physics qn...


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OzOn3

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#4
YEah.. qn1 = A

i a bit rusty ar.. So question 2 might be wrong.. But here goes:

A) The aluminium ring will stay put. Aluminium is not magnetic.

B) No difference. Aluminium is not magnetic.

C) No difference. Try changing a different alloy :)

I hope the answers are correct.. =x
 

Clown

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#5
question2:
A) the aluminum ring will fly off due to change in magnetic flux. aluminum is paramagnetic.
B) if it's thin enuff it'll just run along the iron core.
C) nothing happens if switched to DC cuz the flux will remain the same.
 

HelmetBox

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#6
innovas1 said:
Qn 2 is a little vague.......

How is the circuit, solenoid , the iron core and the aluminium ring connected?

Is the iron core going through the solenoid?
Is the aluminium ring on top or through the iron core?
How is the solenoid wound?

If all these aren't answered, the answer probably has nothing to do with them.

So my guess is, since aluminium cannot be attracted by magnets,

a) nothing will happen as the AC supply will cancel out the magnetic field in the solenoid

b) no difference since we're still on AC supply.

c) A magnetic force will be induced in the solenoid with a DC current. Its direction will depend on how the supply is connected and how the solenoid is wound and determined with the right hand rule. But since Aluminium is not attracted my mgnets, nothing will happen.
 

OzOn3

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#7
eh Clown, Aluminum is Paramagnetic??

HelmetBox: I think, if this follows the standard O level paper, the Soft iron core is in the middle of the Aluminium ring. Well, which way the current goes.. i :dunno: also.. Haha.. The other two question about how the solenoid is wound and whether is it in.. Yeah, i htink it's crucial.. But let's assume it wound in the direction which will cause the "paramagnetic" alloy to go upwards. :p
 

HelmetBox

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#8
OzOn3 said:
eh Clown, Aluminum is Paramagnetic??

HelmetBox: I think, if this follows the standard O level paper, the Soft iron core is in the middle of the Aluminium ring. Well, which way the current goes.. i :dunno: also.. Haha.. The other two question about how the solenoid is wound and whether is it in.. Yeah, i htink it's crucial.. But let's assume it wound in the direction which will cause the "paramagnetic" alloy to go upwards. :p
urr... aluminium Is paramagnetic, but i dun think O levels learn that.......
 

HelmetBox

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OzOn3 said:
eh Clown, Aluminum is Paramagnetic??

HelmetBox: I think, if this follows the standard O level paper, the Soft iron core is in the middle of the Aluminium ring. Well, which way the current goes.. i :dunno: also.. Haha.. The other two question about how the solenoid is wound and whether is it in.. Yeah, i htink it's crucial.. But let's assume it wound in the direction which will cause the "paramagnetic" alloy to go upwards. :p
lews, in the core? then why the drawing like that one....... They expect people to understand?! No wonder I didn't do well for my O's ;p
 

Clown

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#11
aluminum is paramagnetic, thus it will contain magnetic fluxes, but the alignment of electrons wont be such that the become magnetically attractive. (ie ferromagnetic)

there's 3 types
diamagnetic (unable to contain flux)
paramagnetic (able to contain flux but not much)
ferromagnetic (able to contain a lot of flux and cause an induction)

the aluminum ring will fly off if it's thick enough because of the AC current. it will first contain the flux in one orientation, but when the direction changes, the opposing flux will force the ring to fly off.

this experiment was featured in one of my textbooks. =p
 

HelmetBox

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#12
Clown said:
aluminum is paramagnetic, thus it will contain magnetic fluxes, but the alignment of electrons wont be such that the become magnetically attractive. (ie ferromagnetic)

there's 3 types
diamagnetic (unable to contain flux)
paramagnetic (able to contain flux but not much)
ferromagnetic (able to contain a lot of flux and cause an induction)

the aluminum ring will fly off if it's thick enough because of the AC current. it will first contain the flux in one orientation, but when the direction changes, the opposing flux will force the ring to fly off.

this experiment was featured in one of my textbooks. =p
sec 4 textbook? nowadays sec 4 really learn paramagnetism?

Fwah....... secondary schools have come a long way too huh.......
 

Clown

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#13
dun think sec4 textbook. i dun have any sec4 textbooks. haha. dun worry.
 

HelmetBox

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Clown said:
dun think sec4 textbook. i dun have any sec4 textbooks. haha. dun worry.
hehe... then how?

I dun doubt your answer but are sec 4 taught to replicate this understanding? Unless they are taught on the paramagnetism and the effects of free electrons, then I doubt the question requires that answer.

Anyway, thread starter: no ans to refer to meh?
 

innovas1

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#15
I have No given solutions for these qns but theses are o-level prelim qns from certain sec schools.

The diagram does not tell you how it is connected. Anyway, i believe the solenoid and ac supply connection is the standard kind. The soft iron core is in the solenoid and the aluminum ring is slipped onto the iron core. The actual problem states that the thick Al ring moved up and remained halfway up the iron core when the switch was on for the ac circuit. Account for this observation.



i think i read in a textbook, that Alloys of Al is magnetic, can be used as permanent magnet.


