The destructive power behind a spoon

Have you ever sat waiting for someone in a coffee shop, slightly bored? Resisting the urge to check your email or Twitter on the phone (perhaps the battery is dead), you have been stirring your coffee and playing with the vortices that form behind the spoon. Have you wondered why they form? Or played with detaching a vortex from the spoon and getting it to ‘bounce’ off of the side of the cup?

chimney, coffeecupscience, everydayphysics, coffee cup science, vortex

The spiral around this chimney helps to prevent vortex formation in high winds

Such vortices form behind objects in a flow of liquid when either the speed of the liquid, or the size of the object, reaches a critical value. The research about how and why these vortices form is a huge field. From improvements to plane design, through understanding insect flight and even into how wind instruments such as flutes work, understanding these vortices is a challenging topic. It is also useful to know about the behaviour of these vortices when designing chimneys in order to prevent their collapse.

Chimneys are of course stationary, but when they are in high winds, vortices form around the chimney just like the vortices behind the spoon (rather than the spoon moving through the coffee, the wind moves past the chimney). At relatively low speeds, the wind forms small whirlwinds as we see behind the spoon in the coffee. At higher wind speeds, the vortices forming behind the chimney can start to detach and form a pattern known as a Karman vortex sheet. As each vortex detaches from the chimney it subjects the chimney to a small force. Under some conditions and around some objects, this can result in the rather beautiful sounds of the Aeolian harp. Under more extreme conditions, it can result in the collapse of chimneys. The Ferrybridge C cooling towers collapsed in 1965 in high winds as a result of the turbulence around the cooling towers. To minimise the chances of such vortex sheets forming, chimneys are now designed with a spiral pattern (pictured) around them. Far from being an aesthetic feature, this spiral channels the wind so that vortex sheets cannot form behind the chimney.

Something to think about next time you’re waiting for someone in a cafe.

Helium in your coffee?

C-C bond, Esters N16

The sign board at Esters. Note the zig zag underline

As a website based on the physics inside a coffee cup, it was only a matter of time before I visited Esters in Stoke Newington. The name has significance to anyone interested in the physics (or chemistry) of coffee and the signboard outside the shop confirmed it. Under the name, there is a zig zag underline that represents part of a molecular structure. The end of each straight line signifies a carbon atom which is bonded to its neighbouring carbon atom by either a single (one line) or a double (two lines) bond. Inside you can enjoy (as I did) single estate coffees that can be prepared (if there are 2 or 3 of you) in a Chemex, appropriately enough. As I left Esters, I wandered through a local park where, near the entrance to the park, were two helium balloons caught in a tree. One had deflated, the other floated, dejectedly, just beneath the branches. Such a timely observation! The story of the discovery of helium connects the signboard at Esters, a cup of coffee and helium itself, how could that be? You’ll just have to keep reading to find out.

neon sign, light emission

The colours of “neon signs” depend on the particular gas (eg. neon) in the tubes

Helium is the second most plentiful element in the universe but on earth it is relatively rare. It was therefore not discovered on earth but, instead, by looking at the Sun. Its ‘discovery’ in the Sun was due to the way in which atoms interact with light. The atoms in each element emit (or absorb) light at specific frequencies. These frequencies correspond to different colours. It is this property of atoms that creates colours such as the distinctive hue of neon lights. In 1868 two astronomers were observing the same solar eclipse. Independently of each other, they noticed a distinct emission of light from the Sun at a wavelength of 587.49 nanometres (yellow-ish). This emission line corresponded to no element that had been found on earth and so one of them, Norman Lockyer suggested naming this new element helium, after ‘Helios’ the Greek god of the Sun. Helium was not found on earth for another 27 years when William Ramsay isolated it from a uranium based compound. The gas that Ramsay extracted, absorbed and emitted light at the same frequency as the two astronomers had observed for the element in the Sun. Helium had been found on earth.

