Laminar and turbulent flow in Mosedale

 

We climbed up to the bothy on Great Lingy Hill through the old mine in Mosedale, behind Carrock Fell. The mines must have used a lot of water power. The old pipe below clearly fed the workings. Look at the clear water as it runs over the top of the pipe. This is laminar flow. The layers of water run side by side in an orderly way and don't mix. As the water speeds up due to gain of kinetic energy from gravitational potential, the flow becomes turbulent. Eddies form and the water goes opaque - it is white in the picture. The layers intermingle. You can experience this for yourself with a tap. When only turned on a bit you get a thin clear column of water - laminar flow. If you open the tap to increase the flow rate, you get opaque turbulent flow.

Courses for Lower Sixth students

I have mentioned these before but here are some links.

Oxford University run taster courses in the summer term but you need to start thinking about. Applications open in January. You need to be a credible Oxford candidate - ie some A*s at GCSE and likely to be As this year. I can write references but you'd better see me. Asking me with a day to go isn't helpful! http://www.uniq.ox.ac.uk/content/2014-application-information

It's the same with Sutton Trust - applications open in January. Most people who get onto their courses will be the first generation in their family to go to university. Look at the eligibility criteria. http://www.suttontrust.com/students/uk-summer-schools/application-process/

If you don't fit with either of these, you could try Googling for other options. They are bound to be there.

Lower Sixth Estimation Question # 2

 

Estimate the total resistance between the large pylon on the right and the distant pylon just left of centre. Write out your method in full to show me how you did it.

My silver kettle

 

Here's my kettle brewing up for Mrs B at Dash Falls. Electric kettles can be plastic but imagine using a plastic kettle on a gas stove! The metal is shiny because silver emits infra-red heat radiation badly. Less energy is wasted heating the surroundings and thus there is more useful thermal energy in the water. The kettle is more efficient if it wastes less energy.

Flow rates on the Wiza

There was awful flooding on Station Road last May when the Wiza overflowed after a day of torrential rain - see photographs at the bottom. And yes, that is a man canoeing down the road. Since then they have installed a pole for measuring the depth of the water. Fluid dynamics, which covers the flow of water, is a branch of Physics. And the foundations of any good scientific study is good data. So hopefully this new piece of measuring equipment will help to provide the data to prevent further flooding. Note that road level is about 2 metres 20cm. In other words, the water would have been above my head on flood day.

 

 

The picture below shows a tree trapped under the bridge by the station This is very turbulent flow. I'll be posting about the difference between laminar flow and turbulent flow in the future.

Camper van fridges

Camper van fridges work in a different way to the fridges in your kitchen. I'd always thought that it was odd that you could run a fridge by burning gas to heat it. In fact, what you are doing is heating a solution of ammonia in water. The ammonia has a lower boiling point and turns into a gas. Further round the pipe work, it cools and condenses back into a liquid. This liquid is then sent into an evaporator inside the fridge, mixed with hydrogen gas. The food and air inside the fridge gives thermal energy to the ammonia liquid. The fastest (hottest) particles can escape from the liquid, leaving the slower (colder) particles behind. The fact that thermal energy from the contents of the fridge has been used to do this makes the stuff in the fridge colder. I think the gas heating at the beginning is just to maintain the flow of ammonia around the system. It is the evaporation inside the fridge that does the cooling. (Note to Year 10: don't copy all of this - choose the bits which actually answer the question)

 

 

Notice the metal fins around the part where the ammonia evaporates (inside pipes!). The metal fins increase the surface area touching the air. Notice also that it is at the top of the fridge so that when the air loses thermal energy, it cools, shrinks, becomes more dense and sinks over the food, chilling it.

 

And thanks to Mrs B for pointing all of this out to me.

Oxford Science Propaganda

One thing I love about Oxford is the high level of the background art work. I'd love my car to look like this:

 

 

Part of the Bodleian Library was being repaired. This is what was on the hoardings around the building site:

 

My Sixth Form will be familiar with Hooke's Law but in the 17th Century they tended to be all-round scientists. This is from Robert Hooke's work with microscopes.

 

It's coming up to the centenary of the first discovery of X-ray crystallography. A strong Wigton connection there - more on that next year. Dorothy Crowfoot Hodgkin used the technique.

