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    almost like sample assesment paper. only that they change the question, not same as sample assesment.hopefully goes the same as June 2010 paper
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    (Original post by zillyitem90)
    almost like sample assesment paper. only that they change the question, not same as sample assesment.hopefully goes the same as June 2010 paper
    so you have confidence that youll get good resukts
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    I'm doing 6PH08 too. Is it correct that the main official resources are just the specimen paper and the international specification? Is there anything else?

    The specimen paper has only 4 questions. Hard to categorise into planning & improvement, measurement, processing & analysis - because they all seem to contain more than 1 of those, if not all 3!
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    (Original post by Owl_492)
    I'm doing 6PH08 too. Is it correct that the main official resources are just the specimen paper and the international specification? Is there anything else?

    The specimen paper has only 4 questions. Hard to categorise into planning & improvement, measurement, processing & analysis - because they all seem to contain more than 1 of those, if not all 3!
    Yeah thats all we have as to my knowledge, we need to find others!!!
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    Does anyone have any resources for it?
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    Just try to know and understand the suggested practicals in the specification.

    Google some of them if your textbooks don't have them. Anything could come, really.
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    This is a question for those who've been through the 6PH08 specimen paper and mark scheme.

    Does anyone else get the impression that the examiners seem very "uncertain" over the issue of measurement uncertainty??

    Question 1)b)ii). A manufacturer states that the thickness of some foil is 15 X10^-6 metres. You have a micrometer with precision 0.01mm. What do you do? The first bit is easy. Fold the foil over a few times, so you are measuring, say, 8 times its thickness. Make sure you squeeze the air out from between the layers. The second bit is to "Estimate the uncertainty of your value". So you have to divide 0.01mm by 8 x (15 x 10^-6). Multiply by 100% to get 8%. Fine.

    Two problems.

    First, the mark scheme says you can use either 0.01 mm or 0.02 mm as your uncertainty. Well which should it be?

    Second, what does precision of 0.01mm mean? Surely it means that a measurement of say 2.35 mm actually only tells you that a measurement is between 2.345 mm and 2.355 mm. In other words you get an uncertainty of +/- 0.005 mm. In this particular question, you'd then get an answer of +/- 4%.

    You can argue that an uncertainty of 8% means the same as +/- 4%. But I'm not sure that that's true. And why do the examiners say you can use either 0.01mm or 0.02mm? I could understand it if they said you could use a total range of 0.01mm or a +/- figure of 0.005mm (which mean the same thing). But that's not what they say.

    Question 2)b). Measure the extension of a stretched spring. You are obviously supposed to use a metre rule. The mark scheme says you can use an uncertainty of either 1mm or 2mm. Which is best?

    Question 2)c). Here you have to compound uncertainties. The formula is g = 4(pi^2)x/T^2. So the answer is 2*(uncertainty in T)+(uncertainty in x). That's clear enough.

    The mark scheme says you should use an uncertainty in T of somewhere between 0.5 and 1.0 sec. OK, reactions times differ. However I would have thought that 1.0 sec was pretty big for this. Your choice here could have a big effect on what you write for 2)c)iii) when you have to comment on the value you get for g.

    Edexcel Physics for A2 textbook (by Tim Akrill and Graham George), pages 212-217. This is a more complicated question to do with compounding uncertainties, giving an eventual result of 9%. It all seems a bit confused... Here's why...

    For a length measured with a metre rule, they use an uncertainty of +/- 2mm and explain that this is "1mm at either end of the rule". OK, you've got to take measurements at two places on the rule, so I understand why you have to multiply by 2. What I don't understand is why the uncertainty for a single measurement should be +/- 1mm. Surely if a rule measures to precision 1mm, then you can quote the uncertainty for a single measurement as 1mm (whole range) or +/- 0.5 mm? This gives an combined uncertainty for the two measurements as 2mm (whole range) or +/- 1mm. Or am I wrong?? Where does the "+/- 2mm" come from?

    With another measurement, they use another approach. This is the measurement of width. Here you have measured several times to get a mean value, but you take the uncertainty as +/- 0.5 * the range of values. That seems fine. But isn't this inconsistent with what they do with the metre rule?

    Lastly with the vernier scale, they are back to the first approach, and take the uncertainty as +/- the stated precision.

    All other views on this would be much appreciated!

    Basically it comes down to

    1) the relationship between uncertainty and stated precision, and
    2) the correct use of the phrase "+/-" when we are talking about uncertainty.

    owlie
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    I have started another thread here, to ask about % measurement uncertainty in a way that's less tied to the 6PH08 paper.

    owlie
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    I 'm finding this unit impossible does anyone have the practicals please help me!!!
    I normally find physics easy but this unit is so hard!!
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    (Original post by Doughboy)
    Just try to know and understand the suggested practicals in the specification.

