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

Do we just circle the letters for the multiple choice, or is there an answer sheet that we write them on?

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They have boxes with blank crosses and you just cross your intended answer
2. (Original post by MintyMilk)
How can a gravitational field strength be negative? If we're saying that higher field strength means higher density of field lines, how can you have a negative density of field lines?

Does it say in the book that field strength is the negative value of potential gradient?
Gravitational field is a vector quantity, it has a direction and magnitude; in this case you are being asked about the magnitude "gravitational field STRENGTH". The negative sign just signifies that it is acting towards the center of a mass.
magnitude "gravitational field STRENGTH".
Isn't a magnitude always positive by definition?
4. (Original post by MintyMilk)
Isn't a magnitude always positive by definition?
Yes the magnitude is a positive quantity.
5. I seem to be the only one asking questions on here (sorry) haha

but how do you do this?

Topic 7.1 Q3.
A rectangular coil of width 60mm and of length 80mm has 50 turns. The coil was placed horizontally in a uniform horizontal magnetic field of flux density 85mT with its shorter side parallel to the field lines. A current of 8.0A was passed through the coil. SKetch the arrangement and determine the force on each side of the coil.

I have re-read 7.1 a few times but can't seem to find anything in the text to tell me how to do this sort of thing, so if you can help at all that would be great
Yes the magnitude is a positive quantity.
in this case you are being asked about the magnitude "gravitational field STRENGTH". The negative sign just signifies that it is acting towards the center of a mass.
That's fine but if the potential gradient to start with is positive, why would AQA even bother stating that g field strength is always the negative of potential gradient when 50% of the time that logic won't even apply?

Should they have just stated that g strength is always just the magnitude of the potential gradient?
7. (Original post by Euclidean)
They have boxes with blank crosses and you just cross your intended answer
Ok. It's weird because I thought that sheet would be in the Past Paper section, that's why I thought it was just circling

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8. I think you would have to be looking at the question with a bit more overall awareness sometimes... unfortunately for us

For this one what I did was I looked at the period of oscillation ( 0.27s ) and the time given ( 0.20s), since 0.20 is very close to 0.27 you know the oscillation has gone from positive maximum to negative maximum and is on its way back towards positive maximum. use 0.27/2 to get period for half of oscillation = 0.135s, then subtract from time given 0.20-0.135=0.065s substitute this value into displacement equation you should get the correct answer.

I hope it helped
9. (Original post by Music With Rocks)
I seem to be the only one asking questions on here (sorry) haha

but how do you do this?

Topic 7.1 Q3.
A rectangular coil of width 60mm and of length 80mm has 50 turns. The coil was placed horizontally in a uniform horizontal magnetic field of flux density 85mT with its shorter side parallel to the field lines. A current of 8.0A was passed through the coil. SKetch the arrangement and determine the force on each side of the coil.

I have re-read 7.1 a few times but can't seem to find anything in the text to tell me how to do this sort of thing, so if you can help at all that would be great
The short side experiences no force because it is parallel to the field line, not cutting it.

The longer sides do cut the field lines and as there is a current through the coils you can use Fleming's left hand rule to determine direction. Direction of current is not given but that does not matter because if you draw a rectangle wire with current through it, the longer sides will have opposite direction of flow, hence one side is going up, the other side going down.

Hope it helped.
10. I am confused about the equation given for emf induced in a rotating coil, e=BANwsinwt

this equation is obtained from differentiating phi=BANcoswt, so surely there should be a negative sign in front?

11. (Original post by bananamonkeys)
I am confused about the equation given for emf induced in a rotating coil, e=BANwsinwt

this equation is obtained from differentiating phi=BANcoswt, so surely there should be a negative sign in front?

Not entirely sure, but I think its due to Lenz's law where the induced emf has a negative direction and since the differential is also negative then there will be a cancellation.
12. It just gives magnitude, I guess.

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13. Any predictions for the 6 markers?

Also I'm finding Unit 5 the nuclear physics section pretty difficult I haven't evens started revising for astrophysics yet ffs got c4 and c3 and computing in the way
Any predictions for the 6 markers?

Also I'm finding Unit 5 the nuclear physics section pretty difficult I haven't evens started revising for astrophysics yet ffs got c4 and c3 and computing in the way
Still got D1, Unit 4, C3, NM, C4 and FP2 before Astro
15. (Original post by MintyMilk)
Still got D1, Unit 4, C3, NM, C4 and FP2 before Astro
Aww hell naaw, D1 and Unit 4 for me are alright, but FP2 for you must be f***** up, feel for you.
16. Can someone explain why this is 360V instead of 0? surely it is equidistant from each charge, both of which are positive so they cancel out
17. Does anyone know how to do part ii on this question. I cannot get my head around it at all.
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18. (Original post by Alby1234)
Can someone explain why this is 360V instead of 0? surely it is equidistant from each charge, both of which are positive so they cancel out
I assumed that because electric potential is scalar it only has one direction so you are always going to add the two values together; hope that helps.
19. (Original post by cybertron98)
I assumed that because electric potential is scalar it only has one direction so you are always going to add the two values together; hope that helps.
Ah yes, I imagine that is the case, thank you
20. (Original post by Alby1234)
Can someone explain why this is 360V instead of 0? surely it is equidistant from each charge, both of which are positive so they cancel out
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.

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