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

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How many marks was the damping question, thanks
Original post by l1lvink
Just heard back from my school who called AQA; they have realised that its a mistake!!


Does this mean they'll give three marks to everyone for it then?
Original post by DomRauba
How many marks was the damping question, thanks


4 in total
Original post by DomRauba
How many marks was the damping question, thanks


4 marks
Original post by Mehrdad jafari
Good luck with phys5 guys. Im off for core 3 tomorrow


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Yup me too Core 3 (MEI)
I think it's gonna be a walk in the park like the past papers :biggrin:

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Right, thoughts on this?

There's the most damping when the ring is not on the paper. As period and velocity of oscillation only depend on l and g, the velocity is unaffected. As air resistance depends on surface area (unaffected pretty much) and velocity, the damping force stays the same. However a = f/m, so as mass is increased the deceleration of the bob is reduced, so the damping is lighter with the ring and heavier without.
Original post by EmiratesCaptain
Thought it was good, what was the graph for current supposed to look like? I had the initial current below the first graph but just the same shape which im pretty sure is wrong


I didn't really understand what that question wanted me to do. It said the same capacitor was charged and then discharged, but, it never stated if it wanted you to draw only the discharge or the entire thing? Of charging, then discharging. It was three marks, so I assumed you had to draw something more complex than a simple exponential curve. I figured out the current would only go up half-way then what it was originally. But apart from that, there was nothing really worth 3 marks it looked like...
Original post by NEWT0N
Did you get any information how they're going to account for it by any chance?


Original post by CD223
There was a typo?!!? What are AQA doing about it??


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So far only that we won't get penalised for it. Usually when they have a typo they allow the answer that it was meant to be and the answer that was correct as per the question, but with this, I don't see how they could do that. So I'd imagine that any suitable line (?) will get the marks.
Original post by NEWT0N
They'll probably not consider 1 mark, so the paper would be out of 74.


But how can they? when they're question is wrong? it was out of three so surely it would be wrong to expect students to get the wrong answer?
Original post by chizz1889
Q is negatively charged so work is done by the positive test charge

YES - although that's like 1 mark lol
Does anyone actually know what the right answer was for the damping question?
Original post by StarvingAutist
Yeah I see. I didn't write about T in the question anyway, given that the SHM equations are basically irrelevant.


You can read about it in the attached pdf if you like. I did this just before Christmas. You'll need to have done further maths to follow the maths it but I think it's quite interesting.

It's also difficult to believe that there was a typo on one of the questions, given that the papers go through so much quality control! It sounds like not everyone had the typo because looking at some of the first posts they did the sketch at half the other curve at all values. It might have been a printer error.

I ended up redrawing the time axis on the grid in the space above the original curve (partly because I'd cocked up the curve the first time, partly because the curve wouldn't be on the same scale as the other one), drawing what it would have been had I not had the typo, and then wrote notes explaining that the original current should be 1/2000 times the other original current and that the 'half life' should be 2000 times longer!
SHM_.pdf145.2KB
Original post by QueNNch
The question asked for the number of electrons on the NEGATIVE plate ONLY.

As each capacitor plate shares equal number of electrons it is halved.


There is a potential difference between the plates they cant both have the same charge
Original post by JizzaStanger
You can read about it in the attached pdf if you like. I did this just before Christmas. You'll need to have done further maths to follow the maths it but I think it's quite interesting.

It's also difficult to believe that there was a typo on one of the questions, given that the papers go through so much quality control! It sounds like not everyone had the typo because looking at some of the first posts they did the sketch at half the other curve at all values. It might have been a printer error.

I ended up redrawing the time axis on the grid in the space above the original curve (partly because I'd cocked up the curve the first time, partly because the curve wouldn't be on the same scale as the other one), drawing what it would have been had I not had the typo, and then wrote notes explaining that the original current should be 1/2000 times the other original current and that the 'half life' should be 2000 times longer!


I think it's more likely that some people (like me) just saw what they expected to see than only some papers having the typo - and they've accidentally put things that aren't even on the syllabus into an S2 paper before, things like typos do happen somehow! It does seem highly likely that they messed up somehow.
Original post by DanielWall96
There is a potential difference between the plates they cant both have the same charge


Yup, all the electrons move to one plate, the other one is void of conducting electrons.
you don't halve the electrons lol
Original post by Tomh97
Does anyone actually know what the right answer was for the damping question?


Yeah it was: damping is less with the ring.

Explanation: Cross sectional area of the cone is unaffected by the ring, so the drag force due to air resistance experienced by the cone and the cone with the ring is the same. A force results in a change in momentum, so both cones have the same change in momentum due to drag, however the cone with the ring has greater mass. The same change in momentum but a larger mass means a smaller reduction in velocity due to air resistance. This means a lower rate of reduction of velocity for the cone with the ring, and so it loses kinetic energy at a lower rate.

IMHO they didn't give enough space or marks for virtually any of the descriptive type questions.
Ah no I halved the electrons :/
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Does anyone know what were the distances for the work done potential question?
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Original post by NEWT0N
So no halving? :smile:


Woo. One less mistake :biggrin:


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