# Calculating the resistance

1. Nov 20, 2012

### [KNIGHT]

1. The problem statement, all variables and given/known data
Calculate the total resistance of the 2 circuits given below

2. Relevant equations
R = R1+R2+R3..........
(1/R) = (1/R1) + (1/R2) + (1/R3) + .............

3. The attempt at a solution
I am unable to understand how to obtain the final resistance

2. Nov 20, 2012

### Staff: Mentor

General strategy: Look for pairs of resistors that are purely in parallel or in series. Then replace those pairs with their equivalent resistance. Then do it again, until you've simplified it enough.

3. Nov 20, 2012

### [KNIGHT]

After following what you said i got 25/84 Ohms as the answer for the 2nd circuit. Is it correct?

But the 1st one is still confusing because of the connection in between

4. Nov 20, 2012

### Susagep

Rt = R1 * R2 / R1 + R2

Give arbitrary values to each resistance, for example 100 Ohms each. Find a parallel pair and evaluate:Example 2 parallel resistor [Items 1 & 2] 100 Ohms each equals 50 Ohms, that is 100 Ohms divided by 2 resistors. Now add that value to the next resistor in series [ Item 4], you now have 150 Ohms. Find the series branch and add the resistance which equal 200 Ohms Item 7 & 5]. You now have 150 Ohms in parallel with 200 Ohms, evaluate them both in parallel and you arrived at a value of 85.714 Ohms for the entire circuit. Follow the same procedure for circuit # 1

Last edited: Nov 20, 2012
5. Nov 20, 2012

### Staff: Mentor

It looks like you've forgotten to take a reciprocal somewhere. Check your work, or better yet, post it here so others can check.
Can you identify any candidates for parallel or serial connections?

6. Nov 20, 2012

### [KNIGHT]

Thanks for pointing it out. I have made a simplification error. So the answer is 84/25 Ohms. Is it correct now?

Can i consider 3 & 2 as the 1st pair of parallel resistors and 6 & 7 as the 2nd pair of parallel resistors.
Thereafter should i add the 1st and 2nd pair in series?

Btw its the resistance values that are marked in the diagram

Last edited: Nov 20, 2012
7. Nov 20, 2012

### Staff: Mentor

Yes, the result is correct. I cannot comment on your methodology since you did not show it.

8. Nov 20, 2012

### [KNIGHT]

Here is the way i did it
considered 1 & 2 as a parallel pair and found their resistance and added it in series with 4. (group 1)
Then i added 7 & 5 in series and considered it as group 2.
The final answer was obtained by adding group 1 and 2 in parallel

9. Nov 20, 2012

### Staff: Mentor

Yes, that is a correct method.

10. Nov 21, 2012

### [KNIGHT]

What about the method to solve the 1st first? Is it correct?

11. Nov 21, 2012

### Staff: Mentor

You'll have to be more explicit.

Any correct method should arrive at the same result...

12. Nov 21, 2012

### Staff: Mentor

Yes. Assuming that this was your method for the 1st circuit:
That is a good approach.