Category Archives: non-technical

A misconception about microwaves

Or, “how microwaves actually work”

This is a short note on a misconception which I had and which I believe is widely shared.

As you probably know, microwave ovens are named after the type of electromagnetic radiation that they produce. Most microwaves produce radiation with a frequency of 2.45 GHz.

The misconception I had is that this frequency is tuned to one of the vibrational frequencies of the water molecule.

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Does hot water freeze faster than cold? Explaining the Mpemba effect

In this article I would like to lay the following question to rest:

“Does hot water freeze faster than cold?”

The answer is:

“Yes, in some cases.”

In the rest of this article I will explain what these cases are and why the phenomena of hot water freezing faster than cold is not as mysterious as some have made it out to be.

A bit of history
I do not have room here to discuss the long history of this problem, which goes back to Aristotle, was pondered by Thomas Bacon and was more recently revived by a student in Africa named Erasto Mpemba. [For this reason, it is sometimes called the Mpemba effect, even though Mpemba was studying sugared milk during ice cream preperation and not water in his experiments.] While long discussions of the history of this subject are common (see reference 1), I feel that discussing the historical narrative is counterproductive to the point I wish to get across. The standard historical narrative on this subject gives the impression that this is a ‘great mystery’ which has stumped scientists through the ages. The truth is that this phenomena has stumped many great thinkers, but careful experiments on this phenomena have only been carried out very recently. My hope here is to convince you that this phenomena is actually not very mysterious (although it is a bit complicated), and then if interested you can read the historical narrative afterwards from that perspective.

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More is different

For my PhD research I study what happens when water molecules get together.

There is a difference between studying water and studying water molecules. One can study how water flows without ever considering that it is made of zillions of tiny molecules – that is the subject of fluid dynamics. One can also study how water flows and interacts with the environment – that study is called hydrology.

The subject I study is called molecular dynamics and it has to do with understanding what water molecules are actually doing on the microscopic level.

When I tell people I study water I sometimes get a puzzling look. People ask, “isn’t water pretty well understood?”. The popular perception of physicists (as covered in popular science shows) is that they are the people who study the more far flung aspects of the universe, such as dark matter, wormholes, and quantum entanglement. Why would a physicist want to study something banal as water?

There are several reasons I study water. For one thing, I actually like studying things which are commonplace and that I can observe in everyday life. For me, it is exciting and gratifying to have a deeper appreciation of the world that see around me — “my own sensory world”. I guess this is a bit self-centered – most of the universe being outside the limited sphere of my senses — but then again I don’t have to worry about the rest of the universe getting jealous that I’m not interested in it, right? Continue reading

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