What's in a capacitor?

 Whilst ratching in the cupboards, I found a capacitor that one of my predecessors had cut open. I've always been too compliant to wreck things!

I teach capacitors as two metal plates with a gap between usually filled with an insulator. Here we should have two aluminium foil strips with paper in between that has been soaked in an electrolyte. Now if you roll that up, you'd have the aluminium shorting as the strips touched each other and so a second paper strip makes a four layer stack to be rolled. It doesn't look like this one has survived intact. 

There are good pictures and diagrams here.

Motor glider

 

Yesterday afternoon I heard the quiet hum of a motor above but instead of a micro-light, I saw a conventional glider. The picture is confusing - I was too late to get a good picture of the glider and inadvertently photographed an insect as well. https://en.wikipedia.org/wiki/Motor_glider explains what is going on. I will take a closer look next time.

Invisible ink at the wedding

 I was shown this trick at Sarah and Matt's wedding reception. You can't see anything on this paper.

Now you can. Guess who wrote it for me!

What's going on here ties in with what I was telling my Applied Science class. This is how reflection works. A photon of light enters an atom, as shown below. The energy helps an electron to go several energy levels.

When the electron falls back, it loses energy again by firing out a photon, which is what we see.

The electron arrangement in the ink is clearly such that ordinary light does not have enough photon energy to allow emission of photons that we can see. Ultra-violet light has more energy in its photons. However, surely the re-emitted photons would be the same invisible high frequency. What must be happening is that the electron doesn't fall straight down but jumps from one energy level to another, releasing some photons that can be seen by the eye.

Thinking about the diabolo

 

Thomas Trilby did some great diabolo tricks. I know that these are based on the physics of Conservation of Angular Momentum. I found a nice short film that explains what this means as applied to some of the tricks. Apparently the diabolo is related to the yo-yo,

Thomas Trilby's amazing thin film interference patterns

 Thomas Trilby's Circus act at Prospect Farm was the best thing I've seen in years. So skilful and also so funny. He made huge soap bubbles using string on poles.

The thing I noticed about the bubbles was the way the colours graded at the bottom to produce a rainbow spectrum with red at the bottom of the bubble up to violet towards the middle.

I know that bubbles produce colours by thin film interference. Some light bounces off the outside. Some continues through the bubble's liquid shell. When it hits the end of the liquid layer, more bounces back and the rest carries on through the bubble. White light is made of colours - light of different wavelengths that mix. If the precise distance the light has gone from the front of the thin film of bubble mix through to the inner edge of the bubble mix, if that is precisely a multiple of the wavelength of a particular colour, then that's the colour you see. Red has the longest wavelength and at the bottom of the bubble, the liquid is most curved. So from my viewpoint the light will have travelled furthest. This explains the grading of the colours.

Ancient oscillations in Wigton?

 

The initials SHM will always excite a physicist. I've been walking past this stone on Station Road for years. Simple Harmonic Motion is when the acceleration is proportional to the displacement from the equilibrium position and the acceleration is always directed back towards the equilibrium. Pendulum motion is approximately SHM for small displacements.

Absolute and apparent magnitude

 St Mary's in Wigton is famous for having gold stars painted on the ceiling of the chancel.

Some of the stars are bigger than others. It's like that in the night sky - some stars look brighter than others. The Greek astronomer Hipparchus decided that he could discern 6 levels of brightness and this has turned out to be reliable to this day. The brightest stars he called 1 and the dimmest 6. This is called the apparent brightness because it is what it looked like to his eye. But the differences in brightness could be because one identical star is further away than the other - or that two stars of different intrinsic brightness are the same difference away. This intrinsic brightness is called the absolute magnitude. To put this on a scientific setting, it was decided to calculate how bright each star would seem if they could all be moved to the same difference away - 10 parsecs. This distance appears in the equation linking apparent magnitude m and absolute magnitude M. It is m - M = 5log(d/10) where d is the distance to the star in parsecs.