I have a question about the attached RL circuit, which produces the corresponding voltage graph under the influence of a square wave.

As you can see in the voltage graph, there's a discontinuous voltage jump to negative for the Inductor when the step voltage turns off. Is there an implicit current direction change in the circuit occurring with the square wave 0 state? An intuitive reason for the polarity change in the inductor is otherwise escaping me.

Thanks

 

You can see this animator for coil, you will also know why the relay need a diode for back emf.

 

Thanks guys!