• We need your support!

    We are currently struggling to cover the operational costs of Xtremepapers, as a result we might have to shut this website down. Please donate if we have helped you and help make a difference in other students' lives!
    Click here to Donate Now (View Announcement)

physics paper 33

floral123

floral123

Messages
76
Reaction score
17
Points
18
plz tell me that when making the table of six values...do we have to use SI unit of quantity or unit of our measuring device??

my sir said that always use SI unit..but mark schemes give in cm or mm,in case of length.....??
 
A

Almostdoctor

Messages
45
Reaction score
23
Points
8
xxfarhaxx said:
the uncertinty is uslly writtn on da instrument... so u use tat..
fr eg in a metre rule.. da least divion is o.1mm(am i ryt??)..so u wnt da uncertty..
fr ths u tke 0.1 frm da top n 0.1 frm da bottom so ur total UNC will b 0.2...

hav i answrd ur qusetn or hav i misundrstood it:oops: ???plz do tel me
Click to expand...
That's wrong. Uncertainty is least count divided by 2
 
Kayutz Aeschylus Lon 19

Kayutz Aeschylus Lon 19

Messages
5
Reaction score
8
Points
3
hey guys..i heard tht from some trusted source..they said yea,no electricity..for question 1,its about spring...set-up : 2 retorts stands where 2 springs will be on both side...i think they will be a string or wire connected between both springs...then,we will add slotted weights...next,calculated the angle of deflection of the wire/string when weights added... for question 2,its about measurement of the rate of cooling of water (or any liquid) ... this is for p33...All the best for tomorrow!!
 
Kayutz Aeschylus Lon 19

Kayutz Aeschylus Lon 19

Messages
5
Reaction score
8
Points
3
hey guys..i heard tht from some trusted source..they said yea,no electricity..for question 1,its about spring...set-up : 2 retorts stands where 2 springs will be on both side...i think they will be a string or wire connected between both springs...then,we will add slotted weights...next,calculated the angle of deflection of the wire/string when weights added... for question 2,its about measurement of the rate of cooling of water (or any liquid) ... this is for p33...All the best for tomorrow!!
 
floral123

floral123

Messages
76
Reaction score
17
Points
18
floral123 said:
plz tell me that when making the table of six values...do we have to use SI unit of quantity or unit of our measuring device??

my sir said that always use SI unit..but mark schemes give in cm or mm,in case of length.....??
Click to expand...

strange...no one answering!!!
 
A

Almostdoctor

Messages
45
Reaction score
23
Points
8
xxfarhaxx said:
serously!!!:eek:...bt ths is wt i've been doin in all ma pprs..n i;ve been gttn thm ryt!!!! oh no.. :(wt do i do????
Click to expand...
http://www.thestudentroom.co.uk/showthread.php?t=227989
A micrometer screw gauge is readable to the nearest 0.01mm. This means there is a minimum error of +/- 0.005mm. A ruler is generally readable to the nearest 1mm, and thus the minimum error is +/- 0.5mm.

Now, I'm going to explain why I am using the term "minimum error". Someone quoted above that your fractional error is (maximum - minimum)/average. I will illustrate why this is not always true.

If, for example, I take 10 readings from a ruler of the length of something, if I get 502mm EVERYTIME, using the max-min/average formula, my error is zero! This is clearly not true; I must include the minimum error of +/-0.5mm.

Lengths are difficult for illustrating the point because when you measure a static length, you almost always get the same reading. You can imagine that, in some experiment, if you use a thermometer that is readable to 0.1 degrees, it is quite possible that on repeat measurements you may get variations of much more than that, say +/- 1.0degree. In this case you should use the (max - min)/average formula.

If there are markings on an instrument, then it is customary to say that the error is half the division. So if there's divisions every 2mm on a ruler, then the accuracy is +/- 1mm. Of course, you can sometimes interpolate to precision greater than that marked on the instrument, but this is generally unreliable.

So to answer your questions:

Measuring the diameter of a wire is interesting because the diameter of the wire is not constant along the length. You must take many readings along the length that you are considering in order to obtain an average measurement that is useful in calculations. For, say, 1.0m of wire, it wouldn't be a bad idea to take readings at 10cm intervals and average these. If your (max - min) is greater than 0.01mm, then use as your percentage error:

+/- %age error = (max - min)/average x100 x(1/2)

where the half comes in because you are going to quote this as a +/- figure. If it is smaller than 0.01mm (which is fairly unlikely) then use the minimum error of +/-0.005mm. For diameter around 5mm, this will be about 0.1%.

