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

guys I really need your help. I don't know why C is correct and the other answers are invalid

Sort of 20-23, was just wondering if that was good enough, or if everyone was getting 25!


Posted from TSR Mobile

What are you guys generally scoring on the multiple choice section?!

Leaning on a generalization of my first solution. We let the three charges be $q_1,q_2,q_3$ and let the midpoints between $(q_1,q_2),(q_2,q_3),(q_3,q_1)$ be $x,y,z$ respectively. Then letting (wlog) the sides of the triangle be $2$, and as before letting the constant for $V\propto \frac{Q}{r}$ be $k$, we have $k(\frac{q_1}{1}+\frac{q_2}{1})=x\Leftrightarrow k(q_1+q_2)=x$, and similarly $k(q_2+q_3)=y$ and $k(q_3+q_1)=z$. Hence adding up we have $2k(q_1+q_2+q_3)=x+y+z$, giving $q_1+q_2+q_3=\frac{x+y+z}{2k}$. (Call this equation 1)

Now, since the figure is an equilateral triangle, its area is $\frac{s^2\sqrt{3}}{4}$ where $s$ is its side length (I leave this as an exercise to the reader), therefore since our triangle has s=2 then plainly its area is $\sqrt{3}$. Then from the formula $\frac{s^3}{4R}=\frac{s^2\sqrt{3}}{4}$ (where R is the circumradius, and both sides give the area of the triangle) we have $R=\frac{2}{\sqrt{3}}$. Therefore, at the center of the triangle the potential is $k\frac{q_1}{R}+k\frac{q_2}{R}+k\frac{q_3}{R}=\frac{k\sqrt{3}}{2}(q_1+q_2+q_3)$. Then substituting equation (1) into this last expression yields the answer $\frac{k\sqrt{3}}{2}(\frac{x+y+z}{2k})=\frac{\sqrt{3}}{4}(x+y+z)$.

I was doing a past paper, last quesiton on june 2011 and it said that in a transformer a high voltage introduces insulation problems and raises safety issues.
I dont understand how a high voltage can affect insulation can anyone explain?

@CD: Np

This is also why a fuse is used in a transformer.

I see. Is it a fuse in the primary coil I take it? Or both?


Posted from TSR Mobile

~22 at the moment. You?



Thanks for the explanations



I believe the question (albeit ambiguously) is referencing the need for a step down transformer before voltage is supplied to the home. For domestic applications, a much lower voltage is needed to be considered safe and non-hazardous to humans. For many applications, if a much higher voltage surges through them then they will break, making them unsafe.


Posted from TSR Mobile

Yes, primary coil. There was a question about this in a past paper -- from 2013 papers IIRC.

So a fuse is used in the primary coil if the current gets too high in the secondary?

Yes, primary coil. There was a question about this in a past paper -- from 2013 papers IIRC.

Keep an eye on how much time you're spending on each half - you shouldn't be exceeding the recommended 45 minutes on the multiple choice I'd say.

In the multiple choice half, look for proportionality in questions. Also look for contradicting statements (particularly on true or false type questions) - they put these in to speed it up, but they're often missed.

Don't forget to leave time to fill in the multiple choice answer sheet! You might forget about this because in mocks you probably just circle the answer on the paper, but there is in fact a separate sheet to fill in.

Obviously never leave multiple choice questions blank - you've got a one in four chance of getting a question right even if you've got absolutely no clue (if you've got the time though, aim to make educated guesses where possible!).

Decide whether doing multiple choice or written part is best for you, and whether starting or finishing with six markers works better. Remember that you don't have to do the paper in order! (May seem obvious, but a lot of people try and do everything in order and lose a lot of time when they get stuck).

If you get stuck, move on and come back to it. Get all your guaranteed "I can definitely do this right now" marks before you try and figure out anything that's confusing you.

Yeah I thought so, just wanted to make sure I remembered correctly haha.


Posted from TSR Mobile

Yeah, it's also a good idea to do past papers twice (or more if you have time) as questions are repeated each year. (Note: Just checked: it was JAN12, not 2013.)

Thank you!! Did not know there was an answer sheet for mc but makes sense.


Posted from TSR Mobile

Yeah, it's got 1-50 on it for some reason with A, B, C, D circles next to each one and you colour in the right little circle for each question I think.

For those people who insist on doing the paper twice


Posted from TSR Mobile

Really, really hope Milikans oil drop experiment comes up this year as it's my strongest "difficult" topic throughout the whole course

That being said, I bet you guys have no idea what I'm talking about considering I think I'm the only one to be doing option 2D

Also I should have posted this on the PH5 thread but Im not watching that thread

Really, really hope Milikans oil drop experiment comes up this year as it's my strongest "difficult" topic throughout the whole course

That being said, I bet you guys have no idea what I'm talking about considering I think I'm the only one to be doing option 2D

Also I should have posted this on the PH5 thread but Im not watching that thread