Newton and gravity

(This follows on from yesterday's post) ... Well I stood on the trampoline and looked up at this apple tree. It's at the other end of the Vale of Belvoir from Isaac Newton's famous tree - about 20 miles. The way I was told the story at school, you'd think no one had come up with the idea of gravity until he saw an apple fall. I don't think that's true. I understand that it was his linking of the falling of the apple with the force that kept the Moon going round the Earth that was the big leap forward. The Moon is actually falling, but it is falling along and down in a curved path, rather than straight down. This means that it keeps missing the Earth, fortunately. Newton was able to come up with a generalised equation to link gravity on Earth and gravity a long distance away. I think Einstein complained that gravity out into space constituted "spooky action-at-a-distance" whereas his curved space-time at least gave a surface that joined the two.

 

Playing with space-time

There was a trampoline at our friends' house so I took the opportunity to demonstrate Einstein's vision of how gravity works. He thought of the 3 dimensions of space and the 1 dimension of time being inextricably joined to form 4 dimensional space-time. We can't visualise 4 dimensions so it is normal to picture it as a 2 dimensional rubber sheet - the trampoline. I stood in the middle pretending to be a large mass like the Sun. A large mass dents space-time. If you put on another object like the Earth (represented by the basketball) then that second object rolls down the dent. In other words, the Earth is attracted to the Sun. Einstein explains gravitational attraction by saying that masses dent space-time and that other masses roll down the dent towards them. If you fire the ball sideways, it rolls round the dent. This is orbital motion under gravity.

 

 

Upper Sixth Estimation Question #1

I found this dog gate by a stile on the fells at Barf today. I set it swinging. Estimate any sizes you feel necessary from the photographs and then calculate the time period of the swinging gate, stating any assumptions that you made.

My fridge

My fridge is quite an old-fashioned design. These days you are far more likely to have a fridge-freezer. But this design has a small freezer box incorporated in the main fridge. It is at the top because it is good at chilling the air. This makes the air contract and become more dense. Dense air sinks and as it falls, it chills the food on the shelves below. This is what a fridge is supposed to do.

 

 

The back of a fridge is made of twisted black metal. This is because it needs to get rid of the heat that is removed from the contents of the fridge (to cool something you need to take heat away from it). It is black because black is a good emitter of heat radiation. It gets rid of heat easily as infra-red. It is twisted to give a big surface area. Infra-red leaves from the surface and the bigger the surface the more can leave at once. Oh, and if you touch the back of a fridge you'll notice that it is warm.

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The Bishop of Barf is all white

The Bishop of Barf is prominent white painted rock high on a steep scree slope above the A66 by Bassenthwaite Lake. It's worth the scramble for the views. Scroll down for the Physics...

 

 

The clouds in the background are white (and grey) because they scatter all wavelengths of light. The Bishop is white because the paint reflects all wavelengths of light (apart from in the bit where some naughty person has sprayed "AMY :)" ) And note that the fields below don't reflect all wavelengths. The red and blue ends of the spectrum are reflected but the green is reflected.

String theory, gravity and Broad Stand

 I have been sent a link to a marvellous song about String Theory. Danke! You HAVE to see this: it's awesome. http://www.youtube.com/watch?v=2rjbtsX7twc String theory is now an attempt to have a theory of quantum gravity. In other words, to have gravity work on very small scales where the world is inherently lumpy. So you'd think that lumps would be particles. But mathematically a particle is no dimensions. It takes up no space! This leads to problems with the maths because when you divide by zero space, the answer is infinity. The quantum theory of light, Quantum Electrodynamics (QED) for which Feynman, Schwinger and Tomonaga received the Nobel Prize, got round the problem by a technique of effectively cancelling infinities on either side of the equation. This is usually very dodgy but it worked perfectly in this case. The technique is called renormalisation. It doesn't work for quantum gravity. To get round the problem it was suggested that we should give the quantum particles a dimension. Hence they are strings because a line has only one dimension. Your computer screen has two dimensions. It gets round the infinities problem but is controversial because many people say that it is not possible to prove it experimentally.

