potential difference 一問

答案是一定要有potential difference 先有電流流動
potential difference 的存在是電流流動的必要條件

In a normal conductor, flow of electric current is caused by a potential difference. But in a superconductor (one that has no electrical resistance), the flow of current requires practically no potential difference.

This situation is similar to that a force is needed to maintain a body moving at constant speed on a rough surface, but no force is required if the body is moving at a perfectly smooth surface once it starts to move.