A one-dimensional chain consists of a set of N rods each of length a. When stretched by a load, each rod can align either parallel or perpendicular to the length of the chain. The energy of a rod is -ε when aligned parallel to the length of the chain and is +ε when perpendicular to it. When the chain is in thermal equilibrium at temperature T, its average length is:
1
Na/2
2
Na
3
\(Na/\left(1+e^{-2\varepsilon /k_BT}\right)\)
4
\(Na/\left(1+e^{-2\varepsilon /k_BT}\right).\)