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ElektromagnetismRelativistlik füüsika


füüsikaline suuruselektromagnetväljakovariantseltneljamõõtmelineHermann MinkowskierirelatiivsusteooriaLorentzi invariantpseudoskalaarseLevi-Civita sümboldeterminant4-vektorpotentsiaalikovariantneMinkowski meetrikagaMaxwelli võrrandidelektrilaengu4-voolosatuletist










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Elektromagnetvälja tensor




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Elektromagnetvälja tensor on füüsikaline suurus, mis võimaldab elektromagnetvälja kovariantselt kirjeldada. Elektromagnetvälja tensor on neljamõõtmeline tensor, mida hakkas kasutama Hermann Minkowski enda loodud erirelatiivsusteooria formalismis. Elektromagnetvälja tensor võimaldab mõningaid füüsikaseadusi väga kompaktselt kirja panna.




Sisukord





  • 1 Detailid

    • 1.1 Omadused



  • 2 Maxwelli võrrandid


  • 3 Vaata ka




Detailid |


Matemaatiline märkus: selles artiklis kasutatakse abstraktset indeksinotatsiooni.

Elektromagnetvälja tensor Fμνdisplaystyle F^mu nu esitatakse tavaliselt maatriksina:


Fμν=(0−Ex/c−Ey/c−Ez/cEx/c0−BzByEy/cBz0−BxEz/c−ByBx0)displaystyle F^mu nu =beginpmatrix0&-E_x/c&-E_y/c&-E_z/c\E_x/c&0&-B_z&B_y\E_y/c&B_z&0&-B_x\E_z/c&-B_y&B_x&0endpmatrix

või


Fμν=(0Ex/cEy/cEz/c−Ex/c0−BzBy−Ey/cBz0−Bx−Ez/c−ByBx0)displaystyle F_mu nu =beginpmatrix0&E_x/c&E_y/c&E_z/c\-E_x/c&0&-B_z&B_y\-E_y/c&B_z&0&-B_x\-E_z/c&-B_y&B_x&0endpmatrix

kus

E on elektriväli,


B on magnetväli ja


c on valguse kiirus.


Ülaltoodud märgid sõltuvad aegruumi meetrilise tensori valikust. Siin kasutatakse märgikokkulepet +---, millele vastab meetriline tensor

(10000−10000−10000−1)displaystyle beginpmatrix1&0&0&0\0&-1&0&0\0&0&-1&0\0&0&0&-1endpmatrix


Omadused |


Väljatensori maatrikskujust ilmnevad järgmised omadused:



  • antisümmeetria: Fμν=−Fνμdisplaystyle F^mu nu ,=-F^nu mu (millest ka nimi bivektor).

  • elektrivälja tensoril on kuus sõltumatut komponenti.

Et tegemist on tensoriga, siis moodustub kontraktsioonidel Lorentzi invariant:


FμνFμν= 2(B2−E2c2)=invariantdisplaystyle F_mu nu F^mu nu = 2left(B^2-frac E^2c^2right)=mathrm invariant

Tensori Fμνdisplaystyle F^mu nu , korrutis oma duaalse tensoriga annab pseudoskalaarse invariandi:


ϵαβγδFαβFγδ=4c(B→⋅E→)=invariantdisplaystyle epsilon _alpha beta gamma delta F^alpha beta F^gamma delta =frac 4cleft(vec Bcdot vec Eright)=mathrm invariant ,

kus  ϵαβγδdisplaystyle epsilon _alpha beta gamma delta , on neljandat järku täielikult antisümmeetiline pseudotensor ehk Levi-Civita sümbol.


Märkus: ülalasuva avaldise märk sõltub Levi-Civita sümboli jaoks kasutatavast märgikokkuleppest. Siin kasutatud kokkulepe on  ϵ0123displaystyle epsilon _0123, = +1.


Elektromagnetvälja tensori determinant on



det(F)=1c2(B→⋅E→)2displaystyle det left(Fright)=frac 1c^2left(vec Bcdot vec Eright)^2.

Elektromagnetvälja tensori saab kirja panna 4-vektorpotentsiaali Aαdisplaystyle A^alpha , kaudu:


Fαβ =def ∂Aβ∂xα−∂Aα∂xβ =def ∂αAβ−∂βAαdisplaystyle F_alpha beta stackrel mathrm def = frac partial A_beta partial x^alpha -frac partial A_alpha partial x^beta stackrel mathrm def = partial _alpha A_beta -partial _beta A_alpha

Kus nelivektorpotentsiaal on moodustatud magnetvälja vektorpotentsiaalist ja elektrivälja (skalaarsest) potentsiaalist:


Aμ=(ϕc,A→)displaystyle A^mu =left(frac phi c,vec Aright)

ja selle kovariantne kuju leitakse korrutamisel Minkowski meetrikaga ηdisplaystyle eta , :


Aμ=ημνAν=(−ϕc,A→)displaystyle A_mu ,=eta _mu nu A^nu =left(-frac phi c,vec Aright)


Maxwelli võrrandid |



Next.svg Pikemalt artiklis Maxwelli võrrandid.

Maxwelli võrrandid võtavad elektromagnetilise tensori kaudu väljendudes võrdlemisi kompaktse kuju. Elektrivälja jaoks kehtib


∂βFαβ=μ0Jαdisplaystyle partial _beta F^alpha beta =mu _0J^alpha ,

kus


Jα=(cρ,J→)displaystyle J^alpha =(c,rho ,vec J),

on elektrilaengu 4-vool.


Võrrandid magnetismi jaoks taanduvad kujule


Fαβ,γ+Fβγ,α+Fγα,β=0displaystyle F_alpha beta ,gamma +F_beta gamma ,alpha +F_gamma alpha ,beta =0,

kus koma tähistab osatuletist



∂f∂xγ≡∂γf≡f,γdisplaystyle partial f over partial x^gamma equiv partial _gamma fequiv f_,gamma .


Vaata ka |


  • Elektromagnetism



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