Edexcel physics unit 6B (15 May 2014) Watch

jtbteddy
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(Original post by 55 %)
OKAY. Here's whats important.

15 Why are certain instruments suitable? For these questions comment on the precision and range of the instrument eg : "Measured value is many times greater than least count..Therefore less percentage uncertainty"

This is on the specification too:
"identify the most appropriate apparatus, giving essential details-‐essential details may include the range and precision of instruments"

But I don't understand what is meant by the range of instruments, could you explain this please?
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fahim.xd
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(Original post by jtbteddy)
This is on the specification too:
"identify the most appropriate apparatus, giving essential details-‐essential details may include the range and precision of instruments"

But I don't understand what is meant by the range of instruments, could you explain this please?

it means the total range..like a value of 0.36 mm is in the range of a micrometer screw gauge ...but not values like 1 or 2 cm...where a meter rule is more suitable...

hope that helped.
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jtbteddy
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(Original post by fahim.xd)
it means the total range..like a value of 0.36 mm is in the range of a micrometer screw gauge ...but not values like 1 or 2 cm...where a meter rule is more suitable...

hope that helped.
What is the range of a vernier caliper? I know that a caliper is more precise than a metre rule, but I always get confused between them .. Don't know which one to use when
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fahim.xd
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(Original post by jtbteddy)
What is the range of a vernier caliper? I know that a caliper is more precise than a metre rule, but I always get confused between them .. Don't know which one to use when
the most common range is 0 - 150 mm ..

u would use vernier caliper is cases like Diameter of coins, small objects.
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jtbteddy
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(Original post by fahim.xd)
the most common range is 0 - 150 mm ..

u would use vernier caliper is cases like Diameter of coins, small objects.
Okay, thanks
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jtbteddy
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In the Jan 14 paper, Q3 is a nightmare. The line of best fit is weird to draw, and I have problems with this question in particular:

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Why can't an "h" from both sides be cancelled out, and T squared be plotted against h?
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55 %
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(Original post by Relaxedexams)
Thanks a lot! This really helps!

Shouldn't that be "divided"?
so for X+y, when X is 7000 and Y is 7.1, the answer should be 7007 (least d.p), and significant figures do not matter at all? Only decimal places?
Oh yeah! That should be divided. Thanks for pointing that out

Yep. The final answer for that should be 7007.

Note that it's ambiguous as to how much sf 7000 is. Is it 1 sf or is it 4 sf?
Remember if there is a decimal point after the 7000..like in "7000."..then it's 4sf. Otherwise it's ambiguous.

Could you please give more examples of systematic and random errors?

Thank you very much!
Well.. Almost always the zero error exists. I don't think they will specifically ask for random or systematic errors. The most common trap seems to be writing "parallax error" when a micrometer or vernier callipers are involved. This is wrong.

Also forgot to mention.. Don't get mixed up between a rule and a ruler. It's always safer to mention "metre rule". Rule doesn't have any uncalibrated parts at the ends. It immediately starts with 0. Ruler on the other hand has an uncalibrated piece at the end.

I personally think we would see a question where they would ask whether two experimental value are agreeing to each other.
Eg: when measuring something, say x, the percentage uncertainty was 3%. One student got x as 156. Another student got x as 161. Can these results be concordant?
One way of approaching this is by using a range method :
Compair 156 + 3% with 161 - 3%. If the first answer is greater than the second..then the values agree to each other.

Another method could be
*finding the percentage difference
*then finding the double of the percentage uncertainty = 6%
*compare this with the percentage difference

......

Another question I would expect is.. where a multi-variable values are plotted. It won't be x againt y.
Maybe xy against y.
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55 %
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(Original post by jtbteddy)
In the Jan 14 paper, Q3 is a nightmare. The line of best fit is weird to draw, and I have problems with this question in particular:

Name:  Jan 14.png
Views: 136
Size:  24.0 KB
Why can't an "h" from both sides be cancelled out, and T squared be plotted against h?
That graph won't be a straight line.

