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Edexcel Physics Unit 2 "Physics at work" June 2013 watch

  • View Poll Results: The last question - Does resistance increase or decrease?
    It increases ( using V=IR or some other method)
    70.73%
    It decreases using the 'lattice vibrations' theory
    29.27%

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    For Last question I said that they are in series because torch bulb produces less light than mains bulb. as in series the current is same so power dissipated is directly proportional to resistance. for this reason they are in series because in parallel the brightness of bulbs is inversely proportional to resistance. and it was clear in diagrams that mains bulb was brighter. may be I am wrong not too sure.
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    (Original post by jollygood)
    For Last question I said that they are in series because torch bulb produces less light than mains bulb. as in series the current is same so power dissipated is directly proportional to resistance. for this reason they are in series because in parallel the brightness of bulbs is inversely proportional to resistance. and it was clear in diagrams that mains bulb was brighter. may be I am wrong not too sure.
    Yeah that sounds good but thats for the second last question, how about the VERY last question, the one on a very low current passing through the bulb and its effect on the resistance? I put resistance would be less
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    and last past of this question I answered differently. I said that when current is reduced then according to V=IR. V is constant so resistance should increase.incresed resistance would not allow current to flow through.. and I also mentioned that as P=VI so decrease in current would decrease the power dissipation so they will show no light and brightness. do u think it makes some sense?
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    Can u please remind me the second last question ? I cant remember what exactly it asked for?
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    (Original post by jollygood)
    and last past of this question I answered differently. I said that when current is reduced then according to V=IR. V is constant so resistance should increase.incresed resistance would not allow current to flow through.. and I also mentioned that as P=VI so decrease in current would decrease the power dissipation so they will show no light and brightness. do u think it makes some sense?
    To be honest, that makes sense BUT I think a lot of people maybe didn't see the wording of the question. It pointed out that there was a smaller current flowing through, as the bulb was operating below its usual power rating, so it then asked us what effect THAT would have on its resistance?

    That's my interpretation of it anyway.
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    (Original post by jollygood)
    Can u please remind me the second last question ? I cant remember what exactly it asked for?
    About how the bulbs can both operate at their normal ratings even though their pd/power is different. You had to talk about basic series circuit concepts i think (pd being split, current being the same) etc
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    (Original post by jollygood)
    I put it in series too. Okay starting from MCQs what did u put for first two ? I put violin instrument and mgh/VIt
    Surely it was 1/2mv2/ VIt as 1/2mv2 gives you the useful energy whilst mgh is just the potential energy without the loss of heat and what not and the formula for efficiency is: useful/ total energy x100 ----> so leading to my initial statement. Where did the logic behind what u said come from?
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    (Original post by LegendX)
    Surely it was 1/2mv2/ VIt as 1/2mv2 gives you the useful energy whilst mgh is just the potential energy without the loss of heat and what not and the formula for efficiency is: useful/ total energy x100 ----> so leading to my initial statement. Where did the logic behind what u said come from?
    Tbh it's such a confusing concept but I've just seen an example in the edexcel endorsed textbook, and it gives the useful work done by the motor as mgh so im leaning more towards mgh/VIT
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    (Original post by GCSE-help)
    About how the bulbs can both operate at their normal ratings even though their pd/power is different. You had to talk about basic series circuit concepts i think (pd being split, current being the same) etc
    oh thanks GOD I talked about the facr that higher resistance one will have a greater share of pd and vice versa.
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    (Original post by GCSE-help)
    Tbh it's such a confusing concept but I've just seen an example in the edexcel endorsed textbook, and it gives the useful work done by the motor as mgh so im leaning more towards mgh/VIT
    Think about it this way that motor has total energy of VIt and it helps in lifting the object through height h'.. so useful work done is in lifting object to a particular height. and in lifting object to a certain height obviously motor would overcome resistance.
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    What did u guys do about the ultrasound question? how did u tell if corrosion has occurred or not?
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    (Original post by GCSE-help)
    I got mgh/VIT for the efficiency, even though some said it was 0.5mv^2/VIT.

    I also put superposition for the final MC but apparently it was polarisation. Silly me for not reading the q
    The useful energy output in that case was Kinetic energy, so the correct answer was 0.5mv^2/VIt.
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    Corrosion question, I measured the new thickness, and compared to the original thickness (original was 4cm I think) and as the thickness I calculated was less, I deduced that it must have eroded (may have messed up in calc)


    Posted from TSR Mobile
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    it is same to me ^^ , i do not remember exactly the result but the new thinkness is about 3 cm , ??? right ??
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    (Original post by YousufElHabashy)
    Corrosion question, I measured the new thickness, and compared to the original thickness (original was 4cm I think) and as the thickness I calculated was less, I deduced that it must have eroded (may have messed up in calc)


    Posted from TSR Mobile
    Did u divide the time by 2 and using S=Vt got thickness of about 1.5cm.
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    I just remembering it being less, 3 cm sounds good !


    Posted from TSR Mobile
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    (Original post by YousufElHabashy)
    Corrosion question, I measured the new thickness, and compared to the original thickness (original was 4cm I think) and as the thickness I calculated was less, I deduced that it must have eroded (may have messed up in calc)


    Posted from TSR Mobile
    did they ask for new thickness in the question? I thought all they wanted you to figure out was whether or not it had corroded? I put it had corroded but did it in terms of time, as if it had corroded, the time taken for ultrasound pulse to return would be lower...?
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    (Original post by tronyeu)
    it is same to me ^^ , i do not remember exactly the result but the new thinkness is about 3 cm , ??? right ??
    I first got 3cm but then I realized that we have to divide the time by two to just get the time the pulse takes to touch the wall and then I got 1.5cm. Because ultrasound pulses can only be detected when they come back by same distance.
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    (Original post by jollygood)
    Did u divide the time by 2 and using S=Vt got thickness of about 1.5cm.
    Nevermind I suck at ultrasound.
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    (Original post by CharlieTT)
    Nevermind I suck at ultrasound.
    Did the question ask you to work out the amount that it had corroded by? I thought all it asked was whether or not it had corroded? So I measured the time taken which was far less than the time given and stated it must have corroded...
 
 
 
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