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# PHYA1 - Physics Unit 1 Exam - 24th May 2011 watch

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1. (Original post by bubblebuddy)
when it asks that, it tends to say "of the nucleus" so since the electrons aren't included in that, you wouldn't include the mass of the electrons.
Ok, thank you!
2. Has anyone done the specimen paper?

For question 5, where you have to explain why the semiconductor diode protects the ammeter, I really don't understand why the answer is what it says in the mark scheme.

Can anyone help?
3. i dont understand specific charge

can someone give me a few examples and tell me how to calculate it

thanks
4. (Original post by kimmey)
i dont understand specific charge

can someone give me a few examples and tell me how to calculate it

thanks
specific charge is just charge divided by mass.

so to calculate specific charge for an electron you divide the charge of an electron (1.6x10^-19) by the mass of an electron (9.11x10^-31).
an electron has a large specific charge because its mass is so small.

to calculate the specific charge for the nucleus of an atom (so ignore any electrons) you just need to divide the total charge of all the protons by the total mass of all the nucleons.

so for example an atom of 238- uranium:

it has 92 protons and 238 nucleons so specific charge is (92 x charge of proton)/(238 x mass of nucleon).

does that help at all?
5. Any guesses on what the 6 marker will be on?
6. 6 marker will most likely be about using a circuit to draw a I/V graph for a component or something about the photoelectric effect. Just literally write everything you know, as long as it's in a sensible order it won't do any harm, i think one of the papers you got a mark for saying "Put the circuit together", so imagine the examiner is a 2 year old!
7. (Original post by schizopear)
specific charge is just charge divided by mass.

so to calculate specific charge for an electron you divide the charge of an electron (1.6x10^-19) by the mass of an electron (9.11x10^-31).
an electron has a large specific charge because its mass is so small.

to calculate the specific charge for the nucleus of an atom (so ignore any electrons) you just need to divide the total charge of all the protons by the total mass of all the nucleons.

so for example an atom of 238- uranium:

it has 92 protons and 238 nucleons so specific charge is (92 x charge of proton)/(238 x mass of nucleon).

does that help at all?
yes thank you very much
8. (Original post by Aarynn)
6 marker will most likely be about using a circuit to draw a I/V graph for a component or something about the photoelectric effect. Just literally write everything you know, as long as it's in a sensible order it won't do any harm, i think one of the papers you got a mark for saying "Put the circuit together", so imagine the examiner is a 2 year old!
this is my prediction too.

i'd love it to be on the photoelectric effect but since the january paper one was on the photoelectric effect i think it's more likely to be drawing a circuit and explaining how to calculate resistance.
9. I get a little confused with Feynman diagrams. Will it always be a weak nuclear interaction in the exam? That's what the CGP revision guide seems to suggest.
10. (Original post by Thompson93)
I get a little confused with Feynman diagrams. Will it always be a weak nuclear interaction in the exam? That's what the CGP revision guide seems to suggest.
most of the feynman diagrams we have to learn are the weak interaction but we did the electromagnetic force in class as well but I'm not sure if that can actually come up since I haven't seen it on any past papers.
11. (Original post by bubblebuddy)
most of the feynman diagrams we have to learn are the weak interaction but we did the electromagnetic force in class as well but I'm not sure if that can actually come up since I haven't seen it on any past papers.
Yeah, this is what I'm confused about, in class we seemed to cover a few different ones. Now it seems we only have to know the weak interaction diagrams for the exam?
12. hey, can someone please explain to me how you work out the minimum energy of ach photon produced in annihilation and pair production. Please explain this to me as i don't get it at all! Also, what do you have to know for cicuit as in formulas! thanks x
13. I could be wrong, but i'm pretty sure all of the circuit formulae are on the formula sheet you are given
14. (Original post by Minusthevolta)
Work it out via the conservation of charge for that specific reaction...

So for Beta+ decay:

P --> n + e+ + Ve
B 1 1 0 0
L 0 0 1 1
Q 1 0 1 -1

Obviously between the two sides there is not unbalanced charge. So the weak force must be positive - so that the right side has now a charge of +1 to match the proton's charge.

I'm **** at explaining things - this is just how I remember it...

I'm not even sure if this is the right way to remember it - but it seems to work with all the diagrams we need to know...
Thanks, i found out they wont be too complicated so we could probably work them out anyway in the exam.

ANYBODY know about the number of sfs we should leave our answers to? its complicated =/
15. (Original post by Master S P)
Thanks, i found out they wont be too complicated so we could probably work them out anyway in the exam.

ANYBODY know about the number of sfs we should leave our answers to? its complicated =/
From past mark schemes it seems to be 3 s.f. for everything UNLESS they ask you to give it to an appropriate degree of accuracy, in which case, see how many they've left it to in the question
16. (Original post by cobra2k10)
hey, can someone please explain to me how you work out the minimum energy of ach photon produced in annihilation and pair production. Please explain this to me as i don't get it at all! Also, what do you have to know for cicuit as in formulas! thanks x
well in pair production, if it is a electron and positron being produced, minimum energy would be 2x the energy of the positron and electron [same energy], as energy is neither created or destroyed. any remaining energy is kinetic.

In annihilation, a particle and antiparticle have to have the same energy as the 2 photons, their mass is converted into the energy of the pair of gamma ray photons

dunno thats just off the top of my head might not be right
17. Can anyone help me with the prefixes like micro, kilo e.t.c and which are which? I always forget them! also a few examples iof you don't mind Thanks!
18. does anyone know the grade boundaries for the Jan '11 paper? thanks
and good luck for tomorrow everyone!!!
19. The grade boundaries in January were
51/70 A
46/70 B
41/70 C
Which aren't too bad really, but I've done virtually no revision so I've no idea how I'm going to get an A. :/
20. (Original post by Master S P)
Thanks, i found out they wont be too complicated so we could probably work them out anyway in the exam.

ANYBODY know about the number of sfs we should leave our answers to? its complicated =/
Ooh now THAT'S easy. Basically, whatever the number of sfs that are used in the data given to you in the question you should put your answer to. And if they use one value to 2 sfs and one value to 3sfs then put your answer to the lowest amount of sfs used - hence in this case put your answer to 2sfs.

For example -
'Electrons of wavelength 1.2 x 10^-10 are required to investigate the spacing between planes of atoms in a crystal.

Calculate the momentum of an electron of this wavelength.

momentum = planck's constant/wavelength

momentum = 6.63 x 10^-34(3 sfs)/1.2x10^-10(2sfs)

= 5.5 x 10^-24 kgms-1 (2 SFS)

See?

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