Finding the current in a circuit

  • Thread starter kliker
  • Start date
  • #1
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Homework Statement



[PLAIN]http://img156.imageshack.us/img156/2420/physicsm.jpg [Broken]

sorry for the image, i made it very quickly

in the following circuit find

a) the current in R
b) R
c) Emf
d) if we cut the circuit in X, then what would be the current that goes through the 28V battery

Homework Equations



V = I*R

The Attempt at a Solution



a)
i used the first kirchhof law and i found that

I3 = I2 - I1 = 6 - 4 = 2A

b)i used the second law of kirchhof and found that

Emf - 18 - 24 = 0 => Emf = 42 V

c)again i used the second law and found that

28 - 42 + 24 - I3*R = 0 => R = 5 Ω

d) i dont know how to do it, please help
 
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Answers and Replies

  • #2
648
2
If you cut the circuit at X then there will be no current in that branch of the circuit, and you are just left with the top and bottom branch. A much simpler problem because you have already calculated R.
(I haven't checked your answers to the other parts of the question.)
 
  • #3
104
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If you cut the circuit at X then there will be no current in that branch of the circuit, and you are just left with the top and bottom branch. A much simpler problem because you have already calculated R.
(I haven't checked your answers to the other parts of the question.)

hi, thanks for the answer

but if we have only the bottom then it will be

28 - 3*6 - I*5 = 0 => 28 - 18 - I*5 = 0 => 10 = 5I => I = 2

but the book has 3.5 A :frown:
 
  • #4
2,073
332
hi, thanks for the answer

but if we have only the bottom then it will be

28 - 3*6 - I*5 = 0 => 28 - 18 - I*5 = 0 => 10 = 5I => I = 2

but the book has 3.5 A :frown:

The current through the bottom resistance is no longer equal to 6A after you
cut the circuit at X.
 
  • #5
648
2
hi, thanks for the answer

but if we have only the bottom then it will be

28 - 3*6 - I*5 = 0 => 28 - 18 - I*5 = 0 => 10 = 5I => I = 2

but the book has 3.5 A :frown:

Why are you using a current of 6A in the 3ohm resistor, and an unknown current of I? There is only one current now and this is the new current you need to find.
 
  • #6
104
0
ok I understand it now, thanks everyone

so the answer will be

Vab = I*(5+3)

but E = Vab

so I = 28/8 = 3.5

thanks again :)
 

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