21

Radio Show & Podcast Feedback / BBC iPlayer Broken Link

« on: 10/10/2011 18:14:54 »

Thanks I think I have fixed that now

22

Geology, Palaeontology & Archaeology / Does Concrete Melt ?

« on: 22/08/2011 10:13:12 »

Certainly at atmospheric pressure concrete would decompose - it contains calcium hydroxides which would decompose to calcium oxide, essentially it would go back to dry cement. It is possible what is then left may melt eventually as there are quite a lot of silicates in there so would form something like a very basic lava.

Glass fibre won't burn certainly, though fibreglass often refers to GRP (glass reinforce plastic) and the plastic will definitely burn.

23

Science Experiments / Rubens' Tube - waves of fire - Garage Science

« on: 04/05/2011 09:48:36 »

To be honest I haven't done a lot of development work after getting it to work. I have a feeling making the tube slightly larger than 50mm for the 1.8m I used would be a good idea.

My feeling is that if the tube is too narrow, the low frequency resonances will be weakened (I definitely see this) but as the tube gets wide, the high frequencies will start to have a wavelength similar to the radius of the tube, and I imagine that you will get all sorts of exciting modes of vibration across as well as along the tube. I think this will tend to smear the resonances, which is what I see with mine. It works beautifully for short wavelengths but you don't really get the first couple of modes.

24

Science Experiments / How can I make a cloud in a bottle without smoke?

« on: 21/01/2011 20:21:02 »

To do that you need to have a much larger pressure change if you want to do it without smoke. If you can put a car or bicycle valve into the lid of your bottle, and pump it up - I think steve spangler does this. But be very careful about the pressure you pump it to, as I know someone who caused a lemonade bottle to blow up in his face which was very scary. Keep the pressure below about 2 atmospheres and you should be ok if the bottle is undamaged.

25

Science Experiments / Kitchen Sciense Suggestion?

« on: 04/01/2011 09:17:15 »

It is a good experiment. I think you mean something like this
http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/straw-oboe/

26

Science Experiments / Magnetic Fingerprinting - Reading a Credit Card - Kitchen Science

« on: 05/11/2010 12:15:32 »

We were sent some lovely closeup photos form Randy Hirsch
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27

Physics, Astronomy & Cosmology / Neodymium magnet vs high steel carbon in homopolar motor. How?

« on: 15/10/2010 16:28:29 »

They will vary depending on the exact model of magnet you are using even within a type of magnet. In general the rarew earth magnets are a lot stronger, hugely stronger than a conventional carbon steel magnet.

A good way of getting a first approximation of their relative field strengths would be to try pulling them both off the same piece of steel, and see how the forces compare (if they are the same shape and size).

Conductivity you are probably in a better position to measure yourself.

28

Physics, Astronomy & Cosmology / Is spacial expansion really accelerating?

« on: 15/10/2010 13:25:11 »

Please correct the error in my thinking. If there is more redshift as we look through space, and we are looking farther into the past at the same time, doesn't that mean that the past space was moving more quickly than the current?

It means that the further away you look the faster the object was moving relative to you now, when the light was emmitted.

This is entirely consistent with any (sane) expanding universe model, you don't need the universe to be accelerating for it to work. If you are in a normal explosion sitting on a fragment, the further away you look the faster the fragments are moving relative to you, wherever you are sitting, even if all the fragments are going at a constant speed..

29

Physics, Astronomy & Cosmology / What happens to aluminium in a magnetic field?

« on: 08/10/2010 10:31:09 »

Iron would be attracted to the coil - it is a magnet, and incidentally get warm, as you would be reconfiguring its domains all the time which is lossy.

The aluminium is non-magnetic, so the only effects are electromagnetic. The changing magnetic field will induce voltage and therefore current in the aluminium forming an electromagnet which will repel the coil.

The eddy current experiment is very related to this.
http://www.thenakedscientists.com/HTML/content/kitchenscience/garage-science/exp/mysterious-forces-eddy-currents/

In fact you could get the aluminium to levitate as the changing field keeps putting energy into it to overcome resistive losses, and it would levitate like a superconductor.

30

Just Chat! / Server down 7am?

« on: 30/09/2010 23:32:54 »

Our hosting company is taking the server down for less than 30mins tomorrow at 7am (UK time) to try and work out if some errors I have noticed are connected to do with a dodgy hard disc or RAID controller. With any luck this might be related to the random outages you may have noticed recently and they can fix it.
  Sorry for any inconvenience
     Dave

31

Science Experiments / Is condensation in the cloud chamber always due to radiation?

