Evaporation 2: Stafford Services

Here's my mug of coffee on a cold day. You can just make out the cloud of visible water vapour. I know that the rate of heat transfer is greater when there is a bigger temperature difference between inside and outside. I think that applies to conduction: it's a variable in the thermal conductivity equation. I also know that hotter liquids evaporate faster because the particles have higher average kinetic energy and are thus more likely to escape through the surface tension. But does the outside temperature have any bearing on evaporation? Would there be more evaporation of a hot liquid if the air temperature around was colder? Colder air holds less water vapour so perhaps not (that's why I got the "steam") but perhaps colder air might be more conducive to the formation of convection currents which would transport water vapour away from the surface of the liquid. So it hinges on whether the temperature of air affects the formation of convection currents.

How many hairs on a reindeer?

When we visited the Cairngorm reindeer, I was given a figure of 2000 hairs per square inch. If we say that a reindeer is modelled by a cylinder 3 feet long and 1 foot diameter, surface area is pi^2 x radiius x length = pi^2 x  36 = 2131 square inches. That would mean 4.25 million hairs.That's more than I was expecting!

Evaporation 1: Thrupp Lake, Radley

Hard to see in the photograph but the wood on this boardwalk was steaming in the sunshine. The sunlight must have provided enough energy to melt the frost (a latent heat problem) and then cause the liquid water to evaporate. The cold air will not hold much water vapour so it saturates easily and thus the clouds form. However convection effects will then disperse the saturated vapour to unsaturated areas so it does disappear as it moves away from the bridge.

Folded dipole aerial on Cairn Gorm

There is a weather station on the top of Cairn Gorm that sends information back to Herriot Watt university in Eduinburgh http://cairngormweather.eps.hw.ac.uk/  The aerial is of a type called a folded dipole http://www.antenna-theory.com/antennas/foldeddipole.php The two ends of the dipole are joined electrically to form a complete loop circuit. This gives greater bandwidth. The input impedance is higher. Impedance is a word used instead of resistance for an alternating current circuit. AC can attempt to block current by capacitance or inductance. Neither method is to do with vibrating lattice ions so cannot be called resistance. More work needed on this - I never fully understood it during my degree! The top to bottom measurement on the dipole is half a wavelength. Measured against the person in the photograph I'd say 80cm so one wavelength is 1.6m. That gives 188MHz.

Inside the Stevenson Screen on Cairngorm Mountain

We had a peek inside the Stevenson Screen by the ski station on Cairngorm. The screen is designed to ensure that air conditions inside are the same as those outside minus wind and sun. Inside was a wet and dry thermometer. The bulb of one thermometer is wrapped in fabric which is dipped into a pot of water. This wicks up the water to keep the bulb wet. As water evaporates from the wick, it takes energy from the thermometer thus lowering its temperature. Evaporation happens best in dry air and is more difficult in humid air because the air is able to carry less extra vapour. So by comparing the temperatures on the two thermometers, you can tell the humidity. In really humid conditions the temperatures will be the same but in really dry conditions the wet bulb thermometer will have a lower temperature.

Flick the switch and the rods repel

 I put two steel rods side by side inside a solenoid. Then I flicked the switch.

The rods repelled because the magnetic field of the solenoid lines up the domains in the steel. This turns both steel rods into magnets. The domains are aligned identically in each rod so they both have north poles at the same end so they repel.

A circle doesn't cancel a parabola

After I had used the parallel plates to bend the electron beam down in a parabola, I turned on the Helmholtz coils to produce a magnetic force upwards on the beam. To work out the direction of the magnetic field we use Fleming's Left Hand Rule. The middle finger means conventional current, a flow of positives. So for electrons I point my middle finger back to the source of electrons and then my thumb up to show the magnetic force upwards. My first finger points towards me so the north pole is on the back and the south pole is nearest to use. The electron beam should now go straight. It is bent because a magnetic field sends electrons along a circular path and a circle cannot fully cancel out a parabola.

Twinned primary bow: split rainbow at Grune Point

We spotted a Y-shaped rainbow at the end of Grune Point yesterday. Apparently this split rainbow is caused by two different sizes of raindrop. See http://www.itv.com/news/calendar/2015-08-06/very-rare-split-rainbow-spotted-over-hull/ for a better picture and explanation.

