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

I think I might have worked it out. It might just be a matter of semantics. They say 'when the switch is closed', which implies you're working with the circuit at that very moment, when in actuality they mean some length of time has passed since that moment, and thus the current would have stopped flowing and the charges stored on either capacitor would be the same by then.

Do you think that's it?

I'll try explaining it more.

Ok so when the first switch is closed the capacitor 4.5 is charged yes? Forget about the rest of the details focus on the charge. So it has a charge of 2.835 x 10 to the power of minus 5. Now when the first switch is opened again, and the second switch is closed the only way the second capacitor can be charged or have a PD across it is through the first capacitor. But since instantly (like 0.000001 second) when the switch is closed there is no current so the total charge is the charge of the first capacitor +0. And so the pd across both the components will be the total charge divided by the capacitance added (since they are still in parallel).

Hope that helps.

I think it was more the, are they really in parallel?

But thinking about it, there's still a loop behind, just not a closed one, so maybe it is just parallel

I think it was more the, are they really in parallel?

But thinking about it, there's still a loop behind, just not a closed one, so maybe it is just parallel

I'll try explaining it more.

Ok so when the first switch is closed the capacitor 4.5 is charged yes? Forget about the rest of the details focus on the charge. So it has a charge of 2.835 x 10 to the power of minus 5. Now when the first switch is opened again, and the second switch is closed the only way the second capacitor can be charged or have a PD across it is through the first capacitor. But since instantly (like 0.000001 second) when the switch is closed there is no current so the total charge is the charge of the first capacitor +0. And so the pd across both the components will be the total charge divided by the capacitance added (since they are still in parallel).

Hope that helps.

You can't think about series and parallel with capacitors based off of how the circuit looks like you can with normal electronics, you have to think about whether the p.d.s are the same (parallel) or different (series)



But that's like saying when the first capacitor is connected to the the cell, its p.d. is instantly 6.3V, but it takes time for its p.d. to rise.

I mean, looking at it, one part doesn't look like it is but I mean when the question tells you it's parallel you be damn sure it will be parallel. Lol.

Did quarks join to form protons and neutrons before matter and antimatter annihilated or was it the other way round (as far as the spec/mark schemes are concerned)?

Yes ok thats one way of looking at it but I explained it without over complicating. Heres the real reason, when the S2 is closed the charge from the first capacitor is the only charge BOTH of the capacitors have to rely on. Assuming no energy is lost or whatever, even though the charge does get distributed in whatever ratio, the total charge will still be the charge of the first capacitor even before the switch S2 was closed. So V equal total charge divided by 6.

Thanks for everything but I still don't get it. It's my fault, not yours - I'll see if I can understand from a physics teacher tomorrow.

Did quarks join to form protons and neutrons before matter and antimatter annihilated or was it the other way round (as far as the spec/mark schemes are concerned)?

Let me know what your physics teacher says mate I would like to know this haha, hate little things like this that im unsure about.


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Sorry to be a bother but could you send your version for the evolution of the universe? I've got one from a mark scheme but it didn't have about matter and antimatter annihilating, so I'm starting to think there's other things that I may be missing too.

Turns out they are in parallel

I didn't really understand his explanation, so I can't explain it back to you. Sorry.



Universe begins for some bizarre reason
Sea of quarks and leptons
Quarks join to form protons and neutrons
Matter and antimatter annihilate, producing a huge amount of photons and an excess of matter
Protons and neutrons form H and He nuclei
Temp cools enough so that electrons join nuclei and form H and He atoms, Universe becomes 'transparent' and photons can travel through it (origin of CMB)
Stars, galaxies and solar systems form
Universe has cooled to 3K by now, CMB photons' wavelength have increased into microwave spectrum due to expansion of the Universe.

Why would it be that matter and antimatter annihilate after protons and neutrons have formed from quarks? Wouldn't the quarks and antiquarks just annihilate in the quark stage?

PS
In the mark scheme I think this would be irrelevant anyway!

Also
For capacitors thing I think I get it but still not 100%. So once you've charged up the first capacitor, you open the first switch and then close the second switch. Hence initially the two capacitors have different charges on them and so I have concluded this would mean they must be in parallel. Not a very satisfying explanation but I hope you can kind of see what I'm getting at.


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Yeah I thought that but l'Evil Fish said otherwise. I think that's right though, but I don't know if that's even in the spec or if it matters (no pun intended)

But what happens when charge starts to flow? Is it kind of intermediate between series and parallel, becoming parallel when it's reached its final p.d.?

I'd have thought quarks would have to fuse to make matter and anti matter to annihilate

I'd have thought quarks would have to fuse to make matter and anti matter to annihilate

Yeah I thought that but l'Evil Fish said otherwise. I think that's right though, but I don't know if that's even in the spec or if it matters (no pun intended)

But what happens when charge starts to flow? Is it kind of intermediate between series and parallel, becoming parallel when it's reached its final p.d.?

To be honest I don't really understand it properly as to whether it's parallel or series, that was the best sort of explanation I could come up with as to why it's parallel that's all haha.


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Could someone please explain to me the principles of ultrasound imaging?