Simple Power in a circuit problem

[ravenprp]

Homework Statement

Determine the missing quantity in the circuits in figure below.

Homework Equations

P = IV = V^2 / R
V = IR

The Attempt at a Solution

From the diagram, I know the current is flowing opposite to the reference direction (because of the negative current). I used the equation, P = VI.

So,

-20 W = V (-2 A)... I get V is equal to 10 V. It says something is wrong, what am I doing wrong?

I know since the power is negative, and due to Passive Sign Convention, it is supplying power. I'm kind of stuck as to why it doesn't work out the way I did it. Thanks.

 

[ravenprp]

I'm sorry, I re-read the text in the mean while and I figured this out. I forgot to change the polarities and switch V_1 to -(V_1).

 

[piercas]

There could be a few reasons why your solution is not working out. First, make sure you are using the correct signs for the voltage and current values. In this case, the negative sign for the current indicates that it is flowing in the opposite direction of the reference direction, but the voltage should still be positive.

Another possibility is that you are not considering the direction of the current when calculating power. In this circuit, the negative power indicates that the source is absorbing power, not supplying it. This means that the voltage and current values should be multiplied together with the opposite signs, since they are going in opposite directions.

Finally, it could be a simple calculation error. Double-check your calculations to make sure you are using the correct values and performing the operations correctly.

Remember, in science and engineering, it is important to always check your work and make sure your solutions make sense. If you are still having trouble, don't be afraid to ask for help from a teacher or classmate.