From the standpoint of the system used by us, the task also arises to derive the momentum density from relation (18). It follows from (36)

Thus the right-hand side of (18) becomes

(38) |

We express, on the basis of (37), and by the vectors arising in the main equations, and we find

(38a) |

(38b) |

By inserting (38a,b) into (38), we obtain

(38c) |

However, now it follows from (36)

thus the second and third row of the right-hand side of (28c) assume the values

If it indeed holds, as required by (18a)

(39) |

then the second and third row are providing together:

Therefore it eventually follows from (18)

(39a) |

The comparison with (20) gives the important relation

(40) |