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Thread: physics qn...

  1. #21

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    Quote Originally Posted by Clown
    i dun have A levels lor... i N level only..
    I also no A levels! WAKAKAKA!!! Maybe the smarter ones should be commenting here instead....... hehe

  2. #22
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    Quote Originally Posted by NiVleK
    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.
    paramagnetism is not taught in A's... at least not for my time.

  3. #23

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    I can't seem to remember anything i've learnt ...

  4. #24
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    Default Vague question...

    Hiee...

    The Second question is vague as it does not spell out the experiment. I can only think of two possible aim of this setup.....

    1- Comparing if the soft iron core or the aluminium ring is magnetised ???
    2- To see if the aluminium ring will lavitate....??? Was there some flux pattern drawing required for this paper???

    Before you start thinking of what happens to the aluminium ring....you have to look at the electrical characteristics (electrical conductivity) of aluminium.
    Aluminium's electric conductivity is second only to copper. It will not be magnetized.

    Electric capacitance per unit dimensions is large. Due to these favorable characteristics, it is widely applied in capacitors, (paper capacitor, etc.) loud speaker parts, electric cable wrapping and other electric appliances. Also used in basic shielding for EMI EMC problems....

    regards,
    Sulhan

  5. #25

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    Quote Originally Posted by aosis
    I can't seem to remember anything i've learnt ...
    You're not alone.......

  6. #26

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    Quote Originally Posted by innovas1
    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.

    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?)




    any comments?
    I think for qn 1, u need not know the refractive index(they vary)... the critical part is that for plastic, total internal reflection has not taken place. as for glass, total internal reflection has taken place and thus ans = A

    As for 2, since the diagram quite vague, here is my take.

    When the switch is on, a magnetic field is set up in the solenoid. A current will be set up in the aluminum ring (non-magnetic) and flows in such manner to oppose the increase in flux linkage (Lenz's Law). Since it is an AC, the magnetic field in the solenoid is an alternating one. Thus, the current flowing in the ring is an alternating one as well. Hence, the ring will be suspended. (When the solenoid facing the Al ring is a north/south pole, the Al ring facing the solenoid will be a north/south pole as well.)

    Assuming the area of the ring not changed. Thinner implies more resistance in the ring (current induced in ring decreases) and the ring is lighter. cannot conclude. but if u ignore the effect of resistance, with a lighter ring, the ring will levitate at a higher height.

    If DC, no constant change in magnetic flux linkage. Hence, the ring will 'jump' momentarily up, after which it will drop back down and remain at the down position... when off, the same thing happen...

    my 2 cents worth
    Last edited by bearycute; 28th October 2003 at 03:45 PM.

  7. #27
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    Quote Originally Posted by sulhan
    Aluminium's electric conductivity is second only to copper.
    isn't silver no.1 in electrical conductivity?

  8. #28
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    Default side track....the conductivity .....

    Hiee.....

    Here is the conductivity of the metals in comparison...

    Metal Conductivity - Electrical conductivity.
    The values are a measure of how much current will flow in a standard sample when it is attached to 1 volt.

    1 silver 670
    2 copper 640
    3 aluminium 410
    4 mild steel 70

    To answer your question..Yes....no doubt that Silver has the highest...

    rgds...

  9. #29
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    for question 1 i can only think of birefringence. but dunno if O level got teach of this. Birefringence occurs when the light source is seperated in 2 different rays, Odinary and Etraordinary rays.

  10. #30
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    Quote Originally Posted by klumpsier
    for question 1 i can only think of birefringence. but dunno if O level got teach of this. Birefringence occurs when the light source is seperated in 2 different rays, Odinary and Etraordinary rays.
    ohmigosh..
    now we're gonna have some discussion on the quantum level as light is a very touchy subject.
    but then again, i'm lazy

  11. #31

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    huh? Wha, this is getting out of hand! I don't remember getting so much about Paramagnetism in my O's!! What the hell is going on!?? AHHH!!!!

    Clown: Go on leh.. It's getting interesting.. though most don't ring a bell.. =x

  12. #32
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    well.. as much as i'm tempted, this is not birefringence as it's stated it's a monochromatic light, meaning a single wavelength.
    anyway back to the actual question.

    i would say the answer is A, the plastic here has a higher critial angle than glass.
    and innovas, this is a refractive situation. if you do not achieve total internal reFLECtion, u'll most probably end up with some residual light coming out at odd angles outta the plastic prism.

  13. #33

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    Qn: Are we all jumping the gun? Its an O levels question leh....... I'm sure they aren't asking for such 'intense' answers

    If all else fails:

    SKIP THE QUESTION LAH!

    Get the others right still can get A1! kekekekekeke

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