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

1. (Original post by Summerdays)
Did anyone put that the activity would have been higher in the past and thus the calculated value is a minimum?
no idea what they were driving at here, i said cus not all of the nuclei might be radioactive :/
2. (Original post by Ollie901)
Wouldn't that make it a maximum?
No, A = kN so if A is higher in the past then N would have been higher in the past.
3. (Original post by Summerdays)
No, A = kN so if A is higher in the past then N would have been higher in the past.
But if you're using a past activity to calculate N, then that number must be was it was in the past, and therefore it has decreased since then, so is a maximum value..?

What did you get for the mass of uranium btw? You seem to know your stuff.
4. (Original post by Ollie901)
But if you're using a past activity to calculate N, then that number must be was it was in the past, and therefore it has decreased since then, so is a maximum value..?

What did you get for the mass of uranium btw? You seem to know your stuff.
I got 980Kg, so around 1000Kg.

Yeah, but you calculated the value of the number of nucleus using the given activity. It's quite confusing though, so I am not sure anymore. What did you put?
5. (Original post by Ollie901)
But if you're using a past activity to calculate N, then that number must be was it was in the past, and therefore it has decreased since then, so is a maximum value..?

What did you get for the mass of uranium btw? You seem to know your stuff.

(Original post by Summerdays)
I got 980Kg, so around 1000Kg.

Yeah, but you calculated the value of the nucleus using the given activity. It's quite onfusing though, so I am not sure anymore. What did you put?
I reasoned that since radioactive decay = random process, the number of nuclei we calculated that corresponded to the measured activity (that was given to us) would be a minimum - simply because we COULD have more nuclei present that just weren't decaying, therefore still giving us that same measured activity

EDIT// Well, I remember explaining it better in the exam. XD
6. (Original post by arianex)
I reasoned that since radioactive decay = random process, the number of nuclei we calculated that corresponded to the measured activity (that was given to us) would be a minimum - simply because we COULD have more nuclei present that just weren't decaying, therefore still giving us that same measured activity

EDIT// Well, I remember explaining it better in the exam. XD
Yeah, that question was a bit stupid. No explanation found in the book
7. (Original post by arianex)
I reasoned that since radioactive decay = random process, the number of nuclei we calculated that corresponded to the measured activity (that was given to us) would be a minimum - simply because we COULD have more nuclei present that just weren't decaying, therefore still giving us that same measured activity

EDIT// Well, I remember explaining it better in the exam. XD
oo i basically said that, feeling a bit better on that one now
8. arianex, you're method for the uranium mass is exactly what I did I think, what did you get for it appox?

Think I must've done a calculator error.
9. (Original post by Ollie901)
arianex, you're method for the uranium mass is exactly what I did I think, what did you get for it appox?

Think I must've done a calculator error.
Depending on what value of energy released per emission you used, you should get anywhere between 900+ to 1000+ kg (I know Summerdays was in the high 900s, I was just over 1000 or something.)

HOWEVER...I derped and divided it by a thousand because I thought my answer was in grams - so somehow ended up with just over a kilo Then again, considering that you only need a tiny amount of uranium to power up a feckload of things... xD
10. but it was over a year and its OCR so you cant expect them to be realistic
11. (Original post by Summerdays)
Yeah, that question was a bit stupid. No explanation found in the book
Typical OCR, getting you to "use your brain" and a bit of "common sense". In short, a Q that somewhat came outta the butt-end-of-nowhere xD
12. (Original post by arianex)
Typical OCR, getting you to "use your brain" and a bit of "common sense". In short, a Q that somewhat came outta the butt-end-of-nowhere xD
Haha... but there's NO common sense attached to that question. Almost anything could be the damn answer.
13. That wasn't too bad! :P For the radioactive people one i got 435, for the flux one did anybody get 7.1x10^-4wb?? and the 1.8x10^-3 for emf?? also the beta one i put it ionises the air in the gap so postive ions give the sphere positive charge so it moves downwards, that was on a past paper the same question 0.0, for the uranium thingy i got 1024kg?? ring any bells, also the prerelease what did people put for the very last question?? the picked some weird things to ask !!!
14. For the beta particle one I thought that the ball was neutral so I said that the beta particles remove elctrons from the ball and ionise it so it gains a positive charge and therefore is attracted to the bottom plate but apparently this is wrong?? Not sure

Also for the force between the proton and electron did anyone else get about 9 x 10^-8 N I thought this was much too small but I did the calculation about three times and got the same answer every time.
15. (Original post by Birkatron)
That wasn't too bad! :P For the radioactive people one i got 435, for the flux one did anybody get 7.1x10^-4wb?? and the 1.8x10^-3 for emf?? also the beta one i put it ionises the air in the gap so postive ions give the sphere positive charge so it moves downwards, that was on a past paper the same question 0.0, for the uranium thingy i got 1024kg?? ring any bells, also the prerelease what did people put for the very last question?? the picked some weird things to ask !!!
1024kg, great shout, sorted for that one i think
16. (Original post by ChicharitoH)

Also for the force between the proton and electron did anyone else get about 9 x 10^-8 N I thought this was much too small but I did the calculation about three times and got the same answer every time.
yeh think i got that, its just KQq/r^2

for the number of electrons what did people put? this confused me for ages, too many values floating about, quite confident i got it eventually but cant remember my answer haha! something like 9 x 10^9? (1 sf)
17. (Original post by Birkatron)
That wasn't too bad! :P For the radioactive people one i got 435, for the flux one did anybody get 7.1x10^-4wb?? and the 1.8x10^-3 for emf?? also the beta one i put it ionises the air in the gap so postive ions give the sphere positive charge so it moves downwards, that was on a past paper the same question 0.0, for the uranium thingy i got 1024kg?? ring any bells, also the prerelease what did people put for the very last question?? the picked some weird things to ask !!!
Yeah, pretty much except 4.35 is the general consensus. I said what you said about the ball but I forgot that the ball is positive

For the very last quetion I said that you measure the frequency by finding the amount of maxima that is detected after moving the mirror a particular distance. peed of light i teh EXACT value. The second is defined from the frequency of the caesium. Thus lamda calculated is very accurate. Not sure if this is correct though. I blagged, basically.
18. (Original post by Mikkels88)
yeh think i got that, its just KQq/r^2

for the number of electrons what did people put? this confused me for ages, too many values floating about, quite confident i got it eventually but cant remember my answer haha! something like 9 x 10^9? (1 sf)
Yeah, I got that
19. (Original post by Mikkels88)
yeh think i got that, its just KQq/r^2

for the number of electrons what did people put? this confused me for ages, too many values floating about, quite confident i got it eventually but cant remember my answer haha! something like 9 x 10^9? (1 sf)
Somehow I managed to get 8 x 10^9 but a lot of people seem to have got 9 so I think I may have really stupidly rounded wrong.
20. Didn't it say at the beginning of the ball question that is was neutral?

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