Fizikalna predstava
uredi
Divergencija vektorskoga polja u trodimenzionalnom prostoru može da se predstavi ako uzmemo malu okolinu oko neke tačke:
div
F
(
p
)
=
lim
V
→
{
p
}
∬
S
(
V
)
F
⋅
n
|
V
|
d
S
{\displaystyle \operatorname {div} \,\mathbf {F} (p)=\lim _{V\rightarrow \{p\}}\iint _{S(V)}{\mathbf {F} \cdot \mathbf {n} \over |V|}\;dS}
U slučaju da je fluks vektorskoga polja iz te zapremine veći od nula radi se o pozitivnoj divergenciji, a ako je manji od nula o negativnoj divergenciji. Ako je fluks polja nula tada je i divergencija jednaka nuli. Neka vektorsko polje predstavlja, na primer, brzinu širenja vazduha. Ako se vazduh zagrijava oko date tačke tada se širi, pa je divergencija pozitivna. Ako se vazduh hladi tada se skuplja, pa je divergencija negativna.
U Dekartovom sistemu
uredi
Divergencija vektorskoga polja F = U i + V j + W k jednaka je:
div
F
=
∇
⋅
F
=
∂
U
∂
x
+
∂
V
∂
y
+
∂
W
∂
z
.
{\displaystyle \operatorname {div} \,\mathbf {F} =\nabla \cdot \mathbf {F} ={\frac {\partial U}{\partial x}}+{\frac {\partial V}{\partial y}}+{\frac {\partial W}{\partial z}}.}
Definicija u krivolinijskim sistemima
uredi
div
(
A
)
=
div
(
q
1
A
1
+
q
2
A
2
+
q
3
A
3
)
=
{\displaystyle \operatorname {div} (\mathbf {A} )=\operatorname {div} (\mathbf {q_{1}} A_{1}+\mathbf {q_{2}} A_{2}+\mathbf {q_{3}} A_{3})=}
=
1
H
1
H
2
H
3
[
∂
∂
q
1
(
A
1
H
2
H
3
)
+
∂
∂
q
2
(
A
2
H
3
H
1
)
+
∂
∂
q
3
(
A
3
H
1
H
2
)
]
{\displaystyle ={\frac {1}{H_{1}H_{2}H_{3}}}\left[{\frac {\partial }{\partial q_{1}}}(A_{1}H_{2}H_{3})+{\frac {\partial }{\partial q_{2}}}(A_{2}H_{3}H_{1})+{\frac {\partial }{\partial q_{3}}}(A_{3}H_{1}H_{2})\right]}
, gde su
H
i
{\displaystyle H_{i}}
Lameovi koeficijenti .
U slučaju Rimanovoga krivolinijskoga prostora definisanoga metričkim tenzorom
g
i
j
{\displaystyle g_{ij}}
divergencija je dana sa:
div
(
A
)
=
1
|
g
|
∂
∂
x
k
(
|
g
|
A
k
)
{\displaystyle \operatorname {div} (\mathbf {A} )={\frac {1}{\sqrt {|g|}}}{\frac {\partial }{\partial x^{k}}}\left({\sqrt {|g|}}A^{k}\right)}
a metrika prostora definisana je sa:
d
s
2
=
∑
i
,
j
=
1
n
g
i
j
d
x
i
d
x
j
{\displaystyle ds^{2}=\sum _{i,j=1}^{n}g_{ij}dx^{i}dx^{j}}
.
Cilindrične koordinate
uredi
Za cilindrični koordinatni sistem imamo Lameove koeficijente:
H
1
=
1
H
2
=
r
H
3
=
1
{\displaystyle {\begin{matrix}H_{1}=1\\H_{2}=r\\H_{3}=1\end{matrix}}}
.
Dobija se:
div
A
(
r
,
θ
,
z
)
=
1
r
∂
∂
r
(
A
r
r
)
+
1
r
∂
∂
θ
(
A
θ
)
+
∂
∂
z
(
A
z
)
{\displaystyle \operatorname {div} \mathbf {A} (r,\theta ,z)={\frac {1}{r}}{\frac {\partial }{\partial r}}(A_{r}r)+{\frac {1}{r}}{\frac {\partial }{\partial \theta }}(A_{\theta })+{\frac {\partial }{\partial z}}(A_{z})}
Za sferni koordinatni sistem imamo Lameove koeficijente:
H
r
=
1
H
θ
=
r
H
ϕ
=
r
sin
θ
{\displaystyle {\begin{matrix}H_{r}=1\\H_{\theta }=r\\H_{\phi }=r\sin {\theta }\end{matrix}}}
.
