Diffraction through net curtains

I noticed a street light diffracting through net curtains. It makes a cross shape because there are horizontal slits which diffract light vertically and vertical slits that diffract light horizontally. On an enlarged picture you can see that the spikes are subdivided. This shows the orders of diffraction. The colours were visible but haven't photographed well.

Thomas Young does something that isn't Physics in Worthing

Thomas Young is someone who ought to be a household name. I know him as the man who proved that light is a wave by demonstrating diffraction and interference effects. He also worked on the properties of solid materials, giving his name to the Young Modulus, a measure of how difficult a material is to stretch. But I didn't know that he also played a part in translating the Rosetta Stone. Apparently he figured out that the hieroglyphics representing translations of non-Egyptian names were phonetic. Hence the sounds of the hieroglyphs could be worked out. A recent biography described him as "the last man who knew everything". They were exciting times when it was possible to do world class work in many fields.

Components of magnetic field

 We did this experiment to measure components of magnetic flux density B. There was a.c. in the large copper coil which created a rapidly alternating magnetic flux. There was thus a large enough rate of change of flux density in the small yellow search coil to induce an alternating emf. We varied the angle of the search coil so that it was detecting different components of the flux. The largest flux is detected in the position shown below.

We displayed the induced emf on an oscilloscope and measured the peak-to-peak voltage, halving it to read the peak emf. This procedure reduces the percentage uncertainty in the measurement. By Faraday's Law EMF = rate of change of flux linkage = d Nphicos(theta)/dt = d(Nphi)/dt .cos(theta). We plotted peak emf against cos(theta). We got a reasonable straight line. The gradient will be the peak rate of change of flux linkage.

Smartphone compass on Ullscarf

My friend uses a Smartphone compass. I had a theory that it works by GPS comparing the position of the phone with known north. Turns out that this isn't true. The phones have magnetometers in them. These are often Hall probes which are doped semi-conductor. When this is placed in a magnetic field, the charges in the semi-conductor are displaced laterally relative to the current, creating a PD across the Hall probe. PD is proportional to magnetic flux density. I have seen suggestions that small arrays of Hall probes inside measure the flux density differently so allowing software to calculate the direction of greatest flux density - ie north.

Measuring up at Center Parcs

We found this at Center Parcs near Penrith. It was a display of the heights of sports stars. It was as bilingual as ever, giving metric metres and Imperial inches. I've been reading Leonard Susskind's book Quantum Mechanics: The Theoretical Minimum. He comments on why the Planck constant is such a stupidly small number by noting that it is really because humans are too big. The units that we use are based on the size of our bodies, as shown in the picture. The most fundamental scale is really down with the sub-atomic particles. We are just too big.

A Pale Line in the Sky from Scoat Fell

I climbed Scoat Fell on a beautiful day when the cloud mass was gathering in the north. You can make out the cloudy haze above High Stile. The top photograph is as normal and the bottom was taken through a polarising filter. There was a distinct pale level above the cloud. The sky does get paler blue as you get nearer to the horizon as Mie Scattering becomes important but I think this was more pronounced. My hypothesis is that above the cloud there was an increased level of water vapour which led to a more pronounced Mie Scattering effect. This is the scattering of all colours of light by water molecules - all colours so looking white.

Automatic Warning System: magnets on the railway

I spotted this from the railway bridge at Kennington near Oxford. We guessed that it was to do with signals and would be magnetic. I thought it would be a new thing but they have been around for decades. I realise that I have seen them before but that I've never seen a clean, new one. It is an Automatic Warning System AWS ramp. There is a permanent magnet in it which sets up the train's emergency brake system when the train rolls over. I can't find out whether the train detects a magnetic pulse in an electric circuit or whether the magnet works a mechanical switch. There is also an electromagnet which has the opposite polarity. If this is working, it deactivates the emergency brake system. There is human control over the system - the driver has to acknowledge the pulses. If the signal is a stop signal or similar warning, the current to the electromagnet is turned off which means that the emergency braking system is not deactivated. The really clever bit is that the system is set so that if there is a power cut, the emergency brakes will not be deactivated. It is fail-safe. So that's why one magnet is permanent and the other is an electromagnet. https://en.wikipedia.org/wiki/Automatic_Warning_System

Early computer programme for a barrel organ

I liked this barrel organ at St Fagans museum. Each tune is stored on a card that is fed into the organ. There are about 10 rows on holes that must be instructions for 10 parts of the musical accompaniment. It looked as though the whole thing was running through compressed air or something similar. Even the drums were on pressure valves. I'm not sure whether air went through the holes on the card or worked a device to blow air at those times. The first computers worked through cards like these. 

