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Vector fields: Examples

Vector fields arise very naturally in physics and engineering applications from physical forces: gravitational, electrostatic, centrifugal, etc. For example, the vector field defined by the function
where $w_0$ is a real number, is associated with gravity and electrostatic attraction. The gravitational field around a planet and the electric field around a single point charge are similar to this field. The field points towards the origin (when $w_0>0$) and is inversely proportional to the square of the distance from the origin.


Gravitational/Electrostatic field: Click on the image (or link below) to run the simulation.

Link: Here


Another important example is the velocity vector field $\mathbf{v}$ of a steady-state fluid flow. The vector $\mathbf{v}(x, y)$ measures the instantaneous velocity of the fluid particles (molecules or atoms) as they pass through the point $(x, y)$. Steady-state means that the velocity at a point $(x, y)$ does not vary in time -even though the individual fluid particles are in motion. If a fluid particle moves along the curve $\mathbf{x}(t) = (x(t), y(t))$, then its velocity at time $t$ is the derivative
\mathbf{v}= \frac{d\mathbf{x}}{dt}
of its position with respect to $t$. Thus, for a time-independent velocity vector field
\mathbf{v}(x, y) = ( v_1(x, y), v_2(x, y) )
the fluid particles will move in accordance with an autonomous, first order system of ordinary differential equations
\frac{dx}{dt}= v_1(x, y),\qquad \frac{dy}{dt}= v_2(x, y)

According to the basic theory of systems of ordinary differential equations, an individual particle's motion $\mathbf{x}(t)$ will be uniquely determined solely by its initial position $\mathbf{x}(0) = \mathbf{x}_0$. In fluid mechanics, the trajectories of particles are known as the streamlines of the flow. The velocity vector $\mathbf{v}$ is everywhere tangent to the streamlines. When the flow is steady, the streamlines do not change in time. Individual fluid particles experience the same motion as they successively pass through a given point in the domain occupied by the fluid.

Examples of velocity vector fields of steady-state fluids flow are the following:

1. Rigid body rotation: $$\mathbf{v}(x,y)=(-wy,wx),\quad w\in \mathbb R.$$
2. Stagnation point: $$\mathbf{v}(x,y)=(kx,-ky), \quad k\in \mathbb R.$$
3. Vortex: $$\mathbf{v}(x,y)=\left(-\dfrac{y}{x^2+y^2},\dfrac{x}{x^2+y^2}\right).$$
4. Source and Sink: $$\mathbf{v}(x,y)=\dfrac{q}{2\pi}\left(\dfrac{x}{x^2+y^2},\dfrac{y}{x^2+y^2}\right),\quad q\in \mathbb R.$$


The  following simulations show the steady-state fluid flows defined by the above velocity fields. Click on the image (or link below) to access the simulations.

Rigid body rotation

Link: Here
Stagnation point

Link: Here

Link: Here
Source and Sink

Link: Here


Distancia, velocidad y aceleración

Cálculo Diferencial: Método para encontrar la velocidad de un movimiento cuando se conoce la distancia recorrida en un tiempo dado.
Cálculo Integral: Método para encontrar la distancia recorrida cuando se conoce la velocidad.
Para encontrar la distancia recorrida de un objeto, cuando se conoce la velocidad, se recurre al cálculo integral, es decir, se debe calcular el área bajo la curva que representa la dependencia de la velocidad respecto del tiempo.
Para encontrar la velocidad de un movimiento cuando se conoce la distancia recorrida en un tiempo dado, se recurre al cálculo diferencial, es decir, se debe calcular la derivada de la curva que representa la dependencia de la distancia respecto del tiempo.
El problema de la integración es recíproco al problema de derivación y viceversa.
- Al integrar, función velocidad, se calcula distancia. - Al derivar, función distancia, se calcula velocidad
Applets de Geogebra 
Representación del movimiento. En estos applets pueden modificar…

Representaciones en 3D: Espiral y curva paramétrica de pi

Otro uso de proyecciones ortográficas con Geogebra.

1. Curva paramétrica para representar a $\pi$
Para generar la curva que representa a $\pi$ se requiere utilizar una ecuación paramétrica.

En matemáticas, una ecuación paramétrica permite representar una o varias curvas o superficies en el plano o en el espacio, mediante valores arbitrarios o mediante una constante, llamada parámetro, en lugar de mediante una variable independiente de cuyos valores se desprenden los de la variable dependiente.
Por ejemplo: Dada la ecuación $y = x^2$, una parametrización tendrá la forma $$\begin{cases} x = u (t) \\ y = v (t) \end{cases}$$
Una parametrización posible sería $$\begin{cases} x = t \\ y = t^2 \end{cases}$$
Una circunferencia con centro en el origen de coordenadas y radio $r$ verifica que $x^2 + y^2 =r^2$.
Una expresión paramétrica de la circunferencia es $\begin{cases} x = r \cos t  \\ y = r \sin t \end{cases}$
1.1 Curva $\pi$:
En nuestro caso, para generar la curva $\pi$, es necesario defini…

Möbius transformations with stereographic projections

A Möbius transformation of the plane is a rational function of the form $$f(z) = \frac{a z + b}{c z + d}$$ of one complex variable $z$. Here the coefficients $a, b, c, d$ are complex numbers satisfying $ad - bc\neq 0.$
Geometrically, a Möbius transformation can be obtained by stereographic projection of the complex plane onto an admissible sphere in $\mathbb R^3$, followed by a rigid motion of the sphere in $\mathbb R^3$ which maps it to another admissible sphere, followed by stereographic projection back to the plane. 

A Möbius transformation is a combination of dilatation, inversion, translation, and rotation.
The following applet shows the stereographic projection representing different Möbius transformations. Move the sliders to see what happens.

Made with GeoGebra, link here: This applet was made based on the work of D. N. Arnold and J. Rogness.
Further reading:
Arnold, D. N. & Rogness, J. (2008).  Möbius transformations revea…