# How do I find value I immediately after switch is being closed?

#### GopherT

Joined Nov 23, 2012
8,009
The voltage? Which node? Where is the ground reference (I assume the bottom of the schematic).

If at the switch, what do you think the voltage will be if connected to ground? If not zero, you need to add some details on the resistance of your wires and your time-scale definition of "immediately".

#### thexy

Joined Dec 13, 2015
130
The voltage? Which node? Where is the ground reference (I assume the bottom of the schematic.

#### GopherT

Joined Nov 23, 2012
8,009
The voltage at the node between R1 and R2 will be different than the voltage between R1 and R2 (when switch is open).

What point in the circuit do you talk about when you are asking for voltage?

#### thexy

Joined Dec 13, 2015
130
The voltage at the node between R1 and R2 will be different than the voltage between R1 and R2 (when switch is open).

What point in the circuit do you talk about when you are asking for voltage?

#### GopherT

Joined Nov 23, 2012
8,009
Ok, at which point in the circuit do you want to discuss? You have parallel and series in this circuit. Current will not be equal everywhere. So, what point are you interested in?

#### thexy

Joined Dec 13, 2015
130
Ok, at which point in the circuit do you want to discuss? You have parallel and series in this circuit. Current will not be equal everywhere. So, what point are you interested in?
Do you see red I, I want to discuss about current after switch is closed? Immediately after closing.

#### MrAl

Joined Jun 17, 2014
10,621
Hello there,

So you want the current through the switch then, and is the polarity the same as that arrow?
I ask because the normal conventional current flow would be down not up, but either way is ok as long as you understand the difference.

My question is, do you know how to calculate the steady state current or cap voltage BEFORE the switch is closed? That is, assuming the 100ma current had been flowing for a very long time. That would be the starting point.

• anhnha and GopherT

#### thexy

Joined Dec 13, 2015
130
Hello there,

So you want the current through the switch then, and is the polarity the same as that arrow?
I ask because the normal conventional current flow would be down not up, but either way is ok as long as you understand the difference.

My question is, do you know how to calculate the steady state current or cap voltage BEFORE the switch is closed? That is, assuming the 100ma current had been flowing for a very long time. That would be the starting point.
No.

#### GopherT

Joined Nov 23, 2012
8,009
Then assume the capacitor is not there

So, first step is to calculate the
Series resistance of the one parallel leg.
Then calculate the total resistance seen by the current source.

, and then do the calculation of the voltage at each node.

#### WBahn

Joined Mar 31, 2012
29,183
The key things you need to note are:

Before the switch is closed, it must be assumed that everything is in steady state. What does this tell you about the current through the capacitor just before the switch is closed? What are the voltages on all the nodes just before the switch is closed?

Crossing the switch open to switch closed boundary, the voltage on the capacitor cannot change. So what are the voltages on all the nodes just after the switch is closed?

What are the currents in all of the branches consistent with these voltages?

• anhnha

#### thexy

Joined Dec 13, 2015
130
Then assume the capacitor is not there

So, first step is to calculate the
Series resistance of the one parallel leg.
Then calculate the total resistance seen by the current source.

, and then do the calculation of the voltage at each node.
Total resistance is R1||(R2+R3).

#### WBahn

Joined Mar 31, 2012
29,183
This is the total resistance seen by what?

I thought the question was to find the initial current through the switch upon closing it.

Try answering the questions in my prior post.