21. Although signs for these values vary, sign convention usually dictates positive curvature when convex and negative when concave. 22. Throughout this article we work with a metric signature that is mostly positive ( ); see sign convention . 23. Engineering descriptions of the computation of bending moments can be confusing because of unexplained sign conventions and implicit assumptions. 24. In an important sign convention , work that adds to the internal energy of the system is counted as positive. 25. With this now often used sign convention for work, the first law for a closed system may be written: 26. In particular, many undergraduate physics textbooks use an alternate sign convention in which convex surfaces of lenses are always positive. 27. An alternate sign convention is to consider the work performed " on " the system by its surroundings as positive. 28. It is common to take a different sign convention for this equation than one typically does when defining fundamental solutions. 29. This latter convention has the convenient advantage that the s and p sign conventions are the same at normal incidence. 30. The choice of sign for time-dependence, exp (-" i?t " ), dictates the sign convention for the imaginary part of permittivity.
