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

G485, June 2014, Question 1b ii

Lol lets hope that is easy to see...

The point X when its closest to A the force between them is max (since X is also positive i believe) so the repulsive force is max. Also take in account of the direction, it is pushing X to the right so we take right as positive. So the graph starts from max and goes diagonally (curvy, negative gradient) towards 0 at the point where X usually was. (They mention that electric force is zero or something). As X moves away from A it comes in contact with the force from B which is directed LEFT, so negative. Graph follows from that point (still negative gradient) and ends at minimum(max).

Thanks extremely helpful. I will rep you when I can! (Good luck tomorrow for G482 )

No worries and thank you. I edited the post don't forget to read the last line.

Oh I see, yes, it is easier if you visualise it. If you want I can help you with that question?

Only done 2, A*s in both

Even with this post I think I'll get slated lol

You assume everyone is horrible on here haha!
How did you revise for this subject?

.r

Can anyone explain the answer to this question please? Finding the electric field strength between two point charges, at a point which does not lie on the line joining the charges.
Thanks!!

Question 15a)from https://ad57adc4f6eb5ea42b541057f16290e611d4e42b.googledrive.com/host/0B1ZiqBksUHNYcHRSNUJpeFpqNXM/June%202014%20(IAL)%20QP%20-%20Unit%204%20Edexcel%20Physics.pdf

What's the answer? I've got 0.047 N/C and could explain to you how I got that.
EDIT just realised I've done it wrong, I'll have a look again

Hi mate, the way they do it is:

Find the field strength of one of the charges at a distance 33cm (E=kQ/r^2)
Then multiply this by cos49(which is the bit I don't get? Getting the horizontal component?)
And then multiply this by 2 (I guess because the fields add to give a resultant field of double the magnitude)

answer: 0.043 N/C

Tried my best to explain it above, don't really get it myself. Would be good if someone could explain it!

Thanks! That does make sense, about the vertical components cancelling out, but I wouldn't have guessed to do that.

I doubt we would be given one that hard though

For d.i) on that same question, why is the total capacitance just the sum of the capacitors? I'd have thought that because the p.d.s across them are different at t=0 that means they're in series and therefore you have to use the inverse addition formula.

I'm still confused about 1.d.i in June 2010, I understand that the capacitors are in parallel before the first one is charged, because they have the same p.d across them (0V). Then, the first one is charged and has a p.d across it. Then it's connected to the second one and you need to find the total capacitance at this instant. One has 6.3V, the other has 0V. Therefore they are in series. Yet to find the total capacitance you have to simply add them as if they were in parallel. Why?

Yeah I've noticed same problem and I think they are in series too. I realised the examiners wanted parallel however as there was only one voltage in the last question hence same voltage must have been across both.

Examiners report is patronising and says 'some candidates thought the capacitors were in series despite the question clearly stating they are in parallel'. The problem is however that statement and the description afterwards are contradictory! OCR were probably still trying to learn their own specification at this point, it was the first exam after-all and they are usually poor being the first in a series haha.


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Mate OCR are *******s...

I don't care if you tell me what's happening in the question, I need to understand the physics if I'm going to answer the question properly.

I'll ask my physics teacher tomorrow to see if he can make sense of it.

Well for 2 marks and a 1 mark beforehand it was obviously just a quick Q = CV sub in

Didn't think of it your way until you had said, but I see your point

Yeah I've noticed same problem and I think they are in series too. I realised the examiners wanted parallel however as there was only one voltage in the last question hence same voltage must have been across both.

Examiners report is patronising and says 'some candidates thought the capacitors were in series despite the question clearly stating they are in parallel'. The problem is however that statement and the description afterwards are contradictory! OCR were probably still trying to learn their own specification at this point, it was the first exam after-all and they are usually poor being the first in a series haha.


Posted from TSR Mobile

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?