Why Does Metal Spark In The Microwave? (Science Behind It…!)

Almost every microwave oven owner has experienced sparking inside the microwave which looks like a miniature version of a light flashing inside the microwave chamber as you turn it on to get yourself a warm cup of coffee.

If you have accidentally left the spoon inside the cup while you turn the machine on, you will see those above-mentioned sparks.

This happens because the spoon is made of metal which is not a good conductor of electromagnetic waves. Putting metal in the microwave is thus always warned against in every microwave instruction manual. 

Why Does Metal Spark In The Microwave?

Why Does Metal Spark In The Microwave

Microwave ovens run by putting into use the mechanism of electromagnetic radiation, also known as microwaves. The magnetron inside the microwave is driven with a high-frequency voltage diode to generate these microwave radiations The continuous to and fro motion of the electrons inside the magnetron generates a magnetic field.

If metal is placed somewhere on this field, the electrons reflect off the metal surface instead. If the metal in question becomes hot enough for the molecules to start breaking up, it creates a spark.

Metals possess an effective free electrons which responds to externally applied electromagnetic fields as generated within the microwave. The microwaves within this field cannot penetrate the metal but instead create electric currents on the surface of the metal. At every place where the metal bends, the electromagnetic field gets concentrated and the sharper the bend, the more is the concentration of the field in that place.

With very sharp bends as seen in the edges of forks and scrunched up tin and aluminum foils, the electromagnetic field gets so heavily concentrated that it strips electrons off from the air molecules. The ionization of the air, resulting from electron loss results in the spark that we see. 

This phenomenon is often called sputtering and occurs because metals are ultimately made of molecules and eventually the final unit of matter, that is atoms. Heated molecules in the food facilitate warming up the food but when the atoms inside the metals heat up, they have no utility and end up causing sparking.

This is why you will always see strict instructions against microwaving metal from all manufacturers of microwave ovens and the utensils or containers that can be put inside the microwave always have to be either good quality plastic, such as microwave-safe plastic tupperware or microwave-safe glass, or ceramic.

Metal is so dangerous that even glass cups that have metal rims for decoration cannot be put inside the microwave.

What Happens When You Put Metal In The Microwave?

What Happens When You Put Metal In The Microwave

When you put a piece of metal inside the microwave, charges in the metal start moving around. If the metal in question is quite thin and has a sharp bend or edge on its surface then, the electric current gets concentrated on that sharp surface and causes sparks.

The spark is specifically caused by the building up of a high-voltage charge that exceeds the breakdown voltage of air.

The goal of a microwave oven is to heat food up and also to use the same heat to such an extent as to bring chemical and physical changes into the food, thereby cooking it. When you heat food, the water molecules inside the food heat up to become steam and this is why food is always heated from the inside-out when put in a microwave. When the same happens to a metal piece, its heated molecules have nowhere to go and nothing to do.

This is quite a dangerous situation as depending on what kind of metal we are talking about, it may cause fire right inside the microwave! If the metal is a good conductor of electricity then it will not overheat as is the case with most metal spoons one may accidentally put in the microwave.

But if even a small piece of aluminum foil gets stranded inside a microwave, it may get ignited after overheating and start a fire.

One may wonder why then is the interior of the microwave made of metal at times and even parts of items such as a cooking rack can be safely used inside a microwave. While manufacturers are against using metal inside microwaves, we have to remember that the metal interiors are made sure to be good conductors of electricity and are made really smooth without any no sharp points from where the electromagnetic waves can reflect from.

As for the metal parts of a cooking rack, it has to be covered with good quality protective plastic. When this plastic starts to wear off, then the insides of the microwave may start sparking.

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Why Can’t You Put Metal In The Microwave?

Why Can't You Put Metal In The Microwave

You cannot put metal inside the microwave for the simple reason that it is firstly not microwave-safe and secondly and more importantly, it may contain sharp edges that may cause sparking or arcing inside the microwave.

But why does this happen? Let us start at the very beginning. The microwave oven contains a part called the magnetron which is a vacuum tube through which a magnetic field flows.

The magnetron spins electrons around thus producing electromagnetic waves with a frequency of about 2.5 gigahertz which causes vibrations in OH bonds, thus heating the water contained in the food.

But when these charges encounter a metallic surface, it can neither penetrate the metal nor can it convert any present water molecules into steam as it does to food. The charges sort of accumulate at the surface of the metal and generate electric current at the surface.

Nothing really happens if the metallic surface is smooth. But if the metal is thin and it has sharp edges, the electric currents get heavily concentrated and ionize the air around it thus generating sparks. These currents exude large quantities of heat causing the metal to melt and sometimes explode.

So if you see a dented and misshaped metallic bowl after you have microwaved it accidentally, this is what happened to it.

The problem with this is the fact that metal just does not have a surface that can absorb the energy from the microwaves and transfer it to its surroundings.

Although a fire is not likely to happen unless the metal in question is extremely thin and susceptible to overheating to the point of getting ignited.

Is It Safe To Use A Microwave After Putting Metal In It?

Many of us have accidentally put metal in the microwave and then instantly regretted it when seeing the sparks inside the oven. This happens to quite a large number of people, far more than you’d think.

This is because metal and glass are the most common choice of utensils in households. Metals are more so because they are far less costly than glass ones. And people tend to forget to transfer their food into a microwave-safe container before they use it inside the microwave.

But the question is, is it safe to use the microwave after putting metal inside it? It depends on the metal. Aluminum foil, for example, is both thin and sharp, and therefore it is extremely dangerous to put it inside the microwave oven, especially if the foil is bunched up.

The foil will catch fire very soon and if not extracted at once, the fire will spread to the internal circuits of the machine and destroy it. Thin metal heats up very fast and the sharp surface deflects the heat haphazardly onto the oven interior.

As compared to this thick metal like steel plates and spoons are comparatively safer when put inside the microwave and might cause sparking but will not result in a fire as soon as a thin metal like aluminum foil will.

As for how safe a microwave is after a metal has been put inside it, if the metal has been removed and the microwave has stopped sparking then it is safe to use it. But if it caught fire even for a split instant before you extinguished it, it is better to not use the microwave and get it looked at by a professional who can fix damaged parts, if any.

If however, you see arcing, which is like an arch of electric currency between points inside the microwave, followed by a foul smell emitted from the microwave, you should immediately turn off the microwave and disconnect it from the electrical outlet and not use it before getting it repaired.

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The Bottom Line

Microwave-safety of a material depends on how it conducts itself before electromagnetic microwaves and also heat. It has to be a good conductor of microwaves and should be resistant to overheating and thermal shocks, i.e, sudden and extreme changes in temperature.

Since plastics, no matter how good their quality is, release chemical fumes when heated, only glass and ceramic can be considered the most easily available and safe materials for heating food inside the microwave.

Metals are not safe to be used inside the microwave and thus must be always avoided.

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