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

1. Predictions???????

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2. (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.
3. (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...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.

4. 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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5. (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.
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?
6. probably been asked a thousand times, but predictions for the big markers?
7. (Original post by gcsestuff)

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.
8. (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
9. (Original post by oonic0rn)
probably been asked a thousand times, but predictions for the big markers?
Probably something on shm as that hasnt been the topic of a 6 marker as of yet if i'm not wrong
10. (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.

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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11. 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?
12. (Original post by gcsestuff)

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.
13. 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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14. What is coulombs law? i saw it on a paper (it said to define it) but i can't remember which paper
15. (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
The magntitude of electric force between two point or point-like charges is directly proportional to the product of each charge and inversely proportional to the square of their separation.
16. (Original post by gcsestuff)
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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Lenz's law can be used to deduce the direction of current flow in the metal but they are not the same. When there is relative motion between a piece of metal and a magnetic field eddy currents flow in the metal in a direction as to oppose the change in flux that caused them.
17. (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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18. (Original post by _Caz_)
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.
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?)
19. (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?
They're the same thing. Flux is just the amount of flux passing through a loop of wire (one turn), while flux linkage takes into account all the loops that the flux is passing through.

If there was a field passing through a coil with N turns and you were asked for the flux through the coil, you would simply state the flux through one of the coils: BA. On the other hand, if you were asking for the flux linkage (total flux through the coil), you would just add up the flux through each of the N loops to get: .

EDIT: Typo
20. (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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