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Finding the currents in this system

  • Thread starter Dell
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  • #1
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given the following:
http://lh6.ggpht.com/_H4Iz7SmBrbk/Sg04JIIg9vI/AAAAAAAAA-o/xDycbG90Gmc/s720/C.jpg [Broken]

and given that

ε1=3V
ε2=2V
R1=1Ω
R2=2Ω
R3=3Ω

i am asked to find the current flowing throiught R1,R2, R3

what i did was

I3=I2+I1
I1R1-I2R21

but i am short of one equation to solve this, what i would like to do is somehow find R12, as an the sum of R1 and R2, but my problem is that i have ε1 between them and i have no idea how to tackle this,

had ε1 noot been there i would have said R12=(1/R1+1/R2)-1 and then i could make a third equation using this.

how can i solve this?
 
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Answers and Replies

  • #2
tiny-tim
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I3=I2+I1
I1R1-I2R21

but i am short of one equation to solve this, what i would like to do is somehow find R12, as an the sum of R1 and R2, but my problem is that i have ε1 between them and i have no idea how to tackle this,
Hi Dell! :smile:

Stop trying to simplify it … it's simple enough already! :rolleyes:

There are three loops, of which two are independent …

you've already applied Kirchhoff to the R1R2-loop, so now just apply it to the R2R3-loop. :wink:
 
  • #3
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thanks,.. for the second part of the question they ask what the difference in voltage is from pioint A which lies between R3 and ε2, and point B which is between R1 and ε1,

can i use kirchoff here too, and igonre the part with R2 and R3, so I1R1=ε1-v
v=ε1-I1R1=2.8

?
 
  • #4
tiny-tim
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thanks,.. for the second part of the question they ask what the difference in voltage is from pioint A which lies between R3 and ε2, and point B which is between R1 and ε1,

can i use kirchoff here too, and igonre the part with R2 and R3, so I1R1=ε1-v
v=ε1-I1R1=2.8

?
sorry, I don't understand that :confused: … and what happend to E2?

You need the potential difference across E2 and across R1 (or across E1 and across R3):smile:
 
  • #5
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sorry meant to be v=ε2-I1R1=2-0.2=1.8,

but is it okay to just ignore the whole part that includes R2 and I2??
 
  • #6
tiny-tim
Science Advisor
Homework Helper
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sorry meant to be v=ε2-I1R1=2-0.2=1.8,

but is it okay to just ignore the whole part that includes R2 and I2??
Yup … Kirchhoff applies to only one loop at a time! :smile:
 

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