# Uncertainty in area of a circle

by zero13428
Tags: circle, uncertainty
 P: 7 1. The problem statement, all variables and given/known data The radius of a circle is measured to be 14.3+-0.3cm. Find the circle's area and the uncertainty in the area. I don't understand how to correctly apply uncertainty equations with sigma and partial derivatives to these types of problems. 2. Relevant equations A=(pi)(r^2) (pi)(r^2)=642.4cm
 HW Helper P: 6,202 Well then, we have A=πr2. If we take ln of both sides we will get lnA=ln(πr2)=lnπ+2lnr Now just take differentials dA/A = 2*dr/r dA is nothing but the error in A. Same with dr. Just substitute the numbers. I really could not explain it properly without showing you the differentials.
 P: 7 You said to take the ln of both sides. As in the natural log? I didn't know these had anything to logs or am I reading something wrong.
HW Helper
P: 6,202
Uncertainty in area of a circle

 Quote by zero13428 You said to take the ln of both sides. As in the natural log? I didn't know these had anything to logs or am I reading something wrong.
Well normally, to get the error, you would just add the relative errors. I showed you how to do it.

So if you had A=r3 then dA/A = 3*dr/r

It comes out the same if you just differentiate it normally.
 P: 7 I know at the beginning I asked how to use sigma and partial derivatives to solve this type of problem but I don't really know much about them yet. We haven't gotten to them in my math class. This problem is coming from an intro to physics lab course that focuses on propagation of error and uncertainty in measurements made and then using Excel functions like STDEV and (chi^2) to figure out stuff related to uncertainties. Is there a standard formula to use if given a measurement or multiple measurements and the uncertainity in them? "dA/A", is that supposed to be a partial derivative?
HW Helper
P: 6,202
 Quote by zero13428 I know at the beginning I asked how to use sigma and partial derivatives to solve this type of problem but I don't really know much about them yet. We haven't gotten to them in my math class. This problem is coming from an intro to physics lab course that focuses on propagation of error and uncertainty in measurements made and then using Excel functions like STDEV and (chi^2) to figure out stuff related to uncertainties. Is there a standard formula to use if given a measurement or multiple measurements and the uncertainity in them?
In that case, you can just find the areas with the radii given and then find the standard deviation, which would be how much the measurement deviates from the mean. Measuring its error.

 Quote by zero13428 "dA/A", is that supposed to be a partial derivative?
If you had like one value alone and you wanted to get the error,

dA would be the error in A.
A would be the actual measurement.

The relative error in A would then be dA/A
 P: 7 Actually I think I got it worked out. Let me know if this looks right. A=(∏)(r)^2 ∂(A)/∂(r) = 2(∏)(r) sigma_A=√(((∂A/∂r)^2)(sigma_r)^2)) sigma_A=√(((2∏(14.3))^2)(0.3)^2))= 26.9cm Area = 642.4cm Uncertainty = 26.9cm
HW Helper
P: 6,202
 Quote by zero13428 Actually I think I got it worked out. Let me know if this looks right. A=(∏)(r)^2 ∂(A)/∂(r) = 2(∏)(r) sigma_A=√(((∂A/∂r)^2)(sigma_r)^2)) sigma_A=√(((2∏(14.3))^2)(0.3)^2))= 26.9cm Area = 642.4cm Uncertainty = 26.9cm
If you wanted to use the partial derivative ∂A, as the error in A, then it should read like this

∂A/∂r= 2πr or ∂A=2πr ∂r

Now if we divide both sides by A (which is πr2 as well)

∂A/A = 2πr/πr2 ∂r

or ∂A/A = 2∂r/r

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