You are Here: Home

# A2 Physics Unit 5 ~ 18th June 2012 Tweet

Physics exam discussion - share revision tips in preparation for GCSE, A Level and other physics exams and discuss how they went afterwards.

Announcements Posted on
TSR launches Learn Together! - Our new subscription to help improve your learning 16-05-2013
IMPORTANT: You must wait until midnight (morning exams)/4.30AM (afternoon exams) to discuss Edexcel exams and until 1pm/6pm the following day for STEP and IB exams. Please read before posting, including for rules for practical and oral exams. 28-04-2013
1. Re: A2 (Edexcel) Physics Unit 5 6PH05 - Mon 18th June 2012 AM
(Original post by Parthenon93)
Correct me where I am wrong.

Standard candles are variable stars and certain supernova, whose luminosity can be found from other measurements like -
b. the time period for variation in brightness
The distance of these standard candles are calculated using F = L/4pid^2
(b) is true, but I don't think that (a) counts as a standard candle.

(Original post by Parthenon93)
The distance of an unknown star is found out by comparing it with a standard candle of the same luminosity.
I don't think you would ever do this. We normally use a standard candle to determine how far away the galaxy it is in is.
2. Re: A2 Physics Unit 5 ~ 18th June 2012
(Original post by Parthenon93)
Correct me where I am wrong.

Standard candles are variable stars and certain supernova, whose luminosity can be found from other measurements like -
b. the time period for variation in brightness
The distance of these standard candles are calculated using F = L/4pid^2

The distance of an unknown star is found out by comparing it with a standard candle of the same luminosity.
It's all right apart from the last bit, I think. The standard candle provides the distance to a galaxy, AFAIK, and then we can work out that other stars must be within the galaxy, which gives us some idea of a range of distances for the stars.

I may be wrong, though.
3. Re: A2 Physics Unit 5 ~ 18th June 2012
(Original post by Parthenon93)
Correct me where I am wrong.

Standard candles are variable stars and certain supernova, whose luminosity can be found from other measurements like -
b. the time period for variation in brightness
The distance of these standard candles are calculated using F = L/4pid^2

The distance of an unknown star is found out by comparing it with a standard candle of the same luminosity.
To determine the distance to an unknown star, we can use the equation F = L/4πd^2. F can be measured using sophiscated instruments on earth, but we can use λT=2.90x10^-3 (Wein's law) and L=4πσr^2T^4 (Stefan-Boltzmann law) to calculate L.
4. Re: A2 (Edexcel) Physics Unit 5 6PH05 - Mon 18th June 2012 AM
(Original post by fluteflute)
(b) is true, but I don't think that (a) counts as a standard candle.

I don't think you would ever do this. We normally use a standard candle to determine how far away the galaxy it is in is.
I found the distance of the standard candle first. So by comparing unknown star with standard candle of similar luminosity, we get can idea of its distance - because it must be at around the same distance as the standard candle?
5. Re: A2 Physics Unit 5 ~ 18th June 2012
(Original post by dendodge)
It's all right apart from the last bit, I think. The standard candle provides the distance to a galaxy, AFAIK, and then we can work out that other stars must be within the galaxy, which gives us some idea of a range of distances for the stars.

I may be wrong, though.
I found the distance of the standard candle first. So by comparing unknown star with standard candle of similar luminosity, we get can idea of its distance - because it must be at around the same distance as the standard candle?
6. Re: A2 Physics Unit 5 ~ 18th June 2012
(Original post by Parthenon93)
I found the distance of the standard candle first. So by comparing unknown star with standard candle of similar luminosity, we get can idea of its distance - because it must be at around the same distance as the standard candle?
If they're in the same galaxy, and the observed luminosity is the same, then yes.

If they're in different galaxies, the observed luminosity says nothing.
7. Re: A2 Physics Unit 5 ~ 18th June 2012
(Original post by dendodge)
If they're in the same galaxy, and the observed luminosity is the same, then yes.

If they're in different galaxies, the observed luminosity says nothing.
Isn't that why we compare the unknown star to a standard candle of the same luminosity?
8. Re: A2 Physics Unit 5 ~ 18th June 2012
(Original post by Parthenon93)
Isn't that why we compare the unknown star to a standard candle of the same luminosity?
Luminosity changes with distance. So star A could be brighter than star B, but further away, and both stars would have the same apparent luminosity. So the observed luminosity of star A says nothing about the distance of star B.

However, if star A is in the same galaxy as star B, working out the actual luminosity of star A (say it's a variable Cepheid, for example) will tell us how far away the galaxy is. If we can estimate the size of the galaxy (we usually can), we know what range of distances star B must be within.
Last edited by dendodge; 15-06-2012 at 23:26.
9. Re: A2 (Edexcel) Physics Unit 5 6PH05 - Mon 18th June 2012 AM
(Original post by Parthenon93)
I found the distance of the standard candle first. So by comparing unknown star with standard candle of similar luminosity, we get can idea of its distance - because it must be at around the same distance as the standard candle?
Without seeing the question, I'm struggling to completely follow.

But yes, if you have two objects that you are know are the same luminosity, but only know the distance of one, you can work out the distance of the second. (If the flux on Earth was the same for both then the distances would be the same.)
10. Re: A2 (Edexcel) Physics Unit 5 6PH05 - Mon 18th June 2012 AM
Do we need to learn the masses of protons and neutrons - 1.00728 and 1.00867?
11. Re: A2 (Edexcel) Physics Unit 5 6PH05 - Mon 18th June 2012 AM
(Original post by Parthenon93)
Do we need to learn the masses of protons and neutrons - 1.00728 and 1.00867?
No!
12. Re: A2 (Edexcel) Physics Unit 5 6PH05 - Mon 18th June 2012 AM
(Original post by fluteflute)
No!
Will they be given if we are asked to find the mass deficit?
13. Re: A2 (Edexcel) Physics Unit 5 6PH05 - Mon 18th June 2012 AM
(Original post by Parthenon93)
Will they be given if we are asked to find the mass deficit?
They certainly have in every question I've seen.
14. Re: A2 Physics Unit 5 ~ 18th June 2012
what fraction of a radioactive sample remains after 6 half lives ..?!
15. Re: A2 (Edexcel) Physics Unit 5 6PH05 - Mon 18th June 2012 AM
well as the unit 4 waz quite damn tough to me... ne predictions what might come in unit 5 this tym???????
16. Re: A2 Physics Unit 5 ~ 18th June 2012
(Original post by iesians)
what fraction of a radioactive sample remains after 6 half lives ..?!
1/32
17. Re: A2 Physics Unit 5 ~ 18th June 2012
Why is 210Pb (half life 21 years) more suitable for dating bones that 14C (half life 5730 years)?

Edit: Found an answer given by Cora Lindsay on TSR
C-14 can't be used for dating recent bones because its decay rate is too slow to give measurable changes in activity on a timescale of less than a few hundred years.
Last edited by Parthenon93; 16-06-2012 at 09:58.
18. Re: A2 Physics Unit 5 ~ 18th June 2012
(Original post by Parthenon93)
1/32
or 1/64 ?! cuz (1/2)^6 = 1/64
19. Re: A2 Physics Unit 5 ~ 18th June 2012
or 1/64 ?! cuz (1/2)^6 = 1/64
yeh youre right
20. Re: A2 Physics Unit 5 ~ 18th June 2012
(Original post by iesians)
or 1/64 ?! cuz (1/2)^6 = 1/64
Useful resources

## Articles:

Study Help rules and posting guidelinesStudy Help Member of the Month nominations

## Groups associated with this forum:

View associated groups