Physics Curiosities

Why does the microwave oven not heat some objects and why should metallic objects not be placed in it?

The microwave oven, present in most homes, emits microwaves frequently in the 2.5 gigahertz range. The interesting feature of this frequency range is that the radiation excites the asymmetric molecules considerably, such as water, oils and sugars. Thus, when the appliance is used to heat food, only these molecules increase their internal energy, causing an increase in temperature.

The material of the plates and pots is, for the most part, formed by molecules of extremely symmetrical structure, so the heating of them is very small. But when we put food on a plate to be heated, isn’t this plate hot when it is removed from the microwave oven? The answer is yes, it is. However, microwaves are not the reason for this heating, but the direct contact of the dish with the heated food.

And why shouldn’t we put metallic objects in the microwave oven?

For two main reasons: firstly, because metal surfaces reflect microwaves, causing a kind of shielding that prevents waves from reaching liquid molecules. The other reason is that the electric field present inside the oven causes the appearance of electrical currents in the metals, which end up being charged and heating up quickly. Thus, if there is something like a piece of paper or anything else that could catch fire inside the microwave, a fire can be caused.

What cannot be put in the microwave

Metal – We all know that metal cannot be microwaved and this includes aluminum foil, metal lids, stainless steel containers, even tableware decorated with metallic paint (unless the manufacturer says they are microwave safe).

There is, however, an exception to this rule: when heating liquids, we recommend putting a teaspoon in the container to prevent the liquid from starting to boil when you are going to remove it from the microwave. By putting a teaspoon you avoid the risk of burns.

Hermetically sealed containers: Liquids or other foods should never be heated in hermetically sealed containers. You can place the lid on top but without closing it completely.

Food with a hard shell or skin: for example eggs, shellfish, sausages … even some fruits like apples. Before heating them you must prick the skin to prevent them from bursting.

Any tupperware or plastic container that does not specify that it is microwave safe: for example, many of those that come with food at home.

Take a test!

You should never turn on the microwave without food inside, except to perform this test . If you don’t know if a container is microwave safe, follow these steps:

  1. Place the empty container in the appliance for 1⁄2 – 1 minute at maximum power.
  2. Check the temperature from time to time. The container should be cold or warm. If it gets hot or sparks, it is not microwave safe.

With these tips, we hope you have no doubts about which containers can be put in the microwave , to cook safely and satisfactorily. If you want to ask us any questions, leave a comment below. We will reply to you as soon as possible!

What happens if we put metal in the microwave?

We should ask ourselves which metals would be the most dangerous in a microwave oven. Then we should ask ourselves whether the size or shape of the metal object matters or not.

Most of the metal utensils we have in our kitchens are made of aluminum or stainless steel. But it is not the type of metal in the microwave that determines how dangerous it can be, but its shape.

Many prepared meals are presented to us in aluminum foil trays, these trays are normally not dangerous, since they have the right and necessary thickness so as not to generate inconveniences, and in fact favor the heating of the content. 

If we accidentally forget a spoon  when we are going to heat our food, what will probably happen is that the spoon will get a little hot, but nothing much more serious than that. On the other hand, if we leave a fork in the microwave , things can get more dangerous.

It happens that the fork, due to its shape, would cause the impact of the waves between the metal particles to be much faster and more intense. The result: sparks!

Sparks don’t necessarily lead to a fire or the death of your entire family, but they can cause a serious breakdown in your microwave , damaging the magnetron .

Also, we may not die in the explosion, but what is certain is that our food has been cold, since the waves, bouncing so much between the metal elements, have isolated our lunch. 

Very interesting, right? Now you know what happens if you put metal in the microwave , has it ever happened to you?

Why not microwave metal?

If you ever forgot to remove your fork from the plate before using the appliance, do you know what we’re talking about?

It is something that most people know, under no circumstances should you put metal in the microwave. But what happens? Would a microwave fork or spoon blow up your house?

In fact, some metals can be put into the microwave oven. This is evident when you notice that many of the components in this appliance are made of metal.

Microwaves work using electromagnetic radiation. A transmitter within the microwave establishes a magnetic field that emits microwaves of radiation. These waves are directed at the water molecules in the food, causing them to move. This movement generates heat, which ends up heating your food.

The problem with metal is that it is much more solid than food. As food heats up, the water turns into steam and energy is released. When the molecules inside a piece of aluminum foil get hot, they have nowhere to go. Then the metal will heat up at high speed and this will eventually produce fire.

Depending on the shape and smoothness of the surface, some metal objects will reflect microwaves rather than absorb, causing them to bounce off the metal onto the walls and back, until the appliance overheats and ruins.

While nothing generally happens with spoons, forks create sparks by negatively charging your teeth. Depending on what else is microwaved, these sparks can start a fire. Even if they do not catch fire, they can damage the inside of the appliance and make it unsafe to use.

Despite this new information, it is probably not a good idea to put metal objects in the microwave just to see what happens. So in no way do we recommend doing it.

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