It's early morning, and your blurry attention has turned into a serving of oatmeal. You put the bowl into the microwave, press the start button and suddenly panic, while in your kitchen a mini firework is fired. The spoon – you forgot the spoon in the bowl!
While you may believe in movies that this electrical scenario can lead to a fiery explosion the truth is that putting a spoon in the microwave is not necessarily dangerous. But why does metal create sparks when exposed to one of the wonders of mid-20th century technology?
To answer that, we first need to understand how a microwave works. The small oven is based on a device called the magnetron a vacuum tube that causes a magnetic field to flow. The device whirls electrons around and generates electromagnetic waves at a frequency of 2.5 gigahertz (or 2.5 billion times per second), said Aaron Slepkov, a physicist at Trent University in Ontario, to Live Science.
Related: What are microwaves?
For each material there are certain frequencies at which it absorbs light particularly well, and that is 2.5 gigahertz frequency for water. Since most of the things we eat are filled with water, these foods absorb energy from the microwaves and heat up. Interestingly, 2.5 gigahertz is not the most efficient frequency for heating water, Slepkov said. That's because the microwave-invented company Raytheon has noticed that the high-efficiency frequencies are too good at work, he noted. Water molecules in the top layer of something like soup would absorb all the heat so that only the first few millionths of an inch would cook and leave the water under the stone cold.
Now for this sparking metal. When microwaves interact with a metallic material, electrons slosh around on the surface of the material, Slepkov explained. This does not cause any problems if the metal is smooth everywhere. But where there is an edge, such as the tines of a fork, the charges can stack and lead to a high concentration of stress.
"If it is high enough, it can rip an electron off a molecule in the air" and create a spark and an ionized (or charged) molecule, Slepkov said.
Ionized particles absorb microwaves even more than water. So when a spark appears, more microwaves are sucked in and more molecules ionize, making the spark grow like a fireball, he said.
Typically, such an event can only occur in a rough-edged metal object. That's why "if you take aluminum foil and place it in a flat circle, it may not spark at all," said Slepkov. "But if you crumble it into a ball, it will spark quickly."
Although these sparks have the potential to damage the microwave oven, all foods should be properly consumed, according to an article by Mental Floss (just in case you've forgotten the spoonful in your oatmeal). [1
Metals are not the only objects that can produce a light show in a microwave. Viral Internet videos also showed halved grapes producing the spectacular sparks of plasma, a gas of charged particles.
Several detectives had searched for an explanation, suggesting that it was a charge like a metal. However, Slepkov and his colleagues conducted scientific tests to get to the bottom of the phenomenon.
"What we found was much more complicated and interesting," he said.
By filling hydrogel spheres – a superabsorbent polymer used in disposable diapers – with water, researchers found that geometry is the most important factor in generating sparks in grape-like objects. Grape-sized balls happen to be particularly good concentrators for microwaves, said Slepkov.
The size of the grapes caused the microwave radiation to accumulate in the tiny fruits, eventually giving enough energy to rupture an electron. From sodium or potassium in the grape, it produced a spark that grew into a plasma.
The team repeated the experiment with quail eggs – about the size of grapes – first with their natural, egg-yellow interior and then with the effluent liquid. The goose-bumped eggs spawned hotspots, while the empty ones did not, indicating that an aqueous chamber of grape size was needed to mimic the metallic sparkling spectacle.
Originally published on Live Science .