OCR Physics A G485 - Frontiers of Physics - 18th June 2015 Watch

Elcor
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#1121
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(Original post by L'Evil Fish)
Both the voltages would decay at the same rate, because it's the total capacitance of a circuit that dictates the rate of it

So it's just their capacitance that leads to the ratio
Ooh fair enough.

(Original post by sarcastic-sal)
Came up two exams in a row though (June 12 and Jan 13) so probs not gonna make an appearance. You never know though.
Yeah I know

It will probably be CAT, PET and ultrasound or something. There will be little questions on MRI and gamma cameras perhaps. I also predict a weird cosmology question that hasn't come up before.
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fnatic NateDestiel
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(Original post by sarcastic-sal)
Attachment 430807
Basically you use the exponential relationship equation and the value of λ calculated in (ai)
Thank you, but how are you letting the activity in a) be equal to N?
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buxtonarmy
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http://www.ocr.org.uk/Images/58049-q...of-physics.pdf

2bii how do you do it?? I don't understand how to convert it
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fnatic NateDestiel
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(Original post by WishingChaff)
The easiest was of calculating this is to first recall the definition of half-life'

Defn: The half-life is the amount of time, on average, that is takes for half of the active nuclei to decay.

Now, given this, and the fact that the half-life in the question is 12 hours, you can calculate the number of active nuclei left.

Remember, you are told that you leave the nucleus for 36 hours, in other words, 3 times the half life. Therefore, the answer would simply be:
Spoiler:
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 N = 2^{-3} N_0


Hope this helps.
tHANKS i use (1/2)^3
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sarcastic-sal
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(Original post by Elcor)

Yeah I know

It will probably be CAT, PET and ultrasound or something. There will be little questions on MRI and gamma cameras perhaps. I also predict a weird cosmology question that hasn't come up before.
Depends how well PET was done last year as to how likley it is to come up. The examiner's report says
'The majority of the candidates gave decent explanations of the basic principles ofPET scanning and scored two or more marks. The quality of the answers was good,with many candidates showing excellent understanding of the annihilation ofpositron-electron, the production of two gamma photons and the detection of thesephotons to form an image. The question discriminated well.'
So doesn't sound like it was done horrifically. CAT scans are probably more likely to come up.
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L'Evil Fish
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I want MRI

**** the others
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jamesbird18
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(Original post by leroythelost)
how do you know if they move clockwise or anticlockwise? Is it the positive and negative plate from the battery or the capacitors. If it's the capacitor how do we know which plate is positive and which is negative.
Look at battery symbol thats how you know
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MO2898
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That half life of a proton question last year...lol.

What even was the answer to that? The time it takes for the activity of a group of protons to fall to half?

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sarcastic-sal
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(Original post by fnatic NateDestiel)
Thank you, but how are you letting the activity in a) be equal to N?
You don't use the activity but rather the decay constant (but using the decay constant in hours^-1 rather than seconds^-1 to save converting 36 hours into seconds).
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leroythelost
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What AS Physics are guys remembering I keep some stupid 1/2 marks questions on AS knowledge in past papers
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nothepreacher
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(Original post by MO2898)
That half life of a proton question last year...lol.

What even was the answer to that? The time it takes for the activity of a group of protons to fall to half?

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Yaas
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dragozox
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Hello everyone!!
can someone please tell me an explanation of medical tracer and the idea that it should have a short 1/2 life
thanks!!!
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fnatic NateDestiel
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(Original post by sarcastic-sal)
You don't use the activity but rather the decay constant (but using the decay constant in hours^-1 rather than seconds^-1 to save converting 36 hours into seconds).
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Thank you I see.

Thanks for the effort

(Original post by L'Evil Fish)
I want MRI

**** the others
mad
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sarcastic-sal
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(Original post by leroythelost)
What AS Physics are guys remembering I keep some stupid 1/2 marks questions on AS knowledge in past papers
Recurrent themes are energy of a photon, de Broglie wavelength of an electron, resistivity and general circuits stuff. Most are manageable if you know what equation to use, but the examiners reports say that using the wrong equations are often the main mistake in synoptic questions, so make sure you know what all of the equations are used for (especially in the waves topic of G482!)
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simobob16
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Can anyone think of any proofs of equations that we might need to know, like the proof of Kepler's law equation in the first exam?
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actanide
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(Original post by simobob16)
Can anyone think of any proofs of equations that we might need to know, like the proof of Kepler's law equation in the first exam?
they might ask you to find v in terms of B and E, but thats the only one i can think of
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leroythelost
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(Original post by sarcastic-sal)
Recurrent themes are energy of a photon, de Broglie wavelength of an electron, resistivity and general circuits stuff. Most are manageable if you know what equation to use, but the examiners reports say that using the wrong equations are often the main mistake in synoptic questions, so make sure you know what all of the equations are used for (especially in the waves topic of G482!)
I think i am comfortable with equations it's just definitions like electromotive force and line spectra and stuff like that
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fnatic NateDestiel
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(Original post by simobob16)
Can anyone think of any proofs of equations that we might need to know, like the proof of Kepler's law equation in the first exam?
that question was fun.

Loved the question on the proof of E=mcdelta theta.
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JackThorpe7
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(Original post by buxtonarmy)
http://www.ocr.org.uk/Images/58049-q...of-physics.pdf

2bii how do you do it?? I don't understand how to convert it
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kate8
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(Original post by L'Evil Fish)
Both the voltages would decay at the same rate, because it's the total capacitance of a circuit that dictates the rate of it

So it's just their capacitance that leads to the ratio
If the capacitors were in parallel would they still decay at the same rate. Or because the current through them is different the one with the greater capacitance discharges faster?
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