#### cuixinghe

Joined Apr 30, 2016
4
Hi, I was working on this problem, but get confused.

For DC steady-state, inductors become short ckts. But in this problem, there are two inductors in parallel. I am confused about how the current flow through each inductors.

The problem is shown in the attachment.

#### shteii01

Joined Feb 19, 2010
4,644
Step 1.
Assume that the directions shown in the circuit are correct.

Step 2.
Solve what they ask you to solve.

Step 3.
If some of your currents have negative sign, that tells you that assumption you made in Step 1 was wrong, so just switch the current direction and remove the negative sign.

#### cuixinghe

Joined Apr 30, 2016
4
Step 1.
Assume that the directions shown in the circuit are correct.

Step 2.
Solve what they ask you to solve.

Step 3.
If some of your currents have negative sign, that tells you that assumption you made in Step 1 was wrong, so just switch the current direction and remove the negative sign.
Thank you for answering my question. What I don't understand here is when the inductors become short ckt, how the current flow through each one of them. Since they are in parallel, I am not really sure which is the one that current actually flow through.

#### shteii01

Joined Feb 19, 2010
4,644
Thank you for answering my question. What I don't understand here is when the inductors become short ckt, how the current flow through each one of them. Since they are in parallel, I am not really sure which is the one that current actually flow through.
Which part of the hint, that they gave you, you did not understand?

#### cuixinghe

Joined Apr 30, 2016
4
Which part of the hint, that they gave you, you did not understand?
I don't really understand what does "i1 and i2 are time varying at first" mean. Does that information help solving the question?

#### shteii01

Joined Feb 19, 2010
4,644
I don't really understand what does "i1 and i2 are time varying at first" mean. Does that information help solving the question?
It means that current is not constant. At some point in the past you had a circuit that did not have any sources, the inductors were discharged. Then you connected the current sources, the current start to flow out of current sources and entering the inductors, BUT! This process IS NOT instantaneous! It takes time for current to enter the inductor and for inductor to become fully charged. At first there are just a few electrons and inductor starts showing a weak magnetic field, then there is a small stream of electrons and the magnetic field becomes stronger, then there is a torrent of electrons and inductor becomes fully charged, and the magnetic field becomes as strong as it can be. This is what they are talking about. Yes, later, much later, the current sources have been connected to the circuit for a long long time and inductors are fully charged and have been fully charged for minutes or hours or days, yes we know that at this stage they act as short, but they ask you to temporarily suspend this knowledge. Go back, take the circuit that has no sources, that has discharged inductors, connect the source, and now apply the knowledge that inductors are charging, in this state they will start with zero current, progress to some value of current and as the current increases, there will be a voltage across the inductor... this voltage is the clue you need to calculate the values of the resistors.

Also. Since you have two sources in the circuit, you might want to use superposition in order to solve the equations.

#### cuixinghe

Joined Apr 30, 2016
4
It means that current is not constant. At some point in the past you had a circuit that did not have any sources, the inductors were discharged. Then you connected the current sources, the current start to flow out of current sources and entering the inductors, BUT! This process IS NOT instantaneous! It takes time for current to enter the inductor and for inductor to become fully charged. At first there are just a few electrons and inductor starts showing a weak magnetic field, then there is a small stream of electrons and the magnetic field becomes stronger, then there is a torrent of electrons and inductor becomes fully charged, and the magnetic field becomes as strong as it can be. This is what they are talking about. Yes, later, much later, the current sources have been connected to the circuit for a long long time and inductors are fully charged and have been fully charged for minutes or hours or days, yes we know that at this stage they act as short, but they ask you to temporarily suspend this knowledge. Go back, take the circuit that has no sources, that has discharged inductors, connect the source, and now apply the knowledge that inductors are charging, in this state they will start with zero current, progress to some value of current and as the current increases, there will be a voltage across the inductor... this voltage is the clue you need to calculate the values of the resistors.

Also. Since you have two sources in the circuit, you might want to use superposition in order to solve the equations.
I really appreciate your help! I think I know how to do this problem now. Thanks again!

#### MrAl

Joined Jun 17, 2014
10,054
Hi,

I guess they are assuming ideal inductors here.

#### WBahn

Joined Mar 31, 2012
28,465
Hi,

I guess they are assuming ideal inductors here.
Yes, they have to make that assumption because they don't model any parasitic elements.