Multiple Choice (1 Viewer)

[sasso]

All of those formulas relate to motor effect and so also induction, why can't we look at them? This is an induction question.

Reading into the question too much? Are you kidding me, its physics. They would definately make sure they put the right thing on the diagram. The galvanometer means they are expecting or getting very low currents.

Ok please explain how it is D. So far you are telling me that the earth's magnetic field direction and strength is completely irrelevant for the question, making no sense.

To quote you

"although the earth's magnetic field is present, it is not significantly large enough to create any sizeable change in the amount of current produced"

If the earth's field isn't strong enough then it would produce so little current that making the wire bigger wont make any difference at all anyway, so you are supporting my arguement for B.

Picking D is saying that its orientation will affect the current least. Well please look at the attached diagram. Let the top one (generator) be A and bottom one be B, both from top view, the thick line is the axes on which it rotates. Now which one would make a better generator? Obviously A. In B the only change in magnetic field is from the tiny top walls of the coil, and they arnt all going fast because as it gets closer to the rotating axes they get slower due to less radius. So the component perpendicular to the field is very small. In A its quite big, the coil goes along the radius (or rather is the radius), as you can see.

Now apply the diagram to the experiement. The squares are the north and south poles of the earth, the lines are the field, and the coil (the longer side going parallel with the axes) is the wire. Scenario A is where the wire is orientated east to west (the axes is going E to W). And scenario B is where its going North to South. That means that Scenario A is procucing much more current than scenario B. Yes the mag field may be weak but it does have a direction, and there will be a difference, a bigger difference than making the wire thicker.


Why would they specifically state in the question that the wire is 'of very low resisitance'? Its not just of low resisitance, its 'very' low. If a wire is of very low resisitance, then making it thicker wont make hardly any difference to the current. The orientation will. So if thickness affects it least, answer is B.


If you still disagree, please provide an explanation.

 

[jarro_2783]

Here are my teacher's answers.

1A 2D 3B 4D 5C 6B 7A
8A("while" switch is on) or C("as" switch is turned on) 9B 10B 11B 12A 13A
14B 15D

8 is an ambiguous question

However I beg to differ on 9.

For seven which everyone is arguing about.
as the coil turns, the flux linkage is decreasing, since there are less flux lines going through the coil. So the coil will try to oppose that change which means increasing the flux linkage. If you point your right thumb in the direction of the current in A the magnetic field produced increases the flux linkage through the coil.

For 9.
As I have said before, there is no way an eddie current will be produced that will actually produce a magnetic field able to slow down the ring in R. Any currents will cancel themselves out and there will be no net force on the ring making the answer C.

 

[Abtari]

just because the R ring is not complete doesnt mean that eddy currents don't form in it, when the magnetic flux that threads the ring changes...

eddy currents are still formed in R just less so than the full ring, Q. the plastic ring, P has no eddy currents generated in it and so falls the fastest.

 

[haboozin]

just because the R ring is not complete doesnt mean that eddy currents don't form in it, when the magnetic flux that threads the ring changes...

eddy currents are still formed in R just less so than the full ring, Q. the plastic ring, P has no eddy currents generated in it and so falls the fastest.

http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2003exams/pdf_doc/physics_03.pdf

Q20


now marking Guidelines to get 4/4

Fully explains both investigations which could include
– Investigation 1: Since the solenoid is not in a [complete] circuit, no current can be
induced by the magnetic field. No current → no induced magnetic field
→ no motion.
– Investigation 2: The circuit is closed, so magnet induces current in
solenoid. Lenz’s law says the current will produce a magnetic field
opposing the changing field that induced it. So direction of induced
field is opposite to field of magnet. They repel, and solenoid moves to
the LEFT. Includes both a description of Lenz’s law and gets direction
of motion correct



this is the same thing?

have you ever thought that maybe alot of things you say may not be right, so stop telling people they're wrong and simply just give your OPINION.
Try starting your sentenses with "i think", insead of "no it isnt.. the answer should be"

 

[sasso]

just because the R ring is not complete doesnt mean that eddy currents don't form in it, when the magnetic flux that threads the ring changes...

eddy currents are still formed in R just less so than the full ring, Q. the plastic ring, P has no eddy currents generated in it and so falls the fastest.

