Jet engines

This engine was in a museum in Berlin. It reminds me that I have forgotten most of the things I ever learned in engineering. So I've looked it up again. Roughly, the blades at the front compress the air coming into the engine. Fuel is burned in the middle part which sharply heats the air causing it to expand. The expanding air has to go somewhere so it then passes through a turbine. The turbine actually drives the compressor which makes you wonder how you start the whole process in the first process. Then the expanding air passes through a nozzle. This hot expanding air has a higher velocity than the cold air going in. Hence there is a change in momentum and thus an impulse. By Newton's Third Law, there is a reaction to the force pushing the air backwards and this reaction force pushes the plane forwards. Technically the engine shown is a turbojet rather than a turbofan. Clearly since the engine uses air, you can't use jet engines in space. Rocket motors are different.

Recycling in Kilnshaw Chimney

Kilnshaw Chimney is a scramble up a narrow stream bed from the Kirkstone Pass. It featured in Trail magazine recently. This was our second attempt. We got up because it has been so dry recently but the pitches are more tricky than we'd been led to believe. There was a dead sheep in there. It was there when we climbed half way at Easter. Rather a lot more of it has been recycled by nature since then. Only wool is left now. The carbohydrates will have been digested as fuel for respiration by saprophytes and redistributed into the environment as carbon dioxide and water. It's odd to think that the carbon atoms in the food that we eat have probably been through other living organisms, perhaps several times. There used to be a famous question asking how many molecules of air that were in Julius Caesar's last breath end up in one inhaled lungful of air now. It should also be possible to calculate the number of times a carbon atom in my food has been through the carbon cycle previously.

Withnail and I: thinking about entropy again

We visited Sleddale Hall in Wet Sleddale which was Uncle Monty's cottage in the film Withnail and I. It is being restored. It got me thinking about entropy again because it's becoming clear that I'm not totally sorted out in my understanding. I went back to Brian Cox's Wonders of the Universe. In Episode 4 he visited the desert in Namibia and built and sandcastle. That was ordered and therefore had low entropy. They filmed the wind destroying the sand castle and noted that the entropy increases. Sleddale Hall is being put back to a state of order and thus low entropy. But the natural thing to happen is for it to fall apart if untended. This is like the sand castle blown by the wind and thus its entropy will increase. I was also thinking about energy and stability. Energy has to be expended to build these structures. Materials are lifted up and so the structures have higher potential energy. As they fall apart, the potential energy decreases and we would say that things become more stable (in other words, once it's fallen over, it can't fall over again). So stable conditions must have low potential energy and high entropy. A climax community as mentioned in the Greenham Common post two weeks ago is a biologically stable condition. Hence it must have high entropy. It can't be ordered in the way that I suggested - I suppose that it should be homogeneous and any part couldbe swapped with any other without changing anything. Then to what extent would it be meaningful to suggest that a climax community has low potential energy?

Alarm system in the Stasi Jail

The corridors in the Stasi Jail had a bizarre primitive alarm system. It was the long wire shown in the photograph that goes round the building at about chest height. The guards were instructed to raise the alarm by pulling on the wire so that it came apart at the connectors shown. I was thinking about how it works. It looks like a series circuit. It could be a light that is on all the time but pulling the wire breaks the circuit so it goes off. Someone would have to concentrate on the light all the time. A sound alarm would be better. But that would be on all the time and stop when the circuit was broken. It would be irritating. You'd really need a piece of electronics called a NOT gate which would make the loudspeaker do the opposite of the circuit. Or there could be a magnetic relay switch that holds the loudspeaker circuit open in a second circuit. Breaking the first circuit stops the current to the electromagnet and allows a switch to fall back in the second circuit, sounding the alarm. I was just amazed that such a sophisticated and powerful system of oppression had such limited technology.

Rotary dial in the Stasi Jail

The old Stasi Jail in Berlin was an amazing and awful place to visit. Each of the many interview rooms had one of these old-fashioned 1970s telephones. Younger viewers will never have used one. If you want to dial 1, you put your finger in that hole and turn it clockwise. When you hit the barrier in the bottom right, springs pull the dial back round. I never knew how it works but I've just looked it up. So, as the dial springs back it interrupts the current that flows. It does this a specific number of times for each digit: presumably most times for 0 because it will take longer to spring back. Each interruption produces a pulse of current. Each number has a different number of pulses which are decoded by the exchange to work out the number.

