Why metal sparks in a microwave oven

In summary: The reason the sparking happens is because the oven cavity is a resonator -- it's filled with oscillating electric and magnetic fields. The electrons in the metal have to constantly move back and forth to keep the net field inside the conductor zero (conductors always have zero charge inside them, as the electrons are free to slide around to neutralize any field). Sharp point of metal can thus generate very very high voltages on them when the electrons are all sloshed in one direction. The sharper the point of metal, the higher the voltage. (Hence, a fork is more dangerous in a microwave than is a spoon.) When the voltage is large enough to exceed the breakdown voltage of the air, the air becomes a plasma and electrons
  • #1
repugno
78
0
Greetings all,

I've recently been wondering why I see sparks of light when I stick certain metal objects into a microwave. I figured that it has something to do with photons supplying energy to the electrons. When the electrons gain the energy they move to higher energy levels, but when they return they emit light. Is this a viable explanation?
 
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  • #2
No. What you are seeing are not "sparks of light". They are, in fact, actual sparks- that is electrical sparks because the microwaves are stimulating the electrons in the "metal objects" you foolishly put in the microwave. They can burn out the microwave and even conceivably start a fire! The manual that came with your microwave warns against putting metal objects in a microwave. Stop doing it!
 
  • #3
HallsofIvy said:
No. What you are seeing are not "sparks of light". They are, in fact, actual sparks- that is electrical sparks because the microwaves are stimulating the electrons in the "metal objects" you foolishly put in the microwave. They can burn out the microwave and even conceivably start a fire! The manual that came with your microwave warns against putting metal objects in a microwave. Stop doing it!

Thanks for the warning. These sparks must be emitting light otherwise I would not be able to see it? What exactly is the reason for these sparks to occur?
 
  • #4
On the topic of microwave ovens, is anyone able to explain why Ice doesn't melt inside when put a microwave?
 
  • #5
You can only damage the oven if the sparks actually complete a short circuit through the magnetron itself. It would take a very large piece of metal to do this, and probably won't ever happen. The only other danger is sending pieces of molten metal into the walls of the oven, damaging it cosmetically (though probably not functionally).

The reason the sparking happens is because the oven cavity is a resonator -- it's filled with oscillating electric and magnetic fields. The electrons in the metal have to constantly move back and forth to keep the net field inside the conductor zero (conductors always have zero charge inside them, as the electrons are free to slide around to neutralize any field). Sharp point of metal can thus generate very very high voltages on them when the electrons are all sloshed in one direction. The sharper the point of metal, the higher the voltage. (Hence, a fork is more dangerous in a microwave than is a spoon.) When the voltage is large enough to exceed the breakdown voltage of the air, the air becomes a plasma and electrons jump across the air in a spark. Their collision with the atoms in the air produces the light that you see.

- Warren
 
  • #6
dcl said:
On the topic of microwave ovens, is anyone able to explain why Ice doesn't melt inside when put a microwave?

The microwaves in the oven are at a frequency readily absorbed by liquid water. Ice doesn't absorb at that frequency as well and thus doesn't absorb the energy as efficiently.
 
  • #7
dcl said:
On the topic of microwave ovens, is anyone able to explain why Ice doesn't melt inside when put a microwave?

It is because the dielectric loss factor of ice is about 1/3000th that of water. That means that ice is about 1/3000th as able to absorb microwave energy to cause heating. The reason is that water molecules in liquid water are randomly located and essentially are free to rotate, and it is this rotation, as influenced by the microwave field that causes the heating (it's actually much more complicated but that should do for now). In ice, however, the water molecule is caught in a latice of other water molecules and is therefore not free to rotate so it can't generate heat. What is really interesting about this is that there is always some free water unless you are at at temperature of about minus 125 F (I'm not exactly sure of that number but it is close to that). So, what can happen, is that water starts to heat, the microwave field distorts so that area heats favorably, and you can end up with the crazy situation of boiling water inside or on the surface of an ice block.
 

1. Why does metal spark in a microwave oven?

Metal sparks in a microwave oven because the microwaves cause the metal to reflect the waves and create an electric current. This current can create sparks and potentially damage the microwave or even cause a fire.

2. Can any type of metal spark in a microwave?

Yes, any type of metal can potentially spark in a microwave. However, metals with sharp edges or points tend to spark more easily than smooth or rounded metals.

3. Is it safe to put metal in a microwave if it doesn't spark?

No, it is never safe to put metal in a microwave, even if it doesn't spark. The metal can still reflect the microwaves and cause damage to the microwave or create a fire hazard.

4. How can I prevent metal from sparking in a microwave?

The best way to prevent metal from sparking in a microwave is to simply not put any metal objects in the microwave. If you must use metal, make sure it is microwave-safe and follow the instructions carefully.

5. What should I do if metal sparks in a microwave?

If metal sparks in a microwave, immediately turn off the microwave and remove the metal object using oven mitts or tongs. If there is a fire, use a fire extinguisher or baking soda to put it out. Do not use water, as this can create an electric shock. If the microwave is damaged, do not use it and contact a professional for repair.

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