Note that you will have to then devide h from every term in the equation. This would leave you with a graph that's complex if you plot Tsquared against h.
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jtbteddy
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(Original post by 55 %)
That graph won't be a straight line.

Note that you will have to then devide h from every term in the equation. This would leave you with a graph that's complex if you plot Tsquared against h.
OHHH RIGHT! I'd have to divide C by h too and it doesn't have an h! I didn't realise, silly me. Thanks
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Jazelle
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Hey!! How are all of you actually studying apart from doing all of the past papers??


Posted from TSR Mobile
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jtbteddy
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http://pastpapers.edexcel.com/conten...e_20120307.pdf

For Q1c, I really don't understand the mark scheme.

It says:

Measure the coiled length of spring with callipers and divide by the number of
loops (do not credit use of micrometer screw gauge)
Sensible precaution for their choice of instrument
eg repeat and average
measure in different places/orientations


But measuring the coiled length of spring with calliper and then dividing by the number of loops would actually give me the width of each spring, right? And what we're looking for is the diameter ..
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jtbteddy
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http://pastpapers.edexcel.com/conten...e_20120307.pdf

In this paper, Q4bii) says:
"State why this is a suitable instrument for this measurement."

For which the mark scheme says:
"0.01 s « human reaction time (accept 0.1 s) "

But in this thread, people were saying that if the precision of a stopclock is less than reaction time, we have to use the human reaction time as the absolute uncertainty of the measurements, so regardless of the values we get, 0.1 would be the uncertainty... So the mark scheme doesn't make sense to me. Can someone help please?
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crazzysashgirl
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(Original post by 55 %)
OKAY. Here's whats important.

1 a When there is a single reading involved, then the uncertainty is the least count.
b If the reading is measured using a stop watch then the uncertainty is the reaction time. typically 0.1 s

2 a If multiple readings are involved, then the uncertainty is half range.
b If the half range value is less than least count or reaction time, then choose the least count or reaction time as the uncertainty

3 Percentage uncertainty is (Uncertainty/Measured value) * 100%

4 Percentage difference between an experimental value and a standard value is (Measured value - Standard value/ Standard value) * 100%

5 Percentage difference between two experimental values is (Difference between the two/ Average of the two) *100%


OMG!! thank yew so much!! this is so clear n u saved my day... <3 cleared all my doubts... was freaking out... thank yew so much!!!!!!!!
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Relaxedexams
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(Original post by 55 %)
Oh yeah! That should be divided. Thanks for pointing that out

Yep. The final answer for that should be 7007.

Note that it's ambiguous as to how much sf 7000 is. Is it 1 sf or is it 4 sf?
Remember if there is a decimal point after the 7000..like in "7000."..then it's 4sf. Otherwise it's ambiguous.
You are right, it is ambiguous, I meant it as 4 s.f

(Original post by 55 %)
Well.. Almost always the zero error exists. I don't think they will specifically ask for random or systematic errors. The most common trap seems to be writing "parallax error" when a micrometer or vernier callipers are involved. This is wrong.

Also forgot to mention.. Don't get mixed up between a rule and a ruler. It's always safer to mention "metre rule". Rule doesn't have any uncalibrated parts at the ends. It immediately starts with 0. Ruler on the other hand has an uncalibrated piece at the end.

I personally think we would see a question where they would ask whether two experimental value are agreeing to each other.
Eg: when measuring something, say x, the percentage uncertainty was 3%. One student got x as 156. Another student got x as 161. Can these results be concordant?
One way of approaching this is by using a range method :
Compair 156 + 3% with 161 - 3%. If the first answer is greater than the second..then the values agree to each other.

