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# Edexcel A2 Physics Unit 5 'Physics from Creation to Collapse' watch

1. (Original post by definite_maybe)
But why if you know the second object's luminosity and brightness, can't you just apply the inverse square law? Why do you need the standard candle?
A standard candle is an object whose luminosity is constant, so you can easily measure its flux using $F=\frac{L}{4\pi d^2}$ and using its known luminosity, to calculate the distance d away from Earth.
2. (Original post by OL1V3R)
A standard candle is an object whose luminosity is constant, so you can easily measure its flux using $F=\frac{L}{4\pi d^2}$ and using its known luminosity, to calculate the distance d away from Earth.
So it's nothing to do with comparing two different objects?
3. (Original post by definite_maybe)
So it's nothing to do with comparing two different objects?
Well you can use it to find nearby objects as well.
4. (Original post by OL1V3R)
Well you can use it to find nearby objects as well.
OK thank you. THe way the textbook explains it, I thought it meant that. So you can only really use the inverse square law when luminosity remains constant, which makes them standard candles, such as supernova right? lol just checking I understand it.
5. (Original post by definite_maybe)
OK thank you. THe way the textbook explains it, I thought it meant that. So you can only really use the inverse square law when luminosity remains constant, which makes them standard candles, such as supernova right? lol just checking I understand it.
Yup
6. Hey guys, anyone know if we'll need this equation for the thermal physics bit?

Where P = Pressure

p = Density

I got some dodgy text book which I feel is teaching me stuff I don't need to know. It says I need to be able to use and recognize that equation for the exam however I don't need to know how to derive it. Not once in the official text book and my CGP book does it mention that equation.

Also do we need to know about moles and the equation of state of an ideal gas in mole form? (pV = nRT)?
7. (Original post by Sasukekun)
Hey guys, anyone know if we'll need this equation for the thermal physics bit?

Where P = Pressure

p = Density

I got some dodgy text book which I feel is teaching me stuff I don't need to know. It says I need to be able to use and recognize that equation for the exam however I don't need to know how to derive it. Not once in the official text book and my CGP book does it mention that equation.

Also do we need to know about moles and the equation of state of an ideal gas in mole form? (pV = nRT)?
That equation doesn't look familiar - it's not in the Miles Husdon Edexcel textbook so I'm guessing we don't need to know it. Are you using the Edexcel Salters Horners textbook?

We do need to know about the equation pV = nRT, as well as pV = NkT.
8. (Original post by OL1V3R)
That equation doesn't look familiar - it's not in the Miles Husdon Edexcel textbook so I'm guessing we don't need to know it. Are you using the Edexcel Salters Horners textbook?

We do need to know about the equation pV = nRT, as well as pV = NkT.
It's from some text book from Tim Akrill and Graham George which is endorsed from Edexcel. Although I'm revising from like 3 different text books.

I'll make note of it in case.
9. " How many protons are in the Sun if its mass is 2.0 x 10^30 kg? If the Sun fuses all its protons over the course of its estimated 9 billion year lifetime, how many protons are undergoing nuclear fusion every second?"

Can you solve this?

Answer: 1.2 x 10^57 protons; 4.22 x 10^39 protons fuse per second
10. (Original post by Sasukekun)
It's from some text book from Tim Akrill and Graham George which is endorsed from Edexcel. Although I'm revising from like 3 different text books.

I'll make note of it in case.
I recommend the Miles Hudson textbook, but not the Salters Horners one (it's too much context and it's very hard to find the Physics in it).
11. I find Tim Akrill's book way better than Miles Hudson's.
Regarding that equation, it's not in the syllabus, though it's not difficult to recognize the standard symbols in it, is it?
What we really dont need to know, as Doughboy once mentioned - see pages 94-95 of Hudson's book.
12. yadas and OL1V3R gimme your brains for this exam please
13. (Original post by Joann79)
" How many protons are in the Sun if its mass is 2.0 x 10^30 kg? If the Sun fuses all its protons over the course of its estimated 9 billion year lifetime, how many protons are undergoing nuclear fusion every second?"

