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AQA Physics Unit 1 PHYA1 20th May 2013 watch

1. I'm leaving to go and have some lunch before the exam now, good luck to everyone taking this exam, hope you get the grade you want, and I look forward to comparing answers at around 3pm, again, GOOD LUCK!
2. (Original post by x-Sophie-x)
In this interaction, can someone please explain why strangeness is not conserved?

Sigma plus--> pi plus + n

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It decays via the weak interaction.
3. (Original post by jazzynutter)
This one is a bit weird and perhaps a bit tricky to explain, and I'd just like to point out that you are right in terms of Ek of the electron there - but this is a longer explanation as to why that happens:

Using the stair analogy again (I just made that analogy up on the spot btw ), this time think of the "ionisation shell" as the infinite shell, n= - you will most likely have seen this on those energy level diagrams in past papers, maybe without realising. Also on these diagrams, have you ever noticed that the energy levels are labelled as negative numbers? This is also important.
The ground state, usually n=1, will be labelled with a negative number - let's say that an atom has an n=1 of -11eV. This means that in order to leave the atom entirely, the atomic electron has to take at least 11eV from a bombarding electron in order to "get back to 0eV" (shoddy terminology there). From there, any extra energy will be converted to Ek of this electron - just as you said at the beginning! So in a sense, that discrete 11eV is still required, but this time excess energy can still be transferred.
Wow, that cleared things up amazingly, thank you!
Great analogy xD

I can't believe I never realized that!
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4. Hey guys, I am nervous about the exam but recently I've become more confident with circuits but I'm not 100% with the photoelectric effect etc. I also think the 6 marker might be a theory question as we have discovered that over the past exams, the pattern has gone experimental, experimental, theory, theory, experimental experimental etc and so last year was a theory so maybe..? Hope you guys are feeling okay about it too! ALSO look out for Mev instead of Ev in calculations... always catches me out! good luck everyone!
5. (Original post by x-Sophie-x)
So why does the incident photon/electron have to give the atomic electron an exact amount of energy?

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Are you sure it says 'exact'? Could it not mean that it needs 'at least' a specific value of energy ie. the minimum energy required to free the electron?
6. Could someone please give me an example of strong interaction where strangeness is conserves ???

In papers it always seems to weak interactions...

Thanks !
7. (Original post by x-Sophie-x)
Wow, that cleared things up amazingly, thank you!
Great analogy xD

I can't believe I never realized that!
I forgot to mention that n=infinity should actually be labelled as 0eV as that represents all of the space that the electron can occupy without being a part of the atom... hope that clears up my shoddy terminology about "counting back" to 0ev!!!
8. (Original post by simon105)
Are you sure it says 'exact'? Could it not mean that it needs 'at least' a specific value of energy ie. the minimum energy required to free the electron?

I personally believe the mark-scheme is wrong:

For a photon to cause excitation, it must be of specific energies (as a photon can lose all of its energy, not part of its energy).

For an incident electron to cause excitation, its energy must be more than or equal to the energy difference between the final and initial energy levels.

It's wrong to assume that the electrons require an exact/discrete amount of energy to cause excitation, because you'd make the assumption that the electron would come instantaneously to rest after the collision which would be odd for all excitations.
9. (Original post by Sukura)
When you convert MeV into joules, how do you know what to multiply it by? One past paper says 1.6x10-13 and another said 1.6x10-19

The past paper is: jan 2011 question 3

Because MeV is in mega electron volts which is eV times 10 power 6

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10. (Original post by Jorrey10)
Hey guys, I am nervous about the exam but recently I've become more confident with circuits but I'm not 100% with the photoelectric effect etc. I also think the 6 marker might be a theory question as we have discovered that over the past exams, the pattern has gone experimental, experimental, theory, theory, experimental experimental etc and so last year was a theory so maybe..? Hope you guys are feeling okay about it too! ALSO look out for Mev instead of Ev in calculations... always catches me out! good luck everyone!

I'll give it a go at explaining.
You have a metal, which structure is positive ions surronded by a "sea" of free electrons (Not entirely relevant, but nice to know).

If you "shine" some electromagnetic radiation (eg, light) on the metal, and vary the frequency, after you pass a certain frequency, you will begin to see electrons from the metal emitted (If this arrangement was in a circuit, a current would flow)

This is because, the electrons in the metal gained energy from the incident light, enough to leave the metal structure.

The fact that intensity has no effect on whether the photoelectric effect happens (If you shone light of low frequency, no electrons would be emitted no matter how long you shone the light for), it was concluded that light travels as "packets" of energy called photons.

Because emission only happened after a certain frequency, the energy of those "packets" (photons) has a fixed energy dependent on frequency (E=hf, where f is planck's constant)

One electron only interacts with one photon, so can only gain energy equal to hf. The work that is needed to be done to leave the metal structure, is called the work function, if hf > Work function, the electrons have enough energy to leave, if hf < work function, they do not. This is why there is a threshold frequency, as energy of the photon (that is absorbed by the electron) is dependant on frequency.

If hf >> Work function, the electron will have "extra" energy, which is uses as kinetic energy as it leaves the metal. Some electrons will be right on the surface of the structure, so will leave with all of their kinetic energy, while some will have to do extra work to pass through other electrons/ions etc, so will lose some of there kinetic energy. This is why "kinetic energy tends up to a maximum value" the max value being hf
11. (Original post by simon105)
Are you sure it says 'exact'? Could it not mean that it needs 'at least' a specific value of energy ie. the minimum energy required to free the electron?
It says exact.

A poster above cleared it up though
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12. can someone please explain January 2013 6a i) why is it 1/40 + 1/(10+5) for the total resistance??//
13. (Original post by jazzynutter)
I forgot to mention that n=infinity should actually be labelled as 0eV as that represents all of the space that the electron can occupy without being a part of the atom... hope that clears up my shoddy terminology about "counting back" to 0ev!!!
Haha no I got it, thanks very much.

The values are negative just because of the way that the energy is defined, right?

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14. (Original post by posthumus)
Could someone please give me an example of strong interaction where strangeness is conserves ???

In papers it always seems to weak interactions...

Thanks !
π- + p -> Σ + K+
π+ + n -> Σ0 + K+
π- + n -> Σ- + K0

15. (Original post by x-Sophie-x)
Haha no I got it, thanks very much.

The values are negative just because of the way that the energy is defined, right?

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Yeah, that isn't something to get wound up over and probably trips quite a few people up!
16. (Original post by hass456)
can someone please explain January 2013 6a i) why is it 1/40 + 1/(10+5) for the total resistance??//
10 and 5 kohm resistors are in series. 20 and 20 kohm resistors are in series.
10+5=15
20+20=40

But these are in parallel, so use the 1/R rule.

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17. (Original post by x-Sophie-x)
10 and 5 kohm resistors are in series. 20 and 20 kohm resistors are in series.
10+5=15
20+20=40

But these are in parallel, so use the 1/R rule.

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thanks a lot, and how would you work out 6b?, its getting on my nerves
18. (Original post by x-Sophie-x)
Okay thank you surely the voltage would change as well then?

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The p.d. through each component would, but not the overall p.d. of the circuit. If a battery supplies 12v, it will continually supply 12v (unless lost through internal resistance).
19. (Original post by jazzynutter)
π+ + p -> Σ + K+
π- + p -> Σ0 + K+
π- + n -> Σ0 + K-

Thanks !
20. (Original post by posthumus)
Thanks !
I typed them out in the wrong order!!!! Check my original reply, I changed them.

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