At \$t=0\$ switch A opens, switch B closes when there is a voltage, U2. U is a DC voltage source \$> 0 \$ also is the circuit until \$t = 0 \$ in stationary state.
My question is whether switch B is open for \$t < 0\$? If yes, will it stay open after \$t=0\$? I mean the circuit isn't closed at \$t=0\$ or am I wrong?
Best Answer
Since there is no voltage across switch B in steady-state (the resistor discharges the capacitor to 0V) it should be open (the requirement is voltage >0).
Your second question is the point of this exercise-- if switch B is open, will the voltage across it be <0 or >0, given the direction of current U. BTW, this is a representation of a very common circuit configuration where the switches would be a transistor and a diode respectively.