PHYSICS HL MAY 2010 DISCUSSION

[Cesc]

39 points.. so I did well there (only 28 points in chemistry though lol).. what options did you study for p3?

[tdubthebassist]

I kinda messed up paper 3.

I approached some question (luminosity, mass one) in different angle. So hopefully that works too. (I found the slope value k for Sun, which we know is in the main sequence, and then found the slope for star X, and suggested that the slope value k is consistent with that of the Sun. Some mathematical derivation for 2 points ftw). I also made some mistakes in relativity, and relativity itself was a bit hard.

I think it will land on the happy land of 6, possible 7. So I am hoping for overall 7!

[hanniexx]

39 points.. so I did well there (only 28 points in chemistry though lol).. what options did you study for p3?

Ahh that's good. Yeah, that should definitely help. Astrophysics and electromagnetic waves. You?

tdubthebassist, that's a really clever way...never thought of that. I was so happy with the second part of that question, with the O-V limit. Good luck ^^

BTW, random question, but if you went over the dotted lines for an explanation/equation/etc, will that be okay? As in the computer doesn't just pick up what's on the lines, yeah?

Edited May 12, 2010 by hanniexx

[IBer_101]

39 points.. so I did well there (only 28 points in chemistry though lol).. what options did you study for p3?

Ahh that's good. Yeah, that should definitely help. Astrophysics and electromagnetic waves. You?

tdubthebassist, that's a really clever way...never thought of that. I was so happy with the second part of that question, with the O-V limit. Good luck ^^

BTW, random question, but if you went over the dotted lines for an explanation/equation/etc, will that be okay? As in the computer doesn't just pick up what's on the lines, yeah?

I also did Astrophysics and Electromagnetic Waves. I did alright, I don't think there were weird or unusual questions there.

About going over the dotted lines, I don't think they scan this paper, they send the paper itself to the IBO. They only scan Multiple Choice papers. So, the examiner will be handling your original paper, not a scan. That's what I was told at least.

I used the same method as tdubthebassist in that question as well. Did you use another method?

[tdubthebassist]

39 points.. so I did well there (only 28 points in chemistry though lol).. what options did you study for p3?

Ahh that's good. Yeah, that should definitely help. Astrophysics and electromagnetic waves. You?

tdubthebassist, that's a really clever way...never thought of that. I was so happy with the second part of that question, with the O-V limit. Good luck ^^

BTW, random question, but if you went over the dotted lines for an explanation/equation/etc, will that be okay? As in the computer doesn't just pick up what's on the lines, yeah?

I also did Astrophysics and Electromagnetic Waves. I did alright, I don't think there were weird or unusual questions there.

About going over the dotted lines, I don't think they scan this paper, they send the paper itself to the IBO. They only scan Multiple Choice papers. So, the examiner will be handling your original paper, not a scan. That's what I was told at least.

I used the same method as tdubthebassist in that question as well. Did you use another method?

well, what you can simply do is to find the Sun from HR diagram and substitute it.

I kinda messed O-V question too, did not mention the specific mass of O-V limit and the fact that it is most likely to be a blackhole.

[IBer_101]

L is proportional to m^3.5 for main sequence stars

hence, L = k*m^3.5

for main sequence stars L/(m^3.5) = constant

so I found the value of this constant for the sun, then for star X (using the HR diagram and the masses calculated before) Then, since they are almost equal, and the sun is a main sequence star, then star X is a main sequence star too.

That's what I used

[bobo]

I did communication and relativity, those were the most difficult ones.

[hanniexx]

L is proportional to m^3.5 for main sequence stars

hence, L = k*m^3.5

for main sequence stars L/(m^3.5) = constant

so I found the value of this constant for the sun, then for star X (using the HR diagram and the masses calculated before) Then, since they are almost equal, and the sun is a main sequence star, then star X is a main sequence star too.

That's what I used

I started to use that method, using that equation. But then for some unknown and annoying reason, my mind just blanked.

Wow, yeah my teachers were against teaching relativity because they didn't even understand it properly enough! Bobo, how come your teachers picked those two for you?

[zakuropanda]

We did G (Electromagnetic Waves) and I (medical physics). They were both quite fair - no wedge films yay! Although I failed at drawing a spherical aberration of a window as well as the acoustic impedance question. However, paper 3 isn'

t worth much for HL! w00t!

Edited May 14, 2010 by zakuropanda

[hanniexx]

We did G (Electromagnetic Waves) and I (medical physics). They were both quite fair - no wedge films yay! Although I failed at drawing a spherical aberration of a window as well as the acoustic impedance question. However, paper 3 isn'

t worth much for HL! w00t!

