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surface area
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{{pp-pc1|small=yes}}Image:Sphere wireframe 10deg 6r.svg|right|thumb|A spheresphereThe surface area of a solid object is a measure of the total area that the surface of the object occupies.{{MathWorld|title=Surface Area|urlname=SurfaceArea}} The mathematical definition of surface area in the presence of curved surfaces is considerably more involved than the definition of arc length of one-dimensional curves, or of the surface area for polyhedra (i.e., objects with flat polygonal faces), for which 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 surface area is based on methods of infinitesimal calculus and involves partial derivatives and double integration.A 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.

Definition

While the areas of many simple surfaces have been known since antiquity, a rigorous mathematical definition of area requires a great deal of care.This should provide a function
S mapsto A(S)
which assigns 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, â€¦, Sr 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 the 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
with a continuously differentiable function vec{r}. The area of an individual piece is defined by the formula
A(S_D) = iint_Dleft |vec{r}_utimesvec{r}_vright | , du , dv.

Common formulas

{| class="wikitable"|+ Surface areas of common solids!Shape!Equation!Variables
|Cube| 6s^2 , |s = side length
|Cuboid| 2(ell w + ell h + wh) , |â„“ = length, w = width, h = height
|Triangular prism| bh + l(a + b + c) |b = base length of triangle, h = height of triangle, l = distance between triangular bases, a, b, c = sides of triangle
Prism (geometry)>prisms| 2B + Ph , |B = the area of one base, P = the perimeter of one base, h = height
|Sphere| 4pi r^2 = pi d^2, |r = radius of sphere, d = diameter
|Spherical lune| 2r^2theta , |r = radius of sphere, Î¸ = dihedral angle
|Torus| (2pi r)(2pi R) = 4pi^2 Rr|r = minor radius (radius of the tube), R = major radius (distance from center of tube to center of torus)
Cylinder (geometry)>cylinder| 2pi r^2 + 2pi rh = 2pi r(r+h) , |r = radius of the circular base, h = height of the cylinder
cone (geometry)>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 (geometry)>Pyramid|B + frac{PL}{2}|B = area of base, P = perimeter of base, L = slant height
|Square pyramid| b^2 + 2bs = b^2 + 2bsqrt{left(frac{b}{2}right)^2+h^2} |b = base length, s = slant height, h = vertical height
|Rectangular pyramid| lw + lsqrt{left(frac{w}{2}right)^2+h^2} + wsqrt{left(frac{l}{2}right)^2+h^2} |â„“ = length, w = width, h = height
| sqrt{3}a^2 |a = side length

Ratio of surface areas of a sphere and cylinder of the same radius and height

missing image!
- Inscribed cone sphere cylinder.svg|thumb|300px|A cone, sphere and cylinder of radius r and height h.The below given 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 3end{array}The discovery of this ratio is credited to Archimedes.WEB, Chris, Rorres,weblink Tomb of Archimedes: Sources, Courant Institute of Mathematical Sciences, 2007-01-02, no,weblink" title="web.archive.org/web/20061209201723weblink">weblink 2006-12-09, {{clear}}

In chemistry

(File:Surface area.svg|thumb|Surface area of particles of different sizes.){{see also|Accessible surface area}}Surface area is important in chemical kinetics. Increasing the surface area of a substance generally increases the rate of a chemical reaction. For example, iron in a fine powder will combust, while in solid blocks it is stable enough to use in structures. For different applications a minimal or maximal surface area may be desired.{{clear}}

In biology

-
The surface area of an organism is important in several considerations, such as regulation of body temperature and digestion. Animals use their teeth to grind food down into smaller particles, increasing the surface area available for digestion. The epithelial tissue lining the digestive tract contains microvilli, greatly increasing the area available for absorption. Elephants have large ears, allowing them to regulate their own body temperature. In other instances, animals will need to minimize surface area; for example, people will fold their arms over their chest when cold to minimize heat loss.The surface area to volume ratio (SA:V) of a cell imposes upper limits on size, as the volume increases much faster than does the surface area, thus limiting the rate at which substances diffuse from the interior across the cell membrane to interstitial spaces or to other cells. Indeed, representing a cell as an idealized sphere of radius r, the volume and surface area are, respectively, V = 4/3 Ï€ r3; SA = 4 Ï€ r2. The resulting surface area to volume ratio is therefore 3/r. Thus, if a cell has a radius of 1 Î¼m, the SA:V ratio is 3; whereas if the radius of the cell is instead 10 Î¼m, then the SA:V ratio becomes 0.3. With a cell radius of 100, SA:V ratio is 0.03. Thus, the surface area falls off steeply with increasing volume.

References

• {{eom|title=Area|id=A/a013180|author=Yu.D. Burago, V.A. Zalgaller, L.D. Kudryavtsev}}

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