Engineering Basic circuit problem -- Resistor network and a voltage source

[edgarpokemon]

Homework Statement

http://www.chegg.com/homework-help/questions-and-answers/problem-2-variableh-resistor-r-circuit-fig-p224-adjusted-va-equals-60-v-find-value-r-q8083343

Homework Equations

The Attempt at a Solution

Hi, I attached a pdf file with the problem. My question is, for the lower right loop, why is the equation that they have valid? I worked the problem, but the equation that I got for that loop is...

voltage of 180 ohm + voltage of 30 ohm + voltage of 12 ohm + voltage of 18 ohm =0. so the voltage for the 180ohm, is -180V, but they have a positive 180V. How did they got that?

 

[edgarpokemon]

Homework Statement

http://www.chegg.com/homework-help/questions-and-answers/problem-2-variableh-resistor-r-circuit-fig-p224-adjusted-va-equals-60-v-find-value-r-q8083343

Homework Equations

The Attempt at a Solution

Hi, I attached a pdf file with the problem. My question is, for the lower right loop, why is the equation that they have valid? I worked the problem, but the equation that I got for that loop is...

voltage of 180 ohm + voltage of 30 ohm + voltage of 12 ohm + voltage of 18 ohm =0. so the voltage for the 180ohm, is -180V, but they have a positive 180V. How did they got that?

I just noted something. Does the direction of the current affects the signs (+ or -)? For example, if i have a KVL with a loop going clockwise, and when it reaches a resistor, that has a current going in the opposite direction (to the left), does that matter? If a kvl with a loop going clockwise hits the + sign of a resistor, will it always have a + sign regardless of the direction of the current on that resistor?

 

[analogdesign]

Yes the direction of the current affects the sign. Typically you assume a direction and if you're wrong you'll get a negative answer. It's as simple as that.

 

[edgarpokemon]

Yes the direction of the current affects the sign. Typically you assume a direction and if you're wrong you'll get a negative answer. It's as simple as that.

but if a keep the negative answer, I get a different result?

 

[analogdesign]

No, the negative answer just means the current goes in the opposite direction from what you initially assumed. Remember current is a vector quantity with a magnitude and a direction.

 

[edgarpokemon]

Yes the direction of the current affects the sign. Typically you assume a direction and if you're wrong you'll get a negative answer. It's as simple as that.

look at this picture from my book. In this example, what you said is correct, the assumed direction of current affects the sign. But on the other picture that i posted, they are not doing this. I think they are using a counterclockwise loop for the lower left loop in the other picture, but that should not matter. I used a clockwise direction and i got a negative answer, they used a counterclockwise direction, and they got a positive.

 

[cnh1995]

Remember current is a vector quantity with a magnitude and a direction.

I agree with your explanation of the negative current in OP's answer, but current is NOT a vector.