| MadSci Network: Physics |
First, many symbols are used without having a special reason for their use. Scientists try to use a symbol that relates in some way to the name of the quantity represented, but sometimes the symbol that makes the most sense has already been used for another quantity. In that case another symbol will be used, even if it has no real letter connection to the quantity it represents.
Second, the use of certain symbols depends on the book (or other source) that you are using. That is, two books on the same subject may use two different symbols for the exact same quantity. So it is important to be sure of the symbolic convention being used by whatever source you are using.
Third, there is a very good page about symbols at NIST, which you can find here. At that page I do not find the symbol "U" used for electrical tension (voltage). Your book may be one of those which is different than most other sources!
John Link, MadSci Physicist
Try the links in the MadSci Library for more information on Physics.