Why you can’t Microwave Metal
So this question popped into my mind when we installed a microwave at my father’s office and when setting it up, in big letters was the warning of not to microwave metals. So what is it that makes metals so volatile and destructive, you cant microwave them?
Now I was curious to say the least…… Why cant you microwave metal? To find out, I first wanted to know what WOULD happen if I microwave metal. So I plugged in an old microwave and put some water in an old cup with lining of steel. Then I started microwaving.(DONT DO THIS AT HOME PLEASE) To my surprise, there were small sparks which originated out of the lining every five seconds or so. So the first thing that came into my mind was Photoelectric Effect.
The Photoelectric Effect
The Photoelectric effect was first observed by Heinrich Rudolf Hertz in 1887. He observed that when a light of certain frequency falls on a metal, it releases electrons, or in other words, conducts electricity. He wasn’t exactly able to explain why this happened, but did explain why only certain frequencies emitted the now called Photoelectrons. More on that later. This was explained by Albert Einstein in 1905 in the first of his famous Annus Mirabilis Papers(Latin for miracle year, the second paper was on motion of particles in fluids, AKA Brownian Motion, the third on Special Relativity and Time Dilation and the final one was on mass-energy equivalence, or E=mc²) which got him the Nobel Prize. This was explained by Einstein using Planck’s Quantum Theory to which we shall now head to.
Planck’s Quantum Theory
When you heat an iron rod, why does it changes its color from the grey to red to then white and finally blue? That was the question Planck sought to answer. What he came up with is essentially to not only quantum mechanics, but also various other fields in physics such as String Theory and The Standard Model, and even various spectroscopic methods in Chemistry and Biology(in fact, electronic microscopes work on deBroglie’s theory of particle wave duality, which can be thought of as a simple extension of Planck’s theory). He worked out that the rod(or any other material) when heated or cooled, it emits or absorbs energy. But that energy can be of certain frequencies, discrete frequencies, due to which the energy is emitted or absorbed in discrete chunks. Think of it this way-If you want to pay $100 to a shopkeeper in the United States, he wont accept if you say, here keep this £100 and let me take my stuff. He will only accept dollars as it is the currency he wants, he will only accept bank notes in dollars. Similarly, the rod will also be picky and only deal in one currency, or in other words, particular discrete frequencies and hence release or absorb discrete chunks of energy. He called the smallest discrete chunk it will emit or absorb as quanta. He then also imagined a black body, which can radiate and absorb energies of all frequencies. Through this, he arrived at a simple equation:
E ∝ ν
E=(hc)/λ (as ν=c/λ)
where E= Energy, ν=frequency, λ=wavelength of the radiation, c= speed of light and h=Planck’s constant which he used to removed the proportionality sign. (Its value is 6.626 *10-³⁴ J-s or Joule second).
With this small introduction to Planck’s Theory, lets head back to Einstein, Hertz and the Photoelectric effect.
The Photoelectric Effect(continued)
Remember only certain frequencies emitted photoelectrons? Well according to Hertz, the emission was dependent on intensity of light, and not frequency, but according to Planck, it must be related to frequency. Einstein solved this dispute. You see all metals are electron donors, they want to get their electrons removed, but they want to do it right. The electron wants some energy in return and the photoelectric experiments supplies this energy. As the electron flies out, IT MUST have some kinetic energy. And since the electron wants some energy as compensation to leave the atom, it will only leave with some kinetic energy if the energy supplied by the light is greater than the energy required to remove the electron. In other words,
where E⁰ is the minimum energy required to remove the electron and v⁰ is the corresponding frequency. This explains why only certain frequencies emitted the photoelectron. Now let take into account the kinetic energy.
where all symbols have their usual meaning. E⁰ is often referred to as the “Work Function” of the metal, but it means the same thing. ν⁰ is called the “Threshold Frequency” but it means the same thing.
The Hidden Caveat
Now you can check the validity of Planck’s Quantum theory everywhere, with quantum computing being the best example. But it cant explain the phenomenon we are looking for.
Even if you take the BIGGEST frequency in the microwave region, the energy released is about 0.012 eV. This is staggering, why you ask? Its because most Metals have their threshold energy greater than or equal to 1.
What about the spark then?
This one is quite simple to answer if you look at the basic job of a microwave. A microwave heats up food. So the food absorbs some heat. What this means is that the food absorbs energy from the microwaves. Now metals, as we have discussed are capable of transmitting photoelectricity, but if the energy given is not enough to eject the photoelectron, they just reflect it back. In other words, most metals reflect microwaves, and since the microwave is giving energy inside the system in every part of the process which heats up food, there is a lot energy in the main compartment. If there is not enough matter to absorb the energy, the energy is left in the chamber, which builds up and causes it to explode. The spark I saw was a small one because I had a large amount of water that absorbed a lot energy to heat up.
If you find any mistakes, please let me know. I will credit you and correct them as well!