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unit vector
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In mathematics, a unit vector in a normed vector space is a vector (often a spatial vector) of length 1. A unit vector is often denoted by a lowercase letter with a circumflex, or "hat": {hat{imath}} (pronounced "ihat"). The term direction vector is used to describe a unit vector being used to represent spatial direction, and such quantities are commonly denoted as d. Two 2D direction vectors, d1 and d2 are illustrated. 2D spatial directions represented this way are numerically equivalent to points on the unit circle.The same construct is used to specify spatial directions in 3D. As illustrated, each unique direction is equivalent numerically to a point on the unit sphere.(File:2D Direction Vectors.svgthumbExamples of two 2D direction vectors)(File:3D Direction Vectors.tiffthumbExamples of two 3D direction vectors)The normalized vector or versor Ã» of a nonzero vector u is the unit vector in the direction of u, i.e.,
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mathbf{hat{u}} = frac{mathbf{u}}{mathbf{u}}
where u is the norm (or length) of u. The term normalized vector is sometimes used as a synonym for unit vector.Unit vectors are often chosen to form the basis of a vector space. Every vector in the space may be written as a linear combination of unit vectors.By definition, in a Euclidean space the dot product of two unit vectors is a scalar value amounting to the cosine of the smaller subtended angle. In threedimensional Euclidean space, the cross product of two arbitrary unit vectors is a third vector orthogonal to both of them having length equal to the sine of the smaller subtended angle. The normalized cross product corrects for this varying length, and yields the mutually orthogonal unit vector to the two inputs, applying the righthand rule to resolve one of two possible directions.Orthogonal coordinates
Cartesian coordinates
Unit vectors may be used to represent the axes of a Cartesian coordinate system. For instance, the unit vectors in the direction of the x, y, and z axes of a three dimensional Cartesian coordinate system are
mathbf{hat{i}} = begin{bmatrix}1end{bmatrix}, ,, mathbf{hat{j}} = begin{bmatrix}01end{bmatrix}, ,, mathbf{hat{k}} = begin{bmatrix}01end{bmatrix}
They are sometimes referred to as the versors of the coordinate system, and they form a set of mutually orthogonal unit vectors, typically referred to as a standard basis in linear algebra.They are often denoted using normal vector notation (e.g., i or vec{imath}) rather than standard unit vector notation (e.g., mathbf{hat{imath}}). In most contexts it can be assumed that i, j, and k, (or vec{imath}, vec{jmath}, and vec{k}) are versors of a 3D Cartesian coordinate system. The notations (mathbf{hat{x}}, mathbf{hat{y}}, mathbf{hat{z}}), (mathbf{hat{x}}_1, mathbf{hat{x}}_2, mathbf{hat{x}}_3), (mathbf{hat{e}}_x, mathbf{hat{e}}_y, mathbf{hat{e}}_z), or (mathbf{hat{e}}_1, mathbf{hat{e}}_2, mathbf{hat{e}}_3), with or without hat, are also used, particularly in contexts where i, j, k might lead to confusion with another quantity (for instance with index symbols such as i, j, k, used to identify an element of a set or array or sequence of variables).When a unit vector in space is expressed, with Cartesian notation, as a linear combination of i, j, k, its three scalar components can be referred to as direction cosines. The value of each component is equal to the cosine of the angle formed by the unit vector with the respective basis vector. This is one of the methods used to describe the orientation (angular position) of a straight line, segment of straight line, oriented axis, or segment of oriented axis (vector).Cylindrical coordinates
The three orthogonal unit vectors appropriate to cylindrical symmetry are: mathbf{hat{rho}} (also designated mathbf{hat{e}} or boldsymbol{hat s}), representing the direction along which the distance of the point from the axis of symmetry is measured;
 boldsymbol{hat varphi}, representing the direction of the motion that would be observed if the point were rotating counterclockwise about the symmetry axis;
 mathbf{hat{z}}, representing the direction of the symmetry axis;
mathbf{hat{rho}} = cos varphimathbf{hat{x}} + sin varphimathbf{hat{y}}
boldsymbol{hat varphi} = sin varphimathbf{hat{x}} + cos varphimathbf{hat{y}}
mathbf{hat{z}}=mathbf{hat{z}}.
It is important to note that mathbf{hat{rho}} and boldsymbol{hat varphi} are functions of varphi, and are not constant in direction. When differentiating or integrating in cylindrical coordinates, these unit vectors themselves must also be operated on. For a more complete description, see Jacobian matrix. The derivatives with respect to varphi are:
frac{partial mathbf{hat{rho}}} {partial varphi} = sin varphimathbf{hat{x}} + cos varphimathbf{hat{y}} = boldsymbol{hat varphi}
frac{partial boldsymbol{hat varphi}} {partial varphi} = cos varphimathbf{hat{x}}  sin varphimathbf{hat{y}} = mathbf{hat{rho}}
frac{partial mathbf{hat{z}}} {partial varphi} = mathbf{0}.
