What Happens to Metal in the Microwave?

You’ve probably seen it before on Youtube, one of the tens of thousands of videos of people microwaving knives, forks, coins, lumps of metal or (my personal favourite) a CD.  Each time there’s a light show as electricity sparks across the surface, but why is this happening?

Lets find out.

The Short Version (tl;dr)

Microwave photons excite electrons in metals into higher atomic energy levels (orbits) making it easy for their electrons to move to other atoms, hence conducting.  Mutual repulsion forces electrons to cluster at points and edges of the metal, leaving areas of localised positive and negative charge.  Once the charge imbalance builds high enough electrons can ‘arc’ through the air separating the areas of charge imbalance.  The electrons collide with gas molecules, producing more electrons and positively charged molecules (cations), creating a short-lived plasma of charged particles.  When the surface charges are balanced (or the imbalance is significantly lowered) remaining electrons and positively charged (cationic) molecules in the air recombine, releasing a photon of light which is observed as the ‘spark’.

Introduction

When we see a metal object spark in the microwave it means we are producing electricity with our microwave energy source.  What is electricity? Excellent question.

Electricity

Electricity is a flow of electrons from one place to another. 

Electrons are tiny subatomic particles.  How tiny? Noone really knows (read: noone really agrees) to be honest.  They’re definitely so small that we can’t see them even with the most advanced microscopes available.  On a theoretical level, the size of an electron depends on how you define it.  If you’re interested in this idea I’d recommend reading this page, although I’d warn you that the language is a lot more scientific!

Electrons carry a single negative charge and are one of the three building blocks of atoms, the others being protons and neutrons.  Protons hold an opposite, positive, charge whilst neutrons hold no charge at all.  Protons and neutrons stick together to form the nucleus of the atom (they are held together by the strong nuclear force, something we will definitely look at in more detail soon!), whilst electrons orbit the nucleus because opposite charges attract.  Shown in Figure 1 below is a diagram of the atomic structure of helium, one of the smallest atoms in the universe.

Atom

Figure 1. Atomic structure of helium.

Electrons aren’t always stuck to atoms though.  If you give an electron some energy it can break free of its parent atom and hop around between atoms, give it more and it can  break free and move around on it’s own.  If enough electrons are jumping between atoms, it causes an electrical current to flow.

To become ‘free’ an electron needs to escape it’s attraction to the nucleus, and this is not an easy thing to do.  The closer an electron is to the nucleus, the stronger the attraction between them.  This is where microwaves come in.  

Microwaves are photons with wavelengths between 1 mm and 1 m (If this sentence makes no sense to you, please read Eyesight I : Light for a background on photons) and contain just the right amount of energy to push some electrons out of their parent atom and make them hop across to other atoms.  

This effect happens most easily with larger atoms which contain many electrons, for two reasons:

  1. Repulsion between the negative charges in electrons means that not all can electrons can exist in the same ‘orbit’ like we see in Figure 1, some electrons must orbit further away from the nucleus and;
  2. Electrons further out are shielded from the nucleus by electrons that are closer (Imagine you’re at a bonfire, if someone is standing between you and the fire, you’ll feel less heat.  The idea is similar, if there’s a closer-in electron it can shield the further-out electron from the nucleus)

So a larger atom will have electrons further out which are less strongly bound to the nucleus and so would be easy to strip away.  The effect is magnified in metals, as metals are conductors, which just means that little or no energy is required for the outermost electrons to jump between atoms.

Aluminium is a well-known culprit for microwave sparking, so lets have a look at the atomic structure of aluminium.  Aluminium atoms contain 13 electrons arranged in three orbits as shown in Figure 2.

Al

SciGif

Figure 2. Atomic structure of aluminium. (with bonus GIF!)

In Figure 2 we can see the electrons laid out in orbits.  The first orbit contains two electrons (blue) which are closest to the nucleus and will be most strongly bound.  The next orbit can hold eight electrons (orange) for complex reasons we may explore in another post.  The third orbit contains three electrons (red) and is not full, as it can actually hold another 15 electrons.  Importantly, there are also unfilled orbits further out than the filled ones.  To jump to those higher orbits the electrons need to gain energy by absorbing a photon of the right wavelength (i.e. a microwave photon!)

