# How to calculate the equivalent resistance in this Circuit?

## Homework Statement:

Calculate the equivalent resistance. Is it possible to calculate an exact value?

## Relevant Equations:

Series: R1+R2+R3....Rn = Req
Parallel: (1/Req)=(1/R1)+(1/R2)…(1/Rn)
I start from far right:
Green and blue are parallel Connection: This in turn is in series connection with black resistor: and so on ... This is what I'm ending up with My final equivalent resistance is (275/17) Ω. Is this correct?

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Homework Statement:: Calculate the equivalent resistance. Is it possible to calculate an exact value?
Relevant Equations:: Series: R1+R2+R3....Rn = Req
Parallel: (1/Req)=(1/R1)+(1/R2)…(1/Rn)

Green and blue are parallel Connection
What about the black resistor? Either this was a simple oversight, or you need to review what parallel resistors are.

Also, take note that this circuit goes on forever to the right side. There is no "right most" resistor. You said you started at the right; I think you started somewhere in the middle.

Oh right, I get it now. So I will never be able to calculate an exact value because the Circuit is never ending. Maybe I should start from the left and work my way up to at least five more parallel resitors just to show that.

TSny
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Gold Member
Oh right, I get it now. So I will never be able to calculate an exact value because the Circuit is never ending. Maybe I should start from the left and work my way up to at least five more parallel resitors just to show that.
Actually, you can calculate the exact value. There is a trick to getting the answer that might be hard to see if you've never seen the trick before! It's also hard to give a hint without giving the whole thing away. You are interested in the total resistance of the network shown in figure (a) below: Consider the part of this network that is to the right of the brown resistor, as shown in (b).

You can model all of the stuff to the right of some arbitrary point as a single resistor of (initially) unknown value. Then look for patterns. It's sort of like the infinite series calculations that you may have seen in your math classes.

What about the black resistor? Either this was a simple oversight, or you need to review what parallel resistors are.
Actually, you can calculate the exact value. There is a trick to getting the answer that might be hard to see if you've never seen the trick before! It's also hard to give a hint without giving the whole thing away. You are interested in the total resistance of the network shown in figure (a) below:
View attachment 258264

Consider the part of this network that is to the right of the brown resistor, as shown in (b).
Actually, you can calculate the exact value. There is a trick to getting the answer that might be hard to see if you've never seen the trick before! It's also hard to give a hint without giving the whole thing away. You are interested in the total resistance of the network shown in figure (a) below:
View attachment 258264

Consider the part of this network that is to the right of the brown resistor, as shown in (b).
The equivalent resistance for orange and red: The equivalent resistance for 5Ω-resistor and Purple:
5Ω+10Ω = 15Ω

The equivalent resistance for 15Ω-resistor and blue: The equivalent resistance for 6Ω-resistor and yellow:
6Ω+10Ω=16Ω

The equivalent resistance for 16Ω-resistor and green: The equivalent resistance for 80/13Ω-resistor and gray: The equivalent resistance for 210/13Ω-resistor and black: If this calculation is correct , do you mean that this pattern will repeat itself?

TSny
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The equivalent resistance for orange and red:
View attachment 258269
As @DaveE pointed out earlier, the orange and red resistors are not in parallel due to the yellow resistor between them. So, you cannot combine them to get 5Ω equivalence.

Going back to the picture in post #5, how does the total resistance of figure (b) compare to the total resistance of figure (a)?

• scottdave
scottdave
Homework Helper
It is sort of like this puzzle:
$$x = 1+ \frac { 1 } { 1 + \frac {1} {1 + ...}}$$
Can you find the value of ##x## ?
If you can figure how to do that one, then the same type of thinking can be applied to the circuit network.

• TSny
It is sort of like this puzzle:
$$x = 1+ \frac { 1 } { 1 + \frac {1} {1 + ...}}$$
Can you find the value of ##x## ?
If you can figure how to do that one, then the same type of thinking can be applied to the circuit network.
less than 2?
As @DaveE pointed out earlier, the orange and red resistors are not in parallel due to the yellow resistor between them. So, you cannot combine them to get 5Ω equivalence.

Going back to the picture in post #5, how does the total resistance of figure (b) compare to the total resistance of figure (a)?
I guess the total resistance in figure b is less than a?

As @DaveE pointed out earlier, the orange and red resistors are not in parallel due to the yellow resistor between them. So, you cannot combine them to get 5Ω equivalence.

Going back to the picture in post #5, how does the total resistance of figure (b) compare to the total resistance of figure (a)?
Then purple must be in series with orange, yellow with red, grey with blue and finally black with green, or is it wrong?

TSny
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I guess the total resistance in figure b is less than a?
Can you find any difference in the networks of (a) and (b)? [Other than the colors! ]

TSny
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Then purple must be in series with orange, yellow with red, grey with blue and finally black with green, or is it wrong?
None of the individual resistors in the network are in series or parallel.

Perhaps it would help if you first solve the finite circuit that only has the left-most 4 resistors. Then solve it for the left-most 6 resistors, then 8 ... Look for a pattern. How does adding 2 more resistors to the previously solved version change the answer?

scottdave
Homework Helper
Less than 2 is a good guess. I really don't want to give it away @VitaminK . If you can arrive at the method of solving with just a little prompting, it will help your problem solving skills, because you'll probably remember it better.
$$x = 1+ \frac { 1 } { 1 + \frac {1} {1 + ...}}$$

So look at just the denominator of the first 1. How would you compare that expression to the whole expression?

• hutchphd
Less than 2 is a good guess. I really don't want to give it away @VitaminK . If you can arrive at the method of solving with just a little prompting, it will help your problem solving skills, because you'll probably remember it better.
$$x = 1+ \frac { 1 } { 1 + \frac {1} {1 + ...}}$$

So look at just the denominator of the first 1. How would you compare that expression to the whole expression?
The expression in the denominator is the same as the whole expression?

• scottdave
scottdave
Homework Helper
The expression in the denominator is the same as the whole expression?
So using that information, you could solve for an actual value of x.

Now how can you apply that thinking to the resistors. Is there a portion of the whole network, which looks like the whole network. Then we can go about solving for the value of "X" resistance.

Can you find any difference in the networks of (a) and (b)? [Other than the colors! ]
two resistors less

So using that information, you could solve for an actual value of x.

Now how can you apply that thinking to the resistors. Is there a portion of the whole network, which looks like the whole network. Then we can go about solving for the value of "X" resistance.
I guess that would be the resistors in b)

TSny
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Gold Member
I guess that would be the resistors in b)
Yes something like this?

TSny
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If R is the unknown equivalent resistance of (a), what is the equivalent resistance of (b)?

It's R

TSny
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It's R
Yes. Can you redraw network (a) so that it takes this into account and becomes a simpler network? like this?