Is there a physicist in the house?

[carvell]

Third and final A2 Physics paper tomorrow.

Currently struggling with one of the articles we have been given to study before the exam. It's all about neutrinos. I've been given some questions to try, educated guesses by teachers on what could come up on the exam paper.

"If the Sun is 150 Gm away and produces 2 * 10^38 (2E38) neutrinos per second, what is the neutrino flux density at the earth's surface?"

I am confident and familiar with magnetic flux density, but have no idea on this neutrino flux malarky. I doubt anyone will be able to help, as as far as I know there is noone on here either doing A2 physics or a physics degree, but it's worth a try!

I'm doing "Physics B (Advancing Physics)" with OCR incase anyone else is doing the same!

Thanks
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[MarJay]

I imagine that Neutrino flux density can be calculated the same way you would calculate electrical flux density.

I'll try and look it up in my university textbooks.
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[carvell]

Hmm yes.

Magnetic flux density is flux / area.

As flux is a rate of flow, I'd imagine this to be 2E38.

2E38 / area then, I wonder if I can take the area to be 150 Gm? I doubt it, but then the question doesn't give any hints or help for calculating the surface area of the earth, and even then the sun doesn't shine on the whole Earth all the time, so you'd have to calculate the surface area of the earth in sunlight and arrrrgh I've gone cross eyed.

2E38 / 150G is my best guess so far then, which gives 1.3E27.

Annoying that I have no idea what the correct answer is. :/
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[Danny]

I'm so glad I never did Physics A-level.

[DukeRed]

I did physics A2 last year and never even touched upon neutrino stuff.
Think that was EDEXCEL though. Will have a look in txt books etc to see if there's anything in them.

Jan
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[Scratchy]

I could give you the answers, however I think you would benefit from working it out yourself
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[carvell]

Thanks @ DukeRed.

The annoying thing is that there is nothing in my text book and it's not in my course either, but the section of the exam paper that is based on an article can be on anything, and they decided to choose neutrinos.
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[DukeRed]

If you have never studied it then as Marjay said it will be exactlly the same as something you have studied i.e. the equations. That happened in my paper last year, but it turned out it was the same equations as working out the gravity of stuff.

Jan
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[Sparks!]

I thought you said psychiatrist to start with.. then reading the thread, I'd need a psychiatrist after that maths drive me mad...

I've never quite caught onto the point of maths like this, when will you ever need to work out the magnetic flux density thing of the sun?

Answers on a post card!
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[tgabber]

I'd answer that it would be necessary to make too many assumptions about under researched areas of neutrino emissions to provide any answer which was little more than a rough guess.




But then I know nothing about neutrino emissions...
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[Robby]

1.21 jiggerwatts.


So is this for the one where you get a pre release booklet, then some really difficult questions on it in the exams?

I got about 30% on this one last year, also with OCR. The booklet was a good read though.

[carvell]

That's the one robby!

[jay12329]

If the sun is 150gm away from the earth. The total area that the neutrinos are spread over is the area of the surface of a sphere of radius 150gm.
I cant remember how to work out the area of hte surface of a shpear so ill call this S,
So the fulx density is 150 / S * The width of the earth.
Its been a long time since i did physics, so dont blame me if this doesnt make any sence.
J

[carvell]

Yup that made sense - thanks!
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[jay12329]

BTW. I got a D for A-level physics.
But i was ill for the previous 3 years.
J

[hobbitmum]

I expect you have got this sorted by now, but just in case you haven't.... although its a loooong time since i did my physics degree......

neutrino flux density is actually kind of self explanatory. you dont need to know anything about neutrinos. all you need to do is a bit of arithmetic to work out how many of the things will hit a given area in one second, and thats your answer. in other words, divide the number of neutrinos by the area.

you know the radius of the sphere you are considering, you know the formula for the area of a sphere so you have its area. you know how many of the things are being emitted in one second because it tells you that. oh yes, you do have to assume that the things are being emitted evenly in all directions, there's a term for that but i have forgotten what it is.

so all you need to do is to make sure you have got the units right, and there you are.

good luck for tomorrow btw!

[Annabella]

Spot on Hobbitmum!

I'm sure the examination board will want to see exactly that kind of logic, rather than application of taught knowledge, I don't know whether it's still the same, but when I did A' Level Physics we were expected to be able to prove formulae we had never seen from first principle. I would guess if they did ask something that isn't on the syllabus it would be for a demonstration of basic physical and mathematical principles


BEST OF LUCK CARVELL
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[carvell]

Thanks guys/girls!

4 hours to go!
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[carvell]

While we're here, I don't suppose if anyone knows of any problems that would be met in accurately measuring neutrino flux? (apart from background radiation)
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[Annabella]

What hobbitmum said about the distribution. You are assuming that the emmision is consistant - it may not be.

I'll keep thinking.
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[carvell]

Ah yes, thanks!
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[Annabella]

Interesting information:

It seems that the level of flux actually detected is almost half the level that was predicted!

It seems that this may have something to do with the osscilations of neutrinos (their three different states), and possibly the movement of the convective zone of the sun may have some influence over this (though there is still research being done as to whether the magnetic properties are sufficient to cause the level of discrepancy in the detected results).
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[carvell]

[jay12329]

Good luck!
J

[carvell]

Just done the exam (last exam yaaaaaaay!), and they asked this question, only it was rather more spoon fed to me.

They had a very pretty little diagram with the sun in the centre of a sphere, and the radius marked, with the earth on the edge. They also told you surface area of the sphere, then said, "show that the flux is xx".

Couldn't go wrong with that really!

Rest of the paper was ok, lots of "show that" questions which I love, as you know you have got it right!

Thanks for the help with that question!
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