http://www.ohmslaw.com
you really should have a basic understand of electricity and how these BASIC princables work before you begin toying with something as complex and dangerous as a TEC. No with a TEC the resistance is not linear, it is dependant on a lot of other things but for approximations of voltage and current using an approximation of the resistance is good enough. Still, read up people this link will demistify a lot of things for you.