AlPromote

Figure 3. Promotion of an electron to a higher orbit by a photon

Once the electron has moved to this higher orbit the attraction to the nucleus is greatly reduced and one of two things can happen: either the electron drops back down to its previous position (emitting the energy it loses as a photon) or else it escapes and jumps to a different atom, where it can also fall or continue jumping.  It’s important to note here that it’s not just the outermost electrons that can jump to these higher orbits, inner electrons can as well but will more energy (i.e. a shorter wavelength photon) to do so.

The upshot is that once you start firing microwave photons into the metal surface, electrons in almost every metal atom will start bouncing around and jumping between atoms causing electricity to flow.  But we can’t see the electricity in the metal, the sparks happen in the air between two points in the metal, so whats happening there?

The Breakdown of Air

Air is a pretty good insulator, which means it doesn’t allow electrons to pass freely through it.  If you think for a moment at least one reason why should be apparent: air is made of gases, not solids.  The atoms and molecules in air are much further apart, and are moving around a lot more, than the atoms in a solid surface like our aluminium metal.  Because of this its a lot harder for electrons to jump between atoms/molecules in the air, so electricty doesn’t pass very easily.  Another reason is that the molecules in air are generally very stable and their outermost electron orbits are full, so any new electron would have to go into a higher orbit on its own, something which is quite unstable.

However, few things are impossible if you have enough energy!  

With all of the electrons sloshing around in the metal surface they’ll seek a way to get away from one another (remember, electrons repel one another).  This will cause electrons to cluster at the edges of metal.  To understand this lets consider an aluminium fork in the microwave, and think about one prong and the part of the handle it’s attached to.  Electrons at the end of the prong will experience the least electron repulsion, as they ‘have their backs against the wall’.  Meanwhile electrons in the middle of the fork are exposed on all sides and so experience the most repulsion.  You can imagine this like a game of dodgeball: if you’re standing in the middle of the hall you’re vulnerable in all directions, if you’re against the wall your back is safe at least, if you’re at the end of a corridor you only need to worry about a single direction!  This idea is shown in Figure 4 where the electrons are colour coded based on how many repulsions they experience.

Repel

Figure 4. Repulsion of electrons in a fork prong

So why is this significant?  Well all of the electrons will be pushing to get themselves into a good position, causing negative charge to build up in regions around the edges of the metal.  Because some of the electrons have gone, the atoms left behind have an overall positive charge.  This means there are areas of localised positive and negative charge across the metal surface.  Positive and negative charges attract, so the electrons want to return to those positive atoms, but they also don’t want to be repelling one another a lot.  Quite a conundrum!

The solution is for the electrons to jump through the air to the positive area on the metal surface.  This is called an electrical arc.  It starts when some electrons from the negatively charged area of the surface break free and fly towards the positively charged area.  Some of these electrons will hit the gas molecules in the air hard enough to dislodge another electron.  This leaves two electrons and one positively charged molecule.  The positively charged molecule (positive ion or cation) will be attracted to the negative area on the surface, making it easier for more electrons to jump from the surface.  The result is a rapidly formed cloud of cations and electrons.  A cloud of charged particles is called a plasma, so we briefly form a plasma.  The plasma is very short lived, as once the charges in the surface have balanced the remaining electrons and cations recombine to produce molecules again.  As these electrons drop back into their orbits they lose energy as photons, visible ones this time, which is what we see as the ‘spark’.  Now all of that was pretty complex so lets look at it again, step by step with diagrams.

  • Positive and negative charges build up at different points on the metal surface due to the movement of electron.  Figure 1 imagines the space between two prongs on a fork.

Breakdown1

  • Once the charge builds up enough, the first electrons escape the metal surface and accelerate towards the positive part of the metal surface.

Breakdown2

  • These electrons collide with gas molecules in the air, causing them to lose electrons.  The atom left behind becomes positively charged, a cation, and is attracted to the negative area on the metal surface.

Breakdown3

  • The creation of cations, and the drive to balance the charges in the surface, creates a ‘bridge’ for electrons to cross.

Breakdown4

  • Once the charges in the surface balance, the remaining electrons and cations pair up to reform stable gas molecules, releasing photons seen as the ‘spark’.

Breakdown5

So that’s why metal sparks in the microwave!  Interestingly the arcing process that creates lightning is quite similar to the arcing of our metal shown above, although with a number of subtle differences that we’ll explore another time.

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