Problem 4-37


Determine the power delivered by the 30 A source.


First, express the 30 A source in terms of mesh currents.

I3 = 30

Now removing the 30 A source leaves only two meshes, 1 and 2.

KVL around mesh 1.

4 I1 + 16 I1 - 16 I2 + 5.6 I1 - 5.6 I3 - 600 = 0

25.6 I1 - 16 I2 - 5.6 (30) = 600

25.6 I1 - 16 I2 = 768 (Eq. 1)

KVL around mesh 2.

3.2 I2 + 424 + 0.8 I2 - 0.8 I3 + 16 I2 - 16 I1 = 0

- 16 I1 + 20 I2 - 0.8 (30) = - 424

- 16 I1 + 20 I2 = - 400 (Eq. 2)

Solving the two equations yields:

I1 = 35 A

I2 = 8 A


Now, to find the power delivered by the 30 A source, find the voltage across it using KVL.

First find V1 and V1 with Ohm's Law.

V1 = 5.6 (I3 - I1) = 5.6 (- 5) = - 28 V

V2 = 0.8 (I3 - I2) = 0.8 (22) = 17.6 V

KVL yields:

VS = V1 + V2 = - 10.4 V

So the power delivered to the circuit by the 30 A source is

P = 30 (- 10.4) = - 312 W

so the 30 A source is actually absorbing power.


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