## Notation

`Vectors are written in boldface upright type, e.g., r, F; the same`

letters printed in lightface italic type (r, F) denote the modulus of

a vector.

Unit vectors

j, k are the unit vectors of the Cartesian coordinates x, y, z (some-

times the unit vectors are denoted as ex , ey , ez),

ep , eq), ez are the unit vectors of the cylindrical coordinates p, p, z,

n, i are the unit vectors of a normal and a tangent.

Mean values are taken in angle brackets ( ), e.g., (v), (P).

Symbols A, d, and 6 in front of quantities denote:

A, the finite increment of a quantity, e.g. Ar = r 2 — r 1 ; AU =

U2 - U1,

d, the differential (infinitesimal increment), e.g. dr, dU,

8, the elementary value of a quantity, e.g. 6A, the elementary work.

Time derivative of an arbitrary function f is denoted by dfldt,

or by a dot over a letter, f.

Vector operator V ("nabla"). It is used to denote the following

operations:

Vy, the gradient of q) (grad (p).

V •E, the divergence of E (div E),

V X E, the curl of E (curl E).

`Integrals of any multiplicity are denoted by a single sign S and`

differ only by the integration element: dV, a volume element, dS,

a surface element, and dr, a line element. The sign denotes an

integral over a closed surface, or around a closed loop.