Do lines of force only exist in a one wire situation as opposed to an extension cord?

"Lines of force" refers to the electric field that is established between positive and negative charges. Wires don't have lines of force. An extension cord carries a current, and consequently can induce a magnetic field, if that's what you mean.

Tell us what you mean by line of force. All conductors with an A/C electric current have a magnetic field surrounding them. The direction of the magnetic field depends on the direction of the current. Therefore, a hot conductor will have a magnetic field in one direction and the neutral will have a magnetic field in the opposite direction. The two fields cancel each other out. That is the reason that you must always have the neutral in the same cable as the hot in an A/C circuit.

The two fields cancel each other out.

Picking a nit here, but for unshielded cables the fields only cancel each other at points that are equidistant from both conductors - e.g. at the midpoint between the two conductors. In my business (development of high-speed Ethernet over copper pairs) this is a very important issue, as the interacting magnetic fields from a pair of wires always induces crosstalk on adjacent pairs. But we're getting a bit off topic ;)