Homework Statement

I have a general question concerning circuits. I am a bit confused as to how to determine when the current is negative. For instance, on circuit diagrams, we are shown arrows that indicate the positive direction of current. Then, when we open the switch on the circuit, the current through an inductor all of a sudden turns negative. I thought that such a discontinuous change was not possible. In general, how can I determine when the sign of the current turns negative?

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