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# Edexcel Physics Unit 3 9th May Watch

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1. (Original post by Gunner121)
Can anyone give me an idea about uncertainty like how do we calculate it and why sometimes we use the least count as the uncertainty , this is so confusing
Will get back to you 12 hrs from now. I got a great note on it.

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2. (Original post by NilFBosh)
Will get back to you 12 hrs from now. I got a great note on it.

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Thank you but please do get it fast , its driving me crazy.
3. (Original post by Gunner121)
Can anyone give me an idea about uncertainty like how do we calculate it and why sometimes we use the least count as the uncertainty , this is so confusing
Uncertainty of an instrument = least count

Eg:- uncertainty of length measured by meter ruler = +/- 0.1 cm ( 0.1 is least count)

% uncertainty = (uncertainty / measured value) * 100

Uncertainty = +/- 0.5 ( largest value - smallest value)

Using these you can solve any question regarding uncertainty in our syllabus.

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4. (Original post by Relaxedexams)
Uncertainty of an instrument = least count

Eg:- uncertainty of length measured by meter ruler = +/- 0.1 cm ( 0.1 is least count)

% uncertainty = (uncertainty / measured value) * 100

Uncertainty = +/- 0.5 ( largest value - smallest value)

Using these you can solve any question regarding uncertainty in our syllabus.

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Thank you and can please solve this "Question 8 b (ii) " step by step on this link :

http://www.edexcel.com/migrationdocu...e_20110518.pdf
5. (Original post by Gunner121)
Thank you and can please solve this "Question 8 b (ii) " step by step on this link :

http://www.edexcel.com/migrationdocu...e_20110518.pdf
Least count of protractor = 1
Uncertainty = 1

Incidence angle = 60 +/- 1
Refracted angle = 32 +/- 1

(Not sure abt values of angles. U gotta measure em in previous part)

u max = sin (60+1)/sin (32-1)
U min = sin (60-1)/ sin (32+1)

Uncertainty of u = +/- 0.5 (u max - u min)

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6. (Original post by Gunner121)
Can anyone give me an idea about uncertainty like how do we calculate it and why sometimes we use the least count as the uncertainty , this is so confusing
Least Count/Precision

Minimum reading of an instrument which it can record.
Eg.
Meter rule - 1mm
Micrometer screw gauge - 0.01mm
Vernier caliber - 0.1mm or 0.02mm or 0.05mm
*You have to know the above values, except for the vernier caliber one which i think will be mentioned in the question.

Uncertainty

Uncertainty is not an error. It is the inability of an instrument to record more accurate values. Uncertainty of an instrument is equal to its least count. Uncertainty of an analogue instrument is equal to half the least count. Therefore, uncertainty of meter rule is equal to 0.5mm. But when measurements are taken by using meter rule, 2 readings are recorded. One is at zero level and the other is the final reading. Each of the readings have an uncertainty of 0.5mm. Therefore, uncertainty of meter rule measurement is 1mm.

Percentage certainty of a single measurement.

% uncertainty is equal to uncertainty over the measurement times 100

Random error

Inability of the instrument or the experimenter to record the same measurement when the same experiment is repeated. Random error can be reduced through repeated readings.

Systematic error

This is due to the defect of the instrument or the mistake in the method. Systematic error cannot be reduced by repeated measurements. Error correction should be done or instrument has to be replaced.

Eg. Repeated measurement of X1, X2, X3, X4 are taken.

Uncertainty is equal to half times the difference of the maximum measurement and the minimum measurement.
So in this case if X5 is max and X1 is min

the uncertainty is equal to 1/2(X5-X1)

% uncertainty = (1/2(X5-X1)) / (average value) * 100

Pls give me a rep and thumbs up if it helped u.
7. (Original post by Gunner121)
Thank you and can please solve this "Question 8 b (ii) " step by step on this link :

http://www.edexcel.com/migrationdocu...e_20110518.pdf
I got angle of incidence 61 and refraction angle 32.

refractive index max = sin(60+1) / sin(32-1)
refractive index min = sin(60-1) / sin (32+1)

therefore uncertainty = 1/2(refractive index max-refractive index min)
8. (Original post by Relaxedexams)
Least count of protractor = 1
Uncertainty = 1

