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MATH2420 Multiple Integrals and Vector Calculus

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MATH2420 Multiple Integrals and Vector Calculus Prof. F.W. Nijhoff Semester 1, 2007-8. Course Notes and General Information Vector calculus is the normal language used in applied mathematics for solving problems in two and three dimensions. In ordinary differential and integral calculus, you have already seen how derivatives and integrals interrelate. A derivative can be used as the opposite of an integration; it also occurs in changing variables in an integral. The same interrelation applies in multiple dimensions, but with more richness and variety. This module starts with a discussion of different coordinate systems in two and three dimensions. The use of Cartesian, plane polar, cylindrical polar and spherical polar coordinates will run through the whole module. The second section starts with a discussion of vector functions, which are the two- and three- dimensional equivalents of the functions of ordinary calculus. These can be used to describe curves in space. Next we look at functions of several variables: that is, functions of a vector. With these two concepts we can introduce derivatives for fully three-dimensional functions (gradient, divergence and curl). This brings us to the halfway point of the module, and we will pause to review our new understanding before moving on to multiple-dimensional integrals. Here we extend the familiar idea of integration in one dimension to integration over an area or a volume. Finally, with the introduction of line and surface integrals we come to the famous integral theorems of Gauss and Stokes. These encompass beautiful relations between line, surface and volume integrals and the vector derivatives studied at the start of this module. Most real-life problems are not one-dimensional. The amount of heat stored in a piece of metal can be calculated by integrating its temperature in three dimensions; and the diffusion of dye in water is governed by differential equations based on three-dimensional derivatives. This is why a knowledge of vector calculus is essential for further study in many areas of applied mathematics. 1 Chapter 0 REVIEW 0.1 Calculus Differentiation The curve y = f(x) has a slope at point x = a given by the derivative of f with respect to x at a: ′ df f(a + ∆x) − f(a) f (a)= = lim . (1) dx ∆x→0 ∆x a A few particular derivatives are: ′ − f(x)= axn f (x)= anxn 1 ′ f(x)= eλx f (x)= λeλx ′ ′ ′ f(x)= u(x)v(x) f (x)= u (x)v(x)+ u(x)v (x) ′ ′ ′ f(x)= u(x)/v(x) f (x) = [u (x)v(x) − u(x)v (x)]/v2(x). Integration Integration is the opposite of differentiation: b ′ b ′ f (x) dx = [f(x)]a = f(b) − f(a) or f (x) dx = f(x) (2) Za Z b and it may also be seen as giving the area under a curve – so the integral a f(x) dx gives the area under the curve y = f(x) between x = a and x = b. Some specific integrals are: R − xn dx = xn+1/(n +1) for n 6= −1 x 1 dx = ln x Z Z ′ ′ eλx dx = eλx/λ u(x)v (x) dx = [u(x)v(x)] − u (x)v(x) dx Z Z Z 0.2 Lines and Circles The vector equation of a straight line in three-dimensional space is x = a + ub with u ∈ (−∞, ∞) a real scalar. (3) 2 Course Notes 3 2 2 2 The equation of a circle of radius r0 centred about the origin is x + y = r0 and if the circle is centred 2 2 2 on the point P (a,b) the the equation is (x − a) + (y − b) = r0. 