« on: 30/09/2010 18:32:39 »

There are lots of other things which can trigger condensation, but they are almost all local, or random - at least they won't cause a line of condensation. I don't know anything other than charged particles which
will cause these lines of condensation you see in the experiment.

My guess has always been that it is some form of optical illusion, either that or one end of the track is slightly more supersaturated than the other, so that although the whole track was essentially formed at once, the cloud grows faster at one end, and therefore you see it first

32

General Science / Why Aren't These Big Bubbles Round ?

« on: 28/09/2010 13:36:52 »

Surface tension attempts to minimise the surface area of the bubble, the shape with the minimum surface area is a sphere.

Surface tension has a constant strength no matter what size the bubble is, so the larger the bubble is the less important it is compared to effects like turbulence, so the bigger the bubble the less spherical it will be especially if there is a bit of wind.

33

Geology, Palaeontology & Archaeology / Why Is There Gold Just Beneath The Surface Of The Planet ?

« on: 28/09/2010 13:30:11 »

I think a fair amount of it did sink, which is part of the reason that it is so rare in the crust. It is 60 times more abundant in meteorites than in the crust,
http://www.webelements.com/gold/geology.html
so a lot of it has sunk, just some of it has been brought up in mantle convection or has been associated with lumps of rock which have been at the surface

34

Physics, Astronomy & Cosmology / Why does medium wave radio cut out when going through a tunnel?

« on: 28/09/2010 10:19:45 »

In a grill the important scale is the length of the wires, which make it up. These are much longer than the microwave wavelength. In the same way that a long wavelength e/m wave doesn't see a small solid object, it also doesn't see a small hole, so the grill is apparently solid to the microwaves.

To the very short wavelength light, the grill looks like a grill, the diffraction effect is of order a micron, so the grill looks like a grill, light goes through the holes but not the wires.

35

Geology, Palaeontology & Archaeology / Does continental drift keep us on dry land?

« on: 20/09/2010 17:44:17 »

How does on shove relatively light oceanic crust deep into much more dense mantle?

From memory of a course 10 years ago, and a mate of mine who was doing a PhD in the general area:

The crust is basaltic and is the lightest fraction of the hot mantle upwelling at the mid ocean ridges. What is left is therefore denser than most of the mantle (at the same temperature), and makes up the bottom half of the lithosphere and moves with the crust.

At the mid ocean ridges the lithosphere is hot, so buoyant, but by the time it gets to a subduction zone it has cooled over 10s of millions of years, and now the crust+lithosphere is denser than the mantle so should sink into it even though the basaltic crust should float.

I guess some of the basaltic stuff will melt off the slab as it goes down increasing its density.

36

General Science / How DARE you put science in your Science Fiction Novels?

« on: 20/09/2010 13:59:22 »

I don't mind writers changing science if it is for a good reason - if nothing else it is interesting to think what would happen if you could go faster than the speed of light, if genetics worked differently or whatever. But they should be consistent and not do it for trivial plot reasons, that is just lazy

37

Science Experiments / DIY Rainbow - how a rainbow forms - Kitchen Science

« on: 02/09/2010 13:31:06 »

That sounds about right, your shadow should always be at the centre of any rainbow, just normally there is not enough rain between you and the rainbow below you so you can't see it.

38

Science Experiments / What's your kitchen science?

« on: 26/08/2010 21:16:46 »

Some I remember from childhood some i find in books, some from friends, some I invent, but most I find on the net. Though I often  tweak them a bit.

39

Physics, Astronomy & Cosmology / Where did the big bang happen?

« on: 04/08/2010 12:40:50 »

The normal answer given by cosmologists is that everything is getting further apart not by moving through space, but the space is expanding. A 2d model of this can be made if you draw some galaxies on the surface of a balloon and blow it up, everything gets further apart without moving across the balloon
 So the big bang happened everywhere.

40

Physics, Astronomy & Cosmology / How are photons affected by gravity?

« on: 03/08/2010 16:15:47 »

The general relativity answer to this is that space is bent, which means that although the light is going in a straight line locally, when you look at the path from further away it looks bent.

Eg in curved 2d space (on a sphere)  you can draw a triangle with three 90° corners with straight lines between them.