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

1. (Original post by fruity97)
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
2. (Original post by caseyfallon)
do you have the multiple choice paper as well please?
3. (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
4. (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.
5. (Original post by a.a.k)
When you move a wire in a magnetic field.

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how is that different to the left hand rule?????
6. (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.
7. http://filestore.aqa.org.uk/subjects...1-QP-JAN13.PDF for question 22 i have no idea how to work out the frequency?
8. (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.

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9. if they ask all about resonance frequency and the like, what would a 6mark answer be?
10. (Original post by aprocrastinator)
how is that different to the left hand rule?????
Lenz Law, the induced emf/current is in the opposite direction
11. (Original post by fruity97)
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.
12. what on earth is back emf PLEASE
13. (Original post by Dante991)
Lenz Law, the induced emf/current is in the opposite direction
In lH rule tje wire is not moving.

RH rule wire is moving amd you was find out which way is current

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14. (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
I'm pretty sure it's not the last paper. They still have one for next year (those taking their AS this year) and then a resit paper. Hopefully that will mean we avoid the bombshell.
15. (Original post by Jed-Singh)
You would have to bring up certain points. So for example, the amplitude increases sharply as the driving frequency comes close to the natural frequency. My ISA this year was on SHM, and they asked a 4 marker on how a bridge is affected by resonance - like the ones that get absolutely f*****
16. (Original post by oonic0rn)
http://filestore.aqa.org.uk/subjects...1-QP-JAN13.PDF for question 22 i have no idea how to work out the frequency?
Emf = Ban w sin(wt) where w= 2pi*f
So 2pi*f = 100pi
Therefore f = 100pi / 2pi = 50Hz
17. (Original post by Specter)
What do you usually get on papers then?
A/ A*

did 2014 last night and got 1 off an A*.

Not bad considering the MC really caught me off guard.
18. (Original post by Dante991)
You would have to bring up certain points. So for example, the amplitude increases sharply as the driving frequency comes close to the natural frequency. My ISA this year was on SHM, and they asked a 4 marker on how a bridge is affected by resonance - like the ones that get absolutely f*****
haha! yeah exactly, im scared that kinda question will be a 6 marker because I fudged it so bad in the ISA lol
19. (Original post by aprocrastinator)
what on earth is back emf PLEASE
Back EMF, eddy currents, or magnetic damping, is when you are using a transformer and current starts to flow through the core which was meant to be laminated but not everyone can do their job so current flows in it so it is lost
20. Does anyone has a list of past grade boundaries?

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