Hoar Frost

 

As far as I can tell, hoar frost is when water vapour in the air, what is a gaseous state, turns directly into a solid without passing through the liquid phase. This happens when the water vapour touches a cold surface. As this is the opposite of sublimation, which is when a solid is heated and turns straight into a gas without becoming a liquid, it is called desublimation. Rime ice is different because the water vapour turns into a supercooled liquid first before it then turns into a solid on hitting a cold surface. Rime is thus forms on the side of an object exposed to the wind. Now I need to find phase diagrams for H2O to show this.

Bicycle dynamo

 

I found an old bicycle of a type I used in the 80s. It had a dynamo. I used to love these because your batteries never ran out. The little black wheel on it is turned because it sits on the tyre. The turning black wheel is attached to a permanent magnet that spins inside a coil of wire. By Faraday's Law, this induces an EMF and the EMF causes a current to flow. The faster you pedal, the higher the EMF and thus the higher the current and the brighter the light. This is also covered by Faraday's Law which says that induced EMF is proportional to the rate of change of flux linkage. The flux of the magnet links the coils and this changes as it spins round. The faster it spins, the bigger the EMF. Disadvantage is that when you stop at traffic lights, your lights go out, and they are dimmer going uphill!

Sunshine on a globe

 This caught my eye lying abandoned on the verge.

On closer inspection, it turned out to be a globe.

It looks as though it is meant to stand upright with the Earth's axis tilted at the correct 23.5 degrees but discarded on a slope as it is, the axis is more like 40 degrees to the vertical. This led me to think about what would happen if the Earth had a bigger tilt. That would mean that a greater number of countries would have 24 hour sunshine in the summer and 24 darkness in the winter. The seasons would then be more pronounced. This had the best explanation.

Snowflake symmetry

 Mrs B has been making snowflakes as very effective window decorations. The process reminded me that symmetry is used a lot in physics to simplify problems. I came across the idea with tensors, where symmetry across the diagonal makes hideous calculations possible by eliminating a lot of possibilities.

Today I started with a square of paper.

I folded it into four.

This simplified things because I only needed to trim one quarter to make a whole circle.

I then folded it again.

Now I only needed to cut half shapes to get the effect I was looking for.

When opened the symmetry is revealed.

I tried again with a bigger piece of paper and twice as many folds.

I got twice as many patterns.

So using symmetry made it easy to make a very complex design, an analogue for physics computational short cuts.

Car light corona through the windscreen

 We were sat waiting in the car on a rainy day. I noticed that the bright white headlights (or maybe they were LED sidelights) were giving a wonderful diffraction corona.

It proved very difficult to photograph. I think one thing is that I was holding the camera too low. The diffraction depends a lot of droplet size and that was probably variable across the windscreen. It must have been optimal at eye height!

Like Clockwork

We loved this amazing game. You wind up Santa and the reindeer and they are released at the same time. The winner is the one who gets to the middle first.

We were wondering whether anyone will know what clockwork is in the future. Winding the mechanism fills the elastic energy store of the spring by the mechanical working pathway. As the spring unwinds, it itself does mechanical work to turn a crank that turns the legs, filling their kinetic energy store. There must be something eccentric on the crank to make the feet go round in a full circle, probably something like the pedals on a bicycle but driven by the spring. Like pedals so that when one is up, the other is down.

 

The Geminids

 

I saw a shooting star tonight and it was coming from the Gemini direction although all accounts suggest it really should be too late. There is a nice picture here. Researching the Geminids, I was interested to discover that there are some close in asteroids whose orbits cross that of the Earth. I thought all the asteroids were in the asteroid belt.