AQA Physics PHYA4 - 20th June 2016 [Exam Discussion Thread]

Hey!

What do you guys find the most challenging in this unit?
As in specifics not just topics,

Cheers!

Does anyone have any tips on how to revise for 6 mark questions? Whenever I attempt a past paper and then look at the mark scheme I'm absolutely baffled by the points that should be included in the answer, they are usually completely irrelevant and don't even make sense. The instructions are very vague so you could go on about different things for hours and still miss the mark scheme points? The latest question that completely caught me off guard was the 6 marker in Jan 2013 paper. They ask you to compare geosynchronous and low polar orbits. The points they included in ms were as follows
-Earth rotates relative to the orbit (why say something quite as obvious? It clearly orbits planet earth 😶)
-Many orbits with different radii and periods are possible (again don't see the point of saying that it's true but not relevant in any way to the question?)
-Satellite scans the whole surface of the Earth (not sure if that's even true they show the orbit in the diagram as fixed above the earth, by any means not an obvious point to make)

Where does any even find information like this to answer that? The textbook only contains a vague description of geostationary satellites...

Sorry for the rant a man had to get all this off his chest.

I understand what you mean by the first 2 points but the last one is quite essential I think... It is for the low polar orbit right? It rotates about the earth's north and south poles so eventually as it goes north to south, south to north while the earth is spinning, it would have scanned every point of the Earth eventually. It's kind of hard to explain it in words. I think this point is necessary because this is what makes it useful for the jobs it carries out, and a geosynchronous satellite isn't able to do this as it doesn't scan the whole surface.

I didn't actually think of it this way, thanks for explaining.
I really don't like the way knowledge is tested in the exam, even if you make a valid point there's a good chance you won't get the marks because it doesn't overlap with ms.

I didn't actually think of it this way, thanks for explaining.
I really don't like the way knowledge is tested in the exam, even if you make a valid point there's a good chance you won't get the marks because it doesn't overlap with ms.

This question only had a facility of 59%

https://60abffc9b401b1c0936e01291c15112cab0778c4.googledrive.com/host/0B1ZiqBksUHNYVFI1YlJBOWhwRW8/June%202014%20QP%20-%20Unit%204A%20AQA%20Physics.pdf

Could anyone help me with q15 and q19 from here please.
For q15, I tried working out V when its 1m away from P and 3m from Q. Then I subtracted them to find the resultant(is this the right thing to do?) and I'm not too sure what to do from here.

For q19 I tried using simeltaneous equations, but I dont think this is right, it gets way too complicated.

Thanks

Facility?

And am i beng dumb, it is A right?

Facility is the percentage of people who attempted the question who got it right, and yeah it's A. It just seems strange that 41% of people managed not to get it right even though it's probably as common sense as a question gets

For Q.15; Before the distance between the point M and each of the charges was 2m. now; the distance between P and M is 1m and the distance between M and Q is 3m. So, as the relationship between V and r is linear; you can treat this as simple addition. If x/2 + x/2 = 25 then x/1 + x/3 = 33.3v

For 19; the energy stored is being quartered when the voltage across is being decreased by 2. this ONLY happens when you halve the voltage across it; meaning that the voltage across the capacitor when it stores 400 microjoules is 2v. Use this with E=0.5*C*V^2 to find the capacitance; 200 microfarads.

I hope this helps.

Does anyone know where I can find worked answers for the multiple choice section on the exams (PHYA4/1)?

ahh ok thank you that makes sense. So when you need to find the resultant grav. potential, you add them?

Also in 19, when you say the energy stored is being quartered, wouldnt that be the case if it was being halved? Im assuming that when they say decreases by 2, its literally gone down by 2 volts, am I missing something out?

Thank you so much for clearing this up for me

Not necessarily; if at a point halfway between two identical large masses the gravitational potential is X then if you got rid of one of the large masses; the potential would STILL be X. If it is halfway between them, the potential due to each is the same. They don't add together.

For 19; The voltage across has decreased by two, but because of the fact (as the question states) that the energy has quartered; the decrease by 2 volts must have also been halving the original Pd across it (or else the Energy would've decreased by some other amount). So; we can deduce from what the question tells us that the pd before was 4v and the pd after must be 2v.

Not necessarily; if at a point halfway between two identical large masses the gravitational potential is X then if you got rid of one of the large masses; the potential would STILL be X. If it is halfway between them, the potential due to each is the same. They don't add together.

For 19; The voltage across has decreased by two, but because of the fact (as the question states) that the energy has quartered; the decrease by 2 volts must have also been halving the original Pd across it (or else the Energy would've decreased by some other amount). So; we can deduce from what the question tells us that the pd before was 4v and the pd after must be 2v.

Also I have created some more Past paper style questions if anyone has run out of the standard AQA past papers.