Think of energy levels as rungs on a ladder. Image credit © www.artemisworks.co.uk

Think of energy levels as rungs on a ladder. Image credit © www.artemisworks.co.uk

Atoms absorb (and emit) light because of the way that the electrons in the atoms are arranged around the atomic nucleus. The electrons exist in discrete energy states that we can imagine as rungs on a ladder.  Electrons move between the states by absorbing, or emitting, light at specific energies (corresponding to a step up, or a step down on the ladder). As the energy of light depends on its frequency, the colour of an element depends on the spacing of the rungs of this atomic ladder, which is different for different elements. The energy ladder of helium atoms means that helium emits light at 587.49 nanometres. In organic molecules (ie. all the molecules that make up you and I and coffee), it is often the double carbon bonds that provide the energy ‘step’ in the visible range of light. Depending on the number of carbon atoms that are double bonded and the number in the molecule that are not, the energy step is tweaked slightly so that it will absorb in the red region in some materials and in the blue in others. We have our link between the sign at Esters and the observations of the astronomers.

However the explanation above depends on knowing some properties of electrons in atoms and some details of quantum mechanics. Neither electrons (discovered in 1897) nor quantum mechanics were known to the discoverers of helium. How did the astronomers recognise that their observation of a particular colour of light meant that they had identified a new element? Part of the answer must be based on experience. Experimentalists had already found out that different materials absorbed (and emitted) light at different but specific, frequencies. The other part of the answer brings us to our link with coffee.

A milk ring in water. Once it was thought that atoms might look like this.

A milk ring in water. Once it was thought that atoms might look like this (but a lot smaller).

In the video Coffee Smoke rings, we can make rings of milk travel through coffee or water. These rings are vortices which are closed up on themselves to form a doughnut shape. Mathematically, the vortex ring is a completely stable structure, it never decays. You could argue that the reason that it decays in the video is because we live in a non-ideal world with non-ideal liquids (milk and water). Returning to the mathematical world, each vortex ring will vibrate at specific, (resonance) frequencies dependent on its diameter, just like a bell rings with a note dependent on the size of the bell. So, even without knowing about electrons or quantum mechanics, it becomes conceivable that the atoms that go to make up a substance have specific resonance frequencies. If you imagine that atoms are in fact extremely small vortex rings (of the kind you find in a coffee cup), the model even has a predictive power. In 1867 William Thomson proposed such a “vortex atom” model and suggested that the distinct vibrations of the rings led to energy levels, like the ladders of later quantum mechanics and exactly of the sort that were observed by the astronomers. By considering that a sodium atom was made out of two inter-locked vortex rings, the light emission of sodium could even be accounted for. It was therefore entirely conceivable that elements would have distinct fingerprints as the astronomers had observed for this new element, helium.

We have therefore found the connection between the signboard at Esters, milk rings in a coffee cup and the discovery of helium. You would be forgiven for thinking that part of the connection is purely historical, after all, our current models of the atom do not rely on vortex rings at all. However, there is a relatively new theory called “string theory”. More fundamental than atoms, string theory proposes that there exist ‘strings’ that may be closed on themselves and that have specific vibrations that depend on their size and geometry. Sound familiar? Perhaps the connection with the milk rings lives on.

Does nature hate a vacuum?

The problem tea pot

The problem teapot

A few weeks ago, while having lunch with colleagues, one of them was complaining about his problems with his morning tea. So desperate he was to get his cup, he kept tipping the teapot to steeper and steeper angles in an attempt to increase the rate of pouring. Unfortunately, when he did so, the flow out of the spout became chaotic and, rather than having a nice cup of tea, he had a mess on the table. Another colleague suggested (sensibly) that it was a problem with the air-hole at the top of the teapot, not enough air was getting into the pot to enable the tea to flow smoothly out. In fact, my colleague’s tea pot problem turned out to have a different cause that will be featured in the Daily Grind in a few weeks. However, it did get me thinking about the purpose of the air hole in take-away coffee cups.

On the lid of a take-away cup are two holes. One, for drinking from while in a rush to get from A to B, the other, a very small air inlet hole that allows the coffee to flow nicely from the drinking hole. The requirement for such an air inlet has been known for millenia, however it was not understood why it was needed. Traditionally it was explained by saying that “nature abhors a vacuum”, the idea being that the coffee could not leave the cup because if it did so it would leave a vacuum which nature ‘does not allow’.

Take-away cup, plastic lid, equalisation of air pressure

The lid of a take-away cup has two holes. One for drinking from, the other to let air in.

