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

1. (Original post by MintyMilk)
My thought process went like this:

>imagine P and one of the charges as its own system to simplify things initially
>consider the potential at P in this simplified system and what that value would physically represent: a charge differential that causes a force
>now imagine what would happen if another one of those charges were added to the system
>it would double the quantity of stuff acting on P

I didn't do any calculations, but it was clear based on the numbers that 180 was there to represent one charge's potential, and 360 to represent 2 charge's worth of potential, so I guessed 360

As far as what you're saying about forces cancelling goes, I think that would represent the physical, resultant movement of P rather than having an effect on the raw potential the individual charges produce, maybe.
Ahh thats a clever way. How would you do it by calculation? I tried by using
v=-Q/4piE0 r but I didnt get 0
2. (Original post by boyyo)
Ahh thats a clever way. How would you do it by calculation? I tried by using
v=-Q/4piE0 r but I didnt get 0
I thought the point was to get 360?
3. (Original post by MintyMilk)
I thought the point was to get 360?
oh sorry thats what i meant lol. I dont get 360 when I add the electric potential of both charges
4. (Original post by boyyo)
oh sorry thats what i meant lol. I dont get 360 when I add the electric potential of both charges
Tried it for a while and I actually can't get it either haha

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Difference of 1.95 seconds to make up for, and it catches up by 0.05 seconds each time. 0.05 goes into 1.95 39 times, so 39
7. (Original post by MintyMilk)
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Difference of 1.95 seconds to make up for, and it catches up by 0.05 seconds each time. 0.05 goes into 1.95 39 times, so 39
Thank you so much ☺️

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8. (Original post by MintyMilk)
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Difference of 1.95 seconds to make up for, and it catches up by 0.05 seconds each time. 0.05 goes into 1.95 39 times, so 39
ANSWER WAS 38 LOL WHAT DO I DO

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9. (Original post by Cheesecake Ali)
ANSWER WAS 38 LOL WHAT DO I DO

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If Q (1.95s) is moving in phase with P (1.90s) then you must divide 1.90s by the difference between the periods (0.05s) = 38 oscillations
10. (Original post by Cheesecake Ali)
ANSWER WAS 38 LOL WHAT DO I DO

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(Original post by zaybun)
If Q (1.95s) is moving in phase with P (1.90s) then you must divide 1.90s by the difference between the periods (0.05s) = 38 oscillations
Yeah, I was typing from my phone and didn't double check which way around it was. Same logic still applies, just the other way around.

The logic is the important thing
11. Can anyone give a full list of the derivations required for this unit? The specification is quite vague for some of the topics.
12. (Original post by MintyMilk)
My thought process went like this:

>imagine P and one of the charges as its own system to simplify things initially
>consider the potential at P in this simplified system and what that value would physically represent: a charge differential that causes a force
>now imagine what would happen if another one of those charges were added to the system
>it would double the quantity of stuff acting on P

I didn't do any calculations, but it was clear based on the numbers that 180 was there to represent one charge's potential, and 360 to represent 2 charge's worth of potential, so I guessed 360

As far as what you're saying about forces cancelling goes, I think that would represent the physical, resultant movement of P rather than having an effect on the raw potential the individual charges produce, maybe.
Thanks for your reply, yes I think I was thinking about it as I would think about the resultant movement

http://filestore.aqa.org.uk/subjects...2-QP-JUN15.PDF

Why do we not use E=BANwsin(wt) here?!
14. What is the difference between these formulae? I know they are all equations to work out the energy stored in a capacitor but what determines which one you should use? As the energy stored in the second equation is proportional to the capacitance, but the energy stored in the third one is inversely proportional to capacitance, so how are we supposed to know which one applies and in what case?
15. (Original post by Alby1234)
What is the difference between these formulae? I know they are all equations to work out the energy stored in a capacitor but what determines which one you should use? As the energy stored in the second equation is proportional to the capacitance, but the energy stored in the third one is inversely proportional to capacitance, so how are we supposed to know which one applies and in what case?
Use whichever equation that has the values that you have. E.g. If I have values of capacitance and Voltage I'd use E=1/2CV^2. They're all essentially the same, all they've done is re-arranged and substituted C=Q/V into one of the energy equations to get the others.
16. (Original post by Alby1234)
Thanks for your reply, yes I think I was thinking about it as I would think about the resultant movement
hey, for this question you asked, did you try by calculation,using the Electric potential formula? I know mintymilk used a diffrent way.
17. Hi, can someone help me? I'm struggling to know when to use the actual direction of current (- to +) and when to use conventional current (+ to -). From what I can tell, we need to use the actual direction of current in electric field questions, and conventional current in magnetic field questions... can someone verify this or explain what to do? Thanks.
18. (Original post by grace_zzz)

http://filestore.aqa.org.uk/subjects...2-QP-JUN15.PDF

Why do we not use E=BANwsin(wt) here?!
From the examiner's report:

"Calculation of the emf induced when the coil wasrotated by 90° was required in part (b)(ii). This tempted many students to attempt their solution byusing the equation in the data booklet for a uniformly rotating coil, ε = BANω sinωt, which does notapply in this case. Correct solutions should have started from ε = ∆(NΦ) /∆t, and it shouldtherefore have been clear that the induced emf is derived from the change in flux linkage ratherthan just one value of flux linkage."

Not 100% sure what they mean (as usual), but my guess would be that the equation only applies if a coil is consistently rotating through 360 degrees, rather than just 90 degrees once.
19. (Original post by MintyMilk)
From the examiner's report:

"Calculation of the emf induced when the coil wasrotated by 90° was required in part (b)(ii). This tempted many students to attempt their solution byusing the equation in the data booklet for a uniformly rotating coil, ε = BANω sinωt, which does notapply in this case. Correct solutions should have started from ε = ∆(NΦ) /∆t, and it shouldtherefore have been clear that the induced emf is derived from the change in flux linkage ratherthan just one value of flux linkage."

Not 100% sure what they mean (as usual), but my guess would be that the equation only applies if a coil is consistently rotating through 360 degrees, rather than just 90 degrees once.
Ohh it now makes a bit more sense. Thanks alot!!
Another useful you tube channel for revision

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