# separating circuits series/parallel

Discussion in 'Homework Help' started by phantomvs, Jan 21, 2016.

1. ### phantomvs Thread Starter Member

Jan 19, 2016
39
1
Ok so here it goes i got this questions that asks me to find the voltage on R4
It tells me :
R1=100 ohms
R2= 1000 ohms
R3=500 ohms
R4= 1000 ohms
R5=500 ohms

So what i did is find the total resistance of R2 &R5 its 333 ohms
then added R1 + R3+R4 , So 100+333+1500 = 1933 ohms
Find the total amps At=0.10 amps
Then did R1 x At = 1.03 volts
and so on but i think i screwed up somewhere ?

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2. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,990
1,115
Notice that (R3 + R4) are also connected in parallel with R2 & R5.

3. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
You need to be more methodical in documenting your work. For instance, you say

then added R1 + R3+R4 , So 100+333+1500 ...

R1 = 100 Ω, so that's your first term.
R3 = 500 Ω, which is no where on the right.
R4 = 1000 Ω, which is nowhere on the right.
333 Ω, which is no where on the left.

What your equation tells me that you are doing is

100 Ω + 333 Ω + 1500 Ω = R1 + (R2||R5) + (R3 + R4)

Are R1, (R2||R5), and (R3 + R4) in series? Remember, to be in series, whatever current flows in one MUST flow in each of the others.

4. ### phantomvs Thread Starter Member

Jan 19, 2016
39
1
Your right, it thought R3 and R4 were in series but its parallel
So R1 is in series with the rest of the resistors.

So if i do 1/Rt= 1/R2 + 1/R5 + 1/R3 + 1/R4 = 166 ohms

Then add in the R1 166 + 100 = 266 ohms
I/R = 0.075 Amps for I-total ?

5. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
But are R2, R3, R4, and R5 all in parallel? Remember, components are in parallel only if the exact same voltage appears across all of them. Is the voltages across each of these four resistors the same?

You are trying to just throw values at regurgitated formulas without taking the time to think whether or not those formulas apply.

Take it step by step. Identify two components that are either in series or in parallel. Then combine just those two components and replace them with their equivalent. Then repeat this process until you get down to just a single equivalent component.

6. ### phantomvs Thread Starter Member

Jan 19, 2016
39
1
I think thats it , i am not seeing the whole picture.

From what i see, R3 and R4 are in series but parallel to R2 and R5

That would then be in series with R1 ?

Its the R3 and R4 that get me the way they are connected.

7. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
If R3 has some current flowing in it, must that same current also flow through R4? If so, then they are in series. So replace them with a single resistor, perhaps called R34, that is the series equivalent of R3 and R4. Redraw the circuit with that replacement. Then go from there.

8. ### hp1729 Well-Known Member

Nov 23, 2015
2,094
232
It might be easier to understand if you redraw the circuit as you go along step by step.
Figure R2 and R5 in parallel then redraw the circuit with R2||R5 as one resistor.
Figure R3 and R5 in series then redraw the circuit with R3+R4 as one resistor.
Then figure these two resistors in parallel.
Then add R1.

9. ### phantomvs Thread Starter Member

Jan 19, 2016
39
1
So i redrew it and from what i get , doesn't seem to make sense.

I drew R1 as 100 ohms and the put R34 in series with 1500ohms and put R25 in parallel at 333 ohms.

I think i got lost somewhere. Even when i add the R1 with R34.

I will try to post a pic.

10. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
Show us what you did step by step. A schematic is worth a thousand words.

11. ### phantomvs Thread Starter Member

Jan 19, 2016
39
1
this is what i was thinking, sorry for the cheap pic.

But what i did is
R2R5 (333) R34 (1500)

So 333 x1500 / 333+1500 Gives me 272.50 ohms

Then i added in R1 at 100 ohms so 372.50 ohms

At = V/R So 20v / 372.50 = 0.05 Amps Total

Any of this sound right ??

• ###### v2.jpg
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12. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
Again, take it one step at a time.

Draw a circuit in which R3 and R4 have been replaced by their series equivalent. It should look something like this example:

You are trying to do too much at once and it is clear you aren't ready for that yet and that you are making needless mistakes as a result. Don't worry, we were all there at one point and you will quickly move past the need to be so painstakingly careful.

Nov 23, 2015
2,094
232
Sounds good.

14. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
This verbal description doesn't come close to your schematic:

Nov 23, 2015
2,094
232

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16. ### phantomvs Thread Starter Member

Jan 19, 2016
39
1
Sorry about that pic, i uploaded wrong one.

I don`t have access to a pic right now, but this is what i did .

So basically i took R3 and R4 and added them together as its a series circuit.

Then created 3 parallel circuits . One under another .

So R2 1000 ohm parallel to R34 1500 Ohms parallel to R5 500 ohms.

Did the parallel formula and came with 277,77 ohms and added 100 ohms of R1.
Found the total amperage I equals 20v / 377.77 equals 0.05 amps

Now i know parallel circuits voltage is the same and series current is the same.

Original question asks for R4 volts , that is where i am stuck.
Unless i screwed up somewhere on simplifing the circuits.

Thanks again for your patience

17. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
If you could find the current going through R4, could you figure out the voltage across it?

If so, can you figure out a way to find the current going through R4?

18. ### hp1729 Well-Known Member

Nov 23, 2015
2,094
232
Finding R4 voltage.
After you find the total resistance of R2, R3, R4 and R5 consider a simple series circuit made of R1 and R2345. Find the voltage across R2345. Then consider the series circuit of R3 and R4 to find the current through R3 and R4, then the voltage across R4.

19. ### phantomvs Thread Starter Member

Jan 19, 2016
39
1
This is what i had drew

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20. ### hp1729 Well-Known Member

Nov 23, 2015
2,094
232
Super. 20 V did you say? Total resistance divided into voltage gives you current. Current times R2345 ohms gives you voltage across R2345.
R2345 voltage divided by R34 voltage give you R34 current,
R34 current times R4 gives you voltage across R4.

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