Cloud streets at Croome Court

We visited Croome Court in Worcestershire on a day where the cumulus clouds we forming into long lines parallel to the wind direction. These are called cloud streets. Cumulus is formed by convection. As the warm air rises, it cools and can hold less water vapour before becoming saturated. The process of condensation releases thermal energy by latent heat. This further drives the convection at the level of the clouds resulting in the fluffy tops of the clouds. I read a source that said that the neatly spaced rows of cumulus were caused by "longitudinal roll vortices". Hmm... In convection, when warm air rises, colder air sinks to form continuous loop. So the warm air rising at cloud level due to the latent heat must cause some colder air to sink somewhere. My guess is that colder air coming down is what separates the rows making circulation (vortices) that are stretched out longways (longitudinal).

This duck sounds like a Geiger counter

Spotted at the Wildfowl and Wetlands Trust centre at Slimbridge near Gloucester. Is this a Physics duck? And, yes, it does sound a bit like a Geiger counter. We do occasionally get them on the Solway,

Mirrors for X-rays at Diamond

How do you get X-rays to reflect from a mirror? X-rays go through most things and the shadows of bones are caused by X-rays being absorbed, not reflected. The answer is to fire them in at a very tight angle to the surface. In X-ray telescopes, they are called "grazing mirrors" because the X-rays graze the surface. You can see the effect with glass. Looked at straight on, light is transmitted through and you can see what's on the other side, But look along the surface of the glass obliquely and you see a reflection. The poet George Herbert summed it up in a hymn from 400 years ago, before more gender inclusive language I'm afraid: "The man who looks on glass on it may stay his eye, or if he pleaseth through it pass and then the heavens espy".

NPL time signal at Anthorn

I've been looking at these aerials every day for over 10 years but I haven't paid much attention to how they work. It occurred to me today that it isn't the masts that you can see from Wigton that transmit the waves but the wires strung between them. In the picture it is possible to see vertical wires from the ground going up in front of the most distant right hand mast. They then make a T-shaped antenna with horizontal lines. The signal must be being fed from the ground station. By making electrons in the wires vibrate, you can generate an electromagnetic wave. Such dipole antennae have a half wavelength along the top of the T. The masts are 227 metres tall. The distance between the masts is bigger than this - perhaps 500m - which would give 1000m for the shortest wavelength of LF radio waves and would give a frequency of 300 kHz. This sounds right for data on the Rugby masts which used to house the time signal. The radio waves set the time for radio controlled clocks around the country. The time itself is fixed by the NPL National Physical Laboratory in London.

Two TV programmes to watch on the iplayer

There have been two excellent Physics programmes on the BBC this week which will be available to watch on the iplayer for the next month.

1. Jim Al-Khalili did star formation. http://www.bbc.co.uk/programmes/b0754t74 I was really taken with it because he filmed a lot of it in the Canary Islands in places I visited at Christmas! It's a great programme.

2. Last night the Horizon strand on BBC2 covered attempts to produce anti-gravity. It's a good programme for the scientific method as claims are tested rigorously. There's also a nice bit about dark energy in there. http://www.bbc.co.uk/iplayer/episode/b0752f85/horizon-20152016-2-project-greenglow-the-quest-for-gravity-control

Crystal monochromator at Diamond Light Source

You can't see what is happening inside the machine but this is on the other side of the wall to the magnets shown on previous posts. The electrons have been pulled round the ring by the magnet but the X-ray beam carries on in a straight line. Scientists usually want to choose a single wavelength but the X-rays will cover a range of wavelengths. So they are shone at parallel crystals. The planes of atoms act like the slits in a diffraction grating so that the spread of the beam depends on the wavelengths present. With visible light diffraction, I have been using n x lambda = d x sin theta. Different wavelengths are found at different angles theta. So you choose your angle to get your wavelength. William Lawrence Bragg did the maths for X-ray diffraction and put a 2 into a version of the equation I have quoted. His dad was born about 2 miles outside Wigton and the two of them shared the Nobel Prize for Physics for work on this topic.

