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

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Average grade boundary for A*?
Original post by AR_95
Knowing how ***** the exam boards are, they'll probably go out with a bang

Doesn't help that all the other exams people are saying have been the hardest ever. I expect them to drop the same bombshell this time


Too right, I don't understand how uni requirements are getting higher and the exams along with the boundaries are getting harder. I understand the whole thing about separating the smarter students but this is just ridiculous
Original post by Kennethm
literally 30 seconds unless you feel like you're onto something or mid-calculation.
That seems to be the most efficient.


What do you usually get on papers then?
Original post by JIBBYMALIK
Average grade boundary for A*?


Last year it was 61/75, the highest its ever been so I feel it will be 60 or 59
Original post by fruity97
Can someone please help with question 4 c(I) http://filestore.aqa.org.uk/subjects/AQA-PHYA4-2-QP-JAN13.PDF . Why is the force on +4.0 charge going to the right and force on +6.0 charge going to the left, surely because they're alike charges they would go like this...image.jpg


Please anyone?!
Original post by fruity97


do you have the multiple choice paper as well please?
although cyclotrons came up in june 2013, might it come up again?
From CD223's revision notes: "Rate of change of flux linkage is at: Min when coil perpendicular to field, Max when parallel."

But I thought the emf induced was at a maximum when the coil is perpendicular to the field? So shouldn't the rate of flux linkage be at a maximum when cutting the field lines? Could someone clarify this
Original post by fruity97
Please anyone?!


they are point charges so draw radial field lines coming out of both in all directions with more field lines from the larger charge and this will cause the two field lines to reach up and repel going in the upwards and downwards direction
Original post by caseyfallon
do you have the multiple choice paper as well please?


Here you go! http://www.tomred.org/uploads/7/7/8/3/778329/physics_qp_u4a_specimen.pdf
Original post by utvctang
From CD223's revision notes: "Rate of change of flux linkage is at: Min when coil perpendicular to field, Max when parallel."

But I thought the emf induced was at a maximum when the coil is perpendicular to the field? So shouldn't the rate of flux linkage be at a maximum when cutting the field lines? Could someone clarify this


max flux linkage when perpendicular to field lines and min emf induced when this happens as 90degrees out of phase, max emf when parallel to field lines
Original post by 000alex
Can someone explain field lines and equipotentials in electric and magnetic fields?
e.g. when does something exert a force and when is there no work done in a field?


Electric field: force exerted whenever charged particle moves at angle not 90 degrees to field lines (or 0 degrees to the equipotentials). Magnetic field: whenever the angle to the field lines is not 0 degrees. HOWEVER, in a magnetic field in A2 we always assume the angle is 90 degrees. This is why, in the formula booklet, F=BQV and not BQVsin(x). In an electric field the angle doesn't usually have to be considered either, unless there are two forces acting on a charge- but in that case you will usually be asked to calculate the resultant force/potential, not an angle. If you have the Nelson Thornes textbook on you see pg 86.

Work done is just force * distance. As no force is exerted on a particle moving at an equipotential (angle is 90 degrees to field lines), no work is done either. Likewise, as no force is done when a particle moves parallel to a magnetic field, no work is done either.
(edited 8 years ago)
Original post by a.a.k
When you move a wire in a magnetic field.


Posted from TSR Mobile


how is that different to the left hand rule?????
Original post by thedon96
Hi,
So in the first scenario the magnetic force is acting downwards causing there to be a reading of 112g on the mass balance. When the current is reversed, the scale should become negative since the force is now acting upwards (the opposite direction - Fleming's left hand rule). Also, since the current has been doubled and F = BIl (so F is directly proportional to I), the force will also double so that's why the answer is A. I think hahah


Thank you for that but I think you answered thee wrong question. :colondollar:
http://filestore.aqa.org.uk/subjects/AQA-PHYA4-1-QP-JAN13.PDF for question 22 i have no idea how to work out the frequency?:frown:
Reply 3555
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a.a.k
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Original post by utvctang
From CD223's revision notes: "Rate of change of flux linkage is at: Min when coil perpendicular to field, Max when parallel."

But I thought the emf induced was at a maximum when the coil is perpendicular to the field? So shouldn't the rate of flux linkage be at a maximum when cutting the field lines? Could someone clarify this


Its hard to explain and imagine.

But think about where it changrs the most.(not where there are more lines).

Think about at what point the field line increase the most.

I havent used the correct terminology. But i tried explain as best i could.

Posted from TSR Mobile
if they ask all about resonance frequency and the like, what would a 6mark answer be?
Original post by aprocrastinator
how is that different to the left hand rule?????


Lenz Law, the induced emf/current is in the opposite direction
Reply 3558
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Deddy
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Original post by fruity97
Please anyone?!


The force on the 4nC charge isn't to the right, the question is asking for electric field strength, but in terms of forces the 4nC charge is repelling the 6nC charge so the force of repulsion of that charge on the 6nC charge is going to the right. The 6nC is also repelling the 4nC charge, so its force on the 4nC charge is to the left.
what on earth is back emf PLEASE

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