Measuring the length of two points about 50cm apart with a metre rule, where metre rule is precise to +/- 0.5mm:

%age error = 0.0005m/0.5m x100 = +/- 0.1%

Hope this helps.

As a final point, I might say that the +/- figure quoted on scientific measurements is not the (maximum - minimum). This is because there are sometimes points known as "residuals" that lie way outside the range of any of the other readings, and these points are often ignored. If you didn't ignore a ridiculously off-result (which may just have been down to human reading error, or some chance effect that has no bearing on the experiment) the measurement becomes useless. That's why it pays to make as many measurements as possible to be able to identify these residuals.

Also, the error is never (maximum - minimum): it is quoted as the standard deviation of the data. I won't go into details because stats isn't on A-level Physics, but basically it gives you a measure of the spread of the data, which is ultimately what you want to know about your measurement. By standardising an error in this way, scientists can tell from each other's data what kind of probability a measurement has of being out by a certain amount. But this is a bit technical!
 
xxfarhaxx

xxfarhaxx

Messages
249
Reaction score
90
Points
38
Almostdoctor said:
http://www.thestudentroom.co.uk/showthread.php?t=227989
A micrometer screw gauge is readable to the nearest 0.01mm. This means there is a minimum error of +/- 0.005mm. A ruler is generally readable to the nearest 1mm, and thus the minimum error is +/- 0.5mm.

Now, I'm going to explain why I am using the term "minimum error". Someone quoted above that your fractional error is (maximum - minimum)/average. I will illustrate why this is not always true.

If, for example, I take 10 readings from a ruler of the length of something, if I get 502mm EVERYTIME, using the max-min/average formula, my error is zero! This is clearly not true; I must include the minimum error of +/-0.5mm.

Lengths are difficult for illustrating the point because when you measure a static length, you almost always get the same reading. You can imagine that, in some experiment, if you use a thermometer that is readable to 0.1 degrees, it is quite possible that on repeat measurements you may get variations of much more than that, say +/- 1.0degree. In this case you should use the (max - min)/average formula.

If there are markings on an instrument, then it is customary to say that the error is half the division. So if there's divisions every 2mm on a ruler, then the accuracy is +/- 1mm. Of course, you can sometimes interpolate to precision greater than that marked on the instrument, but this is generally unreliable.

So to answer your questions:

Measuring the diameter of a wire is interesting because the diameter of the wire is not constant along the length. You must take many readings along the length that you are considering in order to obtain an average measurement that is useful in calculations. For, say, 1.0m of wire, it wouldn't be a bad idea to take readings at 10cm intervals and average these. If your (max - min) is greater than 0.01mm, then use as your percentage error:

+/- %age error = (max - min)/average x100 x(1/2)

where the half comes in because you are going to quote this as a +/- figure. If it is smaller than 0.01mm (which is fairly unlikely) then use the minimum error of +/-0.005mm. For diameter around 5mm, this will be about 0.1%.

Measuring the length of two points about 50cm apart with a metre rule, where metre rule is precise to +/- 0.5mm:

%age error = 0.0005m/0.5m x100 = +/- 0.1%

Hope this helps.

As a final point, I might say that the +/- figure quoted on scientific measurements is not the (maximum - minimum). This is because there are sometimes points known as "residuals" that lie way outside the range of any of the other readings, and these points are often ignored. If you didn't ignore a ridiculously off-result (which may just have been down to human reading error, or some chance effect that has no bearing on the experiment) the measurement becomes useless. That's why it pays to make as many measurements as possible to be able to identify these residuals.

Also, the error is never (maximum - minimum): it is quoted as the standard deviation of the data. I won't go into details because stats isn't on A-level Physics, but basically it gives you a measure of the spread of the data, which is ultimately what you want to know about your measurement. By standardising an error in this way, scientists can tell from each other's data what kind of probability a measurement has of being out by a certain amount. But this is a bit technical!
Click to expand...
thnx a million..rely did hlp alot:)... n it mst hav been alot to type!!!! :D
 
Usman Ismail

Usman Ismail

Messages
31
Reaction score
12
Points
18
xxfarhaxx said:
​
for eg: ur mass was 25g..n the uncertinty in da measuring clyndr is 0.1 (ths is nt da xact unctrtny of da mesrin cylnde) , then it will b
(0.1/25)*100

the jst an eg... hpe u gt da idea..its alwys da uncrtnty diveded by ur measures value * 100..
hpe u gt da idea!!:)
Click to expand...

isnt this how we calculate uncertainty
0.1/2
=0.05/25 *100
??
 
You must log in or register to reply here.
Top