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# Edexcel Unit 5: Physics from Creation to Collapse 6PH05 (18th June 2015) watch

1. Thread for everyone doing this exam
Unit 4 thread is here: http://www.thestudentroom.co.uk/show....php?t=3114371
2. Subscribed- I thought your name sounded familiar!
3. (Original post by BP_Tranquility)
Subscribed- I thought your name sounded familiar!
How far are you into unit 5? We just have astrophysics left ahh!
4. (Original post by cerlohee)
How far are you into unit 5? We just have astrophysics left ahh!
We've been doing stuff in a weird order :P - we have particle physics and gas laws left (done everything else I think) but they aim for us to be finished by Easter

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5. (Original post by BP_Tranquility)
Subscribed- I thought your name sounded familiar!

(Original post by cerlohee)
How far are you into unit 5? We just have astrophysics left ahh!
Do you guys recommend any revision guides or any particular resources?
6. Hiya everyone! Ugh dreading Physics exams this year! Would really like an A but got a B last year retaking Unit 1 too so *fingers crossed*.

(Original post by sj97)
Do you guys recommend any revision guides or any particular resources?
I highly recommend this revision guide!!! http://www.amazon.co.uk/Edexcel-Phys...4PN1WD00DA4X95
7. Could anyone explain the spark counter and ionizing chamber for use in detecting alpha radiation to me? The explanation in my book is quite confusing and I can't find a decent one online.
8. Hey can anyone help explain these two questions for me? Edexcel physics unit 5 June 2012;

Question 5: Two distant stars are observed through a telescope. Star A is observed to be half as bright as star B. Star A is calculated to be twice as far away as star B.
Which of the following is correct?
A Star A has half the luminosity of star B.
B Star A has the same luminosity as star B.
C Star A has twice the luminosity of star B.
D Star A has 8 times the luminosity of star B.

Can anyone please explain how it is C using equations?

And the second question is-->

19)c)iv) Calculate the kinetic energy, in MeV, of the neutron released by the fusion of deuterium and tritium nuclei. Assume that the net momentum of the nuclei before fusion is zero.

You would have had to calculate that the energy released in the reaction (mass deficit) is 17.5 MeV but other than that, I have no idea! the mark scheme makes a weird assumption that Ek(alpha) + Ek(neutron) = 17.5?

Thank you, I appreciate this if you can explain these issues.
9. (Original post by RaymondG8)
Hey can anyone help explain these two questions for me? Edexcel physics unit 5 June 2012;

Question 5: Two distant stars are observed through a telescope. Star A is observed to be half as bright as star B. Star A is calculated to be twice as far away as star B.
Which of the following is correct?
A Star A has half the luminosity of star B.
B Star A has the same luminosity as star B.
C Star A has twice the luminosity of star B.
D Star A has 8 times the luminosity of star B.

Can anyone please explain how it is C using equations?

And the second question is-->

19)c)iv) Calculate the kinetic energy, in MeV, of the neutron released by the fusion of deuterium and tritium nuclei. Assume that the net momentum of the nuclei before fusion is zero.

You would have had to calculate that the energy released in the reaction (mass deficit) is 17.5 MeV but other than that, I have no idea! the mark scheme makes a weird assumption that Ek(alpha) + Ek(neutron) = 17.5?

Thank you, I appreciate this if you can explain these issues.
If they're as bright as each other then they have the same flux:
F=L/4pid² for star B
So F x 4pid²=Lb where Lb=the luminosity of B

Star A has half the flux of star B
Star A is twice the distance away than star B.

½F x 4pi(2d)²=La where La is the luminosity of A

Do La/Lb and you should get 2.

For your second question, I'm not too sure but:
If the momentum before is 0 and the momentum after is 0, then since some mass is lost (converted to energy), then the final velocity of the particles is higher than expected (to balance the m1u1+m2u2=m1v1+m2v2 equation ) so this increase in velocity corresponds to the energy released (so the sum of kinetic energies of the particles= energy released), or it might say elastic collision somewhere? Not too sure though sorry

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10. I'm dreading this exam lol, unit 4 not so much. I'm doing AS and A2 in one year because of my poor AS performance last yr.
11. (Original post by BP_Tranquility)
If they're as bright as each other then they have the same flux:
F=L/4pid² for star B
So F x 4pid²=Lb where Lb=the luminosity of B

Star A has half the flux of star B
Star A is twice the distance away than star B.

½F x 4pi(2d)²=La where La is the luminosity of A

Do La/Lb and you should get 2.