An immediate problem with such an argument is that it implies that coffee has a will; nature ‘does not want’ a vacuum. Indeed for Rene Descartes (of “I think therefore I am” fame) this was a key problem with the traditional explanation. Descartes died in Stockholm in 1650, although for twenty years before that he had lived in Holland. For Europeans, the Dutch were fairly fast off the mark in terms of the introduction of coffee into their society. They had managed to get hold of a coffee plant in 1616 but only started properly growing coffee for themselves (in Ceylon!) in 1658, a few years after Descartes’ death. It is therefore unlikely that Descartes ever had the opportunity to try much coffee. Instead, when Descartes thought about the importance of air holes, the example that he used was a wine cask. In ‘The World‘, written in about 1632 he states “When the wine in a cask does not flow from the bottom opening because the top is completely closed, it is improper to say, as they ordinarily do, that this takes place through ‘fear of a vacuum’. We are well aware that the wine has no mind to fear anything; and even if it did, I do not know for what reason it could be apprehensive of this vacuum…”

Oranda, fish, Descartes water fish example, air pressure equalisation

The space behind a swimming fish is immediately filled with water as the fish moves forward.

For Descartes, the reason that an air hole was needed in the wine cask was not because nature hated a vacuum but because, on the contrary, nature was completely ‘full’ of matter. Whether that matter was wine, air or the material that made up the barrel, the world was full of ‘stuff’, meaning that if wine came out of the cask the air that it displaced had to go somewhere. Having nowhere else in the universe to go, this displaced air would have to go into the region of the cask that the wine had just vacated. Descartes compared this movement of air into the top of the cask to the displacement of water by fish as they swam through water. We may not notice the water in front of the fish moving to the back as the fish swims through the water but we know that the water must fill the empty space left by the moving fish. In the same way we do not perceive the air to flow from the outlet of the wine cask to the top of the barrel, but we know that it must (because, Descartes thought, it had nowhere else that it could go).

This explanation had far reaching consequences for Descartes world view. He could explain gravity and the motion of the planets as a consequence of the planets moving in a giant vortex of a substance around the Sun. The image of the solar system as a giant cup of coffee being stirred is one that the Daily Grind is sure to return to at some point. For the moment though, we need to step back and think. We know that the universe is not ‘full’ in the sense meant by Descartes and so this part of his explanation must be wrong, but why is it that blocking the air inlet hole stops the flow of water out of the cup?

coffee cup science, coffeecupscience, everydayphysics

Whether coffee leaves the cup or not depends on a balance of forces

Think about the schematic shown here. Gravity is pulling on the mass of coffee in the cup through the drinking hole. Air pressure is acting against this pull, pushing the coffee back into the cup (if you ever wanted a demonstration of how powerful air pressure can be, try sealing an empty water bottle before coming down a mountain or at the start of the descent in a plane). There is also air pressure inside the cup acting downwards on the coffee. With the air hole open, this air pressure is fairly equal to that outside of the cup. The inside air pressure cancels the outside air pressure, gravity wins and the coffee comes out. Imagine now closing the air hole. No air can get into the cup so, after a little coffee leaves, the air pressure inside the cup drops to less than the air pressure outside of the cup. This time, the air pressure outside the cup pushes the coffee back into the cup more than gravity pulls it out and the coffee stays in the cup. Can we test this explanation? One way to test the theory would be to somehow change the pressure inside the cup. Using two identical cups (which I got from the very friendly people with good coffee at Iris and June), the video below shows two experiments. In the first, both cups are filled with the same amount of cold ‘coffee’ (no coffee is ever wasted in these videos, dregs are recycled). The second experiment shows one cup holding cold coffee, one holding steaming coffee. Why might these experiments support the theory that it is air pressure that keeps the coffee in? Perhaps you can think of better experiments, or improvements to this one, let me know in the comments section below, but most of all, enjoy your coffee while you do so.

(note that the cups had got a bit water damaged through practise runs before filming. Note also that for this experiment to be meaningful, you would need to repeat the measurements many times so that you can build up a statistical picture, but that would make the video quite boring).

Brunswick House

Brunswick House, coffee, cortado

Coffee at the Brunswick House cafe

Last Thursday, I had the opportunity to try the coffee at Brunswick House. The old building which houses this cafe/restaurant sits on the corner of a major junction two minutes walk from Vauxhall tube station and feels somewhat out of place with the buildings around it. Inside, the incongruity continues with quirky decor and bookcases stacked with all manner of titles. Coffee beans are supplied by the roasters Coleman Coffee. As it was a lunchtime, I had a very enjoyable cortado (an espresso “cut” with steamed milk in a ratio of 1:1 – 1:2) which was full of flavour but not too bitter. With friendly staff and a spacious interior, this is definitely a place to return to whenever I am next in the Vauxhall area.

