html

try that.

or, don't, because it's quite dangerous.

 

[FredGarvin]

Neat article. Notice they could only get solder to melt. I wonder what kind of power the microwave he was using was? The one thing I take exception to is that the author describes a microwave as an induction furnace. Joule heating and dielectric heating are not the same thing to my knowledge.

This is news to me because I have never had the notion to try to melt metal. I was always afraid of the arcing that would happen.

 

[Mk]

http://www.popsci.com/popsci/how20/15499aa138b84010vgnvcm1000004eecbccdrcrd.html

try that.

or, don't, because it's quite dangerous.

Oh yes! I remember that! I thought it was very cool and I might try it myself in my garage sometime.

 

[Roy Edmundson]

Hi everyone,

Have any of you tried a CD or DVD recordable blank in the microwave?

Its quite spectacular. The thin backing layer of evaporated aluminium only a few tens of atoms thick must be considerable thinner than the skin depth (If anyone really wants I can do the arithmetic and tell you exactly what the skin depth is at 2.4 GHz). The thin Aluminium layer is heated by the surface currents excited by the EM field.

A friend of mine washed some of his CDs and knowing that the 2.45GHz is carefully chosen to be one of the resonant absorbtion peaks of water, thought that just the water would get hot and quickly evaporate. But Horrors of Horrors his precious information exploded into an instant fireworks display.

 

[delta001]

I melted metal once. I wasn't entirely sure what it was, but it melted when put on a stove, and solidified quite hard again afterwards. It didn't glow red-hot, I don't think a stove iron has that much heating power.

 

[ShawnD]

I was always told that microwave ovens send out magnetic waves as well as microwaves. Things capable of soaking up a magnetic field (iron) will do so, and it induces a current. If the metal object in the microwave is uniform and well-connected, like a spoon, it will just heat up. If the object has lots of spaces and is poorly connected to dissipate charge, differences in charge will build up and the item will spark (steel wool).

Put a spoon in the microwave and see what happens. Put steel wool in the microwave and see what happens. Better yet, put steel wool in a mixture of sugar and drano; that's the most fun thing ever! I ruined a microwave diong that because the fire got out of control and it coated the microwave with carbon. It couldn't even heat up a glass of water after that.

 

[Roy Edmundson]

I was always told that microwave ovens send out magnetic waves as well as microwaves. Things capable of soaking up a magnetic field (iron) will do so, and it induces a current. If the metal object in the microwave is uniform and well-connected, like a spoon, it will just heat up. If the object has lots of spaces and is poorly connected to dissipate charge, differences in charge will build up and the item will spark (steel wool).

Put a spoon in the microwave and see what happens. Put steel wool in the microwave and see what happens. Better yet, put steel wool in a mixture of sugar and drano; that's the most fun thing ever! I ruined a microwave diong that because the fire got out of control and it coated the microwave with carbon. It couldn't even heat up a glass of water after that.

Microwaves consist of a stream of photons, just like light or X-rays, but with a very much lower frequency of oscillation. All these are collectively described as electromagnetic radiation, and also include radio waves which are at even lower frequencies. This is because there is both an oscillating magnetic field and an oscillating electric field present. The propagating EM wave can be described mathematically by the Poynting Vector (look up Poynting Vector in Wikipedia, also vector cross product, Maxwells Equations, vector algebra)
If you have only year 10 Math and no knowledge of calculus, vectors, complex numbers or matricies, then the mathematical symbols and concepts used are going to present you with some problems.
I can aim you in the right direction if you post again with any questions.

Poynting Vector = the vector crossproduct of the electric field and the magnetic field. or in symbols :- S = E X B
The X symbol isn't a multiply sign, but the crossproduct operator symbol.

To put it is to plain English the Poynting vector points in the direction the EM radiation is going, and the electric and magnetic fields oscillate in phase with each other at right angles to this as well as at right angles to each other. The displacement current, and the magnetic field it produces, act like a mirror to the microwaves and reflect them - reflection of light or radio waves works the same way. It is impossible to have a propagating electric field without the accompanying magnetic field, and viceversa. The two are opposite sides of the same coin, like love and hate.

