# Realizability

Discussion in 'Homework Help' started by Teszla, Jun 24, 2013.

1. ### Teszla Thread Starter Member

Jun 7, 2013
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How do you check if a circuit is realizable?

Look at the image above. Is this circuit realizable? If not, then why not?

2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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This circuit looks "strange", because two different batteries (two voltage source) are connect in parallel. And because of that we have short in the circuit.

3. ### tshuck Well-Known Member

Oct 18, 2012
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Using a voltage source guarantees that the positive terminal will be X Volts above the negative terminal, regardless of anything else, hence the name "independent source". A current source, likewise guarantees that a given current is guaranteed to flow through it.

With that in mind, do the voltage sources conflict with the given voltage on the shared node?

@ Jony I see no short. A voltage source, by definition, cannot be a short circuit at voltages greater than zero.

Edit: Upon looking at it again- I see what Jony meant, it is, indeed, a short circuit....

Last edited: Jun 24, 2013
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4. ### Jony130 AAC Fanatic!

Feb 17, 2009
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But we have a short between two voltage source. So large current will flow between two voltage sources.

5. ### tshuck Well-Known Member

Oct 18, 2012
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...if these were non-ideal, I would agree with the fact that a large current would flow, however, we are dealing with ideal sources. Look at the definition of the ideal voltage source and whether or not its very definition is violated.

6. ### tshuck Well-Known Member

Oct 18, 2012
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So, disregard the short circuit comment, I had to take a minute to see what you meant, and, yes, it is a short... I don't usually look at it that way, since a short is entirely realizable...

7. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Are you trying to say that there will be now current flow between two voltage sources?
I clearly see infinite amount of current flow between sources. This circuit can "work" only on paper not in real life.

8. ### MrChips Moderator

Oct 2, 2009
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What is your definition of "realizable"?

Can you wire such a circuit? Yes.
Will it give expected results? No.

Reason: Because you have two voltage sources in parallel competing with each other. You need a resistor in series somewhere between the two voltage sources.

If the two voltage sources were given the same value such as both being 2V then there would be no problem.

9. ### tshuck Well-Known Member

Oct 18, 2012
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I see a violation of the definition of a voltage source, therefore, any analysis is moot.

10. ### Teszla Thread Starter Member

Jun 7, 2013
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"Realizable" is not my choice of words but a word used in my exercise book. I assume they mean "give expected results". It's interesting that it could mean two things though. I wonder what would actually happen if you put this circuit together?

11. ### MrChips Moderator

Oct 2, 2009
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In real life, voltage sources have internal resistances.
These will assume the role of the series resistance that is missing in the circuit diagram.
When you include the internal resistance you can solve for current.

12. ### Teszla Thread Starter Member

Jun 7, 2013
43
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I see.

What about in the following case?

Is this configuration realizable? It doesn't look like a short circuit.

13. ### MrChips Moderator

Oct 2, 2009
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This is ok. RD solves the problem.

There is no conflict at the top node.

The 1V and 2V sources are with reference to the voltage drop across each individual voltage source, not with respect to any node in the circuit.

The positive terminal of the 1V source and the positive terminal of the 2V source are both at the same potential.

Edit: You edited your post which said that there was a conflict at the top node.

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14. ### Jony130 AAC Fanatic!

Feb 17, 2009
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I don't see any problem with this circuit

What makes you think so ?

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15. ### Teszla Thread Starter Member

Jun 7, 2013
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Isn't everything marked red here one single node? What would the voltage be for this node? Wouldn't it be determined by both the 1V and the 2V voltage source?

16. ### tshuck Well-Known Member

Oct 18, 2012
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Relative to?

Voltage is a difference in electrical potential so you must specify the reference. Typically, this is called common, or ground.

If, say you were to take the bottom-most node as the reference, the node you indicated is 1V. If, however, you choose the reference to be the right-most node, between Rc and Rd, the node you indicated would be 2V.