Electron deflection tube and parabolic path

This is the electron deflection tube. The electron beam is seen in blue against the fluorescent screen because the electrons give energy to bound electrons in the material, which go up energy levels and then de-excite by emitting blue photons. The beam is bent by parallel plates above and below the beam. The Helmholtz coils are not turned on in this picture. The path is parabolic. We should be able to prove this by looking at the data. Parabolic means that y is proportional to x-squared. In other words, if I double the distance along, I should go twice as far down. The data has really surprised me. I blew the picture up really big. I first noticed that there seem to be eight divisions on the x axis but they are labelled 2, 3 ... up to 10. No zero, no 1. This suggests that they must think that the electric field starts before the plates. I read data off using the numbered scales. Here it is:

If it is parabolic, then the ratio of y to x-squared should be constant. It is.

Coledale Inn filament lamp at Kong Winter Series

We were sat waiting for the results of the Kong Winter Series fell race when I noticed this filament lamp. Unfortunately the individual strands haven't come out in the photograph The light is just on the yellow side of white. Looking at a black body spectrum it puts the temperature at about 4000 degrees Kelvin. http://hyperphysics.phy-astr.gsu.edu/hbase/bbrc.html

No disturbance at the turning on of Wigton Christmas lights

I was stood outside the church whilst the bands were playing on the main stage round the corner and down the street. I could hardly hear them. In part this will be due to attenuation. It is hard to apply the inverse square law here because that implies a perfectly spherical dissipation of the energy. But what about diffraction? Let's say that the gap at the end of the High Street is 10 metres wide. Waves diffract well if their wavelength is the same as the width of the gap. Sound waves have a speed of 340 metres per second. Applying the equation: wave speed = frequency x wavelength gives frequency = 340/10 = 34 Hertz. The best diffracted wavelength is almost too low to hear. Our ears don't detect well at the extreme ends of the spectrum. The most irritating frequency is around 1000 Hertz. A gap to diffract that would be 34 cm across. This gap is too big. It was peaceful.

When an electrolyte really means dissolved salt

I was interested in the use of the word "electrolyte" on this product. An electrolyte is when an ionic compound dissociates when it dissolves in water so that the ions are no longer bonded and are free to move. They can move to the electrodes so that a current flows. I read the details. It seems to me to suggest that they took spring water and distilled it. So they took water with salts dissolved in and distilled it so that the salts were removed. Then they added "electrolytes". The list says the electrolytes are calcium chloride, magnesium chloride and potassium bicarbonate. These are what I'd normally call salts. They do dissociate in water and so could be called electrolytes.

Alpha and omega: Physics in the Bragg family windows??

This is the central panel of the three wonderful windows that Melvyn Bragg put in to Wigton Parish Church. The letters are the capitals for alpha and omega, the first and last letters of the Greek alphabet. Like most Greek letters, they have some Physics pedigree. Back in the day, most physicists had a classical education and were proficient in both Latin and Greek. So if you found three types of radioactivity, it was obvious to call them alpha, beta and gamma. The raises a question that had never before occurred to me: in what order were the three types isolated? Is it in the order they are listed? A capital omega is used to denote the unit Ohms. You couldn't abbreviate 10Ohms as 10O for obvious reasons. Hence the recourse to Greek.

Brilliant Physics puzzle films Advent calendar: Physik im Advent

Who needs a chocolate Advent calendar when you can have http://www.physik-im-advent.de/ You can decide whether to have it in German (DE) or English (EN) using the toggle switch in the top left hand corner. The film of the answer comes up the next day. If you look at days that have already gone, the answer film comes first but scroll down for the original problem.

Magnetic field for parallel wires

Two parallel wires with current running through them exert a force on each other. Each one creates a magnetic field in which the other runs. By Fleming's Left Hand Rule, that means a force. By Newton's Third Law they are equal and opposite forces. It is hard to show bu long parallel foil strips just about do the trick. Here current goes in at the top, down one side and back up the other side. The foil strips repel. The theory is here: http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/wirfor.html

Conical pendulum and skittles at Basildon Park

In the woods at Basildon Park near Reading we found this odd skittles game. There was a central pole with a mass on the end of a string that can go round. The trouble was that the string was too short to let the mass reach the skittles.