Lise Meitner

I noticed Lise Meitner Street as we went through Berlin. This is an advert for a carpet and wallpaper shop! Lise Meitner was one of the greats of nuclear physics. She worked with Otto Hahn and others to discover nuclear fission. He got a Nobel Prize: she didn't. Her family was Jewish and she only just escaped from Nazi Germany. She lived most of the rest of her life in Sweden but is buried near Basingstoke in Hampshire. You should read her story in full. She is an inspiring woman. http://en.wikipedia.org/wiki/Lise_Meitner

Starting A Level Physics next year - things to do in the summer

If you are starting A Level Physics in September, there are various ways of keeping yourself thinking during the holidays.

1. You could watch the legendary physicist Richard Feynman giving lectures http://research.microsoft.com/apps/tools/tuva/index.html (Bill Gates of Microsoft has made the films available free)

2. You could download a book for free: Sustainable Energy: Without the Hot Air has some wonderful Physics in it and it is easy to read. http://www.withouthotair.com/

3. You could watch 10 minute teaching videos from Khan Academy https://www.khanacademy.org/science/physics Any of it would be helpful but perhaps this one would be best https://www.khanacademy.org/science/cosmology-and-astronomy/universe-scale-topic/light-fundamental-forces/v/four-fundamental-forces

4. Interesting topics but at a much higher level: http://www.openculture.com/2013/05/leonard_susskind_teaches_you_the_theoretical_minimum.html These are lectures by another cool physicist.

I will post things on this blog that are relevant to the AS Level course. I am also learning to notice things around me and try to explain them in terms of Physics. In other words, I'm learning to think and to ask questions. You should do that too.

Monoculture and entropy - Hadrian's Wall

These fields are near Harlow Hill on the Hadrian's Wall. The top two pictures show the monoculture wheat. Very uniform so I'd argue low entropy. The field edge in the bottom picture shows what it should be like with a rough mixture. The Second Law of Thermodynamics says that you can only go from lower total entropy to higher total entropy. So the change that made the field have a lower entropy must have resulted in a raising of the entropy elsewhere in the Universe. The entropy is lowered in the field by the use of herbicides to remove the other competing plants. But this may also remove the natural predators of the pest species that might eat the wheat, so pesticides need to be used as well. So where is the increase in entropy? I need to think about that.

Low pressure on The Pewits

 

The Pewits is a notorious bog on the ridge between Watendlath and Thirlmere. The wettest bit of the Lake District. It hasn't rained for two weeks so we tackled it. The peat had dried and cracked for the most part but some bits were still awful. We met a woman who had fallen in up to her waist. We found a helpful section where someone had laid down fence posts. This meant that my weight acted on a much larger area that my feet. Pressure is force per unit area so I put less pressure on the surface of the bog. Pressure is a measure of the damage done by a force. Low pressure meant no damage to the bog surface. In other words, I didn't sink in so High Seat was conquered.

Summer solstice

Here's the view at sunset on the day with the longest daylight. The tilted axis of the Earth means that at the moment our rotation means we spend more hours in the half of the globe lit by the Sun that in the half that is in shadow. Sadly, the days will now get shorter until the winter solstice. But note that it is the opposite in the southern hemisphere at the moment. It's winter in Brazil for the World Cup!

Update on solar power

 I posted a photograph of this display at the RSPB nature reserve at Leighton Moss on 9 January 2014. At that point it said 14306kWh generated so they have generated 578 kWh in five months.

 

I posted an electricity meter reading from about that time in a post on 20 May 2014. Here is a new electricity meter reading. My house has used 836 kWh in 5 months.

 

 

So the solar panels would supply maybe two-thirds of my domestic needs. But note the power of 0.19kW. That isn't high enough for most of appliances in my house. The system will only power these devices by solar panels pooling their output into the National Grid and the us using power from the grid.

Greenham Common: an increase in entropy?

Last time I went to Greenham Common in 1987 it was to follow a cruise missile convoy as part of a protest. I couldn't believe I was stood beside decaying missile silos. The second photograph is taken in the middle of the runway looking at where planes used to land. Nature has reclaimed the land and it is  now a nature reserve. But has the entropy increased? You'd think so because entropy is a measure of disorder. The more disordered a space is, the higher the entropy. So you'd say that a tarmac runway and mown fields would be more ordered and thus have a lower entropy than the heathland that is re-growing. But the gorse bushes themselves are highly ordered organisms. And it is well on the way towards a climax community woodland, so a biologist might argue that it is headed towards something more ordered. In the end, the total entropy in the Universe must increase even if the local entropy decreases. They say that this is what gives us the Arrow of Time.

Cuckoo - the speed of sound on Otmoor

Here's a slightly scary looking Cuckoo on the RSPB reserve at Otmoor. It is a wonderful thing to live in a medium that transmits sound to us like air does. The speed of sound in air depends on temperature. An important reason for this is that the speed of sound in a gas is inversely proportional to the square root of the density of the gas. As the air temperature increases, the air will expand slightly and thus be less dense.

