What is the meaning of a line integral and how is it calculated?

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Discussion Overview

The discussion revolves around the concept and meaning of line integrals in mathematics, particularly in the context of integrating functions along curves. Participants explore various interpretations and applications of line integrals, including their geometric and physical significance.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that a line integral can be understood as finding the area under a curve represented by a function f(x,y,z) along a path C, suggesting a connection to the vertical area covered as one moves along the curve.
  • Others argue that if f(x,y,z) represents mass, then the line integral \(\int_C f(x,y,z) ds\) calculates the total mass along the curve, which is interpreted as a physical application.
  • A participant suggests that if z = f(x,y), then \(\int_C f(x,y) ds\) represents the area of a sheet traced out by the curve in the xy-plane connected to the surface defined by f(x,y).
  • Another participant mentions that the line integral \(\int_C \vec{F} \cdot d\vec{r}\) represents the work done by a force along a path, while \(\int_a^b \vec{E} \cdot d\vec{r}\) represents the potential difference between two points in an electric field.
  • One participant expresses the belief that line integrals represent the "length" of the line between integration points, which is challenged by another who clarifies that this refers to arc length, defined by a different integral.
  • A later reply indicates that line integrals can indeed represent arc length under certain conditions, providing a specific formulation for smooth functions.

Areas of Agreement / Disagreement

Participants express various interpretations of line integrals, with no consensus reached on a singular definition or application. Multiple competing views remain regarding the meaning and implications of line integrals.

Contextual Notes

Some limitations include the dependence on the definitions of the functions involved and the specific conditions under which different interpretations apply. The discussion does not resolve these nuances.

theperthvan
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What does line integral really mean, what is it doing?

Say you have a function f(x,y,z) and you integrate it w.r.t. arc length along some curve C.
Is this like finding the area under C over f? Like if you are walking along C, and the vertical area covered below you is the integral?

It's hard to say what I mean, but is this correct?

Cheers,
 
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theperthvan said:
What does line integral really mean, what is it doing?

Say you have a function f(x,y,z) and you integrate it w.r.t. arc length along some curve C.
Is this like finding the area under C over f? Like if you are walking along C, and the vertical area covered below you is the integral?

It's hard to say what I mean, but is this correct?

Cheers,

Here is an application. Let [tex]f(x,y,z)[/tex] represent the mass at every given point. Then [tex]\int_C f(x,y,z) ds[/tex] along a rectifiable curve [tex]C[/tex] is the total mass of the string/wire (which is represented by the curve).
 
Say you have z = f(x,y). Then [tex]\int_C f(x,y) ds[/tex] represents the area of the sheet that is traces out. That is, the line represented by C in the xy plane, connect it to the surface f(x,y), and the line integral represents the area of this sheet
 
oh right. that makes more sense. cheers,
 
If you are familiar with basic physics, then

[tex]\int_C{\vec{F}\cdot\ d\vec{r}[/tex]

This line integral represents the work done by the force F along the path C.

[tex]\int_a^b{\vec{E}\cdot\ d\vec{r}[/tex]

This line integral represents the potential difference (voltage) between points b and a, where E is the electric field.
 
I thought the line integral in its mathematical sense represented the "length" of the line between the points of integration.
 
jbowers9 said:
I thought the line integral in its mathematical sense represented the "length" of the line between the points of integration.

Nope you are thinking of arclength. The arclength of an integrable function f(x) over [a,b] is given by [tex]\int^b_a \sqrt{ 1+ (f'(x))^2} dx[/tex]
 
OK, thank you.
 
jbowers9 said:
I thought the line integral in its mathematical sense represented the "length" of the line between the points of integration.

It can represent arc length. If [tex]\bold{R} = x(t)\bold{i}+y(t)\bold{j}[/tex] (for smooth functions) then [tex]\int_C 1 \ ds = \int_a^b \sqrt{[x'(t)]^2+[y'(t)]^2} dt[/tex] where [tex]C[/tex] is the path obtained from [tex]\bold{R}[/tex].
 

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