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

Done for the day. Hoping someone can help me with these two

Question 15
The answer is C. Excuse the "May" - using my college planner haha!



Question 18
The answer is C.

$[br]V = \dfrac{Q}{C}[br]$

and

$[br]E= \dfrac{1}{2} CV^2[br]$

Therefore
$[br]E \alpha V^2[br]$

and

$[br]V \alpha Q[br]$

If the charge Q is increased by 50%, it comes $1.5Q$. As such, as V is proportional to Q, V becomes $1.5V$.

This follows that, as E is proportional to $V^2$, E becomes $\left(1.5\right)^{2} E$ which is the same as $2.25E$.

For the electric field one, you gave the right answer but I think you misread the info on the question. You use F=EQ right, because 4200 is the electric field strength? As it's given in Vm^-1 which is the unit for E? F comes out as 2.016x10^-15 doing this so C is still right.

The direction of the magnetic field lines is the direction a free positive charge would move along.

Hence, they point towards a negative charge in this case.

The proton accelerates to the right as it is attracted to the negative charge. It's acceleration increases as the distance between it and the negative charge decreases, and due to the electric field strength equalling:

$[br]\dfrac{Qq}{4\pi {\epsilon_0} r^2}[br]$

This means that the proton's acceleration increases as it gets closer to the negative charge.

Conversely, the electron accelerates to the left as it is repelled by the negative charge. It's acceleration decreases as the distance between it and the negative charge increases, and due to the electric field strength equalling:

$[br]\dfrac{Qq}{4\pi {\epsilon_0} r^2}[br]$

This means that the electron's acceleration decreases as it gets further away from the negative charge.

Hello

I was stuck on this question as well and I wanted to know if you were suppose to write electric field lines instead of magnetic lines or is it really magnetic field lines. If so, why?

Thanks for your time

Lol I'm making mistakes all over the place! Thanks for the correction. Yes, it is electric field lines


Posted from TSR Mobile

Ah its alright

Done for the day. Hoping someone can help me with these two

looks simple but i cant get to the answer

and this one

They have a lot of $J$'s there, so it's probably best to start working from a formula that relates energy to electric field strength. This immediately reminded me of $\delta W=Q\delta V$, so dividing through by some length $L$ gives $\frac{\delta W}{L}=Q\frac{\delta V}{L}$.

Now, $E=\frac{V}{d}$ so we know that the units of $\frac{\delta V}{L}$ are the same as field strength. The units of $\frac{\delta W}{L}$ are just $Jm^{-1}$ and the units of $Q$ are $C$.

So we can think of the formula $\frac{W}{L}=Q\frac{V}{L}$ as $\frac{W}{L}=QE$ where E is field strength. Therefore $E=\frac{W}{QL}$, and plugging in the units on the right hand side we get that $units of field strength = Jm^{-1}C^{-1}$, which contradicts $\bf D$. So the answer is $\bf D$.

Also solvable more quickly (which is what they probably expect, as this is the multiple choice) by noticing that A and D contradict each other, so it must be one of these. E = V/d and voltage can be given as joules per couloumb (V = WQ, from the AS section), so A is correct and D is the answer.

Lol my method was to write out each implied formula like so:



I prefer your "elimination by contradiction" method though. Do you use that method often?


Posted from TSR Mobile

I use it when I see it because it's usually the fastest way to get a question done (but I'm not necessarily great at spotting them!). I do seem to end up just going through the formula sheet when they ask about units though :/

Ah right haha! Just wondered if you ever used that in a true/false question too - must make it much easier.


Posted from TSR Mobile

That is one of the places they're most likely to pop up! Unfortunately for me it always seems to be something like magnetic fields, so I can see one must be wrong but I have no clue which one

I hate how hard some questions can be. There's no uniform difficulty from year to year haha.


Posted from TSR Mobile

Yeah some years you get a pretty nice mc section with only one or two questions that you make an educated guess at, but other years you end up just picking a random letter for five questions or more! I think it more depends on how much it pushes you for time though than them being impossibly hard questions.

Agreed. It's annoying how it takes ~45 minutes for 25 marks.


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