Divergencija je:
div
A
(
r
,
θ
,
ϕ
)
=
1
r
2
∂
∂
r
[
A
r
r
2
]
+
1
r
sin
θ
∂
∂
θ
[
A
θ
sin
θ
]
+
1
r
sin
θ
∂
∂
ϕ
[
A
ϕ
]
{\displaystyle \operatorname {div} \mathbf {A} (r,\theta ,\phi )={\frac {1}{r^{2}}}{\frac {\partial }{\partial r}}\left[A_{r}r^{2}\right]+{\frac {1}{r\sin {\theta }}}{\frac {\partial }{\partial \theta }}\left[A_{\theta }\sin {\theta }\right]+{\frac {1}{r\sin {\theta }}}{\frac {\partial }{\partial \phi }}{\big [}A_{\phi }{\big ]}}
Parabolične koordinate
uredi
Za parabolični koordinatni sistem imamo Lameove koeficijente:
H
1
=
ξ
+
η
2
ξ
H
2
=
ξ
+
η
2
η
H
3
=
η
ξ
{\displaystyle {\begin{matrix}H_{1}={\frac {\sqrt {\xi +\eta }}{2{\sqrt {\xi }}}}\\H_{2}={\frac {\sqrt {\xi +\eta }}{2{\sqrt {\eta }}}}\\H_{3}={\sqrt {\eta \xi }}\end{matrix}}}
.
Divergencija je:
div
A
(
ξ
,
η
,
ϕ
)
=
4
ξ
+
η
∂
∂
ξ
[
A
ξ
ξ
2
+
ξ
η
2
]
+
4
ξ
+
η
∂
∂
η
[
A
η
η
2
+
ξ
η
2
]
+
1
ξ
η
∂
∂
ϕ
[
A
ϕ
]
{\displaystyle \operatorname {div} \mathbf {A} (\xi ,\eta ,\phi )={\frac {4}{\xi +\eta }}{\frac {\partial }{\partial \xi }}\left[A_{\xi }{\frac {\sqrt {\xi ^{2}+\xi \eta }}{2}}\right]+{\frac {4}{\xi +\eta }}{\frac {\partial }{\partial \eta }}\left[A_{\eta }{\frac {\sqrt {\eta ^{2}+\xi \eta }}{2}}\right]+{\frac {1}{\sqrt {\xi \eta }}}{\frac {\partial }{\partial \phi }}{\Big [}A_{\phi }{\Big ]}}
Sferoidne koordinate
uredi
Za izduženi sferoidni koordinatni sistem imamo Lameove koeficijente:
H
1
=
σ
ξ
2
−
η
2
ξ
2
−
1
H
2
=
σ
ξ
2
−
η
2
1
−
η
2
H
3
=
σ
(
ξ
2
−
1
)
(
1
−
η
2
)
{\displaystyle {\begin{matrix}H_{1}=\sigma {\sqrt {\frac {\xi ^{2}-\eta ^{2}}{\xi ^{2}-1}}}\\H_{2}=\sigma {\sqrt {\frac {\xi ^{2}-\eta ^{2}}{1-\eta ^{2}}}}\\H_{3}=\sigma {\sqrt {(\xi ^{2}-1)(1-\eta ^{2})}}\end{matrix}}}
.