Well,
For qn 1, ANyone knows what is the refractive index of plastic? I think it is around 1.5, same as glass, if u taking plastic as perspex. If so, then A does not hold. What is strange is that in the figure for plastic, there are 2 emergent rays. 1 reflected ray(this is refracted too) and 1 refracted ray.

If incidence angle >critical angle, total internal reflection will occur.
Since for both plastic and glass, the critical angle is about 42, so the incident ray of 45degree will be totally reflected. However, due to the rough surface in this case, the light is dispersed. :dunno:

My answer would be:
D) The plastic surface disperses the light

(Sorry, typo error, no 's' after surface)


If plastic's refractive index is 1.3, then incidence angle of 45 <critical angle of 50, no total internal reflection can occur. ANswer =A (but why still have the internally reflected ray?)


I think for qn2a), the Al ring moved up as it was being magnetised and its induced pole repelled that of the solenoid. It would remain halfway up as radial magnetic field was being induced inside the AL ring.. But i wonder why it won't drop down as the circuit is an ac circuit.


I think for 2b), the AL ring will shoot up faster as it is lighter and magnetised faster and repelled more easily.

2c) The question should be: if u change the ac supply to a dc supply, what will happen to the thick AL ring when u on and off the switch? My answer would be that the AL ring will only move slightly upwards at the instant the switch is on and remain there for the duration of closed circuit. It retained its magnetism. At the instant it is switched off, the AL ring will move down back to its original position as it is now demagnetised. (It is anaglous to electromagnetic induction. Imagine the instant of 'ON' as the instant when a solenoid circuit(with no power supply) with a magnet moving in, the galvanometer will deflect. Think of the duration of switch on as the magnet is stationary inside the solenoid, the galvanometer does not deflect. The instant as the switch is off as the instant u are moving the magnet away from the solenoid.
CAn visualise?)


:sweat:

any comments?
 

HelmetBox

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#16
innovas1 said:
ANyone knows what is the refractive index of plastic? I think it is around 1.5, same as glass, if u taking plastic as perspex.
I have No given solutions for these qns but theses are o-level prelim qns from certain sec schools.

The diagram does not tell you how it is connected. Anyway, i believe the solenoid and ac supply connection is the standard kind. The soft iron core is in the solenoid and the aluminum ring is slipped onto the iron core. The actual problem states that the thick Al ring moves up and remain halfway up the iron core. Account for this observation.



i think i read in a textbook, that Alloys of Al is magnetic.
Fwah! nowadays O levels so hard ah? Or are they trying to stop people from going to JCs for the first months by setting such prelim questions? Weird.

Anyway, glass, like plastic, has different refractive indexes. If you are talking about perspex, it should be ~1.49-1.5. If I don't remember wrongly, perspex is somewhat called glass? For glass, generally its 1.5 but then for different types of glass: fuzed quartz, 1.46 and light flint 1.58.

Hmm....... If the question doesn't state which specific type then again I don't think it matters? Anyway, if faced with a dilemma as such, do what SAT teaches: go through the other false choices. Thus analysing, only choice (A) makes most sense.

For your next question, let me go think abit first can? hehe
 

Clown

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#17
i dont really bother about the plastic question but i think the question regarding the aluminum ring has problems.

1: aluminum by itself in its natural state, unless stated otherwise, is PARAmagnetic. to be exact, it as the same permeability as AIR. the electrons are totally too chaotic to hold magnetic orientations.
iron, on the other hand, has a permeability 5000 times that of aluminum. the minimum permeability to qualify for ferromagnetism is 250 times times that of aluminum. so, in this case, it's not magnetism forces within the aluminum ring at work here.

2: so if manetism is not at work here, it has to rely on some other principles, that brings us to faraday's law, inductance, flux, and the such.
here, in this experiment, you must see this setup as 2 seperate coils wound around the iron core. one is the solenoid, the 2nd coil will be your aluminum ring.

3: so here is a setup to prove the presence of flux according to faraday's law of magnetic inductance. as current is delivered to the coil, its changing magnetic fields creates a changing flux in the aluminum ring (so called coil 2) via inductance by the iron core (ferromagnetic). and as you have learnt, emf (electro magnetic force) is created when there is a change in the direction of the current. so when there's opposing emf, between that within the aluminum ring and the coil, repulsion occurs. please do take time to understand the difference and the relationship between magnetic forces, principles and ELECTROmagnetic forces and induction.

that's it for now. anymore problems? :sweat:
 

HelmetBox

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#19
NiVleK said:
If I have taken such questions during my 'O's, I think I will fluke badly.

And hey, I dun even know what is paramagnetic after 'A's. Hehz.
Wakakaka! I so double that! :bsmilie:
 

Clown

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#20
i dun have A levels lor... i N level only..
 

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