I am learning about 1, 2 and 4 quadrant switches, used in inverters etc. I am however struggling to understand how, in the 2nd and 4th quadrant, the voltage can have an opposite direction to current. Using this picture for example
I dont understand here how you can have a positive voltage but current in the opposite direction. For e.g.
On the left, current would flow clockwise, and on the right, current will not flow, but if it did (i.e. if diode was replaced with a resistor) it would be anticlockwise. However, the 2nd and 4th quadrants indicate current in opposite direction to voltage - but I can't imagine how this could happen since (conventionally) current goes from positive to negative - the electromotive force is acting in one direction; how could current ever flow the opposite way?
Any help understanding this would be greatly appreciated; thank you.