Divergencija je:
div
A
(
ξ
,
η
,
ϕ
)
=
1
σ
(
ξ
2
−
η
2
)
∂
∂
ξ
[
A
ξ
(
ξ
2
−
η
2
)
(
ξ
2
−
1
)
]
+
{\displaystyle \operatorname {div} \mathbf {A} (\xi ,\eta ,\phi )={\frac {1}{\sigma (\xi ^{2}-\eta ^{2})}}{\frac {\partial }{\partial \xi }}\left[A_{\xi }{\sqrt {(\xi ^{2}-\eta ^{2})(\xi ^{2}-1)}}\right]+}
1
σ
(
ξ
2
−
η
2
)
∂
∂
η
[
A
η
(
ξ
2
−
η
2
)
(
1
−
η
2
)
]
+
1
σ
(
ξ
2
−
1
)
(
1
−
η
2
)
∂
∂
ϕ
[
A
ϕ
]
{\displaystyle {\frac {1}{\sigma (\xi ^{2}-\eta ^{2})}}{\frac {\partial }{\partial \eta }}\left[A_{\eta }{\sqrt {(\xi ^{2}-\eta ^{2})(1-\eta ^{2})}}\right]+{\frac {1}{\sigma {\sqrt {(\xi ^{2}-1)(1-\eta ^{2})}}}}{\frac {\partial }{\partial \phi }}{\Big [}A_{\phi }{\Big ]}}
div
(
a
F
+
b
G
)
=
a
div
(
F
)
+
b
div
(
G
)
{\displaystyle \operatorname {div} (a\mathbf {F} +b\mathbf {G} )=a\;\operatorname {div} (\mathbf {F} )+b\;\operatorname {div} (\mathbf {G} )}
div
(
φ
F
)
=
grad
(
φ
)
⋅
F
+
φ
div
(
F
)
,
{\displaystyle \operatorname {div} (\varphi \mathbf {F} )=\operatorname {grad} (\varphi )\cdot \mathbf {F} +\varphi \;\operatorname {div} (\mathbf {F} ),}
ili
∇
⋅
(
φ
F
)
=
(
∇
φ
)
⋅
F
+
φ
(
∇
⋅
F
)
.
{\displaystyle \nabla \cdot (\varphi \mathbf {F} )=(\nabla \varphi )\cdot \mathbf {F} +\varphi \;(\nabla \cdot \mathbf {F} ).}
Vektorska polja F i G povezana su sa rotorom
div
(
F
×
G
)
=
rot
(
F
)
⋅
G
−
F
⋅
rot
(
G
)
,
{\displaystyle \operatorname {div} (\mathbf {F} \times \mathbf {G} )=\operatorname {rot} (\mathbf {F} )\cdot \mathbf {G} \;-\;\mathbf {F} \cdot \operatorname {rot} (\mathbf {G} ),}
ili
∇
⋅
(
F
×
G
)
=
(
∇
×
F
)
⋅
G
−
F
⋅
(
∇
×
G
)
.
{\displaystyle \nabla \cdot (\mathbf {F} \times \mathbf {G} )=(\nabla \times \mathbf {F} )\cdot \mathbf {G} -\mathbf {F} \cdot (\nabla \times \mathbf {G} ).}
div
(
grad
(
φ
)
)
=
4
φ
{\displaystyle \operatorname {div} (\operatorname {grad} (\varphi ))={\mathcal {4}}\varphi }
div
(
rot
(
F
)
)
=
0
{\displaystyle \operatorname {div} (\operatorname {rot} (\mathbf {F} ))=0}
Za N-dimenzionalno vektorsko polje:
F
=
(
F
1
,
F
2
,
…
,
F
n
)
,
{\displaystyle \mathbf {F} =(F_{1},F_{2},\dots ,F_{n}),}
divergenciju u N-dimenzionalnom Euklidovom sistemu gde je
x
=
(
x
1
,
x
2
,
…
,
x
n
)
{\displaystyle \mathbf {x} =(x_{1},x_{2},\dots ,x_{n})}
i
d
x
=
(
d
x
1
,
d
x
2
,
…
,
d
x
n
)
{\displaystyle d\mathbf {x} =(dx_{1},dx_{2},\dots ,dx_{n})}
možemo da definišemo kao:
div
F
=
∇
⋅
F
=
∂
F
1
∂
x
1
+
∂
F
2
∂
x
2
+
⋯
+
∂
F
n
∂
x
n
.
{\displaystyle \operatorname {div} \,\mathbf {F} =\nabla \cdot \mathbf {F} ={\frac {\partial F_{1}}{\partial x_{1}}}+{\frac {\partial F_{2}}{\partial x_{2}}}+\cdots +{\frac {\partial F_{n}}{\partial x_{n}}}.}
Korn, Theresa M.; Korn, Granino Arthur. Mathematical Handbook for Scientists and Engineers: Definitions, Theorems, and Formulas for Reference and Review . New York: Dover Publications. str. 157—160. ISBN 978-0-486-41147-7 .