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

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I keep messing up the Multi Choice questions where you have too rearrange and solve, I never know what equations to use... any tips generally for these types of questions?
Does anybody have a concise, simple explanation as to what magnetic flux and magnetic flux linkage actually are, because I don't really know?

Also, for question 23 on http://filestore.aqa.org.uk/subjects/AQA-PHYA4-1-QP-JUN12.PDF why is the magnetic flux able to be 0 when the EMF is 0? Flux linkage is 90 degrees out of phase with the induced EMF so one is max when the other is 0 so how can them both being at 0 be explained? Thanks.
Reply 3002
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CD223
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Original post by _Caz_
I'm sorry everyone! I messed up. Impulse is force multiplied by time.
Apologies. I didn't mean to cause any trouble for anyone.


Lol, it does seem everyone jumped on your back for that :biggrin:


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Predictions???????


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Original post by xela238
Predictions???????


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Mechanics question involving circular motion or conservation of momentum I think. Not been a huge one like that for a while.
Original post by JJBinn
Does anybody have a concise, simple explanation as to what magnetic flux and magnetic flux linkage actually are, because I don't really know?

Also, for question 23 on http://filestore.aqa.org.uk/subjects/AQA-PHYA4-1-QP-JUN12.PDF why is the magnetic flux able to be 0 when the EMF is 0? Flux linkage is 90 degrees out of phase with the induced EMF so one is max when the other is 0 so how can them both being at 0 be explained? Thanks.


I always think of magnetic flux density, B, as how much magnetic field there is per unit area. Almost like the densities of materials. The higher the density of a material, the more mass per unit volume you have. The higher the magnetic flux density, the more magnetic field you have per unit area.

1Tesla =1Wb per square metre.

Magnetic flux is the amount of magnetic field that is passing through the wire/coil. Which is why flux=BAN

I hope this helps a little. :smile:
ImageUploadedByStudent Room1433792712.177655.jpg

Can someone explain 19 I just can't get it!

Also I think there's gonna be a big transformer question ~ well I'm hoping there is now as I spent all afternoon going over them


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Original post by _Caz_
I always think of magnetic flux density, B, as how much magnetic field there is per unit area. Almost like the densities of materials. The higher the density of a material, the more mass per unit volume you have. The higher the magnetic flux density, the more magnetic field you have per unit area.

1Tesla =1Wb per square metre.

Magnetic flux is the amount of magnetic field that is passing through the wire/coil. Which is why flux=BAN

I hope this helps a little. :smile:


That's very useful, thanks! The formula sheet says flux = BA, and flux linkage = BAN. What is the difference of flux and flux linkage? Are they just the same thing except one applies to a coil with turns and the other just applies to a conductor with no turns? Or is that wrong haha?
probably been asked a thousand times, but predictions for the big markers?:smile:
Original post by gcsestuff
ImageUploadedByStudent Room1433792712.177655.jpg

Can someone explain 19 I just can't get it!

Also I think there's gonna be a big transformer question ~ well I'm hoping there is now as I spent all afternoon going over them


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Yeah I'm stuck on this one too
Is it 0? (because the force is going the other way due tot he reversed current it's not acting on the scales)
That's a tough question I'm not sure... If anyone knows and answers could you quote me in the answer too please.
Original post by JJBinn
That's very useful, thanks! The formula sheet says flux = BA, and flux linkage = BAN. What is the difference of flux and flux linkage? Are they just the same thing except one applies to a coil with turns and the other just applies to a conductor with no turns? Or is that wrong haha?


yeah flux linkage is for coils :smile:
Original post by oonic0rn
probably been asked a thousand times, but predictions for the big markers?:smile:


Probably something on shm as that hasnt been the topic of a 6 marker as of yet if i'm not wrong
Original post by _Caz_
Yeah I'm stuck on this one too
Is it 0? (because the force is going the other way due tot he reversed current it's not acting on the scales)
That's a tough question I'm not sure... If anyone knows and answers could you quote me in the answer too please.


The answer was A -244,

I thought that as the current was reversed then the wire would no longer be on the scale and hover above it. Therefore it would be -112 as the scale had been zeroed


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best strategy for the exam?

Finishing mcq's in half an hour MAX and then spending rest of time on section B

OR, start section B first then go onto mcq's finishing latter in around half an hour?
Original post by gcsestuff
The answer was A -244,

I thought that as the current was reversed then the wire would no longer be on the scale and hover above it. Therefore it would be -112 as the scale had been zeroed


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Oh I think what it is getting at then is that it is zeroed before the switch is closed. So when they reverse the current and double it, the force is in the opposite direction (hence the - sign) and is twice as big because force is directly proportional to current.
are eddy currents basically the same as lenzs law. In the fact that an induced emf is caused, the current which makes it flow also opposes is ?


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What is coulombs law? i saw it on a paper (it said to define it) but i can't remember which paper
Original post by sincostanxxx
What is coulombs law? i saw it on a paper (it said to define it) but i can't remember which paper


It's also the equation of force between 2 point charges


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Reply 3018
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a.a.k
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Original post by NEWT0N
I think the problem here is showing that the force is actually felt by the scale. (The wire is hovering above the scale so the force would be acting upwards from the wire so it shouldn't cause the scale to show a reading?)


Before it was hovering(says in question) as the current has been reversed the force acts downwards (left hand rules)



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Reply 3019
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a.a.k
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Original post by NEWT0N
Sorry I don't quite understand what you mean.


It says it is held between poles so it must be hovering which means force was upwaeds

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