Perhaps, in that case then, it would be best to startt right from the beginning, and an analogy might help clinch the picture.
You say you've looked at atoms, and electrons, etc, if we take that idea and simpify it down to a wire being somewhat similar to a piece of garden hose filled with large ball bearings (these are electrons), a battery is a pump for pushing these bearings around, and a heating load is a much thinner bit of pipe, which heats up as the bearings rub aginst the sides of it.
Clearly with this system the 'circuit' must be complete for heat to be generateed, if you disconnect a pipe the ball bearings will go all over the floor! (this is where it breaks down, if you want a better picture, imagine this was a computer similation and we'd set the rules such that a bearing could only move into a place previously occupied by another bearing - then if a pipe were disconnected, they'd just stop moving, which is what happens in reality)
In AC power, the pump changes direction many times a second, the bearings down't move far (just sit there and shuffle back and forth), but energy is still transfered to the load, the bearings going back and forth still generate heat in the load.