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.