Multiple Choice (2 Viewers)

[jarro_2783]

I'll draw a little diagram to illustrate question 9.



Part 1 and 2 are cross sections of a small section side on and the top respectively, both of the cut ring. Since the ring is cut, any currents that may be induced will flow in loops as in the diagram. They will have their associated magnetic fields which as shown in the diagram will cancel each other out with respect to the magnet they are being dropped over. Therefore there will be no eddie current produced that can possibly slow down the ring.
If you look at part 3 it shows the connected ring. This shows how because it is connected it can now have an eddie current that loops completely around the ring. This allows a magnetic field that doesn't cancel itself out to be produced. This magnetic field will loop through the middle of the ring and attempt to push the ring away from the magnet thus slowing its decent.
Therefore the answer is c because ring R has no possible eddie current producable that will slow its decent.

 

[SarahJZ]

Ok I thought I must be doing something wrong in that MC - cos I got a lot of D's and B's and a few A's here and there and no C's...

It was ok, I wasn't sure about the doping one, it was either A or C...put C first, and then crossed it out, cos I thought that if you doped with a group V it would close the energy gap...but I wasn't sure...

Oh and for q9 it has to be B...think about it, the plastic ring falls down first because no eddy currents are induced in plastic, the copper ring with a slit comes down second because it is still copper and eddy currents are still induced, its just they are minimised due to the slit, and then the complete copper ring falls down last because it undergoes full eddy current induction, and thus it is constantly down the magnet having to fight against its own motion (Lenz's Law)...

 

[who_loves_maths]

Originally Posted by jarro_2783
Part 1 and 2 are cross sections of a small section side on and the top respectively, both of the cut ring. Since the ring is cut, any currents that may be induced will flow in loops as in the diagram. They will have their associated magnetic fields which as shown in the diagram will cancel each other out with respect to the magnet they are being dropped over. Therefore there will be no eddie current produced that can possibly slow down the ring.
If you look at part 3 it shows the connected ring. This shows how because it is connected it can now have an eddie current that loops completely around the ring. This allows a magnetic field that doesn't cancel itself out to be produced. This magnetic field will loop through the middle of the ring and attempt to push the ring away from the magnet thus slowing its decent.
Therefore the answer is c because ring R has no possible eddie current producable that will slow its decent.

jarro, your argument only takes into consideration ONE possible way (out of many others) that Eddy Currents may form in the metal loop (cut or not).

1) let's assume that the direction of the induced current matters (which it actually does not because Lenz's law will take care of that), in which case your drawings of the direction of current is not necessarily correct. Eddy currents are swirls of induced current usually formed on the externally surfaces of the conductor.
now, the three rings are dropped above the North pole - this means they generally have a near-uniform magnetic field threading through them up the page. now, as they are released and moves down - this is the same as having the rings stationary and moving the magnets towards them (motion is relative).
so, if the north pole moves toward the rings, the rings will generate it's own north pole to repel the magnet (Lenz's law).

now in order to do that, you can imagine the complete metal ring as a coil - so the current flows around the ring (not the way you drew it to be). so, in theory, with the cut metal ring, this stops the circular current around the ring.
BUT, the currents can be formed in another way to produce smaller localised north poles for the cut ring that act against the magnet's north pole.
namely, on the surface facing the downward direction, you may find discrete circular swirls of eddy currents that produce their own minute, but separate north poles. this, although smaller in magnitude to the field of a full metal ring, is nonetheless counteracting the field of the external magnet.


2) the fact of that question is, you don't even need to know the direction of current flows to determine the order in which they land. Why? because the point of the question is that (i) insulators like plastics do not generate ANY electric motion inside, while (ii) metals will always have electrons moving when moved through a magnetic field.

ALL you need to admit to is that even though the third metal ring is cut, when it passes by the magnet, there will be Eddy currents induced in it {which you know because you drew the currents in cut rings in your last post}.
how does that answer the questions? well:

Lenz's law is about energy conservation. in the plastic, no energy is lost to eddy currents (because there are none) so the plastic falls, having gravity doing work on it. now, in both the full and cut metal rings, eddy currents will be induced.
answer this: where did the Eddy currents get the energy from?
energy cannot be created, the energy comes from the metal rings' KE. KE here is sacrificed so Eddy currents can be generated.
hence, because Eddy currents is induced (in small or large amounts, it doesn't matter) in the cut metal ring, it's KE at every point along its fall will always be smaller than the plastic's. meaning it's velocity at each point is smaller as well.

ie. R will land later than P.