Beamlines at the Diamond Light Source

 I've posted about the electron storage ring. The electrons are pulled round into a rough circle by the big green magnets of a previous post but the X-rays generated are not affected by magnetic fields and carry straight into the sections shown in luminous yellow in the diagram above. These are called beamlines and are the actual experiments. They take place in metal compartments that are like lorry containers. They are painted yellow (see below) and are lead-lined to stop the X-rays escaping.

Some stars shine brighter than others

I found this on the wall of one of the labs today. What an excellent idea! It shows that some stars are actually closer than others. In truth, some stars really do shine brighter than others but a bright star that is a long way off might look dimmer than a star that was not so bright but was closer. This is what they call "apparent magnitude". A fair test would be to line up all the stars at the same distance. Then you'd know which was brightest. They call that "absolute magnitude". The distance chosen is 10 parsecs away and since we can't actually do it, absolute magnitude is calculated.

Dipole bending magnets at Diamond Light Source

 The Diamond Light Source produces X-rays from a beam of very fast electrons going round in a circle. Well, actually, it is not quite a circle. The picture above shows the path of the electrons painted on the shielding on top of the storage ring. Notice that the path is straight and then a bend and then a straight. We went down below...

 There are 3 types of magnet. The green magnets are responsible for the bending.

The bending magnets are dipoles. They have two poles - north and south. One is above and one below. They are electromagnets with two turns above and two turns below. One two turns?! Well, the current is 1500 A and the field strength is 1.4 Teslas. That's huge. So they don't use wires. They use fat lumps of copper. The bigger the cross-sectional area, the lower the resistance. The heating effect would be calculated by current squared x resistance so the resistance needs to be very small. I was told that copper was hollow so that cooling water can flow through it. The electron beam must be travelling away from us to produce light to go into the tangential hutch experiments. To bend the bend left to right in the middle picture, Fleming's Left Hand rule means that the top coil must be a north pole and the bottom a south pole.

Wigglers and undulators at Diamond Light Source

 You can see the alternating magnets in a wiggler insertion device here. See below for an explanation.

I went to an open day at the Diamond Light Source at Harwell in Oxfordshire. I'd been before but this was an even better tour because we got to go into the storage ring. A synchrotron light source is a ring particle accelerator that accelerates the electrons up to speed and then makes them change direction using magnetic fields to put forces on the electrons. Remember that current in a wire is a flow of electrons and creates a moving magnetic field. There is also an electric field around an electron. So by making the electrons change direction, you can make the moving electric and magnetic fields of an electromagnetic wave. Web sources say that what happens with the electrons in the synchrotron is pretty much the same principle as using moving electrons to make radio waves in a radio antenna. The difference is that the speed of the electrons is so big here that relativity theory is needed for the calculations. The synchrotron uses sets of strong magnets about as big as the keys on the piano. They are alternately placed to make the beam bend backwards and forwards. If they are in the horizontal plane they are called wigglers. In the vertical plane they are called undulators. The X-ray light will therefore be polarised either horizontally or vertically.

Belt of Venus at Wigton tip

The Belt of Venus is a phenomenon just after sunset when the sunlight is still lighting the sky above us but a shadow of the Earth is projected into the sky. It was very clear tonight and seemed higher up than usual. There was a high cloud bank perched on the Pennines and perhaps this was why it seemed higher.

Vari-speed on the Nevis Range cable car

 We took the cable car up to the ski station on Aonach Mor. The car accelerated noticeably on leaving the lower station after having crept round slowly to allow us to get in. I wondered how they did that because the cars all seemed to be be joined to the same cable which was moving at a steady speed. On the way back down I inspected the apparatus above our heads. There were wheels on the hanger but they were not touching the cable.

