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
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
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.