3) according to Lenz's Law: "the induced current will act in a fashion as to generate a magnetic field that opposes the motion which initially induced it".
hence, any induced current in the cut ring will move according to that Law. ie. your drawings of the current must be wrong because they do not follow Lenz's Law if they don't oppose the motion of the ring (which you claim they don't).

sorry but you don't get to invent your own laws about eddy currents in exams

the thing most ppl don't get about this law is that: Eddy currents will ONLY ever be induced IF there is potential there for them to oppose the initial motion! otherwise, even if it's a conductor, NO Eddy currents will be produced. it's an "all-or-nothing" characteristic of the Law that ppl don't really understand.

you cannot, ever, get Eddy current to be generated while at the same time having them do nothing to stop the initial motion of the conductor - that would be equivalent to inventing energy!


i hope you understand what i've said jarro

 

[richz]

jarro, your argument only takes into consideration ONE possible way (out of many others) that Eddy Currents may form in the metal loop (cut or not).

1) let's assume that the direction of the induced current matters (which it actually does not because Lenz's law will take care of that), in which case your drawings of the direction of current is not necessarily correct. Eddy currents are swirls of induced current usually formed on the externally surfaces of the conductor.
now, the three rings are dropped above the North pole - this means they generally have a near-uniform magnetic field threading through them up the page. now, as they are released and moves down - this is the same as having the rings stationary and moving the magnets towards them (motion is relative).
so, if the north pole moves toward the rings, the rings will generate it's own north pole to repel the magnet (Lenz's law).

now in order to do that, you can imagine the complete metal ring as a coil - so the current flows around the ring (not the way you drew it to be). so, in theory, with the cut metal ring, this stops the circular current around the ring.
BUT, the currents can be formed in another way to produce smaller localised north poles for the cut ring that act against the magnet's north pole.
namely, on the surface facing the downward direction, you may find discrete circular swirls of eddy currents that produce their own minute, but separate north poles. this, although smaller in magnitude to the field of a full metal ring, is nonetheless counteracting the field of the external magnet.


2) the fact of that question is, you don't even need to know the direction of current flows to determine the order in which they land. Why? because the point of the question is that (i) insulators like plastics do not generate ANY electric motion inside, while (ii) metals will always have electrons moving when moved through a magnetic field.

ALL you need to admit to is that even though the third metal ring is cut, when it passes by the magnet, there will be Eddy currents induced in it {which you know because you drew the currents in cut rings in your last post}.
how does that answer the questions? well:

Lenz's law is about energy conservation. in the plastic, no energy is lost to eddy currents (because there are none) so the plastic falls, having gravity doing work on it. now, in both the full and cut metal rings, eddy currents will be induced.
answer this: where did the Eddy currents get the energy from?
energy cannot be created, the energy comes from the metal rings' KE. KE here is sacrificed so Eddy currents can be generated.
hence, because Eddy currents is induced (in small or large amounts, it doesn't matter) in the cut metal ring, it's KE at every point along its fall will always be smaller than the plastic's. meaning it's velocity at each point is smaller as well.

ie. R will land later than P.


3) according to Lenz's Law: "the induced current will act in a fashion as to generate a magnetic field that opposes the motion which initially induced it".
hence, any induced current in the cut ring will move according to that Law. ie. your drawings of the current must be wrong because they do not follow Lenz's Law if they don't oppose the motion of the ring (which you claim they don't).

sorry but you don't get to invent your own laws about eddy currents in exams

the thing most ppl don't get about this law is that: Eddy currents will ONLY ever be induced IF there is potential there for them to oppose the initial motion! otherwise, even if it's a conductor, NO Eddy currents will be produced. it's an "all-or-nothing" characteristic of the Law that ppl don't really understand.

you cannot, ever, get Eddy current to be generated while at the same time having them do nothing to stop the initial motion of the conductor - that would be equivalent to inventing energy!


i hope you understand what i've said jarro

finally someone has some sense , its just like laminating iron cores of transformers. In books its says lamintating iron cores reduce the size of eddy currents, they do not completely remove them.

 

[who_loves_maths]

^ hey, talk of transformer lamination, since i cbb doing back 10 pages and reading everything through again, can you just tell me what you got for the laminating-core question plz xrtzx?

i believe it was Question 10.

 

[richz]

B
(10char)

 

[who_loves_maths]

yes, B is the model in textbooks, most ppl proli put that.

why not D?

 

[rama_v]

yes, B is the model in textbooks, most ppl proli put that.

why not D?

I thought D because it had the least surface area for eddy currents to form (well, it looked a bit like that anyway), but then again I'm hopeless at these types of questions so it may as well be a random guess for me.

 

[richz]

yes, B is the model in textbooks, most ppl proli put that.

why not D?

if u look at the diagram the coils are on the insulators, how is current ment to be induced on them?? and i also relied on my textbook.