When an EM wave passes within a few wavelengths of a conducting surface it induces a Displacement Current in the metal close to the surface. The current density decreases with depth exponentially - this is known as the skin effect. The phase relationship of the current with respect to the EM wave also varies with depth and the current will be going in different directions at different depths.
The skin depth is the depth at which the current density is 1/e of that at the surface. Skin depth for copper is as follows
60Hz - 8.57mm
10kHz - 0.66 mm
1MHz - 66 micron
1GHz - 2.1 micron
Depth is proportional to the reciprocal of frequency squared

Skin depth for non-magnetic metals is only dependant on conductivity, shallower for better conductors. If the metal is ferromagnetic, then the skin depth is reduced significantly on account of the magnetic properties. Iron although it is a worse conductor than copper by a factor of 5 has a skin depth about 1/6 that copper because of this. Cobalt and Nickel will be similar.

Look up Skin Depth in Wikipedia - there is a full mathematical explanation and some graphs.

Very little displacement current flows deeper than a few skindepths. Waveguides that act as "pipes" for microwaves are often silver plated in order to reduce the resistance that the displacement currents see. The plating only needs to be a few microns thick to reduce the transmission losses by a worhwhile amount. Microwave heating of metal surfaces - eg the metalwork in a microwave oven - is minimal if the metal is thicker than a few skindepths, but if the metal is very thin - eg in the CDs mentioned in my post above - then the same total displacement current must flow, and so the current density higher. In the case of the CDs the aluminium coating is maybe only 10nm thick at best and therefore only ~1/100th the skin depth. The current density will be 100 times higher than if the metal were even only a few microns thick, and the joule heating quickly melts the metal and once the continuous original sheet is fractured by contraction due to surface tension moving the molten metal, the high voltages cause arcing and incandescense. The reason the wirewool mentioned in the above quote bursts into flame is because high voltages are produced between steel filaments that arnt quite touching and arcing ocurrs. Also if there are only small points of contact the high currents flowing there cause considerable heating. Similar sparking occurs if you put an aluminium foil plate in the microwave with a stainless steel spoon on it, but any single solid piece of metal doesnt.

 

[ShawnD]

Similar sparking occurs if you put an aluminium foil plate in the microwave with a stainless steel spoon on it, but any single solid piece of metal doesnt.

Interesting. I would totally try this if I still had a spare microwave.

 

[rashean123]

oh really, i didn't know u could do that if i happen to get an extra microwave ill try that,

thanks for the post

 

[f3nr15]

Well, microwaves induce currents in metals.

Oh OK so microwaves have a separate magnetic flux.

 

[lpfr]

Oh OK so microwaves have a separate magnetic flux.

Not a separate one. All electromagnetic waves, including microwaves, light and gamma rays, have an electric field and a magnetic field, whose amplitudes are related (proportionnaly).

 

[f3nr15]

Not a separate one. All electromagnetic waves, including microwaves, light and gamma rays, have an electric field and a magnetic field, whose amplitudes are related (proportionnaly).

On a EMR graph, there's a graph with a x, y & z axis (for the magnetic field perpendicular to the electric field) ... It's apparently a sine curve, but as soon as the value of x goes past 180 degrees it immediately goes onto the z axis becoming a magnetic field then an electric field once the value once it goes through 180 degrees.
That means, you could say - electromagnetic radiation has changing electric fields & changing magnetic fields, that it ?

But then, if I were to shine this bright light around this piece of metal and just shake it to and fro, would this induce current ?
Because after all, visible light is on the EMR spectrum, or is it the Earth's natural magentic field which cancels it out ?

 

[lpfr]

Yes, I know the graph. You find it in almost every EM textbook. It is, of course, correct, but I found it misleading to most of my students.
So I did another drawing of an electromagnetic plane wave (a wave that comes from far away). Imagine a rectangular chunk of space, chopped in slices as an "American sandwich bread".
In the first slice drawn, the electric field E is vertical and downward and the magnetic field B is horizontal and directed to the right. This is so in all the volume of the slice (the drawing displays just a "square" of the slice which is very much large.
The amplitude of both fields are maximum in middle of the slice (between the two faces) and decreases as a sinusoid to attain zero just at the faces of the slice.
The situation is identical for each other one slice. In the slices in-between the direction of both fields is just reversed.
Now, all the slices move at the speed of light v (or c if it is in vacuum) as the arrow indicates.
The thickness of two slices is the wavelength of the wave.
The real dimensions of the slices depend on the wavelength. For AM radio waves you would be a microscopic spot in the drawing. But, if the EM waves are light, it is the drawing which would be a microscopic spot in your pupil.
Think about this drawing. If you "see" it, then you will be able to understand the other one and realize that, of course, it is also correct.