I had a chance to consider gravity more intimately on Saturday afternoon when I came face to face with Broad Stand, one of the most notorious features in the Lake District. This small cliff blocks the way between Scafell Pike and Scafell. You go in through the narrow cleft in the rock (see detail in the bottom photo). That's as far as I dared. The way forward sees you out over a long drop and many people have been seriously hurt falling. Discretion proved the better part of valour. String theory might explain gravity but it won't stop you falling. It didn't stop Samuel Taylor Coleridge, one of my favourite poets, from falling down over 200 years ago, but he survived to write about it. You have been warned!

Zooming in on Sellafield

We climbed Scafell Pike yesterday and stood on the highest ground in England. Sellafield was prominent in the view - a big employer of physicists in these parts. Here are two photographs taken from the same place. My current camera is a cheap but functional one. It has digital zoom. Essentially, that magnifies the middle of the image on the CCD detector. It makes each pixel bigger but that tends to blur the image as you can see. My last camera had optical zoom. Optical zoom involves moving a concave lens backwards and forwards between two convex lenses. It affects the amount by which the light is bent and you can make the same light cover more pixels. The image is bigger but not blurred. Trouble is that I got sand into the gear wheels that moved the concave lens. My new camera may be cheaper but is more robust!

 

Sixth Form Institute of Physics membership

Free digital membership available if you're in the Sixth Form. See http://www.iop.org/membership/join/student/page_51412.html

Why are rainclouds black or dark grey?

It occurred to me that the fluffy cumulus clouds were white but the rainclouds were looking ominously dark. Previous discussion on this blog has noted that the water particles scatter all wavelengths of light equally well. This adds up to white light and the scattering effect means that what we see is ill-defined. I was thinking that rainclouds would have bigger droplets and so more effective scattering. I've read online that raindrops might absorb light but that the main thing is that rainclouds are thicker so scattering effectively blocks the sunlight coming down from above. Not much light comes to our eyes from the bottom so they look dark.

Convection on the Wall

 

The clouds above the Tyne Valley are typical cumulus clouds. During the day, the Sun heats the land. The air above the warm earth expands and becomes less dense. It floats upwards. There is a certain amount of water vapour in air - usually so little that you don't notice it. However, when the air gets cold, the amount of water vapour that can be held without noticing it decreases. As the warm air rises, it cools. We say that the vapour becomes saturated because the cold air can hold less water vapour. Some water vapour condenses out into water droplets and a cloud forms - all because convection warmed air and sent it upwards to cool.

 

Another thing I noticed is this lovely old barn. I suspect that the idea of it was to put cattle in the left-hand side so they were safe from raiders, but also that you might sleep above your cattle. They are large portable heaters who would heat the air down below and the warm air would rise to heat your room above. A bit smelly perhaps. I've always imagined that this is the case with this barn but if anyone knows differently, please let me know.

Thermal radiation on Hadrian's Wall

 

Here I am on Hadrian's Wall. You can see the vallum ditch in the distance. Aficionados of A Game of Thrones will notice the need to don the black when on the wall. This caused a problem on a bright sunny day. Dull black is an excellent absorber of infra-red thermal radiation and I was soon heating up. Year 10 please note: black absorbs thermal radiation well; it DOESN'T attract heat. No one's ever accused me of looking attractive in this outfit. I'm pictured holding a flask of coffee. Note that it is a shiny silver. Silver is a very poor emitter of infra-red thermal radiation so this minimises the emission from my coffee keeping it hotter for longer.

So note: colder black objects ABSORB infra-red well and heat up. Hot silver objects EMIT infra-red poorly and stay hotter for longer.

Smarter Every Day

If you haven't come across this Youtube video blog http://www.youtube.com/user/destinws2 it's brilliant - you might not like the cat one, though, so try to find the coloured lasers in Dutch.

Einstein isn't the only physicist

 

We found this lovely piece of street art under the bridge over the Thames on the southern by-pass in Oxford. Einstein is such an iconic figure - but I like the phrase "Einstein isn't the only physicist" that I got from a poster in my laboratory. One of the most important living physicist is Leonard Susskind who was a founder of String Theory. He has done a brilliant service in giving public lectures at Stanford University and posting them on the Internet for us all to watch. I have started watching the ones on Einstein's General Theory of Relativity. They are all nearly 2 hours long so I'm going to watch them in bits: http://theoreticalminimum.com/courses

Faraday in Cumbria

 

Today we went up the iconic Nine Standards Rigg at Kirkby Stephen. There were amazing views along the Pennines, over into the Lake District and across to Teeside. Near the summit, we were walking alongside Faraday Gill. Northerners will know that a gill is a stream. Here is a picture of the useless bridge over Faraday Gill.

