The sign of the current and voltage

[525arneh]

I was doing the following problem:

And I was asked to find Iy.

I found Iy to be 2.64 using KCL. However, the right answer was negative 2.64.

Is it negative only because there is a dependent voltage source with "+ -" ? And why must it be negative? Does "-" in the final answer play a significant role? If I put positive 2.64 instead of negative, would that be wrong?

Also, here is another question

I found V1 to be 11.9. However, the right answer was - 11.9.

Can someone clarify for me when do you put negative sign ?

 

[axmls]

If I put positive 2.64 instead of negative, would that be wrong?

Yes, because the problem has chosen which direction the current flows for you. If the current is actually flowing in the other direction, you will get a negative value for the current because of the chosen reference direction.

 

[NascentOxygen]

I found Iy to be 2.64 using KCL.

You made a mistake, but by not showing your working you aren't allowing anyone to point it out.

Quite possibly you didn't solve for I_y, but instead solved for current in the direction opposite to the direction indicated for I_y. If so, that is quite okay, but you must remember before stating your answer to change its sign to correspond with the direction for I_y as specified in the given figure. Achieving the correct polarity is as essential as determining the correct magnitude; direction determines whether the designer needs to provide a power source or a power sink.

 

[CWatters]

+1 Show your working. Sign errors are very common when using KVL. I find it helps to write equations that sum to zero eg write them in the form..

V1 + V2 ...+ Vn = 0