However, The Daily Grind is not so much interested purely in the coffee as in the connections between what we can observe in the coffee cup and the physics of the wider world. At Brunswick House, this came in the form of the link between one way in which we know that space is cold and a seemingly mundane observation, the condensation of water onto cold surfaces. Lifting my glass to appreciate the cortado, I noticed a number of water droplets on the (cold) saucer underneath the (hot) cup. As I kept the cup on the saucer, the saucer became warmer and the water droplets evaporated. By the time I finished my coffee, the saucer was dry. We can observe a similar phenomenon on the inside rim of a cup of steaming hot coffee. As we watch, water droplets form around the cold rim of the cup before starting to evaporate off again as the cup gets warmer. How is this related to the coldness of space? For that, we have to digress to an essay written two hundred years ago about dew.

cortado, Brunswick House, everyday physics, coffee cup science

The cortado on the saucer. 

William Charles Wells published his “Essay on Dew” in 1814 after two years of patient observation of the circumstances under which dew formed in the mornings. By carefully noting the weather conditions of the night preceding the dew fall and the surfaces onto which the dew formed, Wells came to some important conclusions. Firstly, the surfaces onto which dew formed suggested that the earth must be radiating heat into space; space must be cold. Secondly, the earth lost more heat on some nights than on others, it appeared that certain clouds kept the surface of the earth warm. If Wells was right it suggests that there is a natural greenhouse effect which is helpful for life on earth. This in turn suggests that the surface temperature of the earth is the result of a delicate balance between heat transfer to and away from our planet. Upsetting this balance (by introducing more greenhouse gases for example), could have serious consequences. Was Wells right? Perhaps we should start noticing when and where dew forms. So, over the next few weeks, make a note of dew laden mornings. Where did the dew form and under what circumstances? Do you agree with Wells? Let me know in the comments section (below). In a few weeks we will revisit Wells and his essay, in the meanwhile, enjoy your coffee!

A note of thanks

review, referee, thanksThe world of scientific research is supported by a vast volunteer network. Yes, scientists get paid to research their subject but in order to let the wider community know about their results, they have to publish it. To filter out the good papers from the bad or mediocre, working scientists volunteer to review papers submitted for publication. Each ‘reviewing’ scientist is anonymous to the authors of the paper, though the authors are not (yet?) anonymous to the reviewers.

At its best, the scientists reviewing the paper help the authors of the submitted manuscript to refine their papers, suggesting thoughts, other experiments or methods of phrasing that may better capture what the authors had intended. The review process is not just essential for filtering the good from the bad, it is extremely helpful for working scientists to have the input of someone, not involved with their research, commenting on it and trying to improve it. Although it is far from perfect, the review process is indispensable.

Which brings me onto Bean thinking. As a working scientist, I have got very used to the benefits that ‘peer’ review can bring. Therefore, over the past month, I asked 8 physicists and 8 non-physicists to review Bean thinking. These sixteen people were asked to critically read Bean thinking and let me know what they think. Unlike the scientific review process, it wasn’t anonymous, I knew who they were. But I also chose them because I respected each of their opinions and thought that each of these sixteen people would truly tell me what they thought. In this way, I hoped that Bean thinking could be refined to make it more engaging and entertaining while remaining scientifically rigorous. These sixteen got nothing tangible in return for their help which is why they are getting a mention on this page.

You know who you are, I know who you are. I am very grateful to all of you who took the time to read over and comment on Bean thinking. I have tried to take on board as many of your comments as possible (note the number of new photos on the pages!). Obviously, there are some comments that I haven’t been able to incorporate and any errors in the science (of which I hope there aren’t many) remain mine. Nonetheless, I am very grateful to you sixteen (and a few others who came in along the way). I hope you continue to visit Bean thinking, let me know what you think and join the discussion. And to all visitors, please leave any thoughts about the new look Bean thinking including any ideas for experiments that could be included in “Coffee cup science” in  the comments section below.

Footprints

Climate march, greenhouse effect

The People’s Climate March, London, 21st September 2014

A couple of weeks ago, People’s Climate marches were held in cities across the world. Held immediately before the UN climate summit, thousands rallied to emphasise the fact that we all need to work to lower our carbon dioxide emissions before it is too late. It is often said that we can “do our bit” by boiling only enough water to make the amount of tea/coffee that we want. So the question is, how much carbon dioxide is emitted as a result of preparing my morning cafetiere? And a related question, will it really make a difference if I boil the kettle efficiently?

