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

Discussion in 'Homework Help' started by thexy, Dec 26, 2015.

1. ### thexy Thread Starter Member

Dec 13, 2015
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0

There is mistake on photo it's 100mA
Uc(t<0)=Uc(t>0)

But what is next I should do?
I=I1-I2
I=100mA-(U/2kOhm)
Or?

2. ### GopherT AAC Fanatic!

Nov 23, 2012
6,061
3,823
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".

Dec 13, 2015
126
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4. ### GopherT AAC Fanatic!

Nov 23, 2012
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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?

Dec 13, 2015
126
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6. ### GopherT AAC Fanatic!

Nov 23, 2012
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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?

7. ### thexy Thread Starter Member

Dec 13, 2015
126
0
Do you see red I, I want to discuss about current after switch is closed? Immediately after closing.

8. ### MrAl Well-Known Member

Jun 17, 2014
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492
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.

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Dec 13, 2015
126
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No.

10. ### GopherT AAC Fanatic!

Nov 23, 2012
6,061
3,823
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.

11. ### WBahn Moderator

Mar 31, 2012
17,757
4,800
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?

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12. ### thexy Thread Starter Member

Dec 13, 2015
126
0
Total resistance is R1||(R2+R3).

13. ### WBahn Moderator

Mar 31, 2012
17,757
4,800
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.