    Google some of them if your textbooks don't have them. Anything could come, really.
    Do you have any practicals?
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    ...I've done the sample paper, I know what I did wrong the 1st time i did it - I don't want to do it again until the day before...
    any suggestions? A lot of it's common sense, really.
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    (Original post by unamed)
    ...I've done the sample paper, I know what I did wrong the 1st time i did it - I don't want to do it again until the day before...
    any suggestions? A lot of it's common sense, really.
    Do you have any good resourses for it, like the practicals and techmiques to use?
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    (Original post by jenny261)
    Do you have any good resourses for it, like the practicals and techmiques to use?
    I've got a few notes from my teacher last year - how to calculate uncertainties etc. but nothing else.
    Just go through the experiments in the book, and then get your teacher to give you the experiment-papers from the old syllabus and then do them in class (which were amazingly easier than ours)
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    (Original post by unamed)
    I've got a few notes from my teacher last year - how to calculate uncertainties etc. but nothing else.
    Just go through the experiments in the book, and then get your teacher to give you the experiment-papers from the old syllabus and then do them in class (which were amazingly easier than ours)
    will you be able to post them please?
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    (Original post by jenny261)
    will you be able to post them please?
    sure, as soon as I have time!
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    (Original post by unamed)
    sure, as soon as I have time!
    :woo: :woo:
    Would you able to post them today?
    thanks a lot!!!!
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    (Original post by unamed)
    sure, as soon as I have time!
    Please do! Thank you :yep:
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    (Original post by Doughboy)
    Please do! Thank you :yep:
    (Original post by jenny261)
    ...
    Notes: [they're not super-useful]

    random errors:
    • Unpredictable changes in measurements
    • can be effected by the environment
    • nearly always human [parallax] error
    • can be reduced by repetition and averages
    • correct readings will always be between the limits after repeated experiments
    • use better instruments
    • scatter results about value.


    Systematic errors:

    • same change between measurements (+/-2)
    • Affected by flaws in instruments
    • human error only when instruments wrongly used.
    • cannot be reduced without changes in instruments (recalibration etc)
    • accuracy reduced
    • results in all readings being too large or too small


    Uncertainties in some instruments:

    • Stopwatch: 0.2 secs
    • Metre rule: 1mm
    • Vernier caliper: 0.1mm
    • Micrometer: 0.01mm


    You want to measure the length of a wire. The reading is between 374mm and 375mm. You judge it is closer to 374mm. Since you can only judge it to the nearest mm (the smallest on the ruler). You write: l

    length of wire: 375mm +/- 1mm
    To reduce the error, use a different instrument.

    Error is shown by number of sig figs and the +/- signs.

    eg. A voltmeter might read: 1.25v +/-0.01v. This implies voltage between 1.24 and 1.26


    Actual uncertainty therefore would be:
    1mm for the rule
    0.1 for the voltmeter
    0.01mm for the micrometer
    This can also be written as % uncertainty using the following formula:

    uncertainty in the instrument/reading *100

    1/375*100= +/-0.26%

    Combining uncertainty:

    In some cases we have to combine the uncertainties:

    1. Adding or subtracting uncertainty: add together their absolute uncertainty to obtain the absolute uncertainty.
    2. Multiply or dividing uncertainty: add together their % uncertainty to obtain % uncertainty
    3. Raised to the power n: Multiply % uncertainty by n to obtain the % uncertainty.


    Examples:
    1. Finding R using a voltmeter and an ammeter if the reading on the voltmeter is 2.45v and the ammeter is 0.96A:

    V=2.54 +/-0.01v
    % error = 0.01/2.54*100 = 4%

    I= 0.96A +/-A
    0.01/0.96*100= 1%

    combined error: 4%+1%= 5%

    r=v/i

    2.54/0.96 =2.56 +/- 5% ohms

    2.% uncertainty in multiple readings: range/2*100

    eg. 1.96mm, 1.94mm, 1.98mm, 1.95mm, 1.97mm

    (1.98-1.94)/2*100 = 2%

    3. % difference:
    difference/average*100

    Used to compare your answer to examiner's or manufacturer's.

    You find the density of plasticine to be 1.8g/cm3
    The manufacturer gives the density as 1.6g/cm3

    The % difference: 1.8-1.6/[(1.8+1.6)/2] * 100= 11.8%

    If you found the % uncertainty= 5%
    5% of 1.8= 0.1
    your density= 1.8+/- 0.1

    your density is between 1.7 and 1.9 g/cm3
    The manufacturer's density is 1.6g/cm3 which is outside your range.
    Therefore, it is a different plasticine.


    And that's about it. I hope that made sense!
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    Guys, Unit 6 exam's on the 19th! What are you doing for revision???
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    Uhh.. you forgot to mention what subject you're talking about :confused:
 
 
 
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