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Συντηρητική Επίδρασις

Conservative Interaction


Είναι ένα είδος Φυσικής Επίδρασης.

Ετυμολογία[]

Η ονομασία "στυντηρητική" σχετίζεται ετυμολογικά με την λέξη "συντήρηση".

Περιγραφή[]

Ikl Επιδράσεις Ikl
Είδη
Α. Ως προς
το αποτέλεσμα
Β. Ως προς
την αιτία
α. Πεδιακές
(ή θεμελιώδεις)
β. Εφαπτικές
(ή παράγωγες)
(ή αλλιώς, αντίσταση
στερεής επιφάνειας)
(π.χ. τριβή)
Γ. Ως προς την
ύπαρξη «πρόκλησης»
Δ. Ως προς
το περιβάλλον

In vector calculus a conservative vector field is a vector field which is the gradient of a function, known in this context as a scalar potential. Conservative vector fields have the property that the line integral from one point to another is independent of the choice of path connecting the two points: it is path independent. Conversely, path independence is equivalent to the vector field being conservative. Conservative vector fields are also irrotational, meaning that (in three-dimensions) they have vanishing curl. In fact, an irrotational vector field is necessarily conservative provided that a certain condition on the geometry of the domain holds: it must be simply connected.

An irrotational vector field which is also solenoidal is called a Laplacian vector field because it is the gradient of a solution of Laplace's equation.

Definition[]

A vector field is said to be conservative if there exists a scalar field such that

Here denotes the gradient of . When the above equation holds, is called a scalar potential for .

The fundamental theorem of vector calculus states that any vector field can be expressed as the sum of a conservative vector field and a solenoidal field.

Path independence[]

A key property of a conservative vector field is that its integral along a path depends only on the endpoints of that path, not the particular route taken.

Suppose that is a region of three-dimensional space, and that is a rectifiable path in with start point and end point .

If is a conservative vector field then the gradient theorem states that

This holds as a consequence of the Chain Rule and the Fundamental Theorem of Calculus.

An equivalent formulation of this is to say that

for every closed loop in S. The converse is also true: if the circulation of F around every closed loop in an open set S is zero, then F is a conservative vector field.

Irrotational vector fields[]

Αρχείο:Irrotationalfield.svg

The above field F(x,y,z) = (−y/(x2+y2), +x/(x2+y2), 0) includes a vortex at its center, so it is non-irrotational; it is neither conservative, nor does it have path independence. However, any simply connected subset that excludes the vortex line (0,0,z) will have zero curl, ×F = 0. Such vortex-free regions are examples of irrotational vector fields.

A vector field is said to be irrotational if its curl is zero. That is, if

For this reason, such vector fields are sometimes referred to as curl-free vector fields.

It is an identity of vector calculus that for any scalar field :

Therefore every conservative vector field is also an irrotational vector field.

Provided that is a simply-connected region, the converse of this is true: every irrotational vector field is also a conservative vector field.

The above statement is not true if is not simply-connected. Let be the usual 3-dimensional space, except with the -axis removed; that is . Now define a vector field by

Then exists and has zero curl at every point in ; that is is irrotational. However the circulation of around the unit circle in the -plane is equal to . Therefore does not have the path independence property discussed above, and is not conservative.

In a simply-connected region an irrotational vector field has the path independence property. This can be seen by noting that in such a region an irrotational vector field is conservative, and conservative vector fields have the path independence property. The result can also be proved directly by using Stokes' theorem. In a connected region any vector field which has the path independence property must also be irrotational.

More abstractly, a conservative vector field is an exact 1-form. That is, it is a 1-form equal to the exterior derivative of some 0-form (scalar field) . An irrotational vector field is a closed 1-form. Since d2 = 0, any exact form is closed, so any conservative vector field is irrotational. The domain is simply connected if and only if its first homology group is 0, which is equivalent to its first cohomology group being 0. The first de Rham cohomology group is 0 if and only if all closed 1-forms are exact.

Irrotational flows[]

Πρότυπο:Main The flow velocity of a fluid is a vector field, and the vorticity of the flow can be defined by

A common alternative notation for vorticity is .[1]

If is irrotational, with , then the flow is said to be an irrotational flow. The vorticity of an irrotational flow is zero.[2]

Kelvin's circulation theorem states that a fluid that is irrotational in an inviscid flow will remain irrotational. This result can be derived from the vorticity transport equation, obtained by taking the curl of the Navier-stokes equations.

