AQA Physics PHYA4 - Thursday 11th June 2015 [Exam Discussion Thread]

Original post by sykik

So what do you suggest... How to tackle these questions... should i get a new book or something?

Buying a new book is another challenge. I've got more textbooks than i can think. It's better to stick with the official textbook because other textbooks are not linear as the official one and so to find something is time consuming that you end up forgetting what you were looking for. I have the advanced physics textbook which is supposed to have all the content necessary for all the specification but even this fails to include all the details. I think, at the end if the day, you have to put your own reasoning on solving questions.
Again as i said, the answer is always within the question. First think about what you have and then question as to what you need to do to get to that answer. Sometimes it's difficult but that would be the only way to get away with it. And don't worry about it, it's a very rare occurrence

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Reply 1622

Original post by CD223

Me too. I don't know what the correct answer is. About 50% of people I've spoken to said no current, 25% said C2 to Y, 25% said Y to C2.

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Yeah i think no current is the correct answer as when V2 is maximum C2 is fully charged and so there is no current around the circuit. I actually chose no current but i had a different reasoning in mind

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Reply 1623

Original post by CD223

Me too. I don't know what the correct answer is. About 50% of people I've spoken to said no current, 25% said C2 to Y, 25% said Y to C2.

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Pretty much everybody said no current that I spoke to, I had written those marks off but maybe I will get them :biggrin:

The exam just got off to a horrible start for me because my best fit line made it so that my tangent at V1=V2 had already turned and I got a tiny negative answer for the gradient. Wasn't any point changing it all because by that point I'd drawn various lines on my graph and I knew I'd probably end up with a similar result if I redrew the tangent. I don't think they do follow through marks so I've instantly lost about 4-6 marks.

Reply 1624

OP

Original post by JJBinn

Pretty much everybody said no current that I spoke to, I had written those marks off but maybe I will get them :biggrin:

The exam just got off to a horrible start for me because my best fit line made it so that my tangent at V1=V2 had already turned and I got a tiny negative answer for the gradient. Wasn't any point changing it all because by that point I'd drawn various lines on my graph and I knew I'd probably end up with a similar result if I redrew the tangent. I don't think they do follow through marks so I've instantly lost about 4-6 marks.

Hmm. I knew I was wrong on the capacitor questions. Section A was pretty solid in terms of theory questions for the SHM spring experiment too. I just want to see the MS for both sections haha.

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Reply 1625

Original post by CD223

Hmm. I knew I was wrong on the capacitor questions. Section A was pretty solid in terms of theory questions for the SHM spring experiment too. I just want to see the MS for both sections haha.

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I messed up on the 3 mark question in task 1 as i failed to see the how to link two different equations without dealing with spring constant

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Reply 1626

Original post by CD223

Hmm. I knew I was wrong on the capacitor questions. Section A was pretty solid in terms of theory questions for the SHM spring experiment too. I just want to see the MS for both sections haha.

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I didn't think section A was so bad. I might've messed up though on one of the 3 markers where it said to assess the student's prediction or something. I basically did a load of calculations with my own data and compared and wrote some stuff, the numbers all worked but I don't know if that's what they're looking for. Probably wasn't specific enough on describing how I reduced the parallex error at the end either, they're harsh mark schemes.

I'm sure you've done fine, your answers to section B seem reasonable. I got 1.54% for the % error at the end and don't see where I'd've gone wrong though. The uncertainty was l1 and l2, so I worked out the percentage uncertainty in both, added them together and then divided it by 2 as they were square rooted.

Reply 1627

ubisoft

3

Original post by CD223

The capacitor stores half the energy of the supply. Your derivation assumes all the energy from the supply is stored by the capacitor, which isn't the case.

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Why is that?

Reply 1628

sam_97

6

Original post by JJBinn

I didn't think section A was so bad. I might've messed up though on one of the 3 markers where it said to assess the student's prediction or something. I basically did a load of calculations with my own data and compared and wrote some stuff, the numbers all worked but I don't know if that's what they're looking for. Probably wasn't specific enough on describing how I reduced the parallex error at the end either, they're harsh mark schemes.