A small cafetiere holds, roughly, 500ml of water. We need to increase the temperature of the water by approximately 80 C (from room temperature around 20 C to boiling point) in order to make the coffee. It is a property of water that, to raise its temperature by 1 C, we need to supply 4.186 Joules of (heat) energy per gramme (ml) of water (ref). To boil the amount needed for a cafetiere therefore takes 167 440 Joules of energy. (In practise it will take more than this owing to the efficiency, or inefficiency, of the kettle but this gives us a “ball-park” figure and a lower estimate).

How to turn this “energy” into a carbon footprint? Perhaps the simplest estimate would be to use the CO2 emissions guide for different electricity generation methods (link). The amount of CO2 emitted during electricity generation depends on the way that the electricity is generated. A wind farm is clearly going to produce far less CO2 than a coal fired power station. However, as a lot of electricity is still generated by burning fuels, let’s calculate the CO2 emissions for a ‘dirty’ fossil fuel such as (hard) coal and a ‘cleaner’ fossil fuel such as natural gas. According to estimates, (link) hard coal emits 115 kg of CO2 per GJ (ie. per 1 000 000 000 Joules) of energy produced. Natural gas emits 63 kg/GJ. This means that for one cafetiere (167 kJ), 19g of CO2 is emitted if your kettle is powered by a coal burning power station, or 11g (if your electricity supplier largely relies on natural gas).

chemex, coffeeBut what do these figures mean in terms of our carbon footprint? In the UK in 2010, 7900 kg of CO2 was emitted per person, according to the World Bank (link). This means that one cafetiere is the equivalent of 0.1% of an individual’s footprint for one day. This does not seem much but let’s phrase it differently. According to the British Coffee Association, 70 million cups of coffee per day are consumed in the UK and approximately 2 billion per day world wide. For the sake of simplicity in our calculations, let us assume (not unreasonably I think) that one cafetiere is the equivalent of two cups. Then, if we take the worst case (electricity generation from hard coal), each day there are 665 metric tons of CO2 produced in the UK from people enjoying their coffees (19g x 35 million cafetiere equivalents). Worldwide this equates to 19 000 tons of CO2 per day. If each person was boiling twice the amount of water that they needed for their coffee, more CO2 would be emitted each day due to our coffee making than the total annual CO2 emissions of the country of Lesotho (2010 figures, ref).

Something to think about while enjoying a coffee.

A coffee cup loud speaker

In the blog post “Musical Coffee”, I used a loud speaker that I had made out of a coffee cup.  This (hopefully brief) post is just to explain how the speaker was made in order that you can make it at home.  Firstly though I need to thank Jose Pino who had posted instructions for a very similar speaker (link) and from whom I got the idea for this speaker. Secondly, I ought to say that this video is the making of the actual speaker used in Musical Coffee.  Some further optimisation is clearly possible.

To make a loud speaker out of a coffee cup you will need

  1. A take-away coffee cup
  2. Paper and some scissors
  3. Sticky tape
  4. OLD ear-phones
  5. copper wire (0.28mm diameter worked well)
  6. superglue
  7. a magnet (a rare-earth magnet is best)

Instructions

Step 1: Make a paper “jacket” for the magnet. This jacket is there so that, eventually, we can pull the magnet out from the coil that we are going to wrap around the magnet.

Step 2: Make a second jacket with feet.  This jacket is to support the coil.  The ‘feet’ will allow us to easily superglue the jacket to the bottom of the take-away cup.

Step 3: Protect the magnet with a piece of paper. This paper just sits between the magnet and the cup.  It is to stop the magnet from getting glued to the cup in the next step.

Step 4: Glue the jackets to the cup. This is where the feet in step 2 come in.  Glue the outer jacket to the cup, the magnet should still be inside the jackets at this point but DO NOT glue the magnet to the bottom of the cup.

Step 5: Wind the coil. I used 100 turns of 0.28mm diameter (enamel coated) copper wire.  This worked quite well but could easily be optimised.

coffee cup loud speaker, cup speaker

The coffee cup speaker in an improved design

Step 6: Attach the legs. If I were to do this again, I would use cocktail sticks as in the photo here. In the video Musical Coffee though, I used bits of polystyrene cup stuck on with glue after having tried using straws. The aim is to make the cup fairly rigid so that the vibrations from the coil at the bottom of the cup are transmitted into the fluid of the coffee. The legs made out of bits of polystyrene cup worked well enough to hold coffee in the cup while it was being used as a loud-speaker but the cocktail stick design worked much better.

