Metal in microwave-why not?

  • Thread starter MonstersFromTheId
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In summary, putting metal in a microwave can be dangerous because the sharp edges and points on the object can enhance the electric field, leading to a high current density and potential for electrical discharges. This can be caused by either heating effects or emitted electrons from the metal. The magnetic field in the microwave can also contribute to heating effects on the metal.
  • #1
MonstersFromTheId
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Like many I've heard the 'ol rule - DON'T put metal in a microwave oven.
But the thing is, I've neve heard anyone explain WHY you shouldn't put metal in a microwave.
I've seen it done, at least on TV, the metal sparks, but so what?

What exactly is dangerous about putting metal in a microwave?
 
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  • #2
Food absorbs microwave radiation by excitation of molecular bonds, which cause the atoms to vibrate. Typically, microwaves excite the OH bond in water.

The metal will reflect microwaves, and (by itself, i.e. not in contact with a hydrogen-bearing organic substance (usually containing water)) it can cause electrical discharges if the power density is high enough. Any discharge can damage the microwave.

Actually, if one puts a metal object (e.g. spoon) in a piece of food or in liquid containing water (e.g. coffee, tea, soup, stew) there is no problem.

But put a metal object on a plate and one will likely see electrical discharges, like lightning.

If one wraps (completely surrounds) food in Al foil, the food will not heat in the mircrowave, but one will get electrical discharges.

Putting food on a metal plate is OK in some microwave models. The microwaves are simply reflected into the food.
 
  • #3
Hey. I don't get it. How does metal reflecting microwaves cause electrical discharges?
 
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  • #4
A sufficient charge density builds up, usually on points, and IIRC, the atoms in the metal are ionized (electrons knock off) until a potential difference builds up, and then at a critical potential difference, a discharge (like lightning) occurs.

The discharge (electrical surge) can damage sensitive microelectronics.

The issue with reflecting microwave radiation is that microwave ovens are designed with shielding assuming the user (consumer) uses the microwave for the purpose for which it is intended, i.e. heating (cooking) food or drinks. Alternate uses can put the user at risk.
 
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  • #5
Typically, microwaves excite the OH bond in water.

Microwave Ovens normally use 2.4Ghz range and it disrupts Wi-Fi.

http://www.wi-fiplanet.com/tutorials/article.php/3116531

I have read, on a number of websites, that Microwaves cannot be focused, is that true?


Disclaimer Notice:
The issue with reflecting microwave radiation is the microwaves are designed with shielding assuming the user (consumer) uses the microwave for the purpose for which it is intended, i.e. heating (cooking) food or drinks. Alternate uses can put the user at risk. :))
 
  • #6
The sparking and discharging caused by putting metals in the microwave only happens when the metal is not a well-connected solid piece.

Put a spoon in the microwave and run it for 10 seconds. Did it spark? Is the spoon hot?

Put a piece of steel wool in the microwave and run that for 10 seconds. Did it spark? Is the steel wool hot?

Actually test these yourself and report back what you find. It will be ok as long as you put the spoon and the steel wool on several thick glass/porcelain (dinner) plates. This is to prevent any electrical buildup on the spoon/wool from feeding back into the magnetron and damaging the microwave oven.
 
  • #7
I'm pretty sure microwaves can be focused since that's what they use for satellite communications...
 
  • #8
A maser is a microwave frequency laser. I'd consider that focused.
 
  • #9
Danger said:
A maser is a microwave frequency laser. I'd consider that focused.
Yep, and one can do interesting things with a maser.
 
  • #10
Astronuc said:
Yep, and one can do interesting things with a maser.
Fishing comes to mind...:uhh:
 
  • #11
Or hunting squirrels...

http://www.poster.net/eraser/eraser-arnold-schwarzenegger-vanessa-williams-3700089.jpg
 
  • #12
Oh great, Mk... now I'm going to have to rent the damned movie to see what you're talking about.:rolleyes:
 
  • #13
This pertains to focusing microwaves. I wouldn't consider a MASER a focused beam (coherent, not focused). However, you can focus microwaves, you just need something with a different index of refraction than the surrounding material (e.g., teflon) and shape the material into a lens. Many police RADAR units have a lens on the front to limit the beam to about a 6 degree divergence.
 
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  • #14
The microwaves (being EM waves) cause the free electrons in metal to slosh around, energising them. There is no readily available means for this energy to dissipate from the metal object, so it continues to build to the point where electrical discharging can occur.

Claude.
 
  • #15
MonstersFromTheId said:
Like many I've heard the 'ol rule - DON'T put metal in a microwave oven.
But the thing is, I've neve heard anyone explain WHY you shouldn't put metal in a microwave.
I've seen it done, at least on TV, the metal sparks, but so what?

What exactly is dangerous about putting metal in a microwave?

In practically all objects, the "smooth" surface isn't that smooth. This is especially true for most household objects. When you put this object into EM field environment, these pointy parts of the object enhances the electric field at that point. When you have sharp, pointy geometry that can be as small as sub-microns in size, these can enhances the field quite a bit. Now what can happen here is that, depending on how sharp and how high the field is, you can initiate a field emission current - electrons that tunnel out of these tips via tunneling and the large bias electric field. This current can be quite high, or I should say, the current DENSITY can be quite high, since it is coming out of a very small area of the tip, and the current density is higher in metals than in an insulator because of the abundance of conduction electrons.

Because of this current, a number of things can happen in this case (i.e. microwave oven scenario and not in a vacuum): (i) there can be heating effects that could melt the very tip, which in turn causes more adsorbed gas to be liberated and causing a discharge, or (ii) the emitted electrons themselves have enough energy to cause a discharge in the gasses already present in the microwave.

Note that the magnetic field in the microwave can also induce surface currents on the metal, even at the tip, to also add to the heating effects. So the situation isn't that simple if one wants to really dig into this.

Zz.
 
  • #16
The metal reflects the microwaves, and if they get reflected back into the magnetron, which generates them, it can be damaged or destroyed.
 

1. Why can't metal be put in the microwave?

Metal should not be put in the microwave because it can cause sparks and potentially damage the microwave. This is because metal is a good conductor of electricity and can reflect the microwaves, causing them to bounce around and create a dangerous buildup of electrical energy.

2. What happens if you put metal in the microwave?

If metal is put in the microwave, it can create a dangerous situation. The metal can reflect the microwaves and cause sparks, which can lead to fires or damage to the microwave. It is important to never put metal in the microwave.

3. Can any type of metal be put in the microwave?

No, not all types of metal can be put in the microwave. Thin, flat pieces of metal, such as aluminum foil or silverware, should never be put in the microwave. However, some thicker, rounded metal objects, such as a spoon, may be safe to use in the microwave for a short amount of time.

4. Is it safe to put metal in the microwave if it is covered with food?

No, it is still not safe to put metal in the microwave even if it is covered with food. The metal can still reflect the microwaves and cause sparks, which can be dangerous. It is important to always remove any metal objects from food before heating it in the microwave.

5. Can metal containers be used in the microwave?

Some metal containers may be safe to use in the microwave, but it is important to check the manufacturer's instructions first. Some metal containers may have a special coating that makes them safe to use in the microwave, while others may not be safe. It is always best to use microwave-safe containers to avoid any potential hazards.

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