Initial Values in circuit problems

[exis]

While going through some textbook problems I encountered several circuits that had initial conditions like the one attached where there's an initial current of 8mA when the switch is open. What's the source of that current? how can there be a current if the circuit is open?

http://img710.imageshack.us/img710/7132/73399543.jpg

Thanks

 

[berkeman]

While going through some textbook problems I encountered several circuits that had initial conditions like the one attached where there's an initial current of 8mA when the switch is open. What's the source of that current? how can there be a current if the circuit is open?

http://img710.imageshack.us/img710/7132/73399543.jpg

Thanks

That is definitely nonsensical as drawin. The only way you could establish that initial condition for time t = 0- would be if there were another switch that was closed for a time from some other voltage source, and opened just as the other switch were closed.

BTW, the current they mention of 8mA is larger than the final value of the current at t = infinity, right? So for sure that current had to be established by something else not shown in the figure, and presumably disconnected at t = 0 simultaneous with the closing of the switch shown. Weird.

 

[exis]

BTW, the current they mention of 8mA is larger than the final value of the current at t = infinity, right?

Yes it is.. the final current is 4.2mA. But just like you said, it certainly makes more sense that the initial 8mA is a result of some external source that is not being shown.Thanks a lot for replying berkeman :)