@What does this symbol mean

@What does this symbol mean

It used to be a unit of weight and volume; later it became an engineering term; then it became a DOS command, such as the common sentence "@ echo off"; at the beginning, it simply expressed the English word "at". Later, this usage gradually decreased, but there were two more usages: 1. The space between the mailbox user name and the domain name

Find the set of high school physics electrical formulas,

Electric field 1. Two kinds of charge, charge conservation law, elementary charge 2. Coulomb law: F = kq1q2 / R2 (in vacuum) 3. Electric field intensity: e = f / Q (definition formula, calculation formula) 4. Electric field formed by vacuum point (source) charge e = KQ / R25. Electric field intensity of uniform electric field E = UAB / D 6