# Confused about resistance-less wire

Discussion in 'Homework Help' started by epsilon58, Jun 24, 2015.

1. ### epsilon58 Thread Starter New Member

Jun 19, 2015
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I constructed this schematic, so if there are any inconsistencies please let me know . R1 has a voltage of 10V. I2 is equal to zero. R3 and R4 share the same current value and one can apply the voltage divider rule upon them.

Thank you

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2. ### Alec_t AAC Fanatic!

Sep 17, 2013
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Why do you think I2 is zero?

3. ### epsilon58 Thread Starter New Member

Jun 19, 2015
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Because all of the current will go down the the wire with zero resistance.

4. ### tjohnson Active Member

Dec 23, 2014
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No, that's not correct. All wire, unless it is ideal, has a negligible amount of resistance. But regardless of whether or not the wire has zero resistance, I2 ≠ 0.

Last edited: Jun 24, 2015
5. ### epsilon58 Thread Starter New Member

Jun 19, 2015
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I2 will not be zero? And the wire without a resistor will have no current flowing through it?

6. ### tjohnson Active Member

Dec 23, 2014
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Right, and wrong. This is a parallel circuit, and Kirchhoff's Current Law (KCL) states that "The sum of current into a junction equals the sum of current out of the junction." Since the circuit is closed, there is current flowing in each of the three wires that make up the junction.

7. ### epsilon58 Thread Starter New Member

Jun 19, 2015
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Could one say that the wire without the resistance is shorting R2? Also, if a wire meets at a junction it can never have zero current unless it is attached to an open circuit?

8. ### WBahn Moderator

Mar 31, 2012
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By that same reasoning, why isn't R1 equal to zero?

Do you see the similarity? In either case you have a resistor connected to a (potentially) non-zero voltage node on one side and a 0V node on the other side.

9. ### tjohnson Active Member

Dec 23, 2014
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No, because R2 is grounded to the same power source.
I think that's correct, but don't take my word for it. An expert will know.

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10. ### epsilon58 Thread Starter New Member

Jun 19, 2015
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R1 has 10V across it, correct? R2 is in parallel with R4 so it has to have a voltage and therefore a current, so I2 can't be zero. Ok. THank you. So if I were to assign values to all of the resistors I could find the current across the wire without a resistor.

11. ### tjohnson Active Member

Dec 23, 2014
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12. ### epsilon58 Thread Starter New Member

Jun 19, 2015
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I feel like that this PDF better demonstrates my misunderstandings. On page one will I2 be equal to zero because it is being shorted? Also on the second page, does R2 have zero current flowing thought it?

• ###### SCN_0014.pdf
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Last edited: Jun 24, 2015
13. ### WBahn Moderator

Mar 31, 2012
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One does not lead to another. Having current in the wire with the resistor does not mean that there can't be any current in the wire without a resistor.

Use KCL. Label all of your nodes, write expressions for the current into/out of each node, and then apply

No. In order to short R2, the wire would need to be connected across R2 (i.e., one end of the wire to one end of R2 and the other end of the wire to the other end of R2).

No. If a wire connects two junctions (whether or not there is a resistor present), then no current will flow if both junctions are at the same voltage.

If the other end is open-circuited, then this results in both ends having the same voltage.

14. ### WBahn Moderator

Mar 31, 2012
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In this case i2 < 0

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15. ### WBahn Moderator

Mar 31, 2012
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Correct.

Yes.

The fact that they are in parallel does NOT mean that it has to have a voltage or a current. Using the word "so" indicates a cause-effect relationship that is not valid.

Correct. This is a cause-effect relationship: IF R2 has a voltage across it, it has a current through it.

Yes.

You would have i that is going through R1 and you would have -i2 (because the current in R2 is flowing in the direction opposite of that shown for i1).

Also, note in your diagram that you have i3 defined in two ways. Don't do this. Call one of them something else (such as i4).

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16. ### WBahn Moderator

Mar 31, 2012
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Correct.

Correct, but i2 will not be zero. I'm assuming that i2 is the current in the wire coming from R1. While no current will flow in R2, you will have current that comes from R1 and into the junction to the left of R2, flows down through the wire that is shorting R2, and the flows out of the junction to the right of R2 before proceeding on to the junction between R3 and R4.

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Jun 19, 2015
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Thank you!