 

[who_loves_maths]

^ what do you mean how is current supposed to be induce in them?
the diagram shown is only a part of the whole transformer unit. you do not know where the other set of coils are.

 

[richz]

hmm.. after looking at the q, my previous msg is rong because i just saw the wire was insulated. Well then i just remebered the textbook diagram . its in two books, jacaranda and physics contexts

EDIT: check out this page 4

 

[who_loves_maths]

^ hehehe ... well, actually, it wouldn't make sense if it weren't insulated anyways, cause the current would then just leaked into the core and make the transformer unit dysfunctional

but srsli though, i do not think it is B.
i know that the textbooks have B as their models - but i think one need to realise that that's because the textbooks are presenting us with real-life industrial transformer models.
the problem with that is that when building a real transformer, engineering need to weigh the economic factors with energy loss factors. model B is a compromise between reducing energy loss via Eddy Currents and maximising the amount of soft iron in the core so the fluctuating magnetic fields used for induction is strong enough to render the unit economically productive.
ie. B does not show the best way to minimise Eddy currents - think about it, if i made a core from pure plastic, there wouldn't be any Eddy currents at all! (but it's not practical because there would be no soft iron in core to strength the magnetic fields.)

but in the HSC question, we are building a theoretical (not industrial) transformer in which the question asks which one minimises Eddy current generation. here, minimises means the lowest, and there is no need to take a compromise into consideration because the question did not say "and also the one which give the best optional core for magnetic field strengthening, etc..."

also, the term "efficiency" is a percentage, not a sum or total of how much energy is provided. ie. it's a ratio of energy used on total energy input. so to maximise efficiency, we only need to think about maximising energy used and not wasted.

so i believe the answer to be D.

 

[who_loves_maths]

Originally Posted by xrtzx
...two books, jacaranda and physics contexts.

hehe... lucky you you have/use Physics Contexts.

you know, just a week before the HSC i realised that, as two Science books, Chemistry Contexts is proli better than Jacaranda Physics.
i just like its format/content/style more for some reason. too bad i only decided to use it two weeks before the HSC... hehe, it was sitting on my desk collecting dust before that

 

[richz]

ooooooo, oppps, ur saying y not D, i thut u were saying y not C, lol. A and D were the first ones i eliminated, becuz ive never seen them. so it was either B or C but B was most correct.

EDIT: after reading ur above post, yeah maybe, all well just depends on the markers.

 

[richz]

hehe... lucky you you have/use Physics Contexts.

you know, just a week before the HSC i realised that, as two Science books, Chemistry Contexts is proli better than Jacaranda Physics.
i just like its format/content/style more for some reason. too bad i only decided to use it two weeks before the HSC... hehe, it was sitting on my desk collecting dust before that

lol, i have both chem and phys context, bought them at the start of the year, chem contexts is good but i just didnt use it, lol wasted 65 bux cuz conq chem was adequate, (and John Mu's notes (thnx Muuy) .) Phys contexts, i used during the chrissy holis, lol i thut it was shit because the book had no answers for the qs, so i gave up on it. Then i asked acmilan, wat book helped him, he said contexts, even though it built up a little dust until i opened it again in august, it was actually quite useful because it actually spoke my language not like Jacaranda, but still i regret buying it, Spotlight physics is prolly the best phys book.

PS. infact i was a good student in phys in yr 11, it was so much easier but i found yr 12 phys a struggle. Contrastingly, in chem i was an ordinary student in yr 11, but it all changed in yr 12. lol, wierd.

 

[who_loves_maths]

hehe... icic. that's not the case for me though.
i used no textbooks until halfway through the year (our school gave out photocopied notes).
and, unlike you, my ranking for both chem and phys went drastically down in Yr 12!!!

^ (i guess there's a proli a correlation between those two facts eh? lol)

and i hate myself for it right now!

 

[richz]

hehe... icic. that's not the case for me though.
i used no textbooks until halfway through the year (our school gave out photocopied notes).
and, unlike you, my ranking for both chem and phys went drastically down in Yr 12!!!

^ (i guess there's a proli a correlation between those two facts eh? lol)

and i hate myself for it right now!

lol u'll prolly still beat me , wats with me and not reading the questions .

 

[who_loves_maths]

hey you changed your avvy
aww... but the elephant was better ... this one's quality looks a bit shaky

 

[richz]

lol, im working on it

 

[Just James]

Corrections

i dont think 6 is B, thickness of the wire affects the amount of current produced for sure. I recon its D, coz why does it matter which direction its oriented?

You must not forget the Earth's own magnetic flux lines and their density. Depending on the orientation of the wire, the amount of flux cut will be different, hence affecting current produced.