You had aberrations? Oh gosh, I'm so glad I was TZ2, I hate that stuff

[tdubthebassist]

L is proportional to m^3.5 for main sequence stars

hence, L = k*m^3.5

for main sequence stars L/(m^3.5) = constant

so I found the value of this constant for the sun, then for star X (using the HR diagram and the masses calculated before) Then, since they are almost equal, and the sun is a main sequence star, then star X is a main sequence star too.

That's what I used

I started to use that method, using that equation. But then for some unknown and annoying reason, my mind just blanked.

Wow, yeah my teachers were against teaching relativity because they didn't even understand it properly enough! Bobo, how come your teachers picked those two for you?

for the first question of astrophysics, did you made cube or sphere? I did cube and was so sure of getting it right, and then two of my friends used sphere for finding density.

[IBer_101]

I used sphere volume.

But, see, the funny thing is I had no idea what the equation of the volume of a sphere was . I looked through the whole data booklet for it but I couldn't find it.

In the end, I basically used 3/4 instead of 4/3... Epic on my part..

Why did you use the equation for the cube though? I guess it works for the ratio (the third question), since the constants (pie and 4/3) cross out, but I don't think that'd work with the density.

[tdubthebassist]

I used sphere volume.

But, see, the funny thing is I had no idea what the equation of the volume of a sphere was . I looked through the whole data booklet for it but I couldn't find it.

In the end, I basically used 3/4 instead of 4/3... Epic on my part..

Why did you use the equation for the cube though? I guess it works for the ratio (the third question), since the constants (pie and 4/3) cross out, but I don't think that'd work with the density.

why not the cube? It was fairly simple question and when the volume of sphere is not prerequisite of physics syllabus, they are supposed to use the cube. it is still cubic ly.

so I did (no. of star in 17ly)^3/(17^3) and same for galaxy.

[IBer_101]

It's the volume of a sphere because we're taking 17ly in all directions, which draws the boundaries of a sphere. When using a cube, you basically used a cube of a side length of 17ly, which doesn't really represent taking 17ly in all directions from your point of observation.

[tdubthebassist]

It's the volume of a sphere because we're taking 17ly in all directions, which draws the boundaries of a sphere. When using a cube, you basically used a cube of a side length of 17ly, which doesn't really represent taking 17ly in all directions from your point of observation.

even sphere does not mean '17ly from the all dimension'. If there were certain amount of stars in 17ly of 'all direction', then Olber's paradox occurs. there must be infinite number of stars inside, which must not be true.

[IBer_101]

It's the volume of a sphere because we're taking 17ly in all directions, which draws the boundaries of a sphere. When using a cube, you basically used a cube of a side length of 17ly, which doesn't really represent taking 17ly in all directions from your point of observation.

even sphere does not mean '17ly from the all dimension'. If there were certain amount of stars in 17ly of 'all direction', then Olber's paradox occurs. there must be infinite number of stars inside, which must not be true.

It doesn't tell you that this is the density of the stars in the universe man, it just tells you to calculate the density of stars in that portion of the universe, which is included 17ly away from your observation point in all directions.

As you said, it's a fairly simple question. And if it's not supposed to be calculated using the sphere volume equation, then I don't see why it should be calculated by using the cube volume equation.

[tdubthebassist]

It's the volume of a sphere because we're taking 17ly in all directions, which draws the boundaries of a sphere. When using a cube, you basically used a cube of a side length of 17ly, which doesn't really represent taking 17ly in all directions from your point of observation.

even sphere does not mean '17ly from the all dimension'. If there were certain amount of stars in 17ly of 'all direction', then Olber's paradox occurs. there must be infinite number of stars inside, which must not be true.

It doesn't tell you that this is the density of the stars in the universe man, it just tells you to calculate the density of stars in that portion of the universe, which is included 17ly away from your observation point in all directions.

As you said, it's a fairly simple question. And if it's not supposed to be calculated using the sphere volume equation, then I don't see why it should be calculated by using the cube volume equation.

I am not saying that sphere is wrong. I am saying why not cube?

and still, if you were to observe x number of stars in 'all direction', an error occurs because you can turn your eye by 1 degree and then there still be x number of stars. I think that is not true as it implies that there are infinite number of stars. I think observing at most 3 direction would give you the right number of stars in a given space, which can simply be described as cube.

[IBer_101]

It's the volume of a sphere because we're taking 17ly in all directions, which draws the boundaries of a sphere. When using a cube, you basically used a cube of a side length of 17ly, which doesn't really represent taking 17ly in all directions from your point of observation.

even sphere does not mean '17ly from the all dimension'. If there were certain amount of stars in 17ly of 'all direction', then Olber's paradox occurs. there must be infinite number of stars inside, which must not be true.