Can you solve this?

Answer: 1.2 x 10^57 protons; 4.22 x 10^39 protons fuse per second
Yep, sure. The mass of one proton is 1.66 x 10^-27 kg, so to find the number of protons in the Sun, simply divide the mass of the Sun by the mass of a single proton.

$\frac{2.0\times10^{30}}{1.66\times 10^{-27}}=1.2\times10^{57} \; \textup{protons}$

Then for the next question, just convert years into seconds first.

$1\; \textup{year} = 365\times24\times60\times60=3.1536\times10^{8}\; \textup{s}$

Then, multiply this number by 9 billion, and this will give 2.83824 x 10^17 seconds.

So just divide the number of protons the Sun has by this time.

$\frac{1.2\times10^{57}\textup{\; protons}}{2.83824\times10^{17}\textup{\; s}}= 4.22\times10^{39} \textup{\;protons s}^{-1}$

Hope that helps, although to be honest that is a quite basic calculation, just converting from years to seconds, etc.
14. Oh, simple question o_o I guess I just always attempt these questions with a bad attitude.
15. (Original post by OL1V3R)
I recommend the Miles Hudson textbook, but not the Salters Horners one (it's too much context and it's very hard to find the Physics in it).
Definetly agree with you here mate. Kinda annoying that our teachers only started using it halfway through Unit 5 and not anyother units.

Anyways gotta concentrate on biology atm as ive got that on friday. Dammit
16. Going to start my physics revision.
17. FORMATION OF STAR

-Stars are formed from molecular cloud of gases.
-Particles at the edge of the cloud are pulled towards the center due to gravitation attraction.
-As they move towards the center they gain kinetic energy at the expense of gravitational potential energy.
-The cloud collapse under its own gravity.
-The particles move with higher speeds leading to an increase in temperature.
-The pressure and hence density also increases.
-Particles Kinetic energy increases.
-Particles gravitation potential energy decreases.
-The particles start to collide more violently and energetically.
-Formation of a hot plasma of high-speed positively charged nuclei and electrons as the particles break up.
-When the temperature rises high enough for the +ve nuclei to approach fusion starts.
-The energy released due to the fusion halts the collapse and fusion continues.
-A STAR IS BORN.
18. (Original post by darkibm)
FORMATION OF STAR

-Stars are formed from molecular cloud of gases.
-Particles at the edge of the cloud are pulled towards the center due to gravitation attraction.
-As they move towards the center they gain kinetic energy at the expense of gravitational potential energy.
-The cloud collapse under its own gravity.
-The particles move with higher speeds leading to an increase in temperature.
-The pressure and hence density also increases.
-Particles Kinetic energy increases.
-Particles gravitation potential energy decreases.
-The particles start to collide more violently and energetically.
-Formation of a hot plasma of high-speed positively charged nuclei and electrons as the particles break up.
-When the temperature rises high enough for the +ve nuclei to approach fusion starts.
-The energy released due to the fusion halts the collapse and fusion continues.
-A STAR IS BORN.
That's a really good summary! Thanks for sharing that with us!
19. Thanks, but I cant find any part of our syllabus this is related to.
131 Use a simple Hertzsprung-Russell diagram to explain life cycle of stars.
I doubt we would need to remember all those points of star formation.
20. RED GIANT AND SUPER GIANT

-Hydrogen fusion ceases.(Run out of Hydrogen)
-Inner parts of the star cools down.
-Again the star collapse under its own gravity.
-Gaining K.E at the expense of GPE.
-Rise in temperature.
-The material surrounding the newly formed helium core is now at a temperature where hydrogen fusion can take place.
-Now the star have a much cooler core which is collapsing surrounded by a layer where hydrogen fusion is releasing energy.
-The layer near where the hydrogen fusion is taking place expands rapidly, cooling as it does.
-Now if this star has a mass less then Sun then this is a RED GIANT(Star with a cooler exterior and high luminosity due to large mass).
-OR it can be a supergaint if it has a mass greater than sun.

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