Voltage from current across impedance

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SUMMARY

The discussion centers on the relationship between voltage (V), current (I), and impedance (Z) in AC circuits, specifically addressing the formula S=VI*=ZI^2 for calculating power. Participants clarify that the correct expression for voltage is V=ZI*, where I* represents the complex conjugate of the current. The importance of using the RMS (root mean square) value for current is emphasized, as negative values are not valid in this context. Misinterpretations of voltage calculations lead to confusion, particularly when not using the correct form of current.

PREREQUISITES
  • Understanding of AC circuit theory
  • Familiarity with complex numbers in electrical engineering
  • Knowledge of RMS (root mean square) calculations
  • Basic principles of impedance (Z) in electrical circuits
NEXT STEPS
  • Study the application of complex conjugates in electrical engineering
  • Learn about RMS calculations for AC currents
  • Explore the implications of impedance in AC circuit analysis
  • Review power calculations in AC circuits using S=VI*
USEFUL FOR

Electrical engineers, students studying AC circuit theory, and anyone involved in power calculations in electrical systems will benefit from this discussion.

perryben
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So the power is S=VI*=ZI^2.

the question is whether V=ZI* or just ZI. Using V=ZI when I calculate the voltage i get a negative answer.

Thanks!
 
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perryben said:
So the power is S=VI*=ZI^2.

the question is whether V=ZI* or just ZI. Using V=ZI when I calculate the voltage i get a negative answer.

Thanks!

Are you using the rms value for I? You'll need that. RMS (root mean square) cannot be negative here. Also, rms current is either greater than or equal to the DC current (mathematics of rms vs. average).
 
perryben said:
So the power is S=VI*=ZI^2.

the question is whether V=ZI* or just ZI. Using V=ZI when I calculate the voltage i get a negative answer.

Thanks!

Thread moved from EE to homework help.

Why would V = ZI* ? Once you have the complex Z and I, the complex V should just be the product of those two. There are times that you use the complex conjugate of a quantity, but I wouldn't think you would use it here...
 

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