It doesn't tell you that this is the density of the stars in the universe man, it just tells you to calculate the density of stars in that portion of the universe, which is included 17ly away from your observation point in all directions.

As you said, it's a fairly simple question. And if it's not supposed to be calculated using the sphere volume equation, then I don't see why it should be calculated by using the cube volume equation.

I am not saying that sphere is wrong. I am saying why not cube?

and still, if you were to observe x number of stars in 'all direction', an error occurs because you can turn your eye by 1 degree and then there still be x number of stars. I think that is not true as it implies that there are infinite number of stars. I think observing at most 3 direction would give you the right number of stars in a given space, which can simply be described as cube.

The question is not that complicated at all.. It just gives you an observation point. And it is given to you in the question (regardless of how it was obtained) that the number of stars within 17ly of that observation point is X. How do you calculate the density of the stars in that region?

"A perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point." R being 17ly, you find the volume of the sphere and divide the number of stars over it.

A cube doesn't represent, in any way the volume, within 17ly of a point. So, it shouldn't be used.

[tdubthebassist]

It's the volume of a sphere because we're taking 17ly in all directions, which draws the boundaries of a sphere. When using a cube, you basically used a cube of a side length of 17ly, which doesn't really represent taking 17ly in all directions from your point of observation.

even sphere does not mean '17ly from the all dimension'. If there were certain amount of stars in 17ly of 'all direction', then Olber's paradox occurs. there must be infinite number of stars inside, which must not be true.

It doesn't tell you that this is the density of the stars in the universe man, it just tells you to calculate the density of stars in that portion of the universe, which is included 17ly away from your observation point in all directions.

As you said, it's a fairly simple question. And if it's not supposed to be calculated using the sphere volume equation, then I don't see why it should be calculated by using the cube volume equation.

I am not saying that sphere is wrong. I am saying why not cube?

and still, if you were to observe x number of stars in 'all direction', an error occurs because you can turn your eye by 1 degree and then there still be x number of stars. I think that is not true as it implies that there are infinite number of stars. I think observing at most 3 direction would give you the right number of stars in a given space, which can simply be described as cube.

The question is not that complicated at all.. It just gives you an observation point. And it is given to you in the question (regardless of how it was obtained) that the number of stars within 17ly of that observation point is X. How do you calculate the density of the stars in that region?

"A perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point." R being 17ly, you find the volume of the sphere and divide the number of stars over it.

A cube doesn't represent, in any way the volume, within 17ly of a point. So, it shouldn't be used.

cube however can describe the number of stars much simpler.

if you were to find 25 galaxies in 17 ly, then there should be 25^3 galaxies in a cubic universe of 17^3ly^3.

If it were a cube, how can you come up with the number of galaxies?

[IBer_101]

It's the volume of a sphere because we're taking 17ly in all directions, which draws the boundaries of a sphere. When using a cube, you basically used a cube of a side length of 17ly, which doesn't really represent taking 17ly in all directions from your point of observation.

even sphere does not mean '17ly from the all dimension'. If there were certain amount of stars in 17ly of 'all direction', then Olber's paradox occurs. there must be infinite number of stars inside, which must not be true.

It doesn't tell you that this is the density of the stars in the universe man, it just tells you to calculate the density of stars in that portion of the universe, which is included 17ly away from your observation point in all directions.

As you said, it's a fairly simple question. And if it's not supposed to be calculated using the sphere volume equation, then I don't see why it should be calculated by using the cube volume equation.

I am not saying that sphere is wrong. I am saying why not cube?

and still, if you were to observe x number of stars in 'all direction', an error occurs because you can turn your eye by 1 degree and then there still be x number of stars. I think that is not true as it implies that there are infinite number of stars. I think observing at most 3 direction would give you the right number of stars in a given space, which can simply be described as cube.

The question is not that complicated at all.. It just gives you an observation point. And it is given to you in the question (regardless of how it was obtained) that the number of stars within 17ly of that observation point is X. How do you calculate the density of the stars in that region?

"A perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point." R being 17ly, you find the volume of the sphere and divide the number of stars over it.

A cube doesn't represent, in any way the volume, within 17ly of a point. So, it shouldn't be used.

cube however can describe the number of stars much simpler.

if you were to find 25 galaxies in 17 ly, then there should be 25^3 galaxies in a cubic universe of 17^3ly^3.

If it were a cube, how can you come up with the number of galaxies?

I think you're making this way too complicated for the question. It really is not that complicated. the galaxies are within 17ly of the observation point, in all directions. That makes a sphere, not a cube. So to calculate the density, divide the number of galaxies over the volume of the cube, and you'll get the density. I'll be repeating myself once again if I argue more