平面中的线积分

A line integral evaluates quantities distributed along a curve, such as mass, work, or surface area. It is defined as the limit of a Riemann sum, where a curve is partitioned into small segments. Each segment contributes the function value times its arc length. Summing these contributions yields the total accumulation of the function weighted by arc length along the curve.

Parametric equations express a curve using a single parameter t, where x = x(t) and y = y(t) vary over an interval [a, b]. This representation converts geometric and physical quantities into a single variable, facilitating computation. As t changes from a to b, the parametric equations trace the curve from its starting point to its ending point.

The arc length element ds is expressed as the square root of the sum of the squared derivatives: ds = √[(dx/dt)² + (dy/dt)²] dt. This formula captures the curve's geometry and is essential for converting line integrals from abstract form into computable integrals. It weights the function values by the actual distance traveled along the curve.

The wall is divided into vertical strips along its curved base. Each strip has width equal to arc length and height equal to the wall's elevation function. Multiplying these values gives each strip's area. As strip widths become infinitesimal, the sum converges to a line integral that yields the total surface area available for the mural.

The scalar function f(x, y) represents the quantity being accumulated along the curve, such as wall height, wire density, or force magnitude. At each point on the curve, f is multiplied by the arc length element ds. The line integral sums these products, giving the total accumulation of the function weighted by distance along the path.

The curve is partitioned into n small segments, each with arc length Δs. Over each segment, the function is approximately constant, contributing f(xi, yi)Δsi to the sum. As the number of segments increases and their widths approach zero, this Riemann sum converges to the line integral, providing a rigorous limit-based definition.

Line integrals compute distributed quantities including mass of non-uniform wires, work done by force fields along paths, and surface area of vertical surfaces with varying height. In each case, the scalar function represents the quantity's density or intensity, while the curve defines the path or boundary. This versatility makes line integrals fundamental tools in applied mathematics and physics.