Step 7: Remove the inner paper jacket.  This is so that when you turn the coffee cup over and mount it onto a piece of card, the magnet will fall out onto the card centred with the coil.

Step 8: Glue the speaker and magnet to the card. Look again at the picture.  See that the magnet is on the card and directly below the coil? This is how it should be attached. Depending on what you used for legs, either superglue or wax (as used in the photo of the speaker with cocktail stick legs) would be good to hold the speaker in place.

Step 9: Be brutal with your ear-phones. Cut an ear-bud off.  There should be two wires left exposed.

Step 10: Solder the ends of the coil to the wires from the ear-phones. If you are not happy soldering, remove the enamel coating of the wire used for the coil with a sharp blade and then twist this wire with the ear-phone wires before fixing it with tape.

You are ready to go.  Let me know how you get on in the comments section below. Have fun.

Musical Coffee

Tasting notes from Finca San Cayetano coffee

Tasting notes from Hasbean’s Finca San Cayetano coffee

A few weeks ago, I chanced upon an article “Listening to Stars Twinkle” (link) via Mr Gluckin on Twitter. At very nearly the same time, I received in the post, a new coffee from Hasbean (link) which suggested an entertaining coffee (see pic).  A perfect time to have some fun with coffee, I think.

The article was about ‘stellar seismology’: Understanding the inside of a star by watching sound travel through it. We know from daily experience that the way sound travels through air depends on the temperature of the atmosphere.  Sounds can appear to travel further on cold evenings than on warm nights for example (for an explanation of this effect click here). Conventional seismology on earth uses the same principles. By measuring how sound is deflected as it travels through the earth, geologists can work out the type of rock in the interior of the earth (and whether the rock is solid or molten).

Burmese bell, resonating bells, stars

A bell rings with a note that depends on the composition (bronze) and shape of the bell. © Trustees of the British Museum

Unlike these earthly examples though, ‘listening’ to a star is not so easy.  We cannot hear stars vibrate as sound travels through them. We can only view them from a distance.  It is therefore very fortunate that the surface of a star will start to move noticeably as the sound travelling through the star hits one of the star’s ‘resonances’. Just as a bell has a tone depending on its shape and what it is made of, so a star has a series of ‘notes’ that depend on the composition and temperature of the star. These ‘notes’ are the star’s resonances and we can find out what they are by watching the different patterns on the star’s surface. Each resonance has a distinct, signature pattern which is dependent on the ‘tone’ of the resonance, much like the patterns you can see on the surface of a coffee by dragging a take-away cup across a table. The temperature and composition of the interior of the star determine the ‘notes’ of the resonances and so, by looking at the surface vibrate we can work out what is inside a star.

Can we illustrate this with a cup of coffee?  Of course we had fun trying.

In the video, the hot coffee is poured into a take-away cup that I have previously made into a loud speaker.  In the next few days I will upload details of the making of the speaker onto the Daily Grind. Hooking up the speaker to my phone, I could easily play music through the cup (and through the coffee).  But by connecting the cup-speaker to the phone with a tone generator app installed (free and downloadable from the app store for iPhones and probably similar for Android phones) I could generate single ‘notes’ through the speaker from 1Hz to 20 kHz.  Our ears are only sensitive to frequencies from approximately 20 Hz-20 kHz so below 20 Hz we cannot hear the notes being played.

home made loud speaker, coffee cup, kitchen table physics

The coffee cup speaker in an improved design

Nonetheless between 12 and 13 Hz, the surface of the coffee started to show a lot of movement. Although the distinct patterns of a resonance could not be seen (perhaps the speaker, lighting or other experimental conditions needed optimising), we can clearly see the coffee resonate as the surface is vibrating so strongly at these frequencies. As the tone was changed to down to 10Hz or up to 14Hz, the vibrations faded. The ‘resonance’ of the hot coffee filled cup-speaker was 12-13 Hz.  If the cup were to be filled with yoghurt or only half filled, we would expect the ‘note’ at which the surface vibrated to change. Indeed, in this latter case, I could no longer find the resonance anywhere near 12-13 Hz.