I agree with Abtari on this. Besides the fact that I saw it with my very own eyes the magnet fell slower than normal even though it had a slit in the ring. Just think the laminations in a transformer make it more efficient, but still do not stop them, which is the same as the ring with the slit, still eddy currents, just less, they dont have to go around the ring, but can go anywhere they like since its a big conductor they'll find a way to go, they do have ideal paths which is why the complete ring absorbs it more efficiently.


Haboozin, about that question 20 in 03, its a little different, the solenoid has coils which the wire is laminated, otherwise it wouldnt work, in this experiment just think that the solenoid is solid, no laminations means the whole thing is connected regardless of whether the slit is there.

 

[Abtari]

have you ever thought that maybe alot of things you say may not be right, so stop telling people they're wrong and simply just give your OPINION.
Try starting your sentenses with "i think", insead of "no it isnt.. the answer should be"

of course i have. i know when i am wrong, and when to stop debating.

ok look mr grammar-nazi, do u think i need to write 'in my opinion' or 'IMO' at the start of every damn sentence? i don't.. of course, what i write is my opinion and mine alone. and even when i say 'no it isn't...the answer should be', for anyone with half a brain, it is implicit that the words are coming from me, myself and noone else. who said u had to consider these words as the truth? its a free world..

clearly u are misconcepted, IMO

 

[jarro_2783]

my teacher has since said something else about the rings.
She thinks that experimentally there may be some slow down due to some form of eddie currents, and the fact that copper is heavier than plastic air resistance might play some part. But she thinks that it would be hard to determine because it would be negligible. She also says that knowing the board of studies they probably want you to put C because of the fact that the ring is slit. In the syllabus there is nothing about a slit ring still causing some eddie currents and being slowed. It is all or nothing. So going by that, they probably want you to put C, although it could be classified as a stupid question.
In fact, the only dot point on the topic is "explain the production of eddy currents in terms of Lenz's law."
The only way we can find out is just to wait till they publish the answers.
Does anyone know when they do that?

 

[Abtari]

She also says that knowing the board of studies they probably want you to put C because of the fact that the ring is slit. In the syllabus there is nothing about a slit ring still causing some eddie currents and being slowed. It is all or nothing. So going by that, they probably want you to put C, although it could be classified as a stupid question.

knowing the board of studies, they probably don't want u to simply be a syllabus-regurgitator and actually learn the concept of applying concepts within the syllabus to situations they provide in the exam...

and knowing the board of studies, they might want u to know ur experiments, and sometimes go a little further and see what happens if... they might, maybe. Just Maybe.

and who said anything about all or nothing? i am baffled as to where u conjured up this rather misleading idea...

any sensible person wouldn't classify this as a stupid question..

N.B. the above comments are all IMO

 

[sasso]

lol, you dont have to say IMO after everything, the whole point of the forum is to share ideas/opinions and discuss things.

What haboozin is saying is you shouldn't just say, no thats wrong, but explain why you think its wrong. You have to back up your decision/opinion, thats all.

About this question with the rings, If the bos say answer is C then its rigged because i physically (hehe) saw the magnet fall slower down the pipe with a slit than the plastic pipe. There ARE eddy currents present. Wind resistance will not have a visible effect, the eddy currents significantly slow it down, the full looped metal pipe very significantly slowed it down (took 15 seconds to go down or something like that)

 

[haboozin]

if you throw an object in projectile motion, it would never take those exact paths either or those times, range etc..

 

[Abtari]

if you throw an object in projectile motion, it would never take those exact paths either or those times, range etc..

we aren't dealing with the 'real world' in physics exams.. we are just applying the theory we learn in situations that the questions provide.. so even though actual experiments may give differing opinions.. generally they are fairly correct (as in the case of sasso, who himself saw the plastic ring fall faster, note a lot faster, than the split metal ring, which in turn was faster than the complete metal ring)

IMO, we have arrived at a consensus for this question..

 

[jarro_2783]

knowing the board of studies, they probably don't want u to simply be a syllabus-regurgitator and actually learn the concept of applying concepts within the syllabus to situations they provide in the exam...

and knowing the board of studies, they might want u to know ur experiments, and sometimes go a little further and see what happens if... they might, maybe. Just Maybe.

and who said anything about all or nothing? i am baffled as to where u conjured up this rather misleading idea...

any sensible person wouldn't classify this as a stupid question..

N.B. the above comments are all IMO

actually knowing the board of studies they do want you to regurgitate the syllabus. Of course having an understanding of the concepts is much better. If you look at the syllabus you will not find one mention of any experiment or skills outcome that says to identify that a split ring goes slower than a plastic ring or whatever.