It's early morning and your bloodshot alert has helped instant oatmeal. You put the bowl in the microwave, hit the start button, and suddenly panic as a mini fireplace spills into your kitchen. The spoon – you forgot the spoon in the bowl!
While movies may believe that this electric scenario can lead fire explosion, the truth is that putting a spoon in the microwave is not necessarily dangerous. But why exactly does metal generate sparks when subjected to one of the miracles of mid-20th century technology?
To answer that, we must first understand how a microwave oven works. The tiny oven depends on the appliance called a magnetron, a vacuum tube through which a magnetic field flows. The device carries electrons around and produces electromagnetic waves with a frequency of 2.5 gigahertz (or 2.5 billion times per second), Aaron Slepkov, a physicist at Trent University in Ontario, told Live Science.
Related: What Is a Microwave?
For each material, there are particular frequencies at which it absorbs light particularly well, he added, and 2.5 gigahertz happens to be this frequency for water. Because most things we eat are full of water, these foods absorb energy from the microwave and will heat up.
Interestingly, 2.5 gigahertz is not the most effective frequency for hot water, Slepkov said. That's because the company that invented the microwave, Raytheon, realized that the very effective frequencies were too good for their job, he noted. Water molecules in the top layer of something like soup would absorb all the heat, so only the first few million inches would boil and leave the water under a stone cold.
Now about that spinning metal. When microwaves interact with metal material, the electrons on the surface of the material are broken, Slepkov explained. This does not cause problems if the metal is smooth all over. But where there is an edge, as in the rows of a fork, the loads can accumulate and result in a high concentration.
"If it's high enough, it can pull an electron from a molecule in the air," creating a spark and an ionized (or charged) molecule, Slepkov said.
Ionized particles absorb microwaves even more strongly than water, so when a sparkle appears, more microwaves will suck, ionizing even more molecules so that the spark grows like a fireball, he said.
Usually such an event can occur only in a metal object with rough edges. That's why "if you take an aluminum sheet and put it in a flat circle, it may not pop up at all," Slepkov said. "But if you break it into a ball, it will pop up quickly."
While these sparks have the potential to cause damage to the microwave oven, some food should be perfectly edible afterwards (only if you really forgot that spoon in your oats), according to a Mental Floss article.
Metals are not the only objects that can generate a light show in a microwave. Viral internet videos also showed half grapes producing spectacular sparks of plasma, gas of charged particles.
Various sleds have sought clarification, suggesting that it is about building an electric charge as in metal. But Slepkov and his colleagues have made scientific attempts to get to the bottom of the phenomenon.
"What we found was much more complicated and interesting," he said.
Filling hydrogel spheres – a superabsorbent polymer used in disposable diapers – with water, the researchers learned that geometry was the most important factor in generating sparks in grape-like objects. Vine-sized spheres have recently been particularly excellent microwave concentrators, Slepkov said.
The size of the grapes caused the microwave radiation to accumulate inside the tiny fruits, eventually resulting in enough energy to extract electricity from sodium or potassium in the grape, he added, creating a spark that grew in plasma.
The team repeated the experiment with quail eggs – which are about the same as grapes – first with their natural, white insides and then with the liquid poured out. The goo-filled eggs generated points, while the empty ones did not, indicating that imitating the metal-popping spectacle required a watery, grappling room.
Originally published on Life Science.