 When we got off I noticed that the cable went round the lower wheel at the bottom of the photograph but the car was picked up onto a higher cog using the trundle wheels on its support. The cable wheel and the cog had very different angular velocities which allows the car to move slowly for loading and faster when going up the cable. It also explains why the cars are closer together in the loading station and more spaced out on the cable.

Single slit diffraction

This is a ripple tank picture of plane waves (ie long straight wavefronts) coming down to hit a barrier with a gap in it. The gap is about one wavelength wide. The waves fan out below in semi-circles. I have never managed to do it well enough to be able to see the dead spots in the semi-circles in the ripple tank. Try https://johnvagabondscience.wordpress.com/2008/10/25/huygens-and-single-slit-diffraction/ for a better picture. But it did work quite well with a laser.

The single narrow beam is spread out into a blurred line - that's the semi-circle Then we could see two dimmer patches off either end of the main blob. The gap to the dimmer patch is the dead spot that I'm struggling to see in my ripple tank.

Pressure difference: Keswick flood defences

This picture was taken from the path into Keswick from Stormwater Bridge. We were standing outside the flood back on fields that are designed to food. Look closely at the dam. There are flaps in it. Flood water pushing from the outside in would push the flaps flat shut so the water could not get in. Water that had somehow got in over the banks would be able to get out if the pressure on the inside pushing out was greater than the pressure on the outside pushing in. Water did indeed get inside the banks. It has to be let out somehow. The pressure difference can only surely be high enough when the waters on the outside have receded. But there must have been quite a flow of water outwards through the flaps judging by the trail of stones left on the field outside the flaps.

Peter Rabbit is blowing in the wind

We found this Peter Rabbit weather vane near the Lingholm Estate where Beatrix Potter used to take her holidays by Derwent Water. The walled garden was apparently the inspiration for her Peter Rabbit stories. The wind was blowing south to north but notice that the arrow is pointing south. My classes find it hard to get their heads round the idea that the wind is named for the direction from which it comes not to which it goes. So the arrow points in the opposite direction to the air current. It's a bit like an electron in an electric field where the electric force points in the opposite direction to the electric field.

Balance beam platform scales on Corrour Station

We went to the highest mainline station in the UK and probably the most remote. It is 12 miles from the nearest road. The scenery on the edge of Rannoch Moor was amazing. And then I spotted these scales...

I have found pictures of similar on the Internet but I can't find an explanation of how it works. It is clearly a balance bar and must work by the Principle of Moments. The pivot must be somewhere at the top of the main column. One side has a fixed weight near the pivot and the other side has a movable weight for balancing. My main problem is how the luggage based on the flat plate gets to be measured. My guess is that it must connect up the column to the balance bar but very close to the pivot on the side of the fixed mass. If it connected close to the pivot, its moment would be small so could be balanced by a smaller mass further away. More research needed.

A moment of inertia in Grasmere

I spotted the lawn roller at Lancrigg, a house associated with William Wordsworth. It has to be heavy to squash the lawn back flat. Surely that is weight, the force of gravity pulling down. But as it is heavy it has inertia that makes it hard to move forwards - Newton's Second Law shows that acceleration is inversely proportional to inertial mass. But the poor gardeners also had to contend with the rolling motion. Getting something to turn means getting mass to move around a centre of turning. The further away from the centre, the harder it is to move. The effect is called moment of inertia because it is a summing of mass x distance from centre. Where do you put the mass needed to flatten the lawn? For easy movement, you'd want more mass close to the middle. But it seems evenly distributed from the middle outwards as far as a wide rim which must mean the outside has even more mass if density is uniform.

Theremin and capacitance

Today Google were leading with an obscure anniversary for a theremin player. The theremin was the first electronic instrument. Its inventor was trying to come up with a proximity sensor. Watch this to see it being played: https://www.youtube.com/watch?v=MJACNHHuGp0 Notice that she changes the sound without touching the instrument. It turns out that her hands are acting as the Earthed plates of a capacitor. Consider this:

A capacitor is a break in a circuit. It acts to stop current flowing. But with AC current flows backwards and forwards. As the frequency increases, the gap affects the flow of current less and less. Capacitors are used as high pass filters allowing only high frequencies through.