For a two-dimensional flow the vorticity acts as a measure of the local rotation of fluid elements. Note that the vorticity does not imply anything about the global behaviour of a fluid. It is possible for a fluid traveling in a straight line to have vorticity, and it is possible for a fluid which moves in a circle to be irrotational.

Conservative forces[]

Αρχείο:Conservative fields.png

Examples of potential and gradient fields in physics
Scalar fields (scalar potentials) (yellow): VG - gravitational potential; Wpot - potential energy; VC - Coulomb potential; Vector fields (gradient fields) (cyan): aG - gravitational acceleration; F - force; E - electric field strength

If the vector field associated to a force is conservative then the force is said to be a conservative force.

The most prominent examples of conservative forces are the force of gravity and the electric field associated to a static charge. According to Newton's law of gravitation, the gravitational force, , acting on a mass , due to a mass which is a distance away, obeys the equation

where is the Gravitational Constant and is a unit vector pointing from towards . The force of gravity is conservative because , where

is the Gravitational potential energy.

For conservative forces, path independence can be interpreted to mean that the work done in going from a point to a point is independent of the path chosen, and that the work W done in going around a closed loop is zero:

The total energy of a particle moving under the influence of conservative forces is conserved, in the sense that a loss of potential energy is converted to an equal quantity of kinetic energy or vice versa.

Υποσημειώσεις[]

  1. Clancy, L.J., Aerodynamics, Section 7.11, Pitman Publishing Limited, London. ISBN 0-273-01120-0
  2. Liepmann, H.W.; Roshko, A. (1993), Elements of Gas Dynamics, Courier Dover Publications, ISBN 0486419630 , pp. 194–196.

Εσωτερική Αρθρογραφία[]

Βιβλιογραφία[]

  • George B. Arfken and Hans J. Weber, Mathematical Methods for Physicists, 6th edition, Elsevier Academic Press (2005)
  • D. J. Acheson, Elementary Fluid Dynamics, Oxford University Press (2005)

Ιστογραφία[]


Ikl Κίνδυνοι ΧρήσηςIkl

Αν και θα βρείτε εξακριβωμένες πληροφορίες
σε αυτήν την εγκυκλοπαίδεια
ωστόσο, παρακαλούμε να λάβετε σοβαρά υπ' όψη ότι
η "Sciencepedia" δεν μπορεί να εγγυηθεί, από καμιά άποψη,
την εγκυρότητα των πληροφοριών που περιλαμβάνει.

"Οι πληροφορίες αυτές μπορεί πρόσφατα
να έχουν αλλοιωθεί, βανδαλισθεί ή μεταβληθεί από κάποιο άτομο,
η άποψη του οποίου δεν συνάδει με το "επίπεδο γνώσης"
του ιδιαίτερου γνωστικού τομέα που σας ενδιαφέρει."

Πρέπει να λάβετε υπ' όψη ότι
όλα τα άρθρα μπορεί να είναι ακριβή, γενικώς,
και για μακρά χρονική περίοδο,
αλλά να υποστούν κάποιο βανδαλισμό ή ακατάλληλη επεξεργασία,
ελάχιστο χρονικό διάστημα, πριν τα δείτε.



Επίσης,
Οι διάφοροι "Εξωτερικοί Σύνδεσμοι (Links)"
(όχι μόνον, της Sciencepedia
αλλά και κάθε διαδικτυακού ιστότοπου (ή αλλιώς site)),
αν και άκρως απαραίτητοι,
είναι αδύνατον να ελεγχθούν
(λόγω της ρευστής φύσης του Web),
και επομένως είναι ενδεχόμενο να οδηγήσουν
σε παραπλανητικό, κακόβουλο ή άσεμνο περιεχόμενο.
Ο αναγνώστης πρέπει να είναι
εξαιρετικά προσεκτικός όταν τους χρησιμοποιεί.

- Μην κάνετε χρήση του περιεχομένου της παρούσας εγκυκλοπαίδειας
αν διαφωνείτε με όσα αναγράφονται σε αυτήν

IonnKorr-System-00-goog



>>Διαμαρτυρία προς την wikia<<

- Όχι, στις διαφημίσεις που περιέχουν απαράδεκτο περιεχόμενο (άσεμνες εικόνες, ροζ αγγελίες κλπ.)


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