0.3 Trigonometry Recall the functions sin x and cos x, with the identities: sin2 x + cos2 x = 1 (4) tan x = sin x/ cos x (5) and the derivatives (d/dx) sin x = cos x (6) (d/dx)cos x = − sin x (7) (d/dx) tan x = 1 + tan2 x. (8) There are memorable values: x sin x cos x 0 0 1 π/2 1 0 π 0 −1 3π/2 −1 0 2π 0 1 0.4 Determinant The determinant of a 3x3 matrix A is a11 a12 a13 |A| = a a a = a (a a − a a ) − a (a a − a a )+ a (a a − a a ) (9) 21 22 23 11 22 33 32 23 12 21 33 31 23 13 21 32 31 22 a31 a32 a33 0.5 Vector Products We consider two vectors a = (a1,a2,a3) and b = (b1,b2,b3). Their scalar dot product is given by a · b = a1b1 + a2b2 + a3b3 (10) and their vector cross product by i j k a × b = a a a = ([a b − b a ], [a b − b a ], [a b − a b ]). (11) 1 2 3 2 3 2 3 3 1 3 1 1 2 2 1 b b b 1 2 3 Note that although for the scalar product a · b = b · a, for the vector product we have a × b = −b × a. Recall that a · b = ||a|| ||b|| cos θ and ||a × b|| = ||a|| ||b|| sin θ (12) where θ is the angle between the vectors a and b. Chapter 1 COORDINATE SYSTEMS 1.1 Cartesian Coordinates We are familiar with the rectangular Cartesian coordinates x, y in a plane, or x, y, z in space. If the unit vectors in the x, y and z directions are given by i, j and k then we can write the position vector r of a point as r = (x,y,z) (1.1) = xi + yj + zk. (1.2) These two notations will be used interchangeably in this course. 1.2 Plane Polar Coordinates In a fixed frame of reference the rectangular coordinates (x, y) are related to the polar coordinates (r, θ) through the relations x = r cos θ, y = r sin θ, r> 0, θ ∈ [0, 2π) (1.3) y r = (x2 + y2)1/2, θ = arctan . (1.4) x 1.3 Cylindrical Coordinates In a fixed frame of reference the Cartesian coordinates (x,y,z) are related to the cylindrical coordinates (r,θ,z) through the relations: x = r cos θ, y = r sin θ, z = z, r> 0, θ ∈ [0, 2π) (1.5) y r = (x2 + y2)1/2, θ = arctan , z = z. (1.6) x 1.4 Spherical Coordinates In a fixed frame of reference the Cartesian coordinates (x,y,z) are related to the spherical coordinates (ρ,θ,φ) through the relations: x = ρ sin θ cos φ, y = ρ sin θ sin φ, z = ρ cos θ, (1.7) ρ> 0, θ ∈ [0, π), φ ∈ [0, 2π) (1.8) 4 Course Notes 5 θ ρ φ x Figure 1.1: Spherical polar coordinates Course Notes 6 y − ρ = (x2 + y2 + z2)1/2, φ = arctan , θ = arccos[z(x2 + y2 + z2) 1/2] (1.9) x In the above equations, θ is the latitude or polar angle, and φ is the longitude. Remark: Spherical coordinates are commonly used in applications where there is a centre of symmetry. The centre of symmetry is then taken as the origin. Chapter 2 VECTOR CALCULUS 2.1 Vector Functions A vector-valued function f is a vector function whose components are single-valued functions (scalar valued functions). For example, given three single-valued functions f1(t), f2(t), f3(t) we can form the vector-valued function f(t) = (f1(t),f2(t),f3(t)) = f1(t)i + f2(t)j + f3(t)k (2.1) The magnitude of the vector-valued function f(t) is a scalar-valued function and is defined by 2 2 2 1/2 ||f(t)|| = f1 (t)+ f2 (t)+ f3 (t) (2.2) In general, the graph of the vector function f(t)= f1(t)i + f2(t)j + f3(t)k is a curve C, in the sense that, as t varies, the tip of the position vector f(t) traces out C. The equations x = f1(t),y = f2(t),z = f3(t) (2.3) corresponding to the components of f are the parametric equations of C. If one of the components is zero, e.g. f(t)= f1(t)i + f2(t)j, then C is said to be a planar curve, otherwise C is a space curve. 2.1.1 Derivative of a vector function Given the vector function f(t) = (f1(t),f2(t),f3(t)) the derivative of f is defined by ′ ′ ′ ′ f (t) = (f1(t),f2(t),f3(t)). Properties of the derivative ′ • (f + g)′(t)= f (t)+ g′(t) ′ • (αf)′(t)= αf (t) where α is a constant scalar ′ • (uf)′(t)= u(t)f (t)+ u′(t)f(t), where u is a scalar function ′ • (f · g)′(t)= f(t) · g′(t)+ f (t) · g(t). ′ • (f × g)′(t)= f(t) × g′(t)+ f (t) × g(t). ′ • (f(u))′(t)= f (u(t))u′(t). 7 Course Notes 8 2.1.2 Integral of a vector function b Given the vector function f(t) = (f1(t),f2(t),f3(t)), the integral of f is defined by a f(t) dt = b b b ( a f1(t) dt, a f2(t) dt, a f3(t) dt). R R R R Properties of the integral b b b • a (f(t)+ g(t)) dt = a f(t) dt + a g(t) dt R b b R R • a (αf)(t) dt = α a f(t) dt, where α is a constant scalar R b R b • a (c · f)(t) dt = c(t) · a f(t) dt, where c is a constant vector.
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