KVL/KCL FE Electrical Question

[wbishop256]

The question is asking what's the current through the 20 Ohm resistor. To find that, I just divide 100V by 20 Ohm, as explained by the solution.

Could someone explain why the 200V is not factored into the solution? I don't understand how it's not running current into the 20 Ohm loop.

Thanks.

 

[pgheng]

It's a tapped 300V battery. All 300V goes across the 40 ohm, but only 100V goes across the 5, 10, and 20 ohm resistors. The current through the 20 ohm is the same current that flows through the 5 and 10 ohm resistors, because all 3 are connected in series. Current is 100V / (5 + 10 + 20) Ohms = 2.8571A.

 

[wbishop256]

Thanks for the quick response!

Could you please elaborate why only 100V goes across the 5, 10, and 20 ohm resisters in the previous example, yet the two current generators in the following example feed into all resisters in this new circuit:



Is it because they're current sources instead of voltage sources? If so, and you replaced the current sources with voltage sources in the example, do they not reach the far side resister? (In this new example: V1 not reaching R2 and V2 not reaching R3).

Thanks.

 

[pgheng]

Look at the first schematic. The only voltage applied across the three resistors (5, 10, and 20 Ohm) is the 100V battery. The 40 Ohm has 300V applied across it.
This example is different. Each side is balanced with 3A * 1 Ohm = 3V on the left side, and 3 Ohm * 1A = 3V on the right side. 3V on both sides of R1 makes for a zero voltage drop across R1 and zero current flowing through R1.
All of the current from I1 goes through R3, and all of the current from I2 goes through R2.