It was, though, a nice example of a conical pendulum.

The real forces are shown in blue. Weight W=mg downwards and tension along the string. They are not balanced so a resultant force acts. It is a centripetal resultant force as shown in red. The mass accelerates towards the centre of the circle. Vertically, mg = Tcos(theta). Horizontally centripetal force m.vsquared/radius = Tsin(theta). If you put the two together and cancel T, you get vsquared = gr.tan(theta). If the string has length L, then tan(theta) = r/sqrt(Lsquared - rsquared).

Sixth Form homework question: American monument of Islay

This is the American monument on a lonely rocky headland on the island of Islay. It commemorates lives lost on two ships that were wrecked in sight of the point during the First World War. Such a remote and prominent point makes it far more likely to be hit by lightning because the shorter distance between it and the clouds means that the electric field strength above it is bigger than in surrounding places. When the electric field strength reaches a critical value, air will conduct. To deter lightning, there is a lightning conductor up the side.Your job is to look at the photographs to estimate some dimensions, look up one important number on the Internet and then to calculate the resistance of the lightning conductor.

Diffuse reflection at Carraig Fhada, Islay

Two new words have appeared on the GCSE Physics syllabus - speuclar reflection is for when all reflected rays are parallel to each other and make a coherent reflection. Diffuse means that the reflection is from a rough surface. For each ray, angle of incidence = angle of reflection. Even if the incident rays go in parallel to each other they won't all have the same angle of incidence because the of the uneveness of the surface. The reflected rays will not be parallel to each other. The waves were driving in from the left in the middle picture. look how rough the profile of Carraig Fhada is. Diffuse reflection was making for a rough sea that day.

Delezenne Circle

This is an amazing piece of equipment. The Earth's magnetic field passes through the wooden circle, which contains a big coil of wire. Flux from the Earth passes through the coil. Hence there is flux linkage - which is defined as number of turns on the coil x flux. There is a handle at the top which I managed to cut off in the photograph. If I rotate the handle, the wires spin in the flux. Imagine a quarter turn through 90 degrees from full flux linkage with all of the flux passing through the coil to a point at which the coil is parallel to the flux and no flux passes through the coil. Faraday's Law states that induced emf = change in flux linkage/time taken. An emf is a voltage which could be measured by attaching a multimeter to the contacts. Flux changes from N x flux back to zero in time delta t. Delta t is one quarter of the time period so it is T/4. So emf = 4N x flux/T. I rotate the coil at a steady rate and time 20 cycles. I then diivde by 20 to get T. Finally, the Earth's flux = emf x T/4N. If I want magnetic flux density B, then B = flux x area of the circle.

Moments and the slate wine bottle holder at Honnister

I really liked this wine bottle holder in the shop at Honister Slate Mine. It is not bolted down. It really is a balance bar obeying the principle of moments. A 75cl bottle will have a mass of 750g of wine. Let's make it up to 1kg including the glass.Centre of mass looks about 3cm from the pivot on the table top. Centre of mass of slate holder looks maybe 6cm from pivot which would give the piece of slate as 500g.

Diffraction through the net curtain

The diffraction pattern from the neighbour's light was really clear through the net curtain this morning. I'm always amazed by how well net does diffract light. The cross shape is because there are horizontal slits in the net which diffract vertically and vertical slits which diffract horizontally.

I was struck by these things:
1. The bit minimum outside the central cross. There were lesser minima visible to the naked eye beyond.
2. I'm wondering why the central section is also slightly split. I suppose that the diffraction pattern is usually overlaid by an intensity pattern dictated by the single slit. Maybe that effect is just very pronounced here.
3. The colour split is very noticeable. Violet closest to the middle and red further out.

Black Hole at Campfield Marsh RSPB

This pipe on the RSPB reserve at Campfield Marsh reminded me of a black hole. What goes in can't come out - and a bonus is that gravity is the force pulling the water over the edge. The pipe edge would be the event horizon. It is so difficult visualising a black hole - it's a 3-dimensional star but also a singularity.Imagining a hole in the air in front of me is head-wrecking.

