h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

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I think you'll find it was Alfster. And yes, I agree with the cooling air thingy. Although the egg is not sucked in is it?...it's atmospheric pressure pushing it in. That's what I've seen it used to demonstrate.



Alex.

Did anyone mention making pH indicator from red cabbage? You get at least four different colours testing things like cleaning agents. And you realise why red cabbage should be cooked with a little vinegar...

Azara

Thanks Azara.

>>Although the egg is not sucked in is it?...it's atmospheric pressure pushing it in.

If you think about it, that's what all sucking is. Sucking means reducing the pressure in one place so that air pressure in another place can blow something in.

That's right, just like a straw.

It's just that when we xplain it to kids..they think that there is some kind of invisible force pulling the drink up the straw, where in fact it is being pushed up.



Alex.

The egg in a bottle mechanism is in fact differential pressure i.e. the pressure of the gas in the bottle is less than the pressure outside the bottle.

The same would happen if you had a pressure of 2 bara inside the bottle and 3bara outside the bottle. Both pressures are above atomspheric pressure but the differential pressure is -1bar with respect to the inside of the bottle hence higher pressure outside and thus the egg with be pushed into the bottle.

The straw analogy is interesting actually as you could technicallysay the liquid is being pushed into you mouth rather than sucked. It is semantics wehen you get down to it. It depends whether you are using your datum pressure as the high or low pressure area.

As to whether the egg is being sucked into the bottle due to the air contracting, not sure on that one. The way to test that is put the egg on the top of the bottle as quickly as you can to stop the hot air escaping the bottle. I would have to check to see whether co2 and water has a lower volume compared to oxygen to check that point (i.e. the vacuum being caused by the CO2 and water being produced having a smaller over volume than the O2 being used up in the combustion reaction)

No, I'm not convinced by the contracting air argument.

I can't really be nethered to do the caculations, but my gut instinct is that the thermal expansion coefficient of air isn't nearly big enough.

B

Hmmm, maybe I was thinking of the collapsing can experiment but in that one the air is forced out of the can by boiling a bit of water and the steam pushing it out.

Alex.

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Well it may be semantics nut if you had answered in an exam that water was being sucked up the straw, you would have got no marks.

Pressure is no longer on the AQA GCSE Physics syllabus though. How weird is that? We ahve to teach about Earthquakes and Plate Tectonics!! (Geography! Ugh!)

Alex.

What!!!!!!!! Pressure is a fundamental part of engineering and science. Have you stopped teaching temperature, volume and mass/weight?

Before you can understand earthquakes and plate tectonics you need a good understanding of pressure. I suppose you do not need to go into really detailed technical stuff to explain earthquakes - is this more of the trend towards practical examples and real world learning.

It won't be long before physics exams include questions like: Describe how you think Newton felt when he discovered gravity. All that touchy feely rubbish that is creeping into everything.

<< is this more of the trend towards practical examples and real world learning.>>

'fraid so. Pressure is in KS3 though (first to third year) but just not at GCSE. It's returns at A level.

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You're not far wrong, it's creeping in.

Surely Earthquakes and Plate Tectonics is a Chemistry topic on AQA...

I've got a lot of good pracs for that one - I can't stand teaching it either.

B

Well, it may well be on Chemistry as well but I've got to teach seismic waves (fair enough) btu Wegner's plate tectonic theory about similar fossils being found in different continents etc etc. Physics??? I think not.

Any ideas for pracs would be good. I post a message on your PS about it. This thread is getting a bit off topic (due to me I think)..

So, back on track, Home experiments.

What every day objects around the home could be used for scince experiments?

e.g.
fridge magnets,
a torch and a shoe box the make a ray box,
.....

Alex.

This is a site dedicated to home science experiments.

It will not kill the thread as not all the stuff to be made on the site is exactly easily found in the house!

http://www.scitoys.com/

Pinholoe cajera is a good one. I make mine out of a shoe box. It is best to cut a small rectangle out from one end and cover it with kitchen foil into which one makes the pin hole. The opposite end has a large rectangle cut out of it through which one observes; whereas partway down the shoebox one places a screen of tracing paper on which the image is formed.



Other optical ewxperiments include 'Benham's disc' which contains a series of concentric circles and the only two 'colours' in it arte black and white. Throughout the disc (i.e. from centre to outside) there are black arcs of varying lengths. When the disc is spun one obtains the illusion of colours as the frequency (of black arcs)stimulates different optical receptors in the eye.

This helps to explain the pinciple of black and white TV

Another optical experiment (called 'Mental Motion' consists off a disc with a vlack spral onb it going from centre to perimeter.
One spins this and concentrates on it for 30 seconds. One then transfers ones gaze to e.g. a photograph of some clouds, and the clouds appear to be moving i.e. either expnding or contracting. In a variation of this experiment which I call 'The Creeping Flesh' one transfers ones gaze to the back of ones hand.

This expt works because the eyes and brain have receptors for both inward and outward motion. Normally these complementary senses give 'balanced' neural sensations when you look at a static image or scene. Prior exposure to the rotating spiral tires out one set of receptors, creating an imbalance; thus giving rise to the visual effect.

"vlack spral onb" > black spiral on it.

I used to work as a Demonstrator/Explainer at a science centre attached to a military museum. I used to use these demos to explain that helicopter pilots need to be aware of these illusions, created by rotating blades overhead.

Also, one used to be able to make Chemical Gardens (see my GE on the FP this weekend (A4044845); but unfortunately it's becoming increasingly difficult (for the 'home scientist') to procure the chemicals.

Chromatography is also a good one.

1. Of felt tip pens. One uses blotting paper as the stationary phase. I used to get my children to make 'fishy bookmarks. Trace out the outline of a fish. Use a colour fast pen to mark its eye and perhaps lips. Then use for FTP's to put some spots/ streaks of bright colours at one end of the fish (say head end). The stand it head first in a cup of water and allow the chtomatogram to develop. Allow it to dry and then laminate it. Hey presto, you have highly original bookmarks of Angel Fish!

2. Chromatography of leaves. It is interesting to look at the reds, oranges and tellws of autumn leaves. However, one really needs a solvent like propanonme, which may be difficult for the home scientist to procure. However, in the chromatography of grass, yellow xanthophyll travels the furthest, followed by green chlorophyll and finally the orange carotene.

Would I be correct in guessing that Post 35 was written in an advanced state of intoxication?

If not, could someone tell me what a "Pinholoe cajera" is?

B

No, I wasn't intoxicated. Just too early in the morning and too careless.