How did I calculate the power for this source?

Let the circuit be:

Given that Ix = 5A and Vy = 10V, calculate the power absorbed or delivered by each source.

My solution:

10A source delivers 100W (P=UI= (-10 + 20)x10 = 10 x 10 = 100W). Correct

-10V source delivers 50W (P=UI= 10 x 5= 50W). Correct

20V source absorbs 100W (P=UI= 20 x 5= 100W). Correct

0.5Vy source delivers 25W (10 = Ix + Iy, Iy = 10 - Ix = 10 - 5 = 5 and P=UI= (0.5Vy)x Iy= 0.5 x 10 x 5) = 25W). Incorrect

My teacher said that the dependent source 0.5Vy absorbs 50W, while my answer is that it delivers 25W.

I couldn't figure out why my answer is wrong.

• The circuit diagram is misleading. $0.5V_y$ is actually the magnitude of the current from the dependent current source (because of the "A" next to it to indicate Amperes), but you misinterpreted $0.5V_y$ as a voltage.
– Null
Sep 5, 2014 at 19:40
• @Null, yeah. This is what happened. Sep 5, 2014 at 19:44

$$0.5\cdot V_y$$
so if, $V_y = 10$, then the current through the source is 5A, multiplied for 10 V ($V_y$), is 50 W. The voltage polarity of $V_y$ and direction of current $I_y$, indicating that the source is absorbing power.
The current Iy is 5A (5A = 0.5 $\cdot$ Vy A) , so the power is 5A $\cdot$ 10V = 50W.