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OCR Physics B G495 Field and Particle Pictures June 21st 2011 Exam Thread watch

1. (Original post by Jke)
100/(2^n)=6.5
Where n = number of half lifes
2^n=(100/6.5)
n*ln2=ln(100/6.5)
n=(ln(100/6.5))/ln2
n= 3.94
Although you are probably right and your method is far more impressive, surely you would just think 1 half life = 50% of value, 2 half lives = 25%, 3 = 12.5% and then 4 = 6.25, which is about 6.5%? It is only 1 mark so i'm hoping i'm not thinking too simple....
2. (Original post by JoeCarr)
Although you are probably right and your method is far more impressive, surely you would just think 1 half life = 50% of value, 2 half lives = 25%, 3 = 12.5% and then 4 = 6.25, which is about 6.5%? It is only 1 mark so i'm hoping i'm not thinking too simple....
You can, but that method is MUCH faster haha
3. How do you find the electric field strength at the midpoint between two charges? I've seen this question a few times and the method was different so I'm not sure how to do it.
4. (Original post by JoeCarr)
Although you are probably right and your method is far more impressive, surely you would just think 1 half life = 50% of value, 2 half lives = 25%, 3 = 12.5% and then 4 = 6.25, which is about 6.5%? It is only 1 mark so i'm hoping i'm not thinking too simple....
Haha, yeah i think that is what you're meant to do, seeing as it says estimate.
5. http://www.johnbright.conwy.sch.uk/d...64June2006.pdf
http://www.johnbright.conwy.sch.uk/d...06JuneMSA2.pdf
I can't get 11 b & ci) ii) ..where does the half life comes up?
6. (Original post by Raimu)
How do you find the electric field strength at the midpoint between two charges? I've seen this question a few times and the method was different so I'm not sure how to do it.
It's zero if the two charges are equal (and the same polarity.) But it's
E= kq^2/(0.5r)^2 = 4kq^2/r^2 (if the two charges are equal but oppositely charged.)
7. (Original post by Jke)
Haha, yeah i think that is what you're meant to do, seeing as it says estimate.
gotta love the simple methods
8. After one half-life there will be 50% lead-206 and 50% Uranium-238, so the ratio is 1:1

But after TWO half-lifes, there will be 25% Uranium-238 and 75% lead-206, so the ratio is now 3:1

After THREE half lifes, there will be 12.5% Uranium-238 and 87.5% lead-206, so the ratio is now 7:1

Hope this helps
9. (Original post by Birkatron)
Januarys raw mark was 74/100 for an A , anyway if i don't post here again until after the exam good luck everyone!
oh yeah cheers mate! good luck!
10. (Original post by JoeCarr)
gotta love the simple methods
Gotta love physics without mathematics
11. Uranium-238 decays to Lead-206, so whilst the number of U-238 nuclei will fall exponentially, the number of Pb-206 nuclei will increase rapidly then plateau, and as Summerdays has shown, the ratio will therefore change as follows:

After one half life, 50% of U-238 nuclei have decayed to Pb-206, so the ratio is 1:1
After two half lives, 75% of U-238 nuclei have decayed to Pb-206, so the ratio is 1:3
12. (Original post by Summerdays)
Gotta love physics without mathematics
Summerdays, care to give me a crash course/explanation on the stuff related to our dear friend de Broglie? I've seen a few Qs which ask you to use the dB wavelength to explain/account for a few things (a bit fuzzy now though.)

Cheers
13. (Original post by Summerdays)
After one half-life there will be 50% lead-206 and 50% Uranium-238, so the ratio is 1:1

But after TWO half-lifes, there will be 25% Uranium-238 and 75% lead-206, so the ratio is now 3:1

After THREE half lifes, there will be 12.5% Uranium-238 and 87.5% lead-206, so the ratio is now 7:1

Hope this helps
oh I didn't notice this. Thanks mate! that really helps
14. i was also wondering why sometimes E is about kT, and sometimes its 1.5kT ... what the hell do we use? And how much of internal energy and such should we know? thats physics 4 and thanks to OCR B's infinite genius, i havent revised that yet...

also, anyone done the january 2011 paper? Wondering what people got - i found it really weird and the questions were confusing, managed 86/100 but i went over time to be honest :/

-------------------------------

whoever asked earlier about the atomic and nucleus mass ratio being 3, its because the mass ratio is the same as the volume ratio (same density) and as volume is 4/3 pi r^3 (for a sphere) its by a factor of r^3
15. Is there a list of formulae we need to know that are not in the booklet?
16. Is the uncertainty stuff even on this paper? I thought that was just G492? There wasnt any in jan anyway.
17. guysss what should i do, past papers or rev guide!!
18. (Original post by 41jms)
guysss what should i do, past papers or rev guide!!
Both. Rev guide, then past papers to check what you remember from the rev guide. Lather, rinse, repeat.
19. (Original post by arianex)
Summerdays, care to give me a crash course/explanation on the stuff related to our dear friend de Broglie? I've seen a few Qs which ask you to use the dB wavelength to explain/account for a few things (a bit fuzzy now though.)

Cheers
Sure. His thesis says that the momentum of an object detremines their wavelength, the larger the momentum the smaller the wavelength, but it's really the MASS that determines if this effect is noticable or not.

An electron ha a definite wavelength, depending on its speed. If the wavelength is as large a the diameter of the nucleus, diffreaction occurs, sintheta = 1.2lamda/d where d is the diameter of the nucleus. Each nucleus acts as an obstacle (like an aperture) - interference results between the phasors of the paths available to the electron either side of the nucleus. The theta from this equation determine the angle where a MINIMUM occurs. The electron travel all paths. The smaller the wavelength of an electron, the greater it's resolving abilities (based on rayleigh's criterion, th = lambda/d.) If the energy is large enough an electron can cause the gluon field to stretch so much that mesons are released. An electron is used because it is not effected by the strong force, in the nucleus because it is a lepton
20. Anyone want to explain why the Earth bulges at the equator? Other than just 'Cos it's rotating'...just in case it comes up in section C

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