Consider the network in Figure $2$ below :

Figure $2$: A switched DC excited LR network.

The circuit switch is at position $($ A $)$ until steady state is attained, then at time $($ t$=0)$ the switch is moved from position $($ A $)$ to position $($ B $).$ At time $($ t$=1)$ the switch is moved form position B back to position $($ A $).$

Determine an expression for the voltage across the resistor for time t is greater or equal to zero