UnvesedSplash
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#781
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#781
Can anyone explain the difference between using U-235 and U-238 in nuclear fission please and thanks... Also the benefits of using one over the other

Thanks
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Jack93o
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#782
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#782
(Original post by kingm)
Amen to that.

Still got the exam-style questions for chapter 12 to go, then i'm gonna re-do the exam-style q's for astro and then i'm on to the papers - all 3 of them . THEN, i hope to understand what the hell people are talking about on here.
theres exam style question for astrophysics? where?
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UnvesedSplash
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#783
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#783
(Original post by bugsuper)
Don't forget the Specimen papers which are also available
I take it you're talking jsut about the specimen paper on the aqa website or are there others??
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Pinkhead
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#784
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#784
(Original post by kingm)
Amen to that.

Still got the exam-style questions for chapter 12 to go, then i'm gonna re-do the exam-style q's for astro and then i'm on to the papers - all 3 of them . THEN, i hope to understand what the hell people are talking about on here.
Here are more questions for you from the old spec for astro (there are like 75 pages of questions).

I have other topics too. If people ask me for them, I will upload them.
Attached files
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Pinkhead
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#785
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#785
(Original post by UnvesedSplash)
Can anyone explain the difference between using U-235 and U-238 in nuclear fission please and thanks... Also the benefits of using one over the other

Thanks
You can't use U-238 in the fission we learn about in A-level Physics. This is because the nuclear reactors in question use thermal neutrons (slower ones in essence) while U-238 requires fast neutrons for fission to occur.
Also, if you do induce fission in a U-238 isotope, the neutrons which are given off as a result have less energy than the initial neutron so the chain cannot sustain itself for very long.

U-235 requires less energy to get started (not sure about this but since the thermal neutrons are slower I would assume it is true) and produces more energy since an actual chain reaction occurs.
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bugsuper
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#786
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#786
(Original post by UnvesedSplash)
I take it you're talking jsut about the specimen paper on the aqa website or are there others??
Just that one.
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MSI_10
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#787
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#787
(Original post by Pinkhead)
Here are more questions for you from the old spec for astro (there are like 75 pages of questions).

I have other topics too. If people ask me for them, I will upload them.
Wow many thanks for the attachments. Are they any similar ones for nuclear and thermal?
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Pinkhead
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#788
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#788
(Original post by MSI_10)
Wow many thanks for the attachments. Are they any similar ones for nuclear and thermal?
I think I posted them a few pages back but here you go:
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Jack93o
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#789
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#789
Does anyone understand how hubble's law is used to estimate the age of the universe?

I know that v = Hd, where v = speed, H = hubble's constant, d = distance

and that v = d/T (speed = distance / time)

and you're suppose to combine those two equation by subbing in Hd in place of 'v' in v = d/T

but the speed from the hubble's law equation only gives you the speed of a galaxy at a certain distance (i.e. speed would increase with larger distance), whereas the v = d/T equation assumes that speed v is constant throughout the galaxy's travel, so how the does it make sense to combine the two equation?
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bugsuper
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#790
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#790
(Original post by Pinkhead)
Here are more questions for you from the old spec for astro (there are like 75 pages of questions).

I have other topics too. If people ask me for them, I will upload them.
Thanks!

It was depressing looking at this and seeing about two questions I hadn't already done... I'm beginning to think I've done enough
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Pinkhead
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#791
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#791
(Original post by bugsuper)
Thanks!

It was depressing looking at this and seeing about two questions I hadn't already done... I'm beginning to think I've done enough
Looks like it's an easy A* for you then
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bugsuper
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#792
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#792
(Original post by Jack93o)
Does anyone understand how hubble's law is used to estimate the age of the universe?

I know that v = Hd, where v = speed, H = hubble's constant, d = distance

and that v = d/T (speed = distance / time)

and you're suppose to combine those two equation by subbing in Hd in place of 'v' in v = d/T

but the speed from the hubble's law equation only gives you the speed of a galaxy at a certain distance (i.e. speed would increase with larger distance), whereas the v = d/T equation assumes that speed v is constant throughout the galaxy's travel, so how the does it make sense to combine the two equation?
I don't know whether this is correct, but I always took it to mean that the "speed" - in terms of the rate of the Universe's expansion - was constant (if you neglect to take into account Dark Energy.) It's not that the galaxies are actually receding from us, but that they appear to be receding because space is expanding. The more space between us and the galaxies, the faster they appear to be receding, because of the expansion of space. Hubble's constant is kind of a conversion factor, a constant that measures the rate of expansion of space: plug in the distance to the object, and it converts that into the recessive velocities that we think we see, and that we can measure by redshift. In this sense, that does kinda make sense: the "amount the universe has expanded" / "the rate of expansion" = the time the universe has been expanding