Polarised zenith at sunset in Silloth

 Silloth at sunset.

Then I remembered that at sunset, the sky above my head is supposed to be plane polarised. Looking at it with my eyes through polarising sunglasses, it seemed clear to me that the sky became darker as I twisted the filter through 90 degrees. I tried to photograph it. Not that convincing!

Silloth amusement arcade and the Castle Crag slate heap

I love the 2p machines in amusement arcades. Here is one at Silloth. It's the way that the coins manage to build improbable structures by stacking over each other. I know that there are often bits underneath to encourage the overhangs, but even so .....

It's a kind of emergent structure. It must be possible to model it using the laws of mechanics. I was interested in the assertion below that it is not an example of complexity.

The stacked coins remind me of stacked slates in a scree. Here is an example at the former quarry at Castle Crag.

Parabolic reflectors for giant halogen lamps

The car dealership had large halogen lamps - bigger versions of those that I used to put inside an overhead projector (OHP). And they were surrounded by parabolic silver reflectors. A parabola is the curve that you get if you plot y against x-squared. It is an important shape for a reflector because it has a clear focal point. In other words, any parallel beams of light coming in reflect back out through a single point. That is not true for a spherical reflector. Rays that are parallel are said to have come from infinity. So these lamp shades work in reverse. If you put the bulb in at the focal point, the rays that go upwards back into the lamp shade are reflected back out parallel in a beam.

Horse hair icicles on the fence

We were fascinated by these icicles that had formed around horse hairs at Campfield Marsh. Why only on the hairs but not on the barbed wire? We don't know whether they had formed on the wire but already melted. The metal of the wire is a good thermal conductor because of its delocalised electrons. It feels cold to the touch because it conducts thermal energy away from your hand. So maybe water should freeze more quickly on the wire. That might mean that ice would melt more quickly on metal than on an insulator. https://www.nuffieldfoundation.org/sites/default/files/files/Melting%20ice%20-%20merged%20PDF.pdf So perhaps that's the explanation - that there was ice on the fence but that it had melted.

Keeping the lighthouse clock on time

I found this outside the museum at Port Charlotte on Islay. It was from a local lighthouse. The lighthouse was provided with a clock and was expected to keep regular hours. But what if the mechanism ran fast, ran slow or stopped? They were provided with a sundial to give the correct time to set the clock. When a measuring instrument gives the true value, it is said to be ACCURATE. This sundial makes the lighthouse clock accurate.

Strange sky over Wigton

I spotted this interesting sky at sunset. In the bottom picture the pink light from the clouds must be the colour of the sunlight left after the other colours have been scattered away by dust in the atmosphere. Then that light has been able to reflect towards me from certain stand out features on the lower surface of the clouds. The ripple marks are perhaps evidence of turbulence where air masses travelling at different speeds meet.

Ping-pong with static electricity

I connected the parallel plates to a potential difference of 4000V. I hung a table tennis ball between the plates on insulating thread. The ball was painted in conducting paint. The ball is touched on the negative plate. Electrons flood onto the ball and is becomes negative so it is repelled from the negative plate and attached to the positive plate. When it touches the positive plate all of the electrons are pulled from the plate. Judging by the force with which the ball is repelled back, this must include delocalised electrons from the graphite-based paint which will leave the ball depleted of electrons and thus positive. The alternative is that it ends up neutral and gravity pulls it back. I doubt that alternative because damping due to air resistance would leave it short of the negative plate. This ping-pong continues backwards and forwards, shuttling electrons one way across the gap. If the plates are closer, the frequency is much higher. Is this because the distance is less or because the field strength is higher?

Reflecting on partial reflection

I was asked a brilliant question today about why you see a lot of reflection in glass when you look from a bright place out into a dark place but not the other way round. Here's the view this evening. The lights illuminate the inside of the room so there is a lot of light heading towards the window. Some of this is reflected and we can see it. What we can't see is how much of it leaks outside. However, we can see the light from outside transmitted in. We can't see the fraction of the light from outside that hits the outside of the window and is reflected back outside. So we never see the whole picture - we see partial fractions of two different processes. The brighter one will dominate. In this case, it's the internal reflection.