Air source heat pump at St Fagans

We found this at St Fagans museum in Cardiff. It is like a fridge in reverse. They use a liquid that evaporates easily at normal air temperature. Then an electrically powered compressor squeezes the gas. This does work on the gas which by the First Law of Thermodynamics raises the temperature of the gas. The hot gas passes thermal energy into the heating system of the building via a heat exchanger. It isn't really free energy since an electric pump is needed to run it. The question is whether it is more efficient and less polluting than other forms of electrical heating.

Flow rate - Burn O'Vat

Burn O'Vat is an amazing hole in a stream near Braemar. It was created when rocks blocked the flow of water and it backed up, forming a whirlpool which carved out a huge space. The bottom picture shows the area of the downstream hole in the rocks. Area might be 2 square metres. Water is incompressible so even if more pressure builds up, the water will back up. I need to find out a bit more about Bernoulli's theorem to come up with a better analysis. I have tried calculating the speed of the water for a Reynold's number of 50000 which would make the flow turbulent. I get 45 cm per second. This would give a flow rate of about 90 kg per second through the hole. It suggests that the flow rate off the hill must be bigger than that to carve out the hole.

Llyn y Fan Fawr - more foam rays

I posted recently about lines of foam forming on Hayeswater perpendicular to the wavefronts and thus effectively acting as rays. Here it is again, this time from the Black Mountain in South Wales. Again the wind was driving briskly down the length of the lake from left to right. It rucks the water up into waves perpendicular to the wind direction but the foam lines up with the wind direction. I'm trying to come up with a theory as to why.

John Dalton Street

I also met John Dalton in Manchester! He's a local hero from Cumbria - the man who gave the modern world atoms is celebrated in Manchester. He sits opposite James Joule in the entrance hall of the Town Hall. A town that celebrates Physics!

Up close and personal with James Joule

I got to meet James Joule in the lobby at Manchester Town Hall. He was a local brewer who did an experiment where falling weights pulled a paddle wheel in a vat to do mechanical work on the liquid. The temperature of the liquid was raised and thus the First Law of Themrodynamics had its beginnings. I am finding it hard to get used to the convention that it is capital J for the abbreviation as in 100J and small j for the full name as in 100 joules.

Linear air track

The linear air track is a long metal tube with tiny holes in. Air from the outflow of a vacuum cleaner is blown down the tube and comes out through the holes, leaving a layer of upwardly mobile air on which gliders float.

 I use it in conjunction with light gates to try to show conservation of (linear) momentum but also to illustrate Newton's First Law. Once you get the glider moving, it does not require a constant application of force to keep it going. It will keep going in a straight line at a steady speed until it hits the other end and an external resultant force acts on it. Then it will change direction and come back in a straight line at a steady speed etc. Aristotle thought that objects needed force on them all the time just to keep them going at a steady speed. His problem was that he inhabited a world of friction.
The odd thing about our vacuum cleaner is that the tube is kept inside. You have to pop the black handles at the top of the silver drum and list the top off carefully.

Then screw the hose onto the outflow of the vacuum cleaner as shown below. It connects to the linear air track as shown in the top photo. When you put it away, coil the hose so that it doesn't get crushed when you put the lid back on.

Simultaneity on the train to Winchcombe

Sat in the steam train at Cheltenham I was reminded of Einstein's work on special relativity. He suggested that someone stood exactly in the middle of the carriage and had apparatus that was able to shine a light beam instantaneously in both directions. The doors at either end have light activated locks that open the doors when the light beam hits them. The person in the middle of the carriage sees each beam hit each door at the same time so both doors open at the same time. The light has traveled equal distances in equal times. Someone on the platform as the train goes by would see it differently. The back door would be moving forwards towards the backward travelling light beam. The front door would be moving away from the forward travelling light beam. So the person on the platform would see the back door open first and the front door second. Both witnesses of the same event but they see a different order of door opening.

