# PHYA5 ~ 20th June 2013 ~ A2 PhysicsWatch

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6 years ago
#281

I don't understand the mark scheme. Isn't L_0 length measured in frame of reference of the spacecraft? (which is what I'm trying to find?)
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6 years ago
#282
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6 years ago
#283
(Original post by jethacan)
Can anyone doing astrophysics help me with this question?
I'm stuck on (b)(ii) but I can do the rest.

Attachment 220636

I tried looking at the mark scheme and I couldn't understand it at all :/
Have you looked at Wien's displacement law? Put simply, it states that as the temperature of a star increases, the peak wavelength of the emitted e.m radiation decreases, and the intensity of the radiation increases. If you were to plot a graph of intensity against wavelength for black body radiation, the area under the graph will give you the total energy emitted per unit time (power).So from this question, we can see that Sirius B is much hotter than Sirius A, so Sirius B must be have a much shorter peak wavelength (below the visible spectrum) and it must have a higher intensity. This means that Sirius B emits most of it's energy as radiation outside the visible spectrum. As in part 2bii you are comparing the brightness of both stars, you are looking at the visible region of the spectrum, however this does take into account the fact that Sirius B is much hotter and so doesn't emit much energy in the visible spectrum.

This picture should make my explanation make more sense:

Hope that helps!
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6 years ago
#284
Hi there I was wondering were u got that question from

Posted from TSR Mobile
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6 years ago
#285
(Original post by lantern)

I don't understand the mark scheme. Isn't L_0 length measured in frame of reference of the spacecraft? (which is what I'm trying to find?)
I still need to complete the damn syllabus & then do past papers...

how'd you do the first question ?

Did you use the same formula but the lorentz factor "inversed" & with t & t_o replacing the Ls'?

t_o being 16c ?

Also I think for part ii) They would like to know what distance is "observed" (lets say for now) with reference to the spacecraft. You have the distance with reference to the Earth.

But not only that... your using a velocity with reference to the earth also. So you can use hints like that too, to help you.

EDIT: having thought about i) a bit more... do you do v= d/t ... where d=16c & v=0.8c ... so t = 20 years?
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6 years ago
#286
(Original post by posthumus)
EDIT: having thought about i) a bit more... do you do v= d/t ... where d=16c & v=0.8c ... so t = 20 years?
yep

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6 years ago
#287
(Original post by amish123)
Have you looked at Wien's displacement law? Put simply, it states that as the temperature of a star increases, the peak wavelength of the emitted e.m radiation decreases, and the intensity of the radiation increases. If you were to plot a graph of intensity against wavelength for black body radiation, the area under the graph will give you the total energy emitted per unit time (power).So from this question, we can see that Sirius B is much hotter than Sirius A, so Sirius B must be have a much shorter peak wavelength (below the visible spectrum) and it must have a higher intensity. This means that Sirius B emits most of it's energy as radiation outside the visible spectrum. As in part 2bii you are comparing the brightness of both stars, you are looking at the visible region of the spectrum, however this does take into account the fact that Sirius B is much hotter and so doesn't emit much energy in the visible spectrum.

This picture should make my explanation make more sense:

Hope that helps!
Ah yes that helps a lot! See, I thought that because B was hotter, the visible light emitted would be more intense. So does that mean we need to know approximate temperatures for the peak to be in visible light part of the spectrum? From the diagram you attached it shows that the visible light peak would be around 10000K which is Sirius A's temperature, that makes much more sense but I didn't know we needed to know specific temperatures from the Wien's displacement diagram.
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6 years ago
#288
Does anyone understand this 6 marker and could help me out a bit? (It's astrophysics)

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6 years ago
#289
(Original post by jethacan)
Does anyone understand this 6 marker and could help me out a bit? (It's astrophysics)

The absolute magnitude of the supernova was lower than expected indicating its further than it should be if the universe was expanding at a constant rate, so it must be accelerating, I'm struggling to see how I'd 6 marks no matter how I'd structure this answer.
1
6 years ago
#290
(Original post by JRP95)
The absolute magnitude of the supernova was lower than expected indicating its further than it should be if the universe was expanding at a constant rate, so it must be accelerating, I'm struggling to see how I'd 6 marks no matter how I'd structure this answer.
I think you have to say thats its the type 1 A supernova that are observed, white dwarfs i believe. I think they're the ones where you can measure the absolute magnitude.

Posted from TSR Mobile
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6 years ago
#291
(Original post by JayJay95)
I think you have to say thats its the type 1 A supernova that are observed, white dwarfs i believe. I think they're the ones where you can measure the absolute magnitude.

