Lightning in the Kitchen: Why Does Metal Spark in the Microwave?

At the end of the 1970s, as long hair made way for cropped cuts and Michael Jackson decided that 1 could reach higher heights than 5, there were microwave ovens in more than 20% of American homes. By 1993, despite facing sexual abuse allegations, the King of Pop was a household institution; just like microwaves, which had now found a permanent home in 80% of residences (Pittsburgh Post-Gazette, 1990).

Figure 1. Metal in the microwave is bad news. (Olsen, 2008)

It would be fair to say, then, that by 2022 at least 3 generations of children have grown up with the cardinal rule of microwave cooking seared into their mind: don’t put metal in there! You will have heard that it causes sparks and bangs like Bonfire Night squashed inside a cubic foot of metal. You may even have fallen victim to a forgotten fork or an accidental aluminium container and witnessed the fireworks for yourself. The risk of starting a fire is no joke and for the record, please do not try this at home. Still, while we are all aware of the rule and the consequences of breaking it, natural curiosity may lead you to that most interesting of questions: why? Before we delve into the science of why, however, let us first acquaint ourselves with the science of how: how do microwaves work in the first place?

How Do Microwaves Work?

Microwave ovens do what they say on the tin: they use microwave radiation to cook your food. If your first instinct is to panic about the amount of radiation that you’ve put into your dinner over the years, you probably aren’t alone. Scenes of nuclear weapons, Chernobyl and Homer Simpson snoozing through a meltdown have made radiation a somewhat alarming term. However, radiation is simply energy moving from one place to another, either as particles or waves (Galindo, 2022). This can be “ionising” radiation, like the X-rays used to scan your broken bones, which is indeed harmful if you’re exposed for too long. Nonetheless “non-ionising” radiation exists too, has less energy than its damaging counterpart and the exposure threshold for harm is much higher. In this category lives visible light, radio waves and - you guessed it - microwave radiation!

BONUS SCIENCE: visible light, microwaves and the other types of radiation mentioned are all part of the electromagnetic (EM) radiation spectrum (Figure 2). This shows the different types of EM radiation and the inverse relationship between wavelength and frequency. Click here for an EM radiation explainer.

Figure 2. The electromagnetic radiation spectrum shows the relationship between wavelength and energy. (Vargas, 2022)

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So microwaves use microwaves. But how?

Let’s set the scene: your date is waiting on the sofa to watch a Friday night movie when you suggest some delicious popcorn to munch on (salty or sweet as you prefer, it’s your fantasy). They enthusiastically agree, but they also want an explanation on how microwaves work when you get back! You put the bag in the microwave and desperately search the Internet for this article.

What’s going on in there? Well, a piece of machinery called a magneton generates the radiation, which starts bouncing around the inside of your microwave oven as soon as you turn it on. As these waves hit the popcorn kernels, it’s not the corn but actually the water (and other dipolar) molecules inside that absorb the energy! These H2O molecules vibrate furiously with kinetic energy, which is converted to heat. This heat (just a different form of energy) spreads from the water into the food immediately surrounding it and cooks it. The radiation can usually only penetrate about 1 inch into the food, which is why you sometimes find a cold centre despite a piping hot exterior.

DEEP DIVE: What makes the water molecules vibrate and where does the kinetic energy come from? H2O molecules look a bit like an arrowhead, as shown in Fig. 3. They are also dipolar, which means that some parts are a bit more positively charged (δ+) and another part is a bit more negatively charged (δ-).

Figure 3. Dipolar water molecules are partially negative (δ-) at the oxygen atom and partially positive (δ+) at the hydrogen atoms. (SurfGuppy, 2022)

The microwave moves up and down as it snakes through space. This back-and-forth motion is called oscillation. The wave is actually made up of oscillating magnetic and electric fields, the latter having moving areas of electric charge. What this means for our water molecule is that the partially polar parts are attracted to the oscillating electric field; as we already know, positive charges attract negative charges, and vice versa. As we see in Fig. 4 the H2O molecules rotate back and forth to line up with the electric field, converting the electrical energy to kinetic energy. As they're rotated by the electric attraction, the water molecules bump into food molecules next to them, releasing that energy as heat.

