Microwave Popcorn leads to SCIENCE!

Or
“Why Staples Don’t Spark in the Microwave”

The other day I made homemade microwave popcorn, a little trick that people have known about for a while and made more popular by Alton Brown on Good Eats. It’s particularly cheap, costing only about 10 cents compared to $2.50 for pre-made microwave popcorn. The recipe is all over the internet, and is usually a variation of the traditional stove top recipe - the particular one I used worked well, and was as follows:

¼ cup popcorn kernels
1 tsp. popcorn or fine salt
a splash of peanut or olive oil

Place ingredients in a brown paper bag (like the ones you can get cheap for sack lunches), fold over, staple, shake well, push air out and place on a plate in the microwave for 2 ½ to 3 minutes, or until popping occurs only once every 2-3 seconds. Pour into bowl and top with spices of your choosing – I used and recommend a dash of garlic powder, a half teaspoon of chili powder, and about 3 tablespoons of that powdery parmesan.

Notice the instructions call to ‘staple the bag and then microwave it’. Some recipes say to omit the staples, as they might catch the bag on fire, others say that the staples won’t spark, “despite popular belief.”

That caught my eye. I used staples on my bag, and it’s true – they didn’t spark. My bag didn’t catch fire either, another bonus. But I wanted to know why the staples didn’t spark. I know that metal sparks in microwaves – I’ve not only seen it on Mythbusters and Bill Nye, I have also witnessed the effects myself (on accident, I promise), with mugs that contain metal strips and with CD’s that somehow made it into my microwave. I had to find out what was going on. So I went all SCIENCE up in here!

I knew that microwave ovens used microwaves (duh) specifically tuned to a frequency that makes water molecules vibrate, which is how the food you place in them actually gets hot. What I didn’t know was how this led to metal actually releasing electrons as a spark. I hypothesized that either the shape or the type of metal used in a staple prevented it from sparking.

I did some quick web searches to see if anybody else had solved the problem. In short, no. Most of the sites trying to answer the question simply stated that the staples didn’t have sufficient amounts of metal to be able to spark, but this didn’t make any sense to me, and nobody provided any scientific reasons to back up this claim.

I dug deeper. Several physics and chemistry forums talked in fairly complex terms about why metal sparks, but these basically boiled down to ‘the metal blocks the microwaves from passing through, and this eventually leads to a spark.’ I couldn’t find the something that connected the two.

Indiana Public Media put it all into place. They reminded me that microwaves are electromagnetic waves – part of the energy they carry is an electric wave (in addition to, of course, magnetic). Both parts of the wave lead to electrons in a metal becoming mobile, either directly or through induction, and this builds up at specific points on the metal.

Take aluminum foil – if you crumple some around your dinner, there are naturally going to be some sharp points. The electrons migrate to these points and begin to build up. Eventually, the build up of electrons grows quite dense, and this creates an electric field in the air nearby. This electric field eventually breaks through the air, creating a spark.

That makes perfect sense. So, really, anything that is highly electrically conductive and can lead electrons to a point should spark in a microwave. This disproves part of my theory, since staples are usually galvanized steel, a conductive metal.

So, why don’t they? Based off my hypothesis, my conclusion is that it is the shape of the staple that prevents this - if you look at a staple, it is a straight backbone that loops onto itself at the ends. If electrons were to build up, they would transfer down towards the two ends, but this just creates a loop as it touches against the backbone – the electrons flow in two circles at the ends of the staple.

This also explains why the staples might cause the bag to catch fire. As the electrons build and have no place to go, they get more and more excited. Excitation of molecules or electrons is what causes heat – again, this is the principle behind microwave ovens. The excited electrons would eventually cause the staple to get quite hot, and paper’s combustion point is only 451 degrees Fahrenheit. Also, if the staple’s ends don’t quite touch, they might create very small, unnoticeably small sparks back to the backbone , which could ignite paper if there was any in the way.

Further experiments will prove (or disprove) this – if an unbent staple sparks, given enough time in a microwave, while a bent staple next to it does nothing, I would feel fully confident in my theory.

So - anybody want to donate a microwave to science?