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Please help me solve this combo circuit (series-parallel) Check the picture. It has 4...

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supersmartpants | (Level 1) Salutatorian

Posted June 3, 2013 at 3:06 AM via web

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Please help me solve this combo circuit (series-parallel)

Check the picture.

It has 4 resistors. 

Given 

I total = 0.4A

R1 = 10 ohms

R3 = 20 ohms

V4 = 6V

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2 Answers | Add Yours

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pixelgera | High School Teacher | eNotes Newbie

Posted June 3, 2013 at 7:04 AM (Answer #1)

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R2 AND R4 CAN HAVE A SET OF ANY VALUES

eg:- R2 =40 ohm, R4=20 ohm or other values satisfying relationship:-

 

R4( R2+20)

-------------  =  15   

(R2+R4 +20)

hope this helps.

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crmhaske | College Teacher | (Level 3) Associate Educator

Posted June 3, 2013 at 12:51 PM (Answer #2)

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For series resistors, the equivalent resistance is the summation of the resistances. Therefore:

`R_5=R_2+R_3`

For parallel resistors, the equivalent resistance is 1 over the sumation of 1 over the resistances:

`1/R_6=1/R_5+1/R_4`

`R_6=(R_5R_4)/(R_5+R_4)`

`=(R_4(R_2+R_3))/(R_2+R_3+R_4)`

We know that the voltage across R6 is V4. Therefore, according to Ohm's Law:

`I_T = V_4/R_6`

`R_6=V_4/I_T`

`(R_4(R_2+R_3))/(R_2+R_3+R_4) = V_4/I_T`

Now, substitute what we know:

`(R_4(R_2+20))/(20+R_2+R_4)=6/0.4=15`

We don't have enough information to solve any further than that; therefore, `R_2 ` and `R_4` are any such values that satisfy the above equation.

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