I'm basing this mainly on the fact that some galaxies have recessive velocities exceeding that of light (but the galaxy itself, of course, isn't moving faster than light)

That might sound like garbled nonsense so someone please leap in and correct me if it is.
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bugsuper
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#793
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#793
(Original post by Pinkhead)
Looks like it's an easy A* for you then
No such thing! There's always something new they can ask. Hubris is a great way to be taken down about sixty pegs.
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bugsuper
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#794
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#794
http://www.thescienceforum.com/astro...-galaxies.html

interesting discussion about the "true" nature of redshift from another forum which seems to be about 50% physicists and 50% crackpots
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Pinkhead
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#795
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#795
(Original post by bugsuper)
http://www.thescienceforum.com/astro...-galaxies.html

interesting discussion about the "true" nature of redshift from another forum which seems to be about 50% physicists and 50% crackpots
At least it is obvious which ones are the crackpots. I prefer physicsforums although that too has its share of nutters (significantly less but they exist nonetheless).
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bugsuper
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#796
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#796
(Original post by Pinkhead)
At least it is obvious which ones are the crackpots.
In some ways the story of Einstein is kind of a shame for Physics... yes, it's inspirational, but I get the feeling that a lot of people read about how he came up with relativity through a series of thought experiments during the lunch break at a patent office or whatever and think they can do the same thing. Newsflash: you are not actually Einstein. And that's not what actually happened to him, either. Real scientific achievements almost always take place after years and years of dedication and hard work. The "eureka moments" that we hear so much about probably wouldn't happen without them
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Pinkhead
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#797
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#797
(Original post by bugsuper)
In some ways the story of Einstein is kind of a shame for Physics... yes, it's inspirational, but I get the feeling that a lot of people read about how he came up with relativity through a series of thought experiments during the lunch break at a patent office or whatever and think they can do the same thing. Newsflash: you are not actually Einstein. And that's not what actually happened to him, either. Real scientific achievements almost always take place after years and years of dedication and hard work. The "eureka moments" that we hear so much about probably wouldn't happen without them
I agree. People always either underestimate the amount of work required to come up with a solid hypothesis, let alone a theory as brilliant as Einstein's, or they severely overestimate their own ability.
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BearPear
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#798
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#798
For anyone that does applied Physics option

Do you know why there is a resistive force when you are trying to rotate the axis of a spinning wheel? what would happen when the axis is changed, does it stays in the changed axis as long as there is no external forces?
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Lucy-1995
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#799
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#799
(Original post by amish123)
Right first work out the total mass change if one uranium atom disintegrates,

so 390.406 + 1.675 - (149.357 + 239.056 + 2x1.675)

and you get a mass difference of 3.18 x 10^-27kg.

Now, for a uranium atom, there are 235g per mole. So if you want to find the mass change for 0.5kg, you do 500/235 x (6.0 x 10^23) and this will give you the number of uranium atoms in 0.5kg. You'll get an answer of 1.276 x 10^24 atoms.

Then you just multiply the mass difference if one uranium atom disintegrates by the number of uranium atoms you have, so (3.18 x 10^-27) x (1.276 x 10^24) and you'll get a final answer of 4.06 x 10^-4kg, or 406g.

Hope that helps!
What paper was this question from, or was this the whole question? I can't figure out where you got any of the numbers from on the first line of your equation?!

'390.406 + 1.675 - (149.357 + 239.056 + 2x1.675)' why is that so??

Really confused! I hate all the mass defect/binding energy questions with a passion
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Pinkhead
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#800
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#800
(Original post by Lucy-1995)
What paper was this question from, or was this the whole question? I can't figure out where you got any of the numbers from on the first line of your equation?!

'390.406 + 1.675 - (149.357 + 239.056 + 2x1.675)' why is that so??

Really confused! I hate all the mass defect/binding energy questions with a passion
I think they give you that information in the question. The only other way I can see is to work out the mass for each isotope in the equation by looking at the formula booklet but this is obviously far too tedious for a 3-4 mark question.
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