# I don't understand what I'm supposed to do and it's nowhere in my courses.

#### Andreea Blaga ت

Joined Jun 26, 2015
15
Hello,
I have this exercise to solve, but I don't really understand what does it require. I tried to Google it and find a definition to help me start with, but I didn't find anything helpful.
Here it is:
"The circuit, with the switch K closed, is in dc steady state. When t=0, the switch opens. Draw the operational schematic of this circuit (it should include the values of all the elements on the schematic) and write Kirchoff's equations for this schematic. R1=R2=2 ohms, R3=3 ohms, L1=L2=1 H, I3=5 A."
I don't really get what operational schematic really means and what am I supposed to do. As I understand, this is at the transient state regime. I have some exercises with it, but nothing like this.
Here is the circuit also: Any help will be much appreciated. Thanks a lot.

#### Papabravo

Joined Feb 24, 2006
20,369
Before the switch is opened all the voltages and currents are constant and unchanging. It should present no difficulty to figure out what they are. At the moment the switch is opened R3 and the switch are removed from the circuit and the current through them goes to zero. This requires that other parts of the circuit will have a transient response. Your exercise is to figure out what that transient response is. the basic prototype for this circuit is a single RL circuit with a switch that gets opened. What is the first order response of such a circuit. Here you have two circuits in parallel.

#### Andreea Blaga ت

Joined Jun 26, 2015
15
Wow, thanks a lot. I'll try to solve it and see how it goes. #### WBahn

Joined Mar 31, 2012
29,134
I see a big problem here -- and hopefully I'm not giving away the point that the exercise is supposed to have you find on your own.

When the switch it closed there is some non-zero current in R3 meaning that the currents in R1 and R2 do not sum up to the current of the source. When the switch is opened the current in R3 goes to zero and, because of the inductors in the other two branches, the currents in R1 and R2 cannot change instantaneously. Hence KCL cannot be satisfied at t=0. In practice, what would happen would be that the output impedance of the supply would take up the slack and/or the voltage would spike across the switch gap and there would be an arc that would satisfy KCL.