Spherical coordinates
The unit vectors appropriate to spherical symmetry are: mathbf{hat{r}}, the direction in which the radial distance from the origin increases; boldsymbol{hat{varphi}}, the direction in which the angle in the xy plane counterclockwise from the positive xaxis is increasing; and boldsymbol{hat theta}, the direction in which the angle from the positive z axis is increasing. To minimize redundancy of representations, the polar angle theta is usually taken to lie between zero and 180 degrees. It is especially important to note the context of any ordered triplet written in spherical coordinates, as the roles of boldsymbol{hat varphi} and boldsymbol{hat theta} are often reversed. Here, the American "physics" conventionTevian Dray and Corinne A. Manogue,Spherical Coordinates, College Math Journal 34, 168169 (2003). is used. This leaves the azimuthal angle varphi defined the same as in cylindrical coordinates. The Cartesian relations are:
mathbf{hat{r}} = sin theta cos varphimathbf{hat{x}} + sin theta sin varphimathbf{hat{y}} + cos thetamathbf{hat{z}}
boldsymbol{hat theta} = cos theta cos varphimathbf{hat{x}} + cos theta sin varphimathbf{hat{y}}  sin thetamathbf{hat{z}}
boldsymbol{hat varphi} =  sin varphimathbf{hat{x}} + cos varphimathbf{hat{y}}
The spherical unit vectors depend on both varphi and theta, and hence there are 5 possible nonzero derivatives. For a more complete description, see Jacobian matrix and determinant. The nonzero derivatives are:
frac{partial mathbf{hat{r}}} {partial varphi} = sin theta sin varphimathbf{hat{x}} + sin theta cos varphimathbf{hat{y}} = sin thetaboldsymbol{hat varphi}
frac{partial mathbf{hat{r}}} {partial theta} =cos theta cos varphimathbf{hat{x}} + cos theta sin varphimathbf{hat{y}}  sin thetamathbf{hat{z}}= boldsymbol{hat theta}
frac{partial boldsymbol{hat{theta}}} {partial varphi} =cos theta sin varphimathbf{hat{x}} + cos theta cos varphimathbf{hat{y}} = cos thetaboldsymbol{hat varphi}
frac{partial boldsymbol{hat{theta}}} {partial theta} = sin theta cos varphimathbf{hat{x}}  sin theta sin varphimathbf{hat{y}}  cos thetamathbf{hat{z}} = mathbf{hat{r}}
frac{partial boldsymbol{hat{varphi}}} {partial varphi} = cos varphimathbf{hat{x}}  sin varphimathbf{hat{y}} = sin thetamathbf{hat{r}} cos thetaboldsymbol{hat{theta}}
General unit vectors
Common general themes of unit vectors occur throughout physics and geometry:BOOK, Calculus (Schaum's Outlines Series), 5th, Mc Graw Hill, F. Ayres, E. Mandelson, 2009, 9780071508612, { class="wikitable"! scope="col" width="200"  Unit vector! scope="col" width="150"  Nomenclature! scope="col" width="410"  Diagram(File:Tangent normal binormal unit vectors.svg  "200px") (File:Polar coord unit vectors and normal.svg  "200px")A normal vector mathbf{hat{n}} to the plane containing and defined by the radial position vector r mathbf{hat{r}} and angular tangential direction of rotation theta boldsymbol{hat{theta}} is necessary so that the vector equations of angular motion hold. 
mathbf{hat{n}}In terms of polar coordinates;
mathbf{hat{n}} = mathbf{hat{r}} times boldsymbol{hat{theta}}

mathbf{hat{b}} = mathbf{hat  AUTHOR1=M. R. SPIEGEL  AUTHOR3=D. SPELLMAN  ISBN=9780071615457, 
(File:Perpendicular and parallel unit vectors.svg  "200px")One unit vector mathbf{hat{e}}_{parallel} aligned parallel to a principal direction (red line), and a perpendicular unit vector mathbf{hat{e}}_{bot} is in any radial direction relative to the principal line. 
mathbf{hat{e}}_{bot} 
mathbf{hat{e}}_{angle}  (File:Angular unit vector.svg  "200px")Unit vector at acute deviation angle Ï† (including 0 or Ï€/2 rad) relative to a principal direction. 
Curvilinear coordinates
In general, a coordinate system may be uniquely specified using a number of linearly independent unit vectors mathbf{hat{e}}_n equal to the degrees of freedom of the space. For ordinary 3space, these vectors may be denoted mathbf{hat{e}}_1, mathbf{hat{e}}_2, mathbf{hat{e}}_3. It is nearly always convenient to define the system to be orthonormal and righthanded:
mathbf{hat{e}}_i cdot mathbf{hat{e}}_j = delta_{ij}
mathbf{hat{e}}_i cdot (mathbf{hat{e}}_j times mathbf{hat{e}}_k) = varepsilon_{ijk}
where delta_{ij} is the Kronecker delta (which is 1 for i = j and 0 otherwise) and varepsilon_{ijk} is the LeviCivita symbol (which is 1 for permutations ordered as ijk and âˆ’1 for permutations ordered as kji).Right versor
A unit vector in â„3 was called a right versor by W. R. Hamilton as he developed his quaternions â„ âŠ‚ â„4. In fact, he was the originator of the term vector as every quaternion q = s + v has a scalar part s and a vector part v. If v is a unit vector in â„3, then the square of v in quaternions is â€“1. By Euler's formula then, exp (theta v) = cos theta + v sin theta is a versor in the 3sphere. When θ is a right angle, the versor is a right versor: its scalar part is zero and its vector part v is a unit vector in â„3.See also
{{wiktionaryunit vector}} Cartesian coordinate system
 Coordinate system
 Curvilinear coordinates
 Fourvelocity
 Jacobian matrix and determinant
 Polar coordinate system
 Unit interval
 Unit square, cube, circle, sphere, and hyperbola
 Vector of ones
Notes
{{Reflist}}References
 BOOK, G. B. Arfken, H. J. Weber, yes, Mathematical Methods for Physicists, 5th, 2000, Academic Press, 0120598256,
 BOOK, Murray R., Spiegel, Schaum's Outlines: Mathematical Handbook of Formulas and Tables, 2nd, 1998, McGrawHill, 0070382034,
 BOOK, David J., Griffiths, Introduction to Electrodynamics, 3rd, 1998, Prentice Hall, 013805326X, registration,weblink
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