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# 12Vector calculus

In Chapter 8 we introduced the idea of a vector as a quantity with both magnitude and direction and we discussed vector algebra, particularly as applied to analytical geometry, and the differentiation and integration of vectors with respect to a scalar parameter. In this chapter we extend our discussion to include directional derivatives and integration over variables that are themselves vectors. This topic is called vector calculus or vector analysis. It plays a central role in many areas of physics, including fluid mechanics, electromagnetism and potential theory.

## 12.1 Scalar and vector fields

If scalars and vectors can be defined as continuous functions of position throughout a region of space, they are referred to as fields and the region of space in which they are defined is called a domain. An example of a scalar field would be the distribution of temperature T within a fluid. At each point the temperature is represented by a scalar field T(r) whose value depends on the position r at which it is measured. A useful concept when discussing scalar fields is that of an equipotential surface, that is, a surface joining points of equal value. This is somewhat analogous to the contour lines on a two-dimensional map, which join points of equal height. An example of a vector field is the distribution of velocity v(r) in a fluid. At every point r, the velocity is represented by a vector of definite magnitude and direction, both of which can change continuously ...

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