# PHYA5 ~ 20th June 2013 ~ A2 Physics Watch

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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

Thanks

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#782

(Original post by

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.

**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.

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#783

(Original post by

Don't forget the Specimen papers which are also available

**bugsuper**)Don't forget the Specimen papers which are also available

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#784

**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.

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

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#785

(Original post by

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

**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

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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#786

(Original post by

I take it you're talking jsut about the specimen paper on the aqa website or are there others??

**UnvesedSplash**)I take it you're talking jsut about the specimen paper on the aqa website or are there others??

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#787

(Original post by

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.

**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.

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#788

(Original post by

Wow many thanks for the attachments. Are they any similar ones for nuclear and thermal?

**MSI_10**)Wow many thanks for the attachments. Are they any similar ones for nuclear and thermal?

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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?

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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#790

**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.

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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#791

(Original post by

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

**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

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#792

(Original post by

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?

**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'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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#793

(Original post by

Looks like it's an easy A* for you then

**Pinkhead**)Looks like it's an easy A* for you then

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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

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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#795

(Original post by

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

**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**
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#796

(Original post by

At least it is obvious which ones are the crackpots.

**Pinkhead**)At least it is obvious which ones are the crackpots.

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#797

(Original post by

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

**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

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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?

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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#799

(Original post by

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!

**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!

'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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#800

(Original post by

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

**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

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