Geology on Rosthwaite Fell

I found this wonderful paper on the geology of Rosthwaite Fell which explains its place in the Scafell Caldera. http://jncc.defra.gov.uk/pdf/gcrdb/GCRsiteaccount289.pdf The summit rocks pictured yesterday are Crinkle Tuffs and the abstract suggests that these might be ignimbrites. As such it would be the result of a pyroclastic flow. If I ever go again I'll know to look more carefully https://en.wikipedia.org/wiki/Ignimbrite . The rock under Tarn at Leaves is Rosthwaite Rhyolite. Scafell must have been a pretty impressive volcano.

Evidence of ancient earth movements on Rosthwaite Fell

These are the summit rocks on Bessyboot. Notice the diagonal planes from top left to bottom right. These will be layers of ash. I think from previous reading that we are somewhere on the edge of the ancient Scafell Caldera. They may have settled in water perhaps and so they would have been laid horizontally. But the fact that they are at an angle now is evidence that the whole earth must have tilted sideways. Notice that later pressures have split the rock vertically. Evidence of ancient earth movements...

 But in places the rocks have also split along the bedding planes.

Lenticular clouds from Bessyboot

These fabulous lenticular clouds were visible from the summit of Bessyboot at the end of Borrowdale. They stretched almost from Cockermouth to Penrith. The wind was strong and the clouds form in stationary waves after the wind has passed over the mountains. https://en.wikipedia.org/wiki/Lenticular_cloud says that the isolated lens-shaped clouds form when the air is taken to the peak of the wave and the temperature is below the dew point. So the clouds should be at the peaks and the distance between isolated clouds would represent the wavelength. I would estimate a wavelength of at least a mile, more likely 2 miles. It says that the gaps between are when the air goes down to the trough and the temperature is above the dew point. It has reignited my interest in the idea that the wave shape is fixed but that matter moves through the pattern. On the way down, the main pattern from the top picture was still visible - it is a stationary wave after all - although more clouds had filled in.

Back in Wigton, there were some weirdly twisted clouds visible which might be the remains of lenticular clouds as the front comes in and stratus starts to dominate.

The Hill Reaction

We took delivery of centrifuged crushed spinach leaves. They are kept iced so that they last longer away from the plant. There was a solid pellet in the bottom of each tube and a liquid called the supernatant above the pellet. We poured the supernatant into a separate beaker.

 Then we added 3 cubic centimetres of isolation medium to the pellet left at the bottom of each tube and gave it a shake. The isolation medium helps to extract the chloroplasts. We called the resultant medium the LEAF EXTRACT.

 We labelled 5 tubes. We used syringes to measure 5 cubic centimetres of DCPIP into tubes 1, 2, 3 and 5. We used smaller syringes to add 0.5 cubic centimetres of leaf extract to tubes 1, 3 and 4. We wrapped tube 3 in foil. Tube 2 got 0.5 cubic centimetres of isolation medium. Tube 4 got 5 cubic centimetres of distilled water. Tube 5 got 0.5 cubic centimetres of supernatant. We put the tubes 10cm away from a bright light. DCPIP is an electron acceptor.

When light is absorbed by chloroplasts, the energy is used to split water molecules. Two electrons are released. These can be accepted by the DCPIP which loses its colour. Tube 1 is on the left. It is now paler than tube 2. Tube 2 should stay blue because it contains only isolation medium which releases no electrons. Tube 4 is pale green. It contains no DCPIP and is the colour that tube 1 will become when all of the DCPIP has been decoloured. Tube 5 is paler blue/green, showing that the supernatant still contained chloroplasts. The centrifuging hadn't been perfect.

 Every 5 minutes we put samples of tubes 1 and 5 into cuvettes for testing in the colorimeter to quantify the colour change.

Tube 3 in the dark stays blue. This shows that the Hill Reaction is indeed the light dependent reaction. No light: no electrons for the DCPIP to accept.