OCR Physics G485 - June 2013 Unit 5 (OFFICIAL THREAD)

Hmm, maybe we have different textbooks, there are at least 2 main ones for A2 I think. There usually seems to be a mark for mentioning it in the papers so that's why I included it.
My textbook says this:

Principle Contents of the Universe:
- Interstellar Dust & Gas
- Electromagnetic Radiation
- Black Holes
- Dark Matter
- Galaxies & Solar Systems

- The universe is almost uniformly filled with microwave radiation with a corresponding temperature of 2.7K
- These are photons left over from an early stage of the big bang.
- As the universe expanded, the waves were stretched and became cooler.
- There are tiny fluctuations in this temperature which have been predicted and observed. This is where the first galaxies formed.

Hubble's law can explain it. Recessional speed of galaxy is proportional its distance from earth.
- Since the universe is expanding it is not static, and it is not infinite. Also light hasn't reached us from distant galaxies.

ok thanks ]



I asked you "describe the principal contents of the
universe, including stars, galaxies and
radiation;"

You didn't describe, you just stated.


for the question "describe and explain the significance of the
3K microwave background radiation" Are you sure we have to not talk about that it provides evidence the big bang?
And how does your answer, answer my question.



for the question "Interpret Olbers’ paradox to explain why it
suggests that the model of an infinite, static
universe is incorrect", I did not get your answer. Since the universe is expanding, how does it prove that it is not static, and it is not infinite.

Thanks in advance.

EDIT: Who and why negged me?

hrbrox is saying that a build up of electrons at that negative plate repels electrons away in the positive plate.

I said that the positive terminal attracts electrons from the positive plate.

We are both saying the same thing in effect, mine was more or less from the mark scheme but hrbrox explains it in more detail.

Hey, I was wondering if anyone could please explain the difference between the photoelectric effect, the compton effect and pair production? Thank you.

There are 3 ways an X-ray can lose intensity:
Photoelectric effect: Photon gives all of its energy to release an electron from an atom. This is the main one used in CAT scans. It is for photons with energy <0.1 MeV
Compton Scattering: Photon loses some of its energy to remove an electron. The x-ray photon is then scattered with a longer wavelength & the electron goes in a different direction to conserve momentum. This is for photons with energy 0.5-5 MeV
Pair Production: Photon travelling through an electric field of a nucleus suddenly forms an electron/positron pair. This is for photons with energy >1.02 MeV

Could some please help with this question. I understand part i but in ii it asks you to draw the direction of the force. The answer is "towards Q" which has obviously been found out from Fleming's left hand rule. But I was under the assumption that you reverse the direction for current when it involves electrons instead of positive charges which brings the force's direction as downwards.

Could someone please explain why you don't reverse the current's direction in this case?

Sorry if this is a stupid question but this is the part of the spec that I struggle the most with haha.

This question is confusing! I think I see the problem though.
You're correct, you reverse the direction for current, however, the question seems to be asking for the direction of the force on the electrons.
If the current is then going up to P, the direction of the force on the electrons must be pushing them down to Q.

Hey, I was wondering if anyone could help me with the questions I've attached. I somehow managed to get everyone on this page wrong lol. I really don't understand how to get the answers in the mark scheme.

I understand a) i) now. However, I don't understand why a)ii) is 3? I would have thought it would be 0 as well?

Also for b) i I don't know how they got the answers at all.

I'm stuck on some Physics 4 Questions can some of you guys pop to the Physics 4 Thread please. Or am I allowed to ask it here if anyone is willing to help?

Another confusing question Is this an older past paper? Some of this may not be 100% correct but I'll try!:
-Initially, when the capacitor has 0 charge, it acts like a short circuit (with 0 resistance) so all of the current rushes towards the capacitor and ignores the 2 kilo-ohm resistor. This means A1 gets no current so reads 0. Whereas A2 will get all of the current present, since at this point there is effectively a 4 k-ohm resistor and 12 volts this means that A2 reads 12/(4x10^3)=3mA.
- Then when the capacitor has fully charged, the current can no longer do anything at the capacitor since there is nowhere for it to go over there, this means all of the current will be flowing the other way past the capacitor through the 4k-ohm resistor & the 2k-ohm resistor. So A2 will get a reading of 0 and A1 will read 12/[(4x10^3)+(2x10^3)] = 2mA
This page sort of explains it in the charging and discharging sections http://electronicsclub.info/capacitance.htm but there is some really complicated stuff to do with Resistor-Capacitor (CR) circuits so I wouldn't look into it too much.



Yeah sure, either post it here or if you put a link to the unit 4 thread I'll do my best to answer some questions. How did you get banned?

I hate to be a smart arse, but one of you are saying electrons at positive plate are repelled and the other is saying the electrons at positive plate are attracted. Which is it?

I was just wondering if anyone has anyone come across the paper and mark scheme for January 2013?

I have it but uploading it to TSR will get me a warning, if you want (and anyone else for that matter), you can send me a PM and I can provide a download link