For your second question, I'm not too sure but:
If the momentum before is 0 and the momentum after is 0, then since some mass is lost (converted to energy), then the final velocity of the particles is higher than expected (to balance the m1u1+m2u2=m1v1+m2v2 equation ) so this increase in velocity corresponds to the energy released (so the sum of kinetic energies of the particles= energy released), or it might say elastic collision somewhere? Not too sure though sorry

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Thank you so much! Your explanation for question 5 and the second one was really helpful!
12. Anyone know of any good notes for astronomy and cosmology etc

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13. I'm going to fail No matter how much I revise I just cant do the questions when I try a past paper. I got A on coursework and second exam last year but a D in mechanics and materials. I'm retaking the D so hopefully my AS grade will get up to a high B/low A so what could I get in A2s to get a C overall? thankyou in advance
14. (Original post by Rosie:):))
I'm going to fail No matter how much I revise I just cant do the questions when I try a past paper. I got A on coursework and second exam last year but a D in mechanics and materials. I'm retaking the D so hopefully my AS grade will get up to a high B/low A so what could I get in A2s to get a C overall? thankyou in advance
It depends on the exact marks you got last year!
If you get another A in your coursework and a mid B in your resit, you'd only have to average a maximum of a low D in your exams to get a C overall! That increases to a high D for a B but you'll have to average low As to end up with an A
Bear in mind that those are maximum averages!!

Have you been looking at mark schemes and examiners reports?
15. Hey guys, I'm slightly confused on what we need to know for this specification point, does anyone have a good link that i could use to revise this?
135.be aware of the controversy over theage and ultimate fate of the universeassociated with the value of the HubbleConstant and the possible existence ofdark matter
16. (Original post by Hectors)
Hey guys, I'm slightly confused on what we need to know for this specification point, does anyone have a good link that i could use to revise this?
135.be aware of the controversy over theage and ultimate fate of the universeassociated with the value of the HubbleConstant and the possible existence ofdark matter
The actual syllabus point links the age to hubble's constant and the fate of the universe to the existence of dark matter and that both have uncertainties and therefore controversies to their truth.

Firstly the fact that there is uncertainty with the age of the universe. Hubble's constant can determine the age of the universe as
v=H0*d
v/d = H0
t= 1/H0
Yet as Hubble's constant is inaccurate - as distances are difficult to calculate - there is a controversy as to what the value of t is (the age of the universe).

In terms of the ultimate fate of the universe, there are 3 scenarios, which rely on the value of density of the universe.

If density is less than the critical value - expansion will continue forever as an open universe.

If density is more than the critical value - expansion will cease and the universe will contract again.

If density is at critical value then the expansion rate will halt but the universe will not contract again.

Dark matter exists who's behavior is unknown yet must be possible. As the average density of the universe as a whole includes this, it is difficult to calculate therefore the fate of the universe is uncertain so there are controversies.

Hope this is all correct and is helpful.
17. (Original post by iceangel8)
The actual syllabus point links the age to hubble's constant and the fate of the universe to the existence of dark matter and that both have uncertainties and therefore controversies to their truth.Firstly the fact that there is uncertainty with the age of the universe. Hubble's constant can determine the age of the universe as v=H0*d v/d = H0t= 1/H0Yet as Hubble's constant is inaccurate - as distances are difficult to calculate - there is a controversy as to what the value of t is (the age of the universe).In terms of the ultimate fate of the universe, there are 3 scenarios, which rely on the value of density of the universe.If density is less than the critical value - expansion will continue forever as an open universe.If density is more than the critical value - expansion will cease and the universe will contract again.If density is at critical value then the expansion rate will halt but the universe will not contract again.Dark matter exists who's behavior is unknown yet must be possible. As the average density of the universe as a whole includes this, it is difficult to calculate therefore the fate of the universe is uncertain so there are controversies. Hope this is all correct and is helpful.
Very nice, thanks. I assume that the universe mass-energy density must remain constant?
18. (Original post by Hectors)
Very nice, thanks. I assume that the universe mass-energy density must remain constant?
Not quite sure what you mean by that??

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19. (Original post by iceangel8)
Not quite sure what you mean by that??

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The density of the universe must be constant cause If the density changed, none of the explanation that you gave me would make sense, since then an open universe could become a closed universe as the density becomes bigger than the critical value, so the density of the universe must be constant?
20. (Original post by Hectors)
The density of the universe must be constant cause If the density changed, none of the explanation that you gave me would make sense, since then an open universe could become a closed universe as the density becomes bigger than the critical value, so the density of the universe must be constant?
Ah okay that makes sense so the future is uncertain depending on the current density of the universe which is constant but unknown because of dark matter. Cool.

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