Phases of Venus

Venus is low in the southern sky after sunset at the moment. Nigel Henbest's 2024 annual suggests looking through a telescope against the brighter sky before darkness to allow the phase to be seen. On normal settings it over-exposed (above). But a faster shutter speed got something that might be what I was looking for.

Interesting field pattern?

If these arrows were showing the shape of a vector field, it would be a really weird field.

Series circuits with resistors

One thing that was evident with the fairy lights when attempting repair was that the leads had no indication of live or neutral. And that's because it doesn't matter. Resistors like a filament have no preferred direction for current. The fuse will work either way round and there was no Earth lead so it is double insulated.

Filament bulb

This is an old-fashioned outside Christmas fairy light bulb. The filament is the loop shown by the orange arrow. The pink arrow shows a wire strand that holds up the filament. The green arrow is a glass bead to provide insulation. The aim is to keep the wires apart to stop them shorting out.

It is a 12V 3W bulb. Using P=V^2 /R gives resistance of the filament as 48 Ohms. It looks like the filament loop is about 12mm long so resistance per unit length is 4000 Ohms per metre. Tungsten has a resistivity of 5.6 x 10^-8 Ohm.m at 20oC. Using R=rho.L/A and ignoring the temperature rise when turned on, cross-sectional area of such a filament made of tungsten would be 1.4 x 10^-11 square metres so diameter would be 2 x 10^-6 metres. That is surely much thinner than the wire in this bulb.

Thinking about thermodynamics

Mrs B was musing about the thermodynamic implications of this cafe sign. We only talk about heat moving, not cold. And we stress that it is "heat energy" - and modern syllabuses change that to "thermal energy" because we need to stress the difference between heat energy and temperature. Thermal energy is essentially atom wobble. Things with a high temperature have furiously wobbling atoms. We say they are hot. The universe is quite egalitarian and tries to equalise out the amount of atom wobble so ones with more can give to ones with less, but not vice versa. The actual situation is even more complicated because we normally deal with whole objects - collections of millions of atoms. Even an object that feels cold can have a few atoms that wobble massively. So a cold object could pass energy to a hot object but the net flow would be from hot to cold. So physicists tend not to draw arrows labelled "cold"!

Pressure is falling

This was the pressure this morning. You can see from the marker where it was a week ago. What I find odd is that outside the weather is very much like that for high pressure - settled with clear skies. However the barometer reading is not much higher than when Storm Ashley came through last month and I posted about bombogenesis. With Storm Bert on the way, I will need to watch the barometer.

Not quite the Sea of Tranquility...

... but Tranquility on the sea in the marina. It reminded us of the Apollo 11 landings. The Mare Tranquilitatis was named in 1651 by two Jesuit priests who were early physicists. It escaped being a called the Mare Belgicum. It was an assumption by early astronomers that the dark flat areas on the surface of the Moon must be bodies of water like they would be on Earth but they are actually vast basaltic lava flows.