I was trying to explain the dynamics of a DC current in a superconductor -- for example, a ring -- and was asked, essentially,
Cooper pairs are the charge carriers, are all in the same state, and cannot scatter. Well how do they "change direction" to travel in a circle then? Equivalently, why doesn't their inertia cause them to collide with the superconductor edge? What happens in a superconducting wire with a sharp angle?
After some thought, this is what I came up with:
The superfluid/Cooper pairs, since they are all in the same state, can be thought of like a single (rigid?) body with a total charge and total mass, in the "shape" of the superconductor. The initial applied external electric or magnetic field gives this body a "push" and it moves without dissipation. So for example a magnetic field sets it rotating, while an electric field gives it a linear momentum (with Andreev reflection/creating and destroying Cooper pairs at the ends). Therefore there are no worries about how individual Cooper pairs find their direction, they are "pulled along" by the Cooper pair state as a whole.
Leaving aside flux quantisation, is there anything wrong with this analogy, can you add more detail, or can you do better?