  In this paper, we present in detail consistent QED (and scalar QED)
calculations of particle creation effects in external electromagnetic field
that correspond to three most important exactly solvable cases of t-electric
potential steps: Sauter-like electric field, T-constant electric field, and
exponentially growing and decaying electric fields. In all these cases, we
succeeded to obtain new results, such as calculations in modified
configurations of the above mentioned steps and detailed considerations of new
limiting cases in already studied before steps. As was recently discovered by
us, the information derived from considerations of exactly solvable cases
allows one to make some general conclusions about quantum effects in fields for
which no closed form solutions of the Dirac (or Klein-Gordon) equation are
known. In the present article we briefly represent such conclusions about an
universal behavior of vacuum mean values in slowly varying strong electric
fields.