Another method could be
*finding the percentage difference
*then finding the double of the percentage uncertainty = 6%
*compare this with the percentage difference
Was very helpful, thanks

(Original post by 55 %)
Another question I would expect is.. where a multi-variable values are plotted. It won't be x againt y.
Maybe xy against y.
Like in the jan 14 paper, question 3? Hmmm its possible...
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AliNSiddiqui
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(Original post by 55 %)
Oh yeah! That should be divided. Thanks for pointing that out

Yep. The final answer for that should be 7007.

Note that it's ambiguous as to how much sf 7000 is. Is it 1 sf or is it 4 sf?
Remember if there is a decimal point after the 7000..like in "7000."..then it's 4sf. Otherwise it's ambiguous.



Well.. Almost always the zero error exists. I don't think they will specifically ask for random or systematic errors. The most common trap seems to be writing "parallax error" when a micrometer or vernier callipers are involved. This is wrong.

Also forgot to mention.. Don't get mixed up between a rule and a ruler. It's always safer to mention "metre rule". Rule doesn't have any uncalibrated parts at the ends. It immediately starts with 0. Ruler on the other hand has an uncalibrated piece at the end.

I personally think we would see a question where they would ask whether two experimental value are agreeing to each other.
Eg: when measuring something, say x, the percentage uncertainty was 3%. One student got x as 156. Another student got x as 161. Can these results be concordant?
One way of approaching this is by using a range method :
Compair 156 + 3% with 161 - 3%. If the first answer is greater than the second..then the values agree to each other.

Another method could be
*finding the percentage difference
*then finding the double of the percentage uncertainty = 6%
*compare this with the percentage difference

......

Another question I would expect is.. where a multi-variable values are plotted. It won't be x againt y.
Maybe xy against y.
In Jan 2014, Q1, a)ii) it says write the answer to 3 or 4 SF even though the values were subtracted and the lowest d.p. value was 803, so no d.p. but if no d.p. were given then the final answer would be 75, and the required is 75.0 or 74.96.

Am I doing something wrong?
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arshia
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(Original post by AliNSiddiqui)
In Jan 2014, Q1, a)ii) it says write the answer to 3 or 4 SF even though the values were subtracted and the lowest d.p. value was 803, so no d.p. but if no d.p. were given then the final answer would be 75, and the required is 75.0 or 74.96.

Am I doing something wrong?
dont think it makes any diff. how much time left for exam?
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AliNSiddiqui
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(Original post by arshia)
dont think it makes any diff. how much time left for exam?
The S.F. was worth one mark, it does make a difference. 3 hours to go here.
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55 %
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(Original post by jtbteddy)
http://pastpapers.edexcel.com/conten...e_20120307.pdf

In this paper, Q4bii) says:
"State why this is a suitable instrument for this measurement."

For which the mark scheme says:
"0.01 s « human reaction time (accept 0.1 s) "

But in this thread, people were saying that if the precision of a stopclock is less than reaction time, we have to use the human reaction time as the absolute uncertainty of the measurements, so regardless of the values we get, 0.1 would be the uncertainty... So the mark scheme doesn't make sense to me. Can someone help please?
Only a stop clock having a precision less than the reaction time is suitable. Suppose the stop clock had precision 0.5s. Then it wont be suitable as it is increasing the uncertainty. A suitable stop clock is one where the precision is less than the reaction time. So the uncertainty is the reaction time, which is the minimum value of the uncertainty when timing is involved.
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55 %
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Also!

** When values are added or subtracted, their uncertainties are added. REMEMBER! Even if you subtract values you should add the uncertainties!

** And of course, when values are multiplied or divided, their percentage uncertainties are added.
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55 %
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(Original post by AliNSiddiqui)
In Jan 2014, Q1, a)ii) it says write the answer to 3 or 4 SF even though the values were subtracted and the lowest d.p. value was 803, so no d.p. but if no d.p. were given then the final answer would be 75, and the required is 75.0 or 74.96.

Am I doing something wrong?
wait!

First you obtain x/10. So 803/10= 80.3. Then only you subtract. So the least d.p before subtracting is 80.3 which is to 1 dp.
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