# Equivalent resistance between two nodes help

• JasonHathaway
In summary, the conversation is discussing the approach to finding the equivalent resistance between nodes A and B in a circuit with parallel and series resistors. The approach involves short circuiting the voltage source and using parallel and series combinations to find the equivalent resistance. The conversation also mentions the possibility of finding the Thevenin equivalent of the circuit. The correct approach for finding the equivalent resistance may depend on the specific problem statement and circuit diagram provided.
JasonHathaway

## Homework Equations

Parallel-Series resistors combinations.

## The Attempt at a Solution

Using an approach much like the one used to find the Thevinin resistance:

1- Open circuit between A and B (R1, R2, R3), and short circuit the voltage source.
2- (R4//R5)+R6=170
3- Returning R1, R2 and R3 and find their equivalent (R1//R2//R3=30).
4- Rab=170+30=200Is my approach correct? is the resistance between a and b the same as the equivalent resistance?

Edit:
Forgot to add the diagram+the statement

Last edited:
Was there supposed to be a problem statement or at least a circuit diagram?

Okay, the circuit diagram helps but...
What is the problem statement? What are you supposed to determine about the given circuit?

Sorry again, the question is: determine the resistance between A and B

JasonHathaway said:
Sorry again, the question is: determine the resistance between A and B

Okay, presumably that means they want you to find the equivalent resistance that represents the combination of resistors R1, R2, and R3. In other words, the equivalent resistance of this circuit segment:

Unless there's more to the problem such as the resistors between A and B are to be taken as a load and you're to find the Thevenin equivalent for the rest of the circuit that's driving that load...

#### Attachments

• Fig1.gif
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But aren't R4 and R5 also share the node B and should be considered?

P.S. Does the same approach you did applies on this circuit if the problem states: "Find the equivalent resistance between a and b as seen by the source"?

#### Attachments

• circuit2.jpg
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JasonHathaway said:
But aren't R4 and R5 also share the node B and should be considered?
Really it depends on what they mean by "between A and B". Unless they are asking for a Thevenin equivalent , I'd interpret it to mean the resistors as I've shown. It's the simplest interpretation without other information. Perhaps it's a slightly vaguely posed question, but that would be my interpretation.

P.S. Does the same approach you did applies on this circuit if the problem states: "Find the equivalent resistance between a and b as seen by the source"?

No, in that case the nodes a and b are where the source itself connects to the resistor network, so the logical interpretation would be that they want you to reduce the entire network down to a single equivalent resistance that represents the net load on the source.

1 person
Thanks :)

Just to be sure, If A and B were:
http://i.imgur.com/Kilpty3.jpg?1

Then, the equivalent resistance between A and B would be (90//90//90)+(200//2000)=130, isn't?

JasonHathaway said:
Just to be sure, If A and B were:
http://i.imgur.com/Kilpty3.jpg?1

Then, the equivalent resistance between A and B would be (90//90//90)+(200//200[STRIKE]0[/STRIKE])=130, isn't?

Sure. In general it's better to remove any ambiguity by indicating the intended portion of the circuit more specifically, say by drawing a circle around it. When circuits are more complex and other paths between the nodes in question are available, it's important to be able specify the intended path(s) and the excluded ones.

1 person

## 1. What is equivalent resistance?

Equivalent resistance is a measure of the total resistance in a circuit or between two nodes. It is the single resistance that would produce the same amount of current flow as the combination of all individual resistances in the circuit.

## 2. Why is equivalent resistance important?

Equivalent resistance is important because it allows us to simplify complex circuits and calculate the total resistance in a more efficient way. It also helps us understand the overall behavior of a circuit and make predictions about its performance.

## 3. How do you calculate equivalent resistance?

There are several methods for calculating equivalent resistance, depending on the type of circuit. For series circuits, the equivalent resistance is the sum of all individual resistances. For parallel circuits, it is the reciprocal of the sum of the reciprocals of each individual resistance. In more complex circuits, techniques such as Thevenin's theorem or Kirchhoff's laws can be used to calculate equivalent resistance.

## 4. What is the unit of measurement for equivalent resistance?

The unit of measurement for equivalent resistance is the ohm (Ω). This is the same unit used to measure individual resistances in a circuit.

## 5. How does equivalent resistance affect current flow?

The higher the equivalent resistance, the lower the current flow in a circuit. This is because a higher resistance impedes the flow of electrons, resulting in a smaller current. Conversely, a lower equivalent resistance will allow for a higher current flow in a circuit.

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