Physics Midterm Explanation Kirkoff Rule

[Nimmy]

Its the part c question...which states

C) Suppose we made V = 0. Find the new current running through the 5.00 ohm resistor.So I just took the biggest loop and did it this way

Sum of V = 0

So starting at the upper left hand corner of the circuit

-3A(15 Ohms) + 50 V + I (5) = 0

-45 + 50 +5I = 0

5 +5I = 0
I = -1 A

Which is not correct. The answer I think is supposed to be 2.5 Amps but can you tell me what mistake I'm making here.

 

[Doc Al]

I assume the currents labeled in the diagram are from an earlier part of the problem. You can't just assume that the currents remain the same.

 

[nlightner]

The mistake you are making is that you are not taking into account the direction of the current in the 5.00 ohm resistor. Since the voltage is set to 0, the current should flow in the opposite direction of the original current in that section of the circuit. So the correct equation would be:

-3A(15 Ohms) + 50 V - I(5) = 0

-45 + 50 - 5I = 0

5 - 5I = 0

-5I = -5

I = 1 A

This means that the current in the 5.00 ohm resistor is 1 A in the opposite direction of the original current. This makes sense because the voltage is set to 0, so the current should be in the opposite direction to maintain a potential difference of 0. Therefore, the total current in the circuit is 2 A (1 A in the 5.00 ohm resistor and 3 A in the 15 ohm resistor). This is consistent with the answer of 2.5 A given in the question, as there may be some rounding or approximations involved in the calculation. In summary, the mistake you made was not considering the direction of the current in the 5.00 ohm resistor when setting up the equation.