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I am learning about step response in RC and RL circuits and one of the problems i managed to redraw as the image above; however, when I am solving for i the answer is -5mA. I am just confused on why it is negative as soon in the image, I understand that I just use ohms law here to attain i. In other words how do I add the two voltage sources? I keep getting confused because natural i would have put 120-5 which yields a positive. Can anyone explain why it is 5-120?

  • \$\begingroup\$ You used 120V node as your reference, so the current is negative. If you use the 5V node as your reference, the sign would have been positive. Notice that the arrow for i points in the correct way to find -5mA, in that regard your found answer is correct. \$\endgroup\$ – jippie Apr 14 '15 at 6:59
  • \$\begingroup\$ sorry I still do not comprehend, may you please explain a bit further? The way I see it is that the current is entering +120 and then hits -5, which is why I have the urge to have 155, not -155. May you please explain it how you go about it? And is the 120 node the reference because the current is traveling to it first? \$\endgroup\$ – Learn Apr 14 '15 at 7:09

When you nodal analysis, you have to specify the direction of currents before you even begin. In your example you have specified that the current goes from the right side of your schematic to the left. Not a problem.

Conventionally, we think of current going from a positive voltage to a negative voltage. If you look at your circuit, you have a 120V source on the left, and a 5V source on the right, but the direction of your current is the opposite of conventional current flow. The great thing about this, is that it does not matter! The math still holds, and the sign will tell you which way conventional current flows.

If your calculate show yields a negative current, it just means that the current is actually going in in the opposite direction you have defined. If the current is positive, then the direction you have specified initially.


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