Posted from TSR Mobile
You can measure the abs mag of type 2 as well because they both produce consistent light curves
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6 years ago
#292
Guys how many significant figures should we generally use?
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6 years ago
#293
Whatever it is in the question

Posted from TSR Mobile
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6 years ago
#294
(Original post by lantern)

I don't understand the mark scheme. Isn't L_0 length measured in frame of reference of the spacecraft? (which is what I'm trying to find?)
I can usually get the length contraction questions sorted, but I suffer from a similar confusion to your last statement, i.e knowing when to use what as the 'proper length'.

What I prefer to do is use time dilation because I understand that better - maybe you would understand it better that way too so I'll try and give you that point of view (or frame of reference for a relativistic pun). The speed will be the same in both frames of reference. As v=s/t, and v is constant, if s increases then t must increase by the same factor to keep v constant. This factor is the lorentz factor.

So for this question:

First bit is simple, v=s/t=20years.

Second bit is where the time is contracted for those on Earth. Use time dilation to get the time taken as 12years on the rocket for the 20years on Earth. Then v=s/t again yields s=9.6c which agrees with the actual answer. You would get full credit for this answer, but if you want to comply with the mark schemes you could quickly consider in your head how time would be affected, then ensure that the same effect happens to length to keep speed constant. Hope I helped.
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6 years ago
#295
(Original post by jethacan)
Does anyone understand this 6 marker and could help me out a bit? (It's astrophysics)

(Original post by JRP95)
The absolute magnitude of the supernova was lower than expected indicating its further than it should be if the universe was expanding at a constant rate, so it must be accelerating, I'm struggling to see how I'd 6 marks no matter how I'd structure this answer.

(Original post by JayJay95)
I think you have to say thats its the type 1 A supernova that are observed, white dwarfs i believe. I think they're the ones where you can measure the absolute magnitude.

Posted from TSR Mobile
Type 1a supernovae can be used as standard candles as they reach the same peak value of M (absolute magnitude) of around -19.3. From these measurements and using m-M=5log(d/10), we can measure the distance to these galaxies. However, by using red shift measurements and Hubble's Law (which assumes a constant rate of expansion), these supernovae are dimmer than predicted. This must mean that the expansion of the Universe is accelerating and some form of 'dark energy' is driving this expansion.
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6 years ago
#296
(Original post by ehtisham_1)
Turning points is f u c k I n hard, especially the special relativity and considering our teacher can't explain it makes it worse

Posted from TSR Mobile
Turning points is all synoptic until special relativity, so not too bad for me. But special relativity causes me a few problems. Bearing in mind my teacher has just randomly decided to stop teaching us all whatsoever I'm not in a bad state haha.
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6 years ago
#297
(Original post by ehtisham_1)
Turning points is f u c k I n hard, especially the special relativity and considering our teacher can't explain it makes it worse

Posted from TSR Mobile
I don't mind special relativity as its all just maths really with all the equations on the sheet. I never understood the concept of it until someone explained it like:

You're travelling at 0.6c , where c is the speed of light, someone else is travelling at 0.6c in the opposite direction, so relative to you, the other person is travelling at 1.2c

As nothing can travel that speed of light, other things like time and length have to be changed

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6 years ago
#298
Those of you who are doing turning points . I generally get confused between the two microscopes i think our six marker may be based on one of them as that never seems to come up any tips??
Smith
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6 years ago
#299
(Original post by SortYourLife)
I don't mind special relativity as its all just maths really with all the equations on the sheet. I never understood the concept of it until someone explained it like:

You're travelling at 0.6c , where c is the speed of light, someone else is travelling at 0.6c in the opposite direction, so relative to you, the other person is travelling at 1.2c

As nothing can travel that speed of light, other things like time and length have to be changed

Posted from TSR Mobile
I see what you mean here... logically in that respect the other person would be travelling 1.2c relative to you.

What would we then do to find the time relative to the "other person" (to go from A to B)... you could have the time you saw them go from A to B... but if you use 1.2c in the Lorentz factor... you can't square root that?

So they couldn't possibly ask us to do a calculation in situations such as this one?

Maybe they could ask it as a written question instead... where we'd have to quote E=mc^2 & that KE is converted into mass, therefore in reality it would never actually reach that speed?
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6 years ago
#300
What do we think about potential 6 markers for turning points?
I'd say maybe something to do with special relativity or discovery of the electron
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