An inductance is basically an electomagnet. It generates a moving magnetic field with AC. That moving magnetic field is responsible for generating a back emf in the circuit. This is bigger when the frequency is high, by Faraday's Law which says that induced emf is directly proportional to the rate of change of flux linkage. Thus high frequencies are blocked by an inductance. It is a low pass filter.

Now put the two together.They do opposite things. Careful study show that the phase of the current oscillation in them is 180 degrees so when current goes right through the inductance it is going left through the capacitance. Ah! Current will flow in a self-sustaining loop. The frequency of this resonance depends on the values of inductance and capacitance chosen. In the theremin the inductance is fixed. The hand acts as one plate of the capacitor and its size is affected by the distance from the other plate. This changes the frequency and thus the note.

Magic with insulating rods

We took a polythene rod and rubbed one end with a cloth. Electrons were taken from the cloth onto the rod so the rod gained a negative charge overall. We balanced it on a watch glass.

 We took a clear cellulose acetate rod and rubbed it. We were able to use it to pull the white polythene rod round without touching - magic!

 We used another polythene rod to push the first one round without touching.

The conclusions are: negative and negative repel. The clear rod attracts so we conclude that it is an opposite charge and we call it positive. It must have lost electrons to the cloth on being rubbed. Thisis what I teach. It all sounds so definite. But where did the original proof of the electron transfer theory come from? And what makes polythene gain electrons when cellulose acetate loses them?

Young's Double Slit with loudspeakers

 We set up two loudspeakers 2 metres apart on the front bench on the front bench. Each speaker was emitting circular wavefronts of sound of the type shown in the ripple tank picture below.

 When you get two sets of overlapping ripples, you get constructive interference when waves meet in phase - which occurs when the path difference is a whole wavelength. Destructive interference occurs when the waves meet 180 degrees out of phase. This is when the path difference is half a wavelength. The result is the stripey pattern of fringes seen below. The two speakers were correctly cross-wired so that the waves had a fixed phase relationship when they left the loudspeakers - they were COHERENT.
                                     
 We had calculated a frequency that would put the fringes for sound about half a metre apart - the fringes have to be further apart than the distance between your ears! As we moved along we could hear clear maxima of loud sound with minima between. It never does get fully down to zero volume! And lastly, the picture of the surface of the ripple tank.

Centripetal acceleration on turbine blades

 This old power station generator is in a museum in Manchester. It has cut away glass panels so that you can see the turbine blades.

 The board says 3000 revolutions per minute which turns out to be an angular velocity of 314 radians per second.

Estimating a radius of 1 meter for the rotating apparatus, that means that there is a centripetal acceleration of 314 metres per second per second on the blades. Compare that with 10 m/s/s for gravity and you can understand why the blades have to be tough.

Insulation: ice house on Tentsmuir Point

We found this ice house on Tentsmuir Point near St Andrews. These date from the days before freezers. The idea was to have ponds nearby that froze in the winter. They cut ice on the ponds and filled the building. The building must have been very well insulated. The large mass of ice would mean that a lot of energy would be needed to change its temperature. Water has a high specific heat capacity. Ice has about half the SHC but that is still quite high. Since energy needed = mass x SHC x temp difference, a large mass sort of means "safety in numbers" for the ice. Also there is the energy needed to melt the ice once it reaches 0 degrees Celsius - the specific latent heat. Provided the insulation is good, the rate of flow of thermal energy inwards is too small to melt the ice. The ice would stay frozen until next winter. It was used to freeze fish to keep them fresher for export. I would have liked to have been able to get in to see how the door prevents too much hot air getting in every time it is opened.