Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Power and group velocity   Here we make use of the Hellmann-Feynman theorem, with the aim to calculate
macroscopic quantum velocities instead of forces, and we show how it is
possible to derive the expression of the work per unit time, per unit volume
(power density) done by a static external uniform electromagnetic field
interacting with a quantal body.

5
The categories of this paper are: gr-qc
31
The categories of this paper are: ao-sci
17
The categories of this paper are: q-fin
0
The categories of this paper are: hep-ph
11
The categories of this paper are: hep-lat
15
The categories of this paper are: nucl-ex
37
The categories of this paper are: supr-con
33
The categories of this paper are: plasm-ph
32
The categories of this paper are: atom-ph
10
The categories of this paper are: q-bio
22
The categories of this paper are: chao-dyn
29
The categories of this paper are: funct-an
19
The categories of this paper are: eess
12
The categories of this paper are: nucl-th
23
The categories of this paper are: solv-int
6
The categories of this paper are: astro-ph