Consider a freely jointed chain of N bonds of length formula_69 subject to a constant elongational force f applied to its ends along the z axis and an environment temperature formula_70. An example could be a chain with two opposite charges +q and -q at its ends in a constant electric field formula_71 applied along the formula_72 axis as sketched in the figure on the right. If the direct Coulomb interaction between the charges is ignored, then there is a constant force formula_73 at the two ends.