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Let the circuit be:

enter image description here

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.

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  • \$\begingroup\$ 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. \$\endgroup\$
    – Null
    Sep 5, 2014 at 19:40
  • \$\begingroup\$ @Null, yeah. This is what happened. \$\endgroup\$ Sep 5, 2014 at 19:44

2 Answers 2

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There is a small miscalculation. The transfer function for the dependent current source is

$$ 0.5\cdot V_y $$

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.

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  • \$\begingroup\$ I misinterpreted 0.5Vy as a voltage. Thanks. \$\endgroup\$ Sep 5, 2014 at 19:46
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The current Iy is 5A (5A = 0.5 \$\cdot\$ Vy A) , so the power is 5A \$\cdot\$ 10V = 50W.

Since current is flowing into the positive terminal, it is absorbing power.

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