Zero resultant moment in Oxford

This mobile crane was dismantling a tower crane in Oxford. I was watching it remove the concrete counter weight. Notice that the crane has vehicle has extended its legs onto the pavement to increase its base area. The line of action of the concrete weight lies outside the base area, so how come it doesn't topple over? Look at the design of the foot of the extending arm. The motor that drives it overhangs in the opposite direction to the arm. The motor will be heavy. It will exert a moment in the opposite direction to the concrete weight. If the motor is heavier than the concrete load, it can be a smaller distance from the centre and still have the same moment as the concrete load, which is a bigger distance away. Overall the resultant moment will be zero and the crane doesn't topple. The centre of mass of the whole system must be such that the line of action of the gravitational force acting downwards from that point must lie within the base area.

A correction - Hope's Apparatus

Thanks to the people on my email network who identified the equipment in the "odd convection experiment" as Hope's Apparatus. As I understand it, if I kept shovelling ice into the middle bit the bottom thermometer would reach 4 degrees Celsius and no lower whilst the water remained liquid. This is because water is most dense at that temperature and the most dense will sink to the bottom. And Thomas Hope was the scientist who discovered the element Strontium. Some years ago I visited the village of Strontian after which it is named.

An odd convection experiment

I was given this piece of equipment a decade ago but never got round to sorting it out. I looked at it today and it worked! You put tap water into the narrow middle tube and ice in the bit that goes round the middle on the outside. Thermal energy is transferred from the water into the ice by conduction through the metal walls (remember the free electrons!). This means that there is colder water half way down the inner tube. Water contracts and becomes more dense when it gets cold at these temperatures (not true if you are very close to 0 degrees Celsius!). The cold water sinks and thus the thermometer at the bottom reads about 5 degrees Celsius lower than the one at the top. It took about half an hour to work. The aim is show that cold fluid sinking is a type of convection. It's why my fridge has a freezer box inside it at the top not at the bottom (there has been a previous post about this)

Electricity supply for the Cuillin

Here's the view at the start of our walk. We climbed Bruach na Frithe which is just visible on the right hand side. But look at the electricity cables. They lead to the Allt Mor Dearg house. 3 cables take the supply left to right from Sligachan, but only 2 cables go to the house. Is one live and one neutral? Or is the central wire the earth connection? I think the very top wire on 42500V lines is earth. Our house is supplied in a similar manner to this but we only get one cable. Will have to investigate.

Magnetic rocks in the Cuillin

 

The Black Cuillin are regarded as the most amazing hills in the UK. They are quite young and were the magma chamber of a super-volcano. The magma was the mafic type that makes the oceanic crust. The magma cooled slowly and formed big dark crystals on a pale rock. The rock is called gabbro and is very abrasive so it has excellent grip for climbing. It also has a reputation for distorting compass bearings because one of the minerals in it is a compound of iron called magnetite. I was too blown away by the view to remember to check my compass when we went but I did bring back some samples. I have tested them with the Hall probe. The top picture shows the magnetic flux density in my front room. The second reading is when the probe is placed into the box of gabbro. There is quite a difference. The only thing I should warn you is that the readings are very variable and do depend a lot on the direction of the probe. I've tried to be careful but will need to do some further measurements. It seems that there is a measurable effect though.

Pointing the way on Mungrisdale Comman

We found the arrow pointing to the path across the top of Mungrisdale Common. Makes a change from a cairn! There is very little rock visible on the rolling grassy marsh so it was interesting to look at these. They are called spotted slate. The long needle-like crystals are andalucite. The country rock in these parts is Skiddaw slate, formed when the two separate continents bearing England and Scotland collided. This is one form of metamorphism mainly to do with pressure. Later there was an igneous intrusion of magma underground that produced Skiddaw granite. The heat baked the Skiddaw slate in a ring around the magma called a metamorphic aureole. That allowed recrystallization and hence the spotted slate. This is contact metamorphism. The middle photograph seems to show iron in the layers. Could that account for the higher than expected magnetic field strength??

Magnetic field on Mungrisdale Common

Mungrisdale Common might look like an idyllic upland pasture in the bottom photograph but it's an awful swamp. Not sure why Wainwright put it in his Northern Fells book but I'm guessing it was to add a chapter to a thin volume. So this was our third visit. On Friday I remembered that I had access to a portable magnetic field strength meter that hadn't been used for years. So I'm going to start taking readings in different places. The reason for choosing Mungrisdale Common was that it is 5 miles from the nearest civilisation. Surely that will give an unpolluted reading of the Earth's magnetic field strength. The black plastic thin is a coated Hall Probe. I moved it around and got a maximum reading of 8.0 milli Teslas. I'm getting half that in my front room. Hmmm. I need to take more readings in different places. A weird place to go but it has a great view south to the highest fells in England.