Incidence angle = 60 +/- 1
Refracted angle = 32 +/- 1

(Not sure abt values of angles. U gotta measure em in previous part)

u max = sin (60+1)/sin (32-1)
U min = sin (60-1)/ sin (32+1)

Uncertainty of u = +/- 0.5 (u max - u min)

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Thank you so much for the answer.
9. (Original post by NilFBosh)
I got angle of incidence 61 and refraction angle 32.

refractive index max = sin(60+1) / sin(32-1)
refractive index min = sin(60-1) / sin (32+1)

therefore uncertainty = 1/2(refractive index max-refractive index min)
Thank you and i have no idea how to give the rep and one more thing there is a part in the question where you are asked to write an experiment , they ask you an explanation of your choice of measuring instrument for the quantities you measure , can you explain what do we have do over here , is it for example we just right for current i will use ammeter etc. or do we have to write anything else also
10. (Original post by Gunner121)
Thank you and i have no idea how to give the rep and one more thing there is a part in the question where you are asked to write an experiment , they ask you an explanation of your choice of measuring instrument for the quantities you measure , can you explain what do we have do over here , is it for example we just right for current i will use ammeter etc. or do we have to write anything else also
For eg, to measure length u use meter rule cause it has an uncertainty of 1mm.
That's it. The name of instrument and the give a suitable uncertainty.

To give a rep just go to profile. It'll be there

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11. (Original post by NilFBosh)
For eg, to measure length u use meter rule cause it has an uncertainty of 1mm.
That's it. The name of instrument and the give a suitable uncertainty.

To give a rep just go to profile. It'll be there

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Thank you ! and for the graphs , the drawing is it always straight lines (i mean best-fit even if they dont mention it )
12. (Original post by Gunner121)
Thank you ! and for the graphs , the drawing is it always straight lines (i mean best-fit even if they dont mention it )
For physics, its always a line of best fit. (no lines joining the points.)

It depends on the graph too though, but in all the past papers, you have to draw lines of best fit.
13. (Original post by Relaxedexams)
For physics, its always a line of best fit. (no lines joining the points.)

It depends on the graph too though, but in all the past papers, you have to draw lines of best fit.
Thank you and do you any resources for the experiment.
14. hey do you guys know the emf and internal resistance expt. im confused on how to go about it. any help?
oh make sure that you all know how to calculate the uncertainty
15. (Original post by rockstar101)
hey do you guys know the emf and internal resistance expt. im confused on how to go about it. any help?
oh make sure that you all know how to calculate the uncertainty

set circuit ;; the cell/battery. ammeter connected in series. volt meter parallel to variable resistor.

by changing the value of R, measure current(I) and pd(v)
plot a graph pd against I
(the graph would be linear)
the y-intercept is the emf.

the equations is "E"= Ir - IR(which is V, since V=IR)

r= internal resistance.

y=mx+c --> V=-r I + "E"

i.e; m=-r
16. (Original post by Relaxedexams)
Least count of protractor = 1
Uncertainty = 1

Incidence angle = 60 +/- 1
Refracted angle = 32 +/- 1

(Not sure abt values of angles. U gotta measure em in previous part)

u max = sin (60+1)/sin (32-1)
U min = sin (60-1)/ sin (32+1)

Uncertainty of u = +/- 0.5 (u max - u min)

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why is one +ve(1) and -ve (1)
and the other one is the opposite?
17. i was just wondering , do we need to know how to measure the refractive index using the real and apparent depth method?
18. Can someone plz give the link for the physics 3B specification?
19. (Original post by Meera.S.)
why is one +ve(1) and -ve (1)
and the other one is the opposite?
When calculating the max you divide largest by smallest. (therefore one +, the other -)

When calculating min you divide smallest by largest (one -, the other +)
20. (Original post by *Mariya*)
i was just wondering , do we need to know how to measure the refractive index using the real and apparent depth method?
Are you sure its in the specification? Never heard of measuring refractive index using apparent depth method.

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