Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How can microwaves melt metal?

  1. Apr 18, 2007 #1

    Mk

    User Avatar

    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?
     
  2. jcsd
  3. Apr 18, 2007 #2

    FredGarvin

    User Avatar
    Science Advisor

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

    From wiki:
     
    Last edited: Apr 18, 2007
  4. Apr 18, 2007 #3

    Mk

    User Avatar

    Yes, but there's no possibility of them melting some kind of metal?
     
  5. Apr 18, 2007 #4

    vanesch

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Well, microwaves induce currents in metals.
     
  6. Apr 18, 2007 #5

    Mk

    User Avatar

    And the currents could for sure cause resistive Joule heating, which could heat and melt the metal?
     
  7. Apr 18, 2007 #6
    Gold foil on plates burns in a microwave.
     
  8. Apr 19, 2007 #7
    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.
     
  9. Apr 19, 2007 #8
  10. Apr 19, 2007 #9

    FredGarvin

    User Avatar
    Science Advisor

    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.
     
    Last edited: Apr 19, 2007
  11. Apr 20, 2007 #10

    Mk

    User Avatar

  12. Apr 23, 2007 #11
    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 realy 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.
     
  13. Apr 24, 2007 #12
    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.
     
  14. Apr 24, 2007 #13

    ShawnD

    User Avatar
    Science Advisor

    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.
     
    Last edited: Apr 24, 2007
  15. Apr 24, 2007 #14
    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 isnt 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.
     
  16. Apr 25, 2007 #15

    ShawnD

    User Avatar
    Science Advisor

    Interesting. I would totally try this if I still had a spare microwave.
     
  17. Apr 25, 2007 #16
    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
     
  18. Apr 26, 2007 #17
    Oh OK so microwaves have a separate magnetic flux.
     
  19. Apr 26, 2007 #18
    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).
     
  20. Apr 26, 2007 #19
    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 ?
     
  21. Apr 27, 2007 #20
    Yes, I know the graph. You find it in almost every EM text book. 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.
     

    Attached Files:

    Last edited: Apr 27, 2007
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: How can microwaves melt metal?
Loading...