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circuit

Knowing that I1 = 100mA, I3 = 300mA, UR2 = 10V, determine R2.

So I tried it like this: we know that in parallel connection, the voltage stays the same so UR2 = UR1 = 10V. Then, R1 = U1 / I1 = 100 Ohm. Also, I3 = I2 + I1 so I2 = 0.3A - 0.1A = 0.2A and so R2 = 10 / 0.2 = 50 Ohm.

Is this correct? I'm trying to learn for the exam but something doesn't seem right to me here.

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  • \$\begingroup\$ What is "U" in this context? \$\endgroup\$ – JYelton Sep 5 '13 at 17:17
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    \$\begingroup\$ @JYelton In Europe U is commonly used for voltage, to prevent confusion between the variable and the unit. \$\endgroup\$ – jippie Sep 5 '13 at 17:18
  • \$\begingroup\$ Edited, per meta.electronics.stackexchange.com/a/1453/2028 \$\endgroup\$ – JYelton Sep 5 '13 at 17:25
  • \$\begingroup\$ seems to be star network \$\endgroup\$ – AKR Sep 5 '13 at 17:43
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    \$\begingroup\$ Yes, use Kirchoff's current law and see immediately that I2 is 200mA. Since the voltage across R2 is given, Ohm's law tells you its resistance. Jippie gave you the right answer. Don't assume that it's a proper Y or that resistors are in parallel just because of the shape of the schematic. \$\endgroup\$ – Joe Hass Sep 6 '13 at 10:32
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Your answer is correct but your method too complicated. If you know I1 and I3, then the resistors R1 and R3 really don't matter. Apply Kirchoff's current law for I1, I2 and I3.

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  • \$\begingroup\$ I see. So by the Kirchoff's law, I can just skip the U_R2 and R_1 part and just calculate I2=I3-I1 which leads straight to the answer, right? Thank you a lot. \$\endgroup\$ – Straightfw Sep 5 '13 at 17:16
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If I2= 300-100= 200mA and V2=10V then R2= 10/.2= 50.

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