# Help with problem 7.26 please.

Discussion in 'Homework Help' started by thisonedude, May 28, 2014.

1. ### thisonedude Thread Starter Member

Apr 20, 2014
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0
My Impedance, from what i calculated, was 20+40j. I know power is V^2/2R. and my w = 2pi(400) = 800pi. Can i get some help?

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2. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,516
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Power=V*I
I=V/R
Power=V*(V/R)=(V^2)/R

Conclusion: You don't know power.

Also. Since you have current source, you are better of using current to find power:
Power=V*I
V=I*R
Power=(I*R)*I=R*I^2
This way you know the Equvalent Impedance of the circuit (R=20.166+40.124j) and you know the Current since it is given by the problem.

Last edited: May 28, 2014
3. ### thisonedude Thread Starter Member

Apr 20, 2014
52
0
That's what the problem is asking to find.

4. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,516
515
You have the impedance (R) in complex form. This part is done.

You have current (I) in time domain form. Change it to complex form. This is needed to be done.

Then use Power=R*I^2. This is needed to be done.

5. ### thisonedude Thread Starter Member

Apr 20, 2014
52
0
so if my impedence is 20+40j. And my current is 10cos(wt+27°) = 10e^(j27°). so then my current^2 is 100e^2(j27°) and my power is (20+40j)(100e^2(27°)) Does this seem/sound right?

6. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,516
515
10e^(j27°) this is Phasor Representation.

You have three choices:
* Time domain
* Phasor
* Complex

10cos(wt+27°) is Time domain

10e^(j27°) is Phasor

Complex is 10*cos(27°)+j*10*sin(27°) Once you find cos and sin, you will have current in the form of complex number (X+jY). So now the form that represents current matches the form that represents impedance. Since they are in the same form, you can do math with them.

7. ### thisonedude Thread Starter Member

Apr 20, 2014
52
0
Oh i see. So then if i do the math, my current is 8.91+j4.5. if i square this, i get 58.86+80.72j. So then i muliply that by 20+j40?

8. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,516
515
Yes.
The result is Complex Power: S=P+jQ
S is Complex Power
P is Real Power, unit W
Q is Reactive Power, unit VAR