线积分、面积分和体积积分

A line integral is the integral of the dot product of a vector function with an infinitesimal displacement vector along a prescribed path. When applied to a force vector, the line integral calculates work done by or against the field. For closed paths, the net work is zero in conservative fields, where circulation around every closed path equals zero.

A surface integral integrates the dot product of a vector function with an infinitesimal area vector over a prescribed surface, whereas a line integral operates along a path. The area vector is always perpendicular to the surface. Surface integrals calculate flux through surfaces, making them essential for analyzing fluid flow and energy transport in physics.

The area vector is always directed perpendicular to the surface at each location. For closed surfaces, it points outward from the enclosed region. For open surfaces, its direction follows the right-hand rule: if fingers curl along the contour direction, the thumb points along the positive area vector.

Flux measures the flow of a vector quantity through a surface. It is calculated as the surface integral of a vector function, such as the product of fluid density and velocity. This integral quantifies how much of the vector field passes through the surface, providing critical information about mass or energy transport.

A volume integral is the integral of the product of a scalar function and an infinitesimal volume element throughout a region. For example, integrating density over a volume yields total mass, while integrating energy density gives total stored energy. When a vector function replaces the scalar, it reduces to a combination of scalar integrals.

A conservative field has zero circulation around every closed path, meaning the line integral around any closed loop equals zero. A non-conservative field has non-zero circulation around at least some closed paths. This distinction determines whether work done by the field depends only on endpoints or also on the path taken.

Vector field integrals have direct physical interpretations related to energy and matter flow, such as work, flux, and circulation. Scalar field integrals, like temperature or pressure along a path, lack this direct physical meaning. This makes vector integrals essential for understanding fundamental physics phenomena and engineering applications.