SigFigs in Volume and Uncertainty?

In summary: Before we worry about such niceties, let's get the answer basically right. I can see one definite problem with your answer and another possible one.You added an uncertainty percentage in one distance (diameter) to an uncertainty percentage in another distance (length) to get an uncertainty percentage in volume. Does anything strike you as rather doubtful in that?Secondly, there are two approaches to adding up independent uncertainties. The approach you have used, simply adding them, finds the worst case result; the other finds a more likely range by using a root-sum-square rule. Which have you been taught to use?The first approach would be to use the Rule of Thumb which is the "sigfig
  • #1
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Homework Statement


A car engine moves a piston with a circular cross section of 7.500 ± 0.005cm diameter a distance of
3.250 ± 0.001cm to compress the gas in the cylinder.
(a) By what amount is the gas decreased in volume in cubic centimeters?
(b) Find the uncertainty in this volume.

Homework Equations


Area = πr2
ΔVolume = Area × ΔDistance
%unc = (ΔA/A) × 100%

The Attempt at a Solution


The radius of the cross section of the piston is 3.75cm so the area comes out to 44.18 cm2
The change in volume then comes out to 143.6cm3
-This value is measured to 4 significant figures because both the diameter and the distance were given to 4 significant figures.​
The percent uncertainty in the diameter comes out to 0.0267%
The percent uncertainty in the distance comes out to 0.0308%
The percent uncertainty in the change in volume should then be these values added together, giving 0.0575%
-With the correct number of significant figures this should be 0.06% because the original uncertainties (0.005 and 0.001) both had only 1 significant figure.​
Then, to get the uncertainty for the change in volume you use %unc = (ΔA/A) × 100% and solve for ΔA
-For this I got 0.08618 which comes to ± 0.09cm3

My question is: If the change in volume (143.6cm3) has the correct number of significant figures but is measured only to the nearest tenth of a centimeter, then how can the uncertainty (±0.09cm3) be in hundredths of a centimeter?
 
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  • #2
The "sigfig" rule is only a rule of thumb and should not be used if you have the option.
If you have actual uncertainty values, as is the case here, those are what you use.
 
  • #3
Simon Bridge said:
The "sigfig" rule is only a rule of thumb and should not be used if you have the option.
If you have actual uncertainty values, as is the case here, those are what you use.
I agree, but would express it a little differently. The sig fig system is a convention for implying the accuracy. When it is given explicitly, the convention does not apply.
 
  • #4
haruspex said:
I agree, but would express it a little differently. The sig fig system is a convention for implying the accuracy. When it is given explicitly, the convention does not apply.

Thanks, so just to be clear that would mean that I should express the change in volume and the uncertainty in the change in volume to the nearest thousandth of a centimeter because they were measured to that degree of accuracy in the question, right?
 
  • #5
maxhersch said:
Thanks, so just to be clear that would mean that I should express the change in volume and the uncertainty in the change in volume to the nearest thousandth of a centimeter because they were measured to that degree of accuracy in the question, right?
Before we worry about such niceties, let's get the answer basically right. I can see one definite problem with your answer and another possible one.
You added an uncertainty percentage in one distance (diameter) to an uncertainty percentage in another distance (length) to get an uncertainty percentage in volume. Does anything strike you as rather doubtful in that?
Secondly, there are two approaches to adding up independent uncertainties. The approach you have used, simply adding them, finds the worst case result; the other finds a more likely range by using a root-sum-square rule. Which have you been taught to use?

Edit: one more problem maybe... how did you calculate the 0.0267%?
 

What are significant figures (SigFigs) and why are they important in volume measurements?

Significant figures, also known as SigFigs, are digits in a numerical value that indicate the precision of the measurement. They are important in volume measurements because they help to communicate the accuracy and precision of the measurement. SigFigs also ensure that calculations involving the measurement are carried out correctly.

How do you determine the number of SigFigs in a volume measurement?

The number of SigFigs in a volume measurement is determined by counting all the digits from the first non-zero digit to the last digit. Zeros at the end of a number after a decimal point are also considered significant. Zeros at the beginning of a number or at the end of a whole number without a decimal point are not significant.

What is the rule for rounding off volume measurements with SigFigs?

The rule for rounding off volume measurements with SigFigs is to round the final answer to the least number of SigFigs in any of the values used in the calculation. If the first digit to be dropped is 5 or above, round up. If the first digit to be dropped is below 5, round down.

How do you calculate uncertainty in volume measurements?

Uncertainty in volume measurements is calculated by finding the difference between the highest and lowest values in a set of measurements. The average of these values is then calculated and the difference between the average and the lowest value is the uncertainty. This uncertainty is then rounded off to the same number of SigFigs as the measurement.

What is the significance of expressing uncertainty in volume measurements?

Expressing uncertainty in volume measurements helps to communicate the range of values that a measurement can fall within due to limitations in the precision of the measuring instrument or technique. This can provide a more accurate and reliable representation of the measurement and help to avoid misinterpretation of the results.

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