Albedo and drying roads

There was plenty of snow here yesterday and slush on some roads out of town even this morning which could have caused problems with a forecast of freezing temperatures.

But where the dark colour of the roads has been exposed, thermal energy is more easily absorbed. We say that the road has a low albedo because it is a good absorber. The thermal energy helps to evaporate the standing water, drying the roads and hopefully keeping us free from ice in the coming freeze.

Data about Solar Panels

 

This article from the Oxford Mail gives some useful figures about solar panel output. 1400 square metres is giving 260,000kWh per year. My south-facing roof is maybe 30 square metres and further north so might give 5000kWh per year.

LED Christmas Lights

The proprietor said that his smart meter shows it uses less power than the microwave, so maybe 600W. Suppose 1000 LEDs in the display so 0.6W each. 

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.