‘Listening’ to the coffee by watching its surface means that we can, in principle, work out the properties of the coffee, its temperature, density etc.  And it is in this way that we ‘listen’ to stars ‘twinkle’ so as to understand our universe more.  So thank you MrGluckin and Hasbean for providing an entertaining couple of weeks for me!  Please try this at home and let me know what you discover in the comments section below.

Important Disclaimer: No coffee was wasted in this experiment! I had already finished drinking the contents of the cafetiere and just used the old grounds to provide the ‘coffee’ in the video.

Extra thanks: Becky Ramotowski and Gardensafari.net for the photos. The photos from Garden Safari are © www.gardensafari.net

Dappled with Dew

Part of my morning routine can involve a walk through a local park. Each day reveals how the seasons are affecting the plants, bird life etc. This morning on walking through the park, I was treated to the spectacle of a thick layer of dew, shimmering and spectacular, glinting in the sunlight.

dew, surface tension, everyday physics, slow morvement

The dew this morning

Taking out my phone, I tried to take a picture of the scene for later and yet, what came out in the image was not the brilliant scene before me but instead some blurry grass. The ‘immediacy’ of the sight struck home. As with so many of the gifts that nature provides, attempting to take a photograph of it somehow just doesn’t quite capture the beauty of the moment. There are some great photographs of sunsets or sunrises, but part of the attraction of the image is not the photograph itself but our memory of those brilliant sunsets that we have experienced. The photograph is suggestive of the beauty that the photographer saw but somehow, the fullness of that beauty has not translated into the photograph.

As we stop to enjoy the moment, rather than photograph it and rush off to our morning appointment, we can start to notice what it is about it that captivates us. From my viewpoint, the majority of the dew this morning formed a silver blanket on the grass. It was this that caught my eye initially. Yet as I observed the dew, individual droplets came into focus and, because of the angle at which I was viewing them, they appeared as blue, as a slightly different blue and then other different colours. The physics of the rainbow was being revealed before me, one metre away on the grass. If I moved, the clues to these mysteries would disappear.

It was a reminder to slow down and notice things, who knows what we’ll see.  Perhaps you will disagree and say that it is just my poor photography skills that are the problem.  Please disagree in the comments section below!  Alternatively, if you agree and want to share a moment of beauty and everyday physics, please also share that in the comments section below.  I’ll finish this post however with an excerpt from the thoughts of someone who obviously did stop, slow down and observe his world.  The excerpt is from “Inversnaid” by Gerard Manley Hopkins:

Dew, surface tension, everyday physicsDegged with dew, dappled with dew,
Are the groins of the braes that the brook treads through,
Wiry heathpacks, flitches of fern,
And the beadbonny ash that sits over the burn.

What would the world be, once bereft,
Of wet and of wildness? Let them be left,
O let them be left, wildness and wet;
Long live the weeds and the wilderness yet.

 

 

From a Caravan to the Grecian

It is a Saturday morning as I write this while sitting in Granary Square in Kings Cross, London. I’ve just enjoyed an Ethiopean filter coffee at Caravan. If only more cafes offered the possibility of sampling single estate coffees rather than the espressos that are otherwise so popular in London.

Caravan, Granary Square, coffee, single estate, good cafes in London

The fountains in front of Caravan

In the square outside, people are laughing (and dancing!) in front of the old warehouses that accomodate Caravan. Amongst them all, four sets of ground-level fountains push jets of foaming water 50cm into the air, in patterns that change as you watch. There is so much physics here to observe: The white colour of the water foam, the dance of the water droplets as they emerge from the main jet of the fountain and then fall back to earth, the fact that the wet concrete around the fountains is darker than the dry concrete nearby.

Consider though one more observation. As the water shoots upwards, it is pushed by occasional gusts of wind from west to east making the fountains appear as loops rather than columns of dancing liquid. Although the direction of the wind is determined by local weather patterns, over the UK the prevailing wind direction is Westerly, that is flowing from west to east.

People have wondered about the origin of the winds from ancient times. The Greeks had four wind Gods who had authority over the winds from each direction: Boreas, god of the North wind, Notus of the South, Euros of the East and Zephryos of the West. Pliny the Elder speculated at length on the causes of the winds and yet the start of the modern conversation regarding the origin of the winds had to wait until 1686 with the publication of a work by Edmund Halley.