Figure 4. The dipolar water molecule rotates to line up its partial electric charges (δ+/-) with the oscillating electric field. (Insider, 2014)

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What’s the Problem with Metal?

Still with me? Good, because we have arrived at the “why” of it all. What is the science behind safe microwave cooking 101? Why does an unremembered utensil in your leftover spaghetti lead to WW3 in the kitchen? To understand, we have to look quickly at the structure of metals.

Figure 5. Metals are made up of positively charged ions and negatively charged electrons, which can move around.

At the atomic level, we have a block of positively charged metal ions which are held in place. All around them, there is a cloud of much smaller, negatively charged electrons which can move about freely. These two components attract each other strongly and usually the electron (e-) cloud is spread out evenly across the matrix of metal ions. However, when we hit it with some microwave radiation, something interesting happens. The electrons move towards an edge or point on the metal structure and they start to bunch up there.

Although misery loves company, similar charges repel each other and all these negative e-s hate being so close to each other. More and more of them crowd the same point on the metal, until all the negativity and complaining reaches a critical point. When they can’t stand it any longer, all that electrical energy has to be released and the e-s start to discharge - violently. These are the sparks and arcs of lightning that you see when you leave metal in there: electrons jumping ship to the walls of the microwave.

Finally, you may well be wondering why those metal walls of the microwave oven don’t cause the same static electricity build-up. This is related to their smooth shape: there aren’t any sharp points at which the e-s can collect and discharge. For the same reason, metal spoons are often round and smooth enough to sit in a microwave without an explosive reaction, while twisted aluminium foil or knives wouldn’t fare so well.

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What Did We Learn?

To review, we can say that microwaves work by heating water molecules in food using microwave radiation. When that same radiation is applied to metals with sharp points or edges, it messes with the distribution of the free electron cloud and makes a lot of those negatively charged e-s bunch up in one spot. They hate this and eventually they make a jump for it, releasing static electricity which we see as sparks inside the microwave. Although it might look like a Thriller, putting metal in your microwave is a Bad idea and if you start a fire, you can only blame the Man in the Mirror.

Figure 6. Tony Stark's early arc reactor tests were not successful. (Brettman02, 2008)

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Reference List

1. Brettman02, 2008. THE BEST CD In Microwave. [image] Available at: <https://www.youtube.com/watch?v=0JkCIfLE_-M&ab_channel=brettman02> [Accessed 28 July 2022].

2. Insider, 2014. A single water molecule has a negative and positive charge at both ends.. [image] Available at: <https://www.businessinsider.com/how-do-microwaves-work-2014-6?r=US&IR=T> [Accessed 26 July 2022].

3. Galindo, A., 2022. What is Radiation?. [online] Iaea.org. Available at: <https://www.iaea.org/newscenter/news/what-is-radiation> [Accessed 25 July 2022].

4. Olsen, E., 2008. Do-it-yourself ball lightning in the microwave? Not exactly.. [image] Available at: <https://scienceline.org/2008/07/ask-olson-balllightningmicrowave/> [Accessed 26 July 2022].

5. Pittsburgh Post-Gazette, 1990. Campbell Microwave Institute Studies Reveal Microwave Usage Trends. [online] p.43. Available at: <https://news.google.com/newspapers?nid=1129&dat=19900307&id=x65RAAAAIBAJ&sjid=Y24DAAAAIBAJ&pg=4118,1862379&hl=en> [Accessed 25 July 2022].

6. SurfGuppy, 2022. Polar covalent bonds exist in the water molcule. [image] Available at: <https://surfguppy.com/matter/why-is-water-polar/> [Accessed 26 July 2022].

7. Vargas, A., 2022. Non-ionizing and ionizing radiation have different wavelength, which directly relate to its energy.. [image] Available at: <https://www.iaea.org/newscenter/news/what-is-radiation> [Accessed 25 July 2022].