# Dazed and confused..

Discussion in 'Homework Help' started by TwoPlusTwo, Feb 16, 2011.

1. ### TwoPlusTwo Thread Starter Member

Oct 14, 2010
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Hi, I'm having trouble understanding this problem.

The voltage Va for the network below is -1 V. If it suddenly jumped to 20 V, what could have happened to the circuit structure?

Since the chapter introduces short and open circuits, I'm assuming it has something to do with that but I don't really understand the drawing. Is this supposed to be a branch of some larger network with some unknowns that I have to determine first?

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2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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No, you do not need to determine any unknown.
You need to think what could have happened to 4K and 3K resistors to get Va equal 20V

3. ### beenthere Retired Moderator

Apr 20, 2004
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Label the resistors Ra, Rb, And Rc. What happens to the voltage at point a when any one of them changes? Do you know how a voltage divider works? That the voltage sources will cause currents in the resistors, and that those currents can be calculated to find the voltage dropped over each resistor?

4. ### TwoPlusTwo Thread Starter Member

Oct 14, 2010
51
0
Ok, how is this?

Total voltage=20 V - 4 V=16 V

Current=16 V/(4+3+1)kΩ=2mA

U1=2mA*4kΩ=8 V

U2=2mA*3kΩ=6 V

U3=2mA*1kΩ=2 V

5. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Wrong
Total voltage is equal
20V - (-4V) = 24V

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6. ### TwoPlusTwo Thread Starter Member

Oct 14, 2010
51
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Ok. I fixed it. So is the rest of it ok?

7. ### TwoPlusTwo Thread Starter Member

Oct 14, 2010
51
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And why exactly is Va = - 1 V?

8. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Yes, looks ok

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9. ### TwoPlusTwo Thread Starter Member

Oct 14, 2010
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I still don't understand why Va has to be -1 V in this configuration. But if I were to guess at a solution to the original problem it would be that Va has jumped from -1 V to 20 V because the 4k and 3k resistors have been shorted out. Does that make sense?

10. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Becaues Va voltage in not a voltage drop across 1K resistor.
Va is a voltage between Va and GND.

Yes, you right

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11. ### TwoPlusTwo Thread Starter Member

Oct 14, 2010
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Great! I think I understand. Va is given by

- 4 V + U3 = - 4 + 3 V = - 1 V

12. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Great job

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13. ### BillB3857 AAC Fanatic!

Feb 28, 2009
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Overly thought out problem. What caused the voltage drops? What if the current somehow became zero (series circuit)

14. ### Ron H AAC Fanatic!

Apr 14, 2005
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You also need to consider a fault in the 1k resistor.