Loch Cluanie and hydro-electric power

We stopped for lunch beside Loch Cluanie. You can see the dam in the top picture. Apparently the dam is 675m long and 40m high. It feeds the Ceannacroc power station which is 20MW but this is also fed by Loch Loyne. Given that the wind farm at Bothel close to Wigton is quoted as 10MW this hydro-electric scheme doesn't seem that much. It must surely have been more expensive than the 10 turbines at Bothel. It could be argued that it has disfigured the landscape as much. It's advantage is that the energy supply is more consistent. I do need to look further into these figures because 20MW doesn't sound very much.

Knife-edge effect diffraction and Flodigarry Island

These are pictures of Flodigarry Island off the north end of Skye taken from the Quaraing. The diffraction of the waves bothered me and I have had to do a lot of research to explain it. I teach that waves will only diffract around an object if their wavelength is the same size as the object. That's why FM radio reception is poor in mountainous areas. But look at the waves in the picture. Their wavelength is tiny in comparison to the length of the island and yet they diffract round both ends of the island. The answer is called the knife-edge effect and owes a lot to Huygens' understanding of how waves propagate. He came up with idea that you divide the wavefront up into points that act as sources of semi-circular waves. Where these semi-circles overlap, you get the new wave. On a plane wave (a flat fronted wave) the points are so close together that the result is actually another plane wave. But a knife edge can act as a distinct secondary source of waves with none inside it. The key to my example is that the island is not round. It has pointed ends and these pointed ends are about the same width as the wavelength. So the ends of the island act as secondary sources, diffracting circular waves into the space behind the island. A mountain does not act as a knife edge to FM radio waves. The size of the mountain is too big for that.

Rotation and phase difference

I use this experiment to show the phase difference in cyclical motion. I rotate the turntable and the shadow of the ball moves from the middle to the side, back to the middle across to the other side and back to the middle. If we set up a pendulum as well, its shadow would move in the same way. So motion is a circle is linked to pendulum motion, and is linked to wave motion, because waves go middle to side to middle to other side and back to middle. One quarter of a rotation moves the shadow from the middle to one side. So I say that the phase difference between the middle and the side in a wave motion is 90 degrees or pi/2 radians. Half a rotation will bring the shadow back to the middle but it will be moving in the opposite direction. So the half way point in a wave motion is 180 degrees or pi radians out from the starting point.

The Quaraing

The Quaraing is an amazing area at the north end of Skye. If you look at the bottom photograph you will see that it is made of tilted layers. It is the result of basalt lava flows from a massive volcano where the Cuillin mountains now stand, 20 miles to the south. These can just be seen in the far distance on the right of the picture. My earlier pictures from the Old Man of Storr show that the lava was very gassy because you can still see the bubbles. With the gas being less dense that the liquid, the bubbles float to the top. I have read that this makes the top of each successive layer more crumbly. There was a massive landslide resulting in the weird features like The Table, which is hard to find but just as good as it looks. The top picture shows the miniature features resulting from the landslide. The rock has been split in two perpendicular planes. The corners of the original rock are still sharp because they were not transported and subject to erosion. Instead, mineral rich fluid penetrated the cracks, The water evaporated and left the mineral veins. The rock was frighteningly crumbly and bits were coming off in my hand as we negotiated that gully!

Amygdales in Trotternish

We found rocks with white flecks all over them on the way up to the Old Man of Storr. I knew that we were crossing old lava flows and figured out that the holes were where there had been gas bubbles in the lava. I know also that when you get mineral layers in rocks it is because very hot water with dissolved minerals inside go down the cracks where the water evaporates causing the minerals to crystallise out. I was pleased to find out that I was basically correct but found also that when it is in bubbles in basalt lava flows they have a technical term. They are AMYGDALES. It comes from the Greek word for almonds because they are that shape. The same is true of the amygdala, part of your brain.

Coastal convection at the Old Man of Storr

 

We were on the Isle of Skye and visited the amazing rock pinnacles of Trotternish, including the Old Man of Storr. We were caught in a shower when the photographs were taken and sheltered behind a rock because the wind was driving the rain in. I looked up to see which way the clouds were moving and noticed that they were moving in the opposite direction to the rain. The wind seemed to be going in the opposite direction higher up. I wondered if this was evidence for coastal convection that I remember learning about at school. The sun heats the land faster than the sea during the day because the sea has a much higher specific heat capacity. The land becomes hotter than the sea, so warm air rises over the land and cold air sinks over the sea. This sets up a cycle that is completed by air moving off the sea onto land at ground level and off the land out to sea at a higher level. This was roughly what I observed.