 

What got me excited is that Faraday is such an unusual name. Michael Faraday was one of the most amazing physicists ever, having discovered so much about electromagnetism. He was a great experimenter: I have a lot of respect for experimental physicists. Looking up the connection, Wikipedia informs me that there is a road Kirkby Stephen named after Michael Faraday's uncle who traded in the town. Michael Faraday's father was a blacksmith in a hamlet very close to Kirkby Stephen but moved to London before Michael Faraday was born. The naming of the gill must surely have something to do with the family. Another Cumbrian Physics connection. Hurrah!

 

Below is a picture of the shale beds uncovered by Faraday Gill. How much shale gas is locked up in here??

A detail I hadn't spotted

 

I posted this picture of a transformer in August. It is front of the iconic Scottish mountain, Buchaille Etive Mor. I was looking at it again today and I noticed that the insulators holding the wires onto the wooden frame are not equal lengths. It has finally dawned on me that the higher the voltage, the more insulators are needed. Logic would suggest that the transformer is stepping down the voltage from right to left. The Kingshouse Hotel is out of shot on the left. If the insulators show that the voltage is 2/3 the size on the left, 2/3 of 11kV is 7300V. I think that's a bit high. I wonder how they choose the number of insulators? My calculations in a post on the subject in September suggests that they use a large safety factor.

Street lights

One of the things about city life is the orange glow in the night sky from the large number of street lights. In Nottingham I could never see the Milky Way because of the light pollution. In Wigton it is possible to pick it out. The orange street lights are sodium. The metal is vaporised and placed in a glass bulb between two electrodes. A beam of electrons then courses through the vapour. When an electron hits a sodium atom, it knocks one of the electrons in that atom up several energy levels. As that excited electron falls back down, it emits a photon of light. The frequency (colour) of the photons is fixed by the size of the energy level jump. As these are unique to a particular element, sodium has photons of a different frequency to the mercury street lamps. In sodium this adds up to an orange glow.

 

Newton in suburbia

 

Here's what it looks like in the suburbs of a big city. But I was thinking about the forces under a car's tyres. Typically, I draw friction as a backwards force under the wheels of a car - a counter force to its motion. But think - the car engine provides torque to turn the wheels. The contact between the rubber and the road means that the tyre tries to push the road backwards. By Newton's Third Law the road must try to push the car forward. The road is heavier and it doesn't move. So you can argue that it is friction that is the force pushing the car forward. Try this Durham University page as an example: http://www.dur.ac.uk/r.g.bower/public_html_old/PoM/pom/node16.html

Flying school?

 

The above picture shows a classroom at a school in Nottingham - yes, it 's the aeroplane. The Headteacher had the plane brought over from Liverpool Airport and refitted for lessons! It reminds me that there is a controversy in Physics about the way in which planes fly. I was taught the Bernoulli Effect explanation which focuses on the aerofoil shape of the wing - basically flat along the bottom and curved over the top. You then say that the wing splits the air. Some goes over the top, some goes over the bottom but the air joins up again at the back. It has to go further over the curved top so this means it has to go faster to keep its appointment to rejoin the air from the bottom at the back of the wing. Faster air has molecules that are more widely spread and thus at lower pressure. Higher pressure below the wing and lower pressure above: the wing is pushed up. Now it seems more acceptable to use Newton's Third Law as we did with the rowing boat last week. The wing is angled as it goes through the air to push air downwards. The air has mass (inertia) but obviously less than water. However, it means that the air pushes back up on the wing. There's the lift. If you search the Internet, you'll find that people get quite passionate about this debate. Here's one example: http://amasci.com/wing/airfoil.html 

Incidentally, viewers in rural areas like Wigton might take note of the picture below. Even schools in nicer parts of the city have fences around them to keep you out of the grounds when school is closed.