 

[f3nr15]

Yes, I know the graph. You find it in almost every EM textbook. It is, of course, correct, but I found it misleading to most of my students.
So I did another drawing of an electromagnetic plane wave (a wave that comes from far away). Imagine a rectangular chunk of space, chopped in slices as an "American sandwich bread".
In the first slice drawn, the electric field E is vertical and downward and the magnetic field B is horizontal and directed to the right. This is so in all the volume of the slice (the drawing displays just a "square" of the slice which is very much large.
The amplitude of both fields are maximum in middle of the slice (between the two faces) and decreases as a sinusoid to attain zero just at the faces of the slice.
The situation is identical for each other one slice. In the slices in-between the direction of both fields is just reversed.
Now, all the slices move at the speed of light v (or c if it is in vacuum) as the arrow indicates.
The thickness of two slices is the wavelength of the wave.
The real dimensions of the slices depend on the wavelength. For AM radio waves you would be a microscopic spot in the drawing. But, if the EM waves are light, it is the drawing which would be a microscopic spot in your pupil.
Think about this drawing. If you "see" it, then you will be able to understand the other one and realize that, of course, it is also correct.

I saw one that looked like a sine curve I mean, each time it completed a 90⁰ period (clarify my wave terminology) it would go to the z-axis which is the magnetic field. Another 90⁰ later it would go back to the y-axis which is the electric field.

And because it was a sine curve, as x was increasing, the amplitude also changed hence a changing electric led then changing magnetic field and vice versa that's how I perceived it.

 

[Roy Edmundson]

I think the last 2 posts by f3nr15 are explained by his misinterpretation of what he was looking at. I suspect he thinks it is just one graph he's looking at wherase it is two that are superimposed using the same set of coordinates. One for the electric field and another for the magnetic field. It is always dificult to convey 3 dimensional information in a 2D drawing, and I think this has produced the confusion in the present situation.

Suppose a microwave EM wave is traveling straight at you, then , if your eyes could see the electric and magnetic fields existing as the wave reaches you, this is what you would see. The electric field would be oscillating, say, up and down in a vertical straight line and the magnetic field would be oscillating at exactlyn90 degrees to it, from side to side in a horizontal straight line. Both oscillations will be simple harmonic and in exact phase with each other. If we freeze the wave in time and now travel towards the microwave transmitter, measure both fields and plot a 3D graph of each on the same set of coordinates we will see two sine waves, and the graph that is commonly shown. The electric field sine wave will be vertical and the magnetic, horizontal. The distance in space between sucessive crests of either wave is the wavelength. Frequency and wavelength are related by the following equations.

Frequency = Speed of light/Wavelength

or Wavelength = Speed of light/Frequency

or Speed of light = Frequency x Wavelength

Speed in m per second, Wavelength in m, Frequency in Hz

The frequency may be counted electronically with a frequency counter to frequencies of a few GHz easily and with increasing difficulty and rapidly increasing cost to a few 100GHZ. Beyond high microwave frequencies when direct counting is no longer possible, frequencies are infered by measuring the wavelength. Other options include beating with a known frequency to get a difference frequency that is low enough to be counted.

So for infrared of 1 micron wavelength, just outside the visible spectrum the frequency is:-

299,792,458/(1 x 10^-6) = 299,792,458,000,000 Hz = ~300THz


The above is a description of a continuous plain EM wave which consists of a continuous stream of phase locked photons such as would be emitted from a simple dipole antenna. Near to the antenna the wave will be spherically expanding, but at a distance of a few hundred wavelengths is an approximation to a plane wave.

By using a helical antenna it is possible to produce a circularly polarised wave. Each microwave photon emitted still has the electric and magnetic fields at right angles to each other, but as each sucessive photon is emitted the plane of polarization is rotated slightly so resulting in a rapidly rotating plane of oscillation for the entire wave.

 

[lpfr]

I saw one that looked like a sine curve I mean, each time it completed a 90⁰ period (clarify my wave terminology) it would go to the z-axis which is the magnetic field. Another 90⁰ later it would go back to the y-axis which is the electric field.

And because it was a sine curve, as x was increasing, the amplitude also changed hence a changing electric led then changing magnetic field and vice versa that's how I perceived it.

In an EM wave, the variations in time of electric field create magnetic field and the variations in time of magnetic field create electric field.

Understanding the graphs and making a mental idea of what is an EM wave asks for reflection and mental endeavor. And it is you who must do it. No one can think on your behalf. Think again about the two graphs.

 

[Ashlyn]

can someone tell me how i can use or convert a domestic microwave oven to burn off accumulated dirt in used gold(gold content 91.6%) jewelry

 

[Ashlyn]

while i don't plan to melt metals someone has tried melting silver http://home.c2i.net/metaphor/mvpage.html