At a recent meeting I bumped into the joint author of a very popular graduate textbook. It has gone through a couple of editions over 20 plus years and sold 10,000 plus copies in total. The current ebook version sells for about £50. Apparently the two authors get about a pound per copy each. This leaves £48 left. And total income over 20 plus years is maybe half a million, of which authors get around £20,000, leaving £480,000.
von Karman streets
Theodore von Karman was a Hungarian-American fluid mechanics scientist/engineer active from early to the middle of the twentieth century. He was very smart and pioneered understanding the fluid flows needed for jet-powered flight. He was also the first to develop a theory for what are called von Karman streets. These can be rather beautiful, an example is shown above, in an image from a NASA satellite. At the top left of the image, the little green splodge is the island of Tristan da Cunha, an island in the South Atlantic. The von Karman street is the line of the swirls and holes in the clouds, that goes from top left to bottom right of the image. They have formed in the wake of the air of the atmosphere flowing around the peak of this island. The winds are blowing from top left to bottom right.
Is labelling foods “ultraprocessed” junk science?
There have been stories suggesting “ultraprocessed” foods are bad for you. This made me curious about what actually is a ultraprocessed food? How are ultraprocessed foods defined? So I did Googling* and, oddly, the British Heart Foundation has a detailed page on them that links to this document on the “NOVA classification”. The NOVA classification seems to be a standard*. I am no food scientist but this is a weird document.
Do A-level grades matter?
Depends on what you want to do, is the short answer.
The rest of this post is a short motivation followed by a longer but still partial answer*. On Saturday I read an article on that day’s coronation of Charles III, written by an A-level student already worrying about her exams. Students worrying about exams always makes me a bit sad. I like to see students working at learning physics – it was a pleasure to teach a computational physics class last week – but thinking about students hunched over books or laptops revising makes me a bit sad. This raises the question: Is this revision a necessary evil or an unnecessary burden on young people who could be doing more fun and more useful things with their young lives.
Which is harder to kill? Bad news wrapped in protein, or in soap?
Viruses have been described* as “bad news wrapped in protein” because many of them are just the genes of the virus – the bad news bit which takes over an infected cell – wrapped in a protein shell**. The wrapping is an elastic shell a few nanometres thick that is assembled from proteins. The image just below shows a reconstruction of such a protein shell:
The colours are false, they just use different colours for different proteins. This protein shell protects the “bad news” inside. But not all viruses are like this. Some, such as SAR-CoV-2 shown at the top of this post, are “bad news wrapped in soap”. Instead of a shell made of an array of protein molecules, it is (mostly) made of soapy/fatty molecules called lipids (shown in grey).
Health risks of elevated carbon dioxide in the air we breathe
On Sunday the MailOnline published an alarmist article on what they say are the risks of wearing a face mask, due to elevated carbon dioxide (CO2). Masks are an obstacle to breathing and so although when you wear a mask you just breathe a little bit harder and so get all the oxygen you need, they do retain a bit of the carbon dioxide you breathe out. When you breathe in, you tend to breathe in more carbon dioxide then if you are not wearing a mask. This is a well-known problem and the FFP2 standard for masks specifies that the carbon dioxide measured should be no more than 1%, which is 10,000 ppm* (parts per million). If you look at an FFP2 standard certificate you see they measure the carbon dioxide (via a method I don’t quite follow but I assume people have worked out a good way). For that case they measured carbon dioxide at 0.6% or 6,000 ppm.
Viruses can infect animals across tens of kilometres, and the English channel, can they infect humans across these distances?
Particularly in the early stages of the COVID-19 pandemic, there were still medics who thought that because the virus was confined to some sort of mythical fast dropping, droplets, the virus could only be transmitted over distances of a metre or two. The virus could not go further. This can’t be true as it conflicts with basic aerosol physics. And there is pretty strong, pretty direct, evidence for transmission of COVID-19 across large rooms. But what I had not appreciated until this week was that people studying transmission of the virus that causes foot-and-mouth disease*, have pretty good evidence that the virus can carry over tens of kilometres. For example, from northern France across the English Channel to the Isle of Wight. The distance from the Isle of Wight to France is of course a lot more than two metres.
Viruses need to survive a large but slow shock, to spread across a room
Someone infected with COVID-19, flu, measles, maybe mpox, or any one of a number of a bunch of viruses, breathes out the virus in tiny (micrometre) droplets of mucus. The droplets start in the infected person’s breath, which is saturated with water, i.e., at 100 % relative humidity (RH), because the breath has come straight from the infected person’s lungs. As the breath mixes with the air of a room (or of the surrounding air if they are outdoors), the humidity drops from 100% to the typical 40 to 60% of the air in and outdoors. This mixing takes a few seconds, and driven by this change of humidity the droplet dries out. The virus needs to survive this sudden drying intact, in order to go on to infect a new person. Drying in a few seconds may sound quick, but viruses are small so this may actually be felt as slow drying by the virus.
Masks when you are breathing: A hostile environment for viruses?
Early on in the pandemic there were all sorts of schemes going around to allow FFP2 and N95 masks to be reused; in hospitals they are traditionally worn only once (for say a single shift) and discarded but then there was a shortage so people wanted to extend the period a mask could be used. Were these schemes to try and clean a mask needed?
Tyndall showed us that dust is repelled by heat and attracted by cold
In 1870 John Tyndall was a prominent Irish scientist; he is now perhaps less known, possibly because he studied unglamorous things like dust. In a presentation that year he noted that a heated wire repelled dust particles. Note that this is separate from the convection of the air itself that the heated wire also causes. In addition to the convection the particles move relative to the air. Shortly afterwards his contemporary James Clerk Maxwell gave the following explanation, which is probably mostly right*.
Chance & Necessity 
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