Learn and talk about Surface area, Area

Surface area is the total area of the faces and curved surface of a solid figure. Mathematical description of the surface area is considerably more involved than the definition of arc length or polyhedra (objects with flat polygonal faces) the surface area is the sum of the areas of its faces. Smooth surfaces, such as a sphere, are assigned surface area using their representation as parametric surfaces. This definition of the surface area is based on methods of infinitesimal calculus and involves partial derivatives and double integration.

General definition of surface area was sought by Henri Lebesgue and Hermann Minkowski at the turn of the twentieth century. Their work led to the development of geometric measure theory which studies various notions of surface area for irregular objects of any dimension. An important example is the Minkowski content of a surface.

1 Definition of surface area
2 Common formulas
- 2.1 Ratio of surface areas of a sphere and cylinder of the same Radius and Volume
3 In chemistry
4 In biology
5 References
6 External links

Definition of surface area [edit]

While areas of many simple surfaces have been known since antiquity, a rigorous mathematical definition of the area requires a lot of care.

$S \mapsto A(S)$

of a positive real number to a certain class of surfaces that satisfies several natural requirements. The most fundamental property of the surface area is its additivity: the area of the whole is the sum of the areas of the parts. More rigorously, if a surface S is a union of finitely many pieces S₁, …, S_r which do not overlap except at their boundaries then

$A(S) = A(S_1) + \cdots + A(S_r).$

Surface areas of flat polygonal shapes must agree with their geometrically defined area. Since surface area is a geometric notion, areas of congruent surfaces must be the same and area must depend only on the shape of the surface, but not on its position and orientation in space. This means that surface area is invariant under the group of Euclidean motions. These properties uniquely characterize surface area for a wide class of geometric surfaces called piecewise smooth. Such surfaces consist of finitely many pieces that can be represented in the parametric form

$S_D: \vec{r}=\vec{r}(u,v), \quad (u,v)\in D$

with continuously differentiable function $\vec{r}.$ The area of an individual piece is defined by the formula

$A(S_D) = \iint_D\left |\vec{r}_u\times\vec{r}_v\right | \, du \, dv.$

Thus the area of S_D is obtained by integrating the length of the normal vector $\vec{r}_u\times\vec{r}_v$ to the surface over the appropriate region D in the parametric uv plane. The area of the whole surface is then obtained by adding together the areas of the pieces, using additivity of surface area. The main formula can be specialized to different classes of surfaces, giving, in particular, formulas for areas of graphs z = f(x,y) and surfaces of revolution.

One of the subtleties of surface area, as compared to arc length of curves, is that surface area cannot be defined simply as the limit of areas of polyhedral shapes approximating a given smooth surface. It was demonstrated by Hermann Schwarz that already for the cylinder, different choices of approximating flat surfaces can lead to different limiting values of the area (Known as Schwarz's paradox.) ^[1] .^[2]

Various approaches to general definition of surface area were developed in the late nineteenth and the early twentieth century by Henri Lebesguè and Hermann Minkowski. While for piecewise smooth surfaces there is a unique natural notion of surface area, if a surface is very irregular, or rough, then it may not be possible to assign any area at all to it. A typical example is given by a surface with spikes spread throughout in a dense fashion. Many surfaces of this type occur in the theory of fractals. Extensions of the notion of area which partially fulfill its function and may be defined even for very badly irregular surfaces are studied in the geometric measure theory. A specific example of such an extension is the Minkowski content of the surface.

Common formulas [edit]

Surface areas of common solids
Shape	Equation	Variables
Cube	$6s^2 \,$	s = side length
Rectangular prism	$2(\ell w + \ell h + wh) \,$	ℓ = length, w = width, h = height
All Prisms	$2B + Ph \,$	B = the area of one base, P = the perimeter of one base, h = height
Sphere	$4\pi r^2 \,$	r = radius of sphere
Spherical lune	$2r^2\theta \,$	r = radius of sphere, θ = dihedral angle
Closed cylinder	$2\pi r(h+r) \,$	r = radius of the circular base, h = height of the cylinder
Lateral surface area of a cone	$\pi r \left(\sqrt{r^2+h^2}\right) = \pi rs \,$	$s = \sqrt{r^2+h^2}$ s = slant height of the cone, r = radius of the circular base, h = height of the cone
Full surface area of a cone	$\pi r \left(r + \sqrt{r^2+h^2}\right) = \pi r(r + s) \,$	s = slant height of the cone, r = radius of the circular base, h = height of the cone
Pyramid	$B + \frac{PL}{2}$	B = area of base, P = perimeter of base, L = slant height

Ratio of surface areas of a sphere and cylinder of the same Radius and Volume [edit]

A cone, sphere and cylinder of radius r and height h.

The above formulas can be used to show that the surface area of a sphere and cylinder of the same radius and height are in the ratio 2 : 3, as follows.

Let the radius be r and the height be h (which is 2r for the sphere).

$\begin{array}{rlll} \text{Sphere surface area} & = 4 \pi r^2 & & = (2 \pi r^2) \times 2 \\ \text{Cylinder surface area} & = 2 \pi r (h + r) & = 2 \pi r (2r + r) & = (2 \pi r^2) \times 3 \end{array}$

The discovery of this ratio is credited to Archimedes.^[3]

In chemistry [edit]

In biology [edit]

References [edit]

^ http://www.math.usma.edu/people/Rickey/hm/CalcNotes/schwarz-paradox.pdf
^ http://mathdl.maa.org/images/upload_library/22/Polya/00494925.di020678.02p0385w.pdf
^ Rorres, Chris. "Tomb of Archimedes: Sources". Courant Institute of Mathematical Sciences. Retrieved 2007-01-02.

Yu.D. Burago, V.A. Zalgaller, L.D. Kudryavtsev (2001), "Area", in Hazewinkel, Michiel, Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4

External links [edit]

Surface Area Video at Thinkwell

Original courtesy of Wikipedia: http://en.wikipedia.org/wiki/Surface_area — Please support Wikipedia.
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Surface area

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