Grecian, Coffee House, London Coffee House

The Devereux now stands where the Grecian once was

Halley (1656-1742) is now more famous for the comet that is named after him rather than his meteorological work but, as with many scientists of the time, he had his finger in many pies. He also seems to have been a keen coffee drinker, or at least, he regularly spent time in one of London’s coffee houses, the Grecian, discussing science with Isaac Newton, Hans Sloane and others. A pub, the Devereux, now stands on the site of the old Grecian in a little side street off of Fleet Street.

Did Halley ponder cloud formation, rain and the origin of the winds while contemplating his steaming coffee cup on cold days in 17th Century London? Regardless, Halley did recognise that the heat from the Sun was the driving force for the wind system. Halley surmised that as a parcel of air was heated by the Sun and rose upwards, the cold air surrounding it would have to flow in to its place so as to replace the risen air so “..by a kind of Circulation N.E. Trade Winds below will be attended by a S.W. above, and the S.E. with a N.W. Wind above”* The problem for Halley was that his explanation of the wind system could account for a North-South wind direction owing to the Sun’s heating the air at the equator, but not the Easterly direction of the Trade Winds near the equator nor the Westerly direction of the winds over the UK.

A few years later, George Hadley (1685-1768) suggested that it was the rotation of the earth that was responsible for the east-west component; the mass of air, being detached from the earth, would appear to flow in a particular direction as a consequence of the earth spinning below it. The idea was not new, Galileo had proposed it some years earlier while similar arguments were made later by the philosopher (and scientist) Immanuel Kant (1724-1804). At first sight, such an argument looks appealing but there are problems, as John Herschel (1792-1871) pointed out. If this were the explanation for the wind direction, the effect would be “so great as to produce not merely a wind, but a tempest of the most destructive violence”.

Herschel suggested, as had Hadley before him, that friction could slow the wind to the speeds that we normally observe, but while this may explain the wind speed at ground level, what about the upper circulatory patterns noted by Halley: What friction could slow these down?

Grecian, Devereux, Coffee house London

A plaque outside the Devereux pub

It turns out that this is not the reason for the discrepancy in the wind speed. Hadley’s theory was wrong on a number of issues (if you are interested, I suggest reading this article). The real driving force for the Trade Winds is the Coriolis effect which deflects the warm air rising at the equator towards the right as it travels to the North pole. The majority of this air then cools and descends at about 30 degrees latitude, circling back on itself (as per Halley) as the Easterly trade winds. However the air that continues in the westerly direction north (or south) of 30 degrees latitude becomes those prevailing westerlies of the sort that batter the shores of the UK (see here for more information).

Even if Hadley’s simple model was wrong, its contemplation did lead to an important discovery that is still relevant for us today. The question was: What was it in the upper atmosphere that could cause a friction effect that could slow the winds? The person contemplating this question was taking a walking holiday in the Alps in the first half of September in 1886. Hermann von Helmholtz (1821-1894) observed a layer of clouds which showed “whirls formed by perturbation and rolling up” of the surfaces of two neighbouring layers of air. Helmholtz had observed what became known as “Kelvin Helmholtz clouds”, a beautiful but very rare cloud type, for an example click here. Helmholtz realised that the formation of these clouds required that two layers of air rubbed against each other. In the region between the two layers, the air became unstable, wavy and finally showed the whirls which are actually a series of vortices. As these vortices developed, the two layers of air would get more thoroughly mixed and it was in this way that friction could develop in the upper atmosphere.

Such vortices and “surfaces of discontinuity” are now an important concept in many places including the coffee cup. The video “Coffee Rings” presents another manifestation of the effects of surfaces of discontinuity. So we have returned from contemplation of the wind in a late summer square in London, through a famous Coffee House and back to the coffee.

I have not yet had the opportunity for myself to see a Kelvin Helmholtz cloud. If any reader has been so fortunate please share photos with @thinking_bean. Let me know what you think and what you see around you in the comments section below and most importantly, enjoy your coffee!

*from E. Halley, An Historical Account of the Trade Winds, Transactions of the Royal Society, 1686, p. 133, via “From Watt to Clausius”, DSL Cardwell, Cornell University Press, 1971

†Quotes taken from Anders O Persson, “Hadley’s Principle: Understanding and Misunderstanding the Trade Winds”, History of Meteorology, 3, (2006) p. 17 (linked in article)

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