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Find the total amount of current flowing through R3 in the circuit below.

I mainly have doubts whether the answer given in the book is correct, to me it doesnt seem correct. Heres my attempt anyway.

http://img409.imageshack.us/img409/7584/superposition2iu4.png [Broken]

Make VS1 a short.

Rt = R1//(R2//(R3+R4)

(R1 is in parallel with R2 which is in parallel with the series combination of R3 and R4).

Rt = 680//(220//(330+470) = 680//(220//800) = 680//172.54 = 138.89ohms

Total current flowing from current source = 100mA

IR3 = Rt/(R3+R4) * IS = 139/800 * 100mA = 17mA

Current is flowing left to right over R3.

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Make IS an open as it is a current source.

Rt = R2 + ((R3+R4)//R1)

(R3 and R4 are in series, this combination is in parallel with R1, R2 is in series with the rest of the circuit.)

Rt = 220 + (800//680) = 587

It = Vs1/Rt = 20/587 = 0.034A or 34mA

IR3 = Rt / (R3+R4) * It = 587/800 * 0.034 = 0.025A or 25mA

(i chose R3+R4 as the current flowing through a series combination is the same.)

As the polarity of both sources is the same (current flowing in same directions). The total current across R3 = 42mA

If there are any errors could someone please indicate them to me, any help would be very much appreciated.

I mainly have doubts whether the answer given in the book is correct, to me it doesnt seem correct. Heres my attempt anyway.

http://img409.imageshack.us/img409/7584/superposition2iu4.png [Broken]

Make VS1 a short.

Rt = R1//(R2//(R3+R4)

(R1 is in parallel with R2 which is in parallel with the series combination of R3 and R4).

Rt = 680//(220//(330+470) = 680//(220//800) = 680//172.54 = 138.89ohms

Total current flowing from current source = 100mA

IR3 = Rt/(R3+R4) * IS = 139/800 * 100mA = 17mA

Current is flowing left to right over R3.

-------------------------------------------------------------------------

Make IS an open as it is a current source.

Rt = R2 + ((R3+R4)//R1)

(R3 and R4 are in series, this combination is in parallel with R1, R2 is in series with the rest of the circuit.)

Rt = 220 + (800//680) = 587

It = Vs1/Rt = 20/587 = 0.034A or 34mA

IR3 = Rt / (R3+R4) * It = 587/800 * 0.034 = 0.025A or 25mA

(i chose R3+R4 as the current flowing through a series combination is the same.)

As the polarity of both sources is the same (current flowing in same directions). The total current across R3 = 42mA

If there are any errors could someone please indicate them to me, any help would be very much appreciated.

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