I'm sure you've done fine, your answers to section B seem reasonable. I got 1.54% for the % error at the end and don't see where I'd've gone wrong though. The uncertainty was l1 and l2, so I worked out the percentage uncertainty in both, added them together and then divided it by 2 as they were square rooted.

For the question about how the student calculated the DVD mass I did exactly the same as you, I just used my data to show what the student would do with theirs. The DVD mass with my data came out to be 12.5g (it said to show it was about 13g).

For section B, I think you had to add the absolute uncertainties of l1 and l2 (you can't add % uncertainties unless you are multiplying or dividing).

Reply 1629

Original post by sam_97

For the question about how the student calculated the DVD mass I did exactly the same as you, I just used my data to show what the student would do with theirs. The DVD mass with my data came out to be 12.5g (it said to show it was about 13g).

For section B, I think you had to add the absolute uncertainties of l1 and l2 (you can't add % uncertainties unless you are multiplying or dividing).

Yeah mine came out similarly to yours, it definitely rounded to 13 too. I hope we get the marks haha.

Hmmm I see, I probably did get it wrong because lots of people got a value of 2-3. There was too much room for error in that paper and not enough marks imo.

Reply 1630

Original post by JJBinn

Yeah mine came out similarly to yours, it definitely rounded to 13 too. I hope we get the marks haha.

Hmmm I see, I probably did get it wrong because lots of people got a value of 2-3. There was too much room for error in that paper and not enough marks imo.

There was a completely theoretical approach to that question with which you didn't need any measurements from yours to calculate the mass of the DVD.

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Reply 1631

OP

Original post by Mehrdad jafari

There was a completely theoretical approach to that question with which you didn't need any measurements from yours to calculate the mass of the DVD.

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Yeah my calculation for the 3.1% and 3.3% increase was cancelled down into:

[br]\left(\left(1.031\right)^{2} \times m\right) - m[br]

And

[br]\left(\left(1.033\right)^{2} \times m\right) - m[br]

Where m = 200g

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Reply 1632

Original post by Mehrdad jafari

There was a completely theoretical approach to that question with which you didn't need any measurements from yours to calculate the mass of the DVD.

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I know but it's possible they might accept either method

Reply 1633

OP

Original post by ubisoft

Why is that?

Although you don't need to know..

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capeng2.html

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Reply 1634

Somniare

Original post by CD223

Ah I just use past papers. I've done all bar 2013 papers now.

Have you got any predictions for the six marker?

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Wow =O even though the exam is still so (relatively) far away?

Hmmm, no particular predictions, but our physics teacher gave us a nice list of past six markers with a few possible future ones. I'm just writing out answers to those and comparing with friends which I find is quite helpful. How about yourself?

Reply 1635

sykik

Original post by Mehrdad jafari

There was a completely theoretical approach to that question with which you didn't need any measurements from yours to calculate the mass of the DVD.

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Cheers bro... Yeah you're right it must be rare to ask these kind of questions as all the past papers i have done seem reasonably easy

Reply 1636

OP

Original post by Somniare

Wow =O even though the exam is still so (relatively) far away?

Hmmm, no particular predictions, but our physics teacher gave us a nice list of past six markers with a few possible future ones. I'm just writing out answers to those and comparing with friends which I find is quite helpful. How about yourself?

Oh nice! Would you mind sharing what his predictions were?

I'm left with PPs and the revision guide :smile:

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Reply 1637

OP

Usually I'm okay with these questions, but this one has stumped me:

Jan 2013 - any takers?

ImageUploadedByStudent Room1431939265.527632.jpg

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Reply 1638

Somniare

using M = Mass of moon, m = mass of earth, R = radius of moon and r = radius of earth

Gm/(r^2) = 6 GM/(R^2) (gravitational field strength)

0.6 m/(4/3*pi*r^3) = M/(4/3*pi*R^3) (density)

We can then cancel out to get

m/r^3 = 0.6 M/R^3
m/r^2 = 6 M/R^2

Dividing the second set by the